Misplaced Pages

#214785

171-499: The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals . It consists of nervous tissue and is typically located in the head ( cephalization ), usually near organs for special senses such as vision , hearing and olfaction . Being the most specialized organ, it is responsible for receiving information from the sensory nervous system , processing those information ( thought , cognition , and intelligence ) and

342-408: A central lobe , a limbic lobe , and an insular lobe . The central lobe comprises the precentral gyrus and the postcentral gyrus and is included since it forms a distinct functional role. The brainstem , resembling a stalk, attaches to and leaves the cerebrum at the start of the midbrain area. The brainstem includes the midbrain, the pons , and the medulla oblongata . Behind the brainstem

513-455: A cephalic flexure . This flexed part becomes the forebrain (prosencephalon); the adjoining curving part becomes the midbrain (mesencephalon) and the part caudal to the flexure becomes the hindbrain (rhombencephalon). These areas are formed as swellings known as the three primary brain vesicles . In the fifth week of development five secondary brain vesicles have formed. The forebrain separates into two vesicles – an anterior telencephalon and

684-462: A common ancestor that appeared late in the Cryogenian period, 700–650 million years ago, and it has been hypothesized that this common ancestor had the shape of a simple tubeworm with a segmented body. At a schematic level, that basic worm-shape continues to be reflected in the body and nervous system architecture of all modern bilaterians, including vertebrates. The fundamental bilateral body form

855-409: A complex developmentally predetermined process which generates patterns of folds that are consistent between individuals and most species. The first groove to appear in the fourth month is the lateral cerebral fossa. The expanding caudal end of the hemisphere has to curve over in a forward direction to fit into the restricted space. This covers the fossa and turns it into a much deeper ridge known as

1026-470: A gradient of size and complexity that roughly follows the evolutionary sequence. All of these brains contain the same set of basic anatomical structures, but many are rudimentary in the hagfish, whereas in mammals the foremost part ( forebrain , especially the telencephalon ) is greatly developed and expanded. Brains are most commonly compared in terms of their mass . The relationship between brain size , body size and other variables has been studied across

1197-568: A lighter color. Further information can be gained by staining slices of brain tissue with a variety of chemicals that bring out areas where specific types of molecules are present in high concentrations. It is also possible to examine the microstructure of brain tissue using a microscope, and to trace the pattern of connections from one brain area to another. The brains of all species are composed primarily of two broad classes of brain cells : neurons and glial cells . Glial cells (also known as glia or neuroglia ) come in several types, and perform

1368-533: A massive expansion of the cerebral cortex, especially the prefrontal cortex and the parts of the cortex involved in vision . The visual processing network of primates includes at least 30 distinguishable brain areas, with a complex web of interconnections. It has been estimated that visual processing areas occupy more than half of the total surface of the primate neocortex. The prefrontal cortex carries out functions that include planning , working memory , motivation , attention , and executive control . It takes up

1539-419: A microscope . The cortex is divided into two main functional areas – a motor cortex and a sensory cortex . The primary motor cortex , which sends axons down to motor neurons in the brainstem and spinal cord, occupies the rear portion of the frontal lobe, directly in front of the somatosensory area. The primary sensory areas receive signals from the sensory nerves and tracts by way of relay nuclei in

1710-441: A much larger proportion of the brain for primates than for other species, and an especially large fraction of the human brain. The brain develops in an intricately orchestrated sequence of stages. It changes in shape from a simple swelling at the front of the nerve cord in the earliest embryonic stages, to a complex array of areas and connections. Neurons are created in special zones that contain stem cells , and then migrate through

1881-491: A neuron is in place, it extends dendrites and an axon into the area around it. Axons, because they commonly extend a great distance from the cell body and need to reach specific targets, grow in a particularly complex way. The tip of a growing axon consists of a blob of protoplasm called a growth cone , studded with chemical receptors. These receptors sense the local environment, causing the growth cone to be attracted or repelled by various cellular elements, and thus to be pulled in

SECTION 10

#1732764945215

2052-467: A number of critical functions, including structural support, metabolic support, insulation, and guidance of development. Neurons, however, are usually considered the most important cells in the brain. In humans, the cerebral cortex contains approximately 14–16 billion neurons, and the estimated number of neurons in the cerebellum is 55–70 billion. Each neuron is connected by synapses to several thousand other neurons. The property that makes neurons unique

2223-454: A particular direction at each point along its path. The result of this pathfinding process is that the growth cone navigates through the brain until it reaches its destination area, where other chemical cues cause it to begin generating synapses. Considering the entire brain, thousands of genes create products that influence axonal pathfinding. The synaptic network that finally emerges is only partly determined by genes, though. In many parts of

2394-412: A posterior diencephalon . The telencephalon gives rise to the cerebral cortex, basal ganglia, and related structures. The diencephalon gives rise to the thalamus and hypothalamus. The hindbrain also splits into two areas – the metencephalon and the myelencephalon . The metencephalon gives rise to the cerebellum and pons. The myelencephalon gives rise to the medulla oblongata. Also during the fifth week,

2565-510: A previously existing brain structure. This category includes tardigrades , arthropods , molluscs , and numerous types of worms. The diversity of invertebrate body plans is matched by an equal diversity in brain structures. Two groups of invertebrates have notably complex brains: arthropods (insects, crustaceans , arachnids , and others), and cephalopods (octopuses, squids , and similar molluscs). The brains of arthropods and cephalopods arise from twin parallel nerve cords that extend through

2736-415: A set of structures deep within the hemispheres involved in behaviour and movement regulation. The largest component is the striatum , others are the globus pallidus , the substantia nigra and the subthalamic nucleus . The striatum is divided into a ventral striatum, and dorsal striatum, subdivisions that are based upon function and connections. The ventral striatum consists of the nucleus accumbens and

2907-443: A subtype of oligodendrocyte progenitor cells . Astrocytes are the largest of the glial cells. They are stellate cells with many processes radiating from their cell bodies . Some of these processes end as perivascular endfeet on capillary walls. The glia limitans of the cortex is made up of astrocyte endfeet processes that serve in part to contain the cells of the brain. Mast cells are white blood cells that interact in

3078-416: A thickened strip called the neural plate . By the fourth week of development the neural plate has widened to give a broad cephalic end, a less broad middle part and a narrow caudal end. These swellings are known as the primary brain vesicles and represent the beginnings of the forebrain (prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon). Neural crest cells (derived from

3249-416: A variety of organ systems . These specific systems are widely studied in human anatomy . The functions of these organ systems often share significant overlap. For instance, the nervous and endocrine system both operate via a shared organ, the hypothalamus . For this reason, the two systems are combined and studied as the neuroendocrine system . The same is true for the musculoskeletal system because of

3420-485: A way that reflects in part the complexity of their behavior. For example, primates have brains 5 to 10 times larger than the formula predicts. Predators , who have to implement various hunting strategies against the ever changing anti-predator adaptations , tend to have larger brains relative to body size than their prey. All vertebrate brains share a common underlying form, which appears most clearly during early stages of embryonic development . In its earliest form,

3591-687: A whole, are referred to as the Organon because logic is a tool for philosophical thinking. Earlier thinkers, such as those who wrote texts in the Hippocratic corpus , generally did not believe that there were organs of the body but only different parts of the body. Some alchemists (e.g. Paracelsus ) adopted the Hermetic Qabalah assignment between the seven vital organs and the seven classical planets as follows: Chinese traditional medicine recognizes eleven organs, associated with

SECTION 20

#1732764945215

3762-482: A wide range of vertebrate species. As a rule of thumb , brain size increases with body size, but not in a simple linear proportion. In general, smaller animals tend to have proportionally larger brains, measured as a fraction of body size. For mammals, the relationship between brain volume and body mass essentially follows a power law with an exponent of about 0.75. This formula describes the central tendency, but every family of mammals departs from it to some degree, in

3933-410: A wide variety of biochemical and metabolic processes, most notably the interactions between neurotransmitters and receptors that take place at synapses. Neurotransmitters are chemicals that are released at synapses when the local membrane is depolarised and Ca enters into the cell, typically when an action potential arrives at the synapse – neurotransmitters attach themselves to receptor molecules on

4104-488: Is 2 to 4 millimetres (0.079 to 0.157 in) thick, and deeply folded to give a convoluted appearance. Beneath the cortex is the cerebral white matter . The largest part of the cerebral cortex is the neocortex , which has six neuronal layers. The rest of the cortex is of allocortex , which has three or four layers. The cortex is mapped by divisions into about fifty different functional areas known as Brodmann's areas . These areas are distinctly different when seen under

4275-402: Is a list of some of the most important vertebrate brain components, along with a brief description of their functions as currently understood: Modern reptiles and mammals diverged from a common ancestor around 320 million years ago. The number of extant reptiles far exceeds the number of mammalian species, with 11,733 recognized species of reptiles compared to 5,884 extant mammals. Along with

4446-400: Is a major focus of current research in neurophysiology . Organ (biology) An organ's tissues can be broadly categorized as parenchyma , the functional tissue, and stroma , the structural tissue with supportive, connective, or ancillary functions. For example, the gland 's tissue that makes the hormones is the parenchyma , whereas the stroma includes the nerves that innervate

4617-435: Is a tube with a hollow gut cavity running from the mouth to the anus, and a nerve cord with an enlargement (a ganglion ) for each body segment, with an especially large ganglion at the front, called the brain. The brain is small and simple in some species, such as nematode worms; in other species, such as vertebrates, it is a large and very complex organ. Some types of worms, such as leeches , also have an enlarged ganglion at

4788-458: Is almost always inhibitory. Neurons using these transmitters can be found in nearly every part of the brain. Because of their ubiquity, drugs that act on glutamate or GABA tend to have broad and powerful effects. Some general anesthetics act by reducing the effects of glutamate; most tranquilizers exert their sedative effects by enhancing the effects of GABA. There are dozens of other chemical neurotransmitters that are used in more limited areas of

4959-412: Is by visual inspection, but many more sophisticated techniques have been developed. Brain tissue in its natural state is too soft to work with, but it can be hardened by immersion in alcohol or other fixatives , and then sliced apart for examination of the interior. Visually, the interior of the brain consists of areas of so-called grey matter , with a dark color, separated by areas of white matter , with

5130-580: Is critical at key periods of development. Additionally, the quantity and quality of experience are important. For example, animals raised in enriched environments demonstrate thick cerebral cortices, indicating a high density of synaptic connections, compared to animals with restricted levels of stimulation. The functions of the brain depend on the ability of neurons to transmit electrochemical signals to other cells, and their ability to respond appropriately to electrochemical signals received from other cells. The electrical properties of neurons are controlled by

5301-664: Is driven by the process of neurotransmission . The brain is protected by the skull , suspended in cerebrospinal fluid , and isolated from the bloodstream by the blood–brain barrier . However, the brain is still susceptible to damage , disease , and infection . Damage can be caused by trauma , or a loss of blood supply known as a stroke . The brain is susceptible to degenerative disorders , such as Parkinson's disease , dementias including Alzheimer's disease , and multiple sclerosis . Psychiatric conditions , including schizophrenia and clinical depression , are thought to be associated with brain dysfunctions. The brain can also be

Brain - Misplaced Pages Continue

5472-405: Is evident. Why the cortex wrinkles and folds is not well-understood, but gyrification has been linked to intelligence and neurological disorders , and a number of gyrification theories have been proposed. These theories include those based on mechanical buckling , axonal tension , and differential tangential expansion . What is clear is that gyrification is not a random process, but rather

5643-400: Is expressed in interneurons. Proteins expressed in glial cells include astrocyte markers GFAP and S100B whereas myelin basic protein and the transcription factor OLIG2 are expressed in oligodendrocytes. Cerebrospinal fluid is a clear, colourless transcellular fluid that circulates around the brain in the subarachnoid space , in the ventricular system , and in the central canal of

5814-401: Is generated by baroreceptors in aortic bodies in the aortic arch , and passed to the brain along the afferent fibres of the vagus nerve. Information about the pressure changes in the carotid sinus comes from carotid bodies located near the carotid artery and this is passed via a nerve joining with the glossopharyngeal nerve . This information travels up to the solitary nucleus in

5985-449: Is involved in the sense of smell, and the dentate gyrus of the hippocampus, where there is evidence that the new neurons play a role in storing newly acquired memories. With these exceptions, however, the set of neurons that is present in early childhood is the set that is present for life. Glial cells are different: as with most types of cells in the body, they are generated throughout the lifespan. There has long been debate about whether

6156-450: Is made up of six neuronal layers , while the allocortex has three or four. Each hemisphere is divided into four lobes – the frontal , temporal , parietal , and occipital lobes . The frontal lobe is associated with executive functions including self-control , planning , reasoning , and abstract thought , while the occipital lobe is dedicated to vision. Within each lobe, cortical areas are associated with specific functions, such as

6327-455: Is narrowly furrowed into numerous curved transverse fissures. Viewed from underneath between the two lobes is the third lobe the flocculonodular lobe. The cerebellum rests at the back of the cranial cavity , lying beneath the occipital lobes, and is separated from these by the cerebellar tentorium , a sheet of fibre. It is connected to the brainstem by three pairs of nerve tracts called cerebellar peduncles . The superior pair connects to

6498-404: Is passed to the olfactory cortex . Taste is generated from receptors on the tongue and passed along the facial and glossopharyngeal nerves into the solitary nucleus in the brainstem. Some taste information is also passed from the pharynx into this area via the vagus nerve . Information is then passed from here through the thalamus into the gustatory cortex . Autonomic functions of

6669-416: Is passed up a series of neurons through tracts in the spinal cord. The dorsal column–medial lemniscus pathway contains information about fine touch, vibration and position of joints. The pathway fibres travel up the back part of the spinal cord to the back part of the medulla, where they connect with second-order neurons that immediately send fibres across the midline . These fibres then travel upwards into

6840-431: Is related to control of movements, neurotransmitters and neuromodulators responsible for integrating inputs and transmitting outputs are present, sensory systems, and cognitive functions. The avian brain is the central organ of the nervous system in birds. Birds possess large, complex brains, which process , integrate , and coordinate information received from the environment and make decisions on how to respond with

7011-491: Is responsible for the cognitive functions of birds. The pallium is made up of several major structures: the hyperpallium, a dorsal bulge of the pallium found only in birds, as well as the nidopallium, mesopallium, and archipallium. The bird telencephalon nuclear structure, wherein neurons are distributed in three-dimensionally arranged clusters, with no large-scale separation of white matter and grey matter , though there exist layer-like and column-like connections. Structures in

Brain - Misplaced Pages Continue

7182-437: Is the cerebellum ( Latin : little brain ). The cerebrum, brainstem, cerebellum, and spinal cord are covered by three membranes called meninges . The membranes are the tough dura mater ; the middle arachnoid mater and the more delicate inner pia mater . Between the arachnoid mater and the pia mater is the subarachnoid space and subarachnoid cisterns , which contain the cerebrospinal fluid . The outermost membrane of

7353-406: Is the placenta , which has evolved more than 100 times independently in vertebrates, has evolved relatively recently in some lineages, and exists in intermediate forms in extant taxa. Studies on the evolution of the placenta have identified a variety of genetic and physiological processes that contribute to the origin and evolution of organs, these include the re-purposing of existing animal tissues,

7524-495: Is the brain's primary mechanism for learning and memory. Most of the space in the brain is taken up by axons, which are often bundled together in what are called nerve fiber tracts . A myelinated axon is wrapped in a fatty insulating sheath of myelin , which serves to greatly increase the speed of signal propagation. (There are also unmyelinated axons). Myelin is white, making parts of the brain filled exclusively with nerve fibers appear as light-colored white matter , in contrast to

7695-400: Is the central organ of the human nervous system , and with the spinal cord , comprises the central nervous system . It consists of the cerebrum , the brainstem and the cerebellum . The brain controls most of the activities of the body , processing, integrating, and coordinating the information it receives from the sensory nervous system . The brain integrates the instructions sent to

7866-437: Is their ability to send signals to specific target cells, sometimes over long distances. They send these signals by means of an axon , which is a thin protoplasmic fiber that extends from the cell body and projects, usually with numerous branches, to other areas, sometimes nearby, sometimes in distant parts of the brain or body. The length of an axon can be extraordinary: for example, if a pyramidal cell (an excitatory neuron) of

8037-427: Is used to compare brain sizes across species. It takes into account the nonlinearity of the brain-to-body relationship. Humans have an average EQ in the 7-to-8 range, while most other primates have an EQ in the 2-to-3 range. Dolphins have values higher than those of primates other than humans, but nearly all other mammals have EQ values that are substantially lower. Most of the enlargement of the primate brain comes from

8208-438: Is yet to be solved. Recent models in modern neuroscience treat the brain as a biological computer , very different in mechanism from a digital computer , but similar in the sense that it acquires information from the surrounding world, stores it, and processes it in a variety of ways. This article compares the properties of brains across the entire range of animal species, with the greatest attention to vertebrates. It deals with

8379-470: The Cambrian period , and may have resembled the modern jawless fish ( hagfish and lamprey ) in form. Jawed vertebrates appeared by 445 Mya, tetrapods by 350 Mya, amniotes by 310 Mya and mammaliaforms by 200 Mya (approximately). Each vertebrate clade has an equally long evolutionary history, but the brains of modern fish , amphibians , reptiles , birds and mammals show

8550-446: The blood–brain barrier . Pericytes play a major role in the formation of the tight junctions. The barrier is less permeable to larger molecules, but is still permeable to water, carbon dioxide, oxygen, and most fat-soluble substances (including anaesthetics and alcohol). The blood-brain barrier is not present in the circumventricular organs —which are structures in the brain that may need to respond to changes in body fluids—such as

8721-455: The cerebral cortex and the cerebellar cortex, are folded into convoluted gyri and sulci in order to maximize surface area within the available intracranial space . Other parts, such as the thalamus and hypothalamus, consist of many small clusters of nuclei known as "ganglia". Thousands of distinguishable areas can be identified within the vertebrate brain based on fine distinctions of neural structure, chemistry, and connectivity. Although

SECTION 50

#1732764945215

8892-415: The cerebral hemispheres , forms the largest part of the brain and overlies the other brain structures. The outer region of the hemispheres, the cerebral cortex , is grey matter , consisting of cortical layers of neurons . Each hemisphere is divided into four main lobes – the frontal lobe , parietal lobe , temporal lobe , and occipital lobe . Three other lobes are included by some sources which are

9063-451: The common carotid arteries . They enter the cranium through the carotid canal , travel through the cavernous sinus and enter the subarachnoid space . They then enter the circle of Willis , with two branches, the anterior cerebral arteries emerging. These branches travel forward and then upward along the longitudinal fissure , and supply the front and midline parts of the brain. One or more small anterior communicating arteries join

9234-447: The diencephalon (which will contain the thalamus and hypothalamus). At about the same time, the hindbrain splits into the metencephalon (which will contain the cerebellum and pons) and the myelencephalon (which will contain the medulla oblongata ). Each of these areas contains proliferative zones where neurons and glial cells are generated; the resulting cells then migrate, sometimes for long distances, to their final positions. Once

9405-423: The environment . Some basic types of responsiveness such as reflexes can be mediated by the spinal cord or peripheral ganglia , but sophisticated purposeful control of behavior based on complex sensory input requires the information integrating capabilities of a centralized brain. The operations of individual brain cells are now understood in considerable detail but the way they cooperate in ensembles of millions

9576-415: The eukaryotes , the functional analogue of an organ is known as an organelle . In plants, there are three main organs. The number of organs in any organism depends on the definition used. There are approxiamately 79 Organs in the human body,but it is something that is debated as not all scientist agree on what counts as an organ. Except for placozoans , multicellular animals including humans have

9747-470: The extrapyramidal system . The sensory nervous system is involved with the reception and processing of sensory information . This information is received through the cranial nerves, through tracts in the spinal cord, and directly at centres of the brain exposed to the blood. The brain also receives and interprets information from the special senses of vision , smell , hearing , and taste . Mixed motor and sensory signals are also integrated. From

9918-486: The five Chinese traditional elements and with yin and yang , as follows: The Chinese associated the five elements with the five planets (Jupiter, Mars, Venus, Saturn, and Mercury) similar to the way the classical planets were associated with different metals. The yin and yang distinction approximates the modern notion of solid and hollow organs. Human brain This is an accepted version of this page The brain

10089-399: The forebrain (prosencephalon, subdivided into telencephalon and diencephalon ), midbrain ( mesencephalon ) and hindbrain ( rhombencephalon , subdivided into metencephalon and myelencephalon ). The spinal cord , which directly interacts with somatic functions below the head, can be considered a caudal extension of the myelencephalon enclosed inside the vertebral column . Together,

10260-418: The haruspices or the augurs in order to divine the future by their shape, dimensions or other factors. This practice remains an important ritual in some remote, tribal societies. The term "visceral" is contrasted with the term " parietal ", meaning "of or relating to the wall of a body part, organ or cavity " The two terms are often used in describing a membrane or piece of connective tissue, referring to

10431-441: The human brain insofar as it shares the properties of other brains. The ways in which the human brain differs from other brains are covered in the human brain article. Several topics that might be covered here are instead covered there because much more can be said about them in a human context. The most important that are covered in the human brain article are brain disease and the effects of brain damage . The shape and size of

SECTION 60

#1732764945215

10602-443: The lateral sulcus and this marks out the temporal lobe. By the sixth month other sulci have formed that demarcate the frontal, parietal, and occipital lobes. A gene present in the human genome ( ARHGAP11B ) may play a major role in gyrification and encephalisation. The frontal lobe is involved in reasoning, motor control, emotion, and language. It contains the motor cortex , which is involved in planning and coordinating movement;

10773-428: The medial geniculate nucleus , and finally the auditory radiation to the auditory cortex . The sense of smell is generated by receptor cells in the epithelium of the olfactory mucosa in the nasal cavity . This information passes via the olfactory nerve which goes into the skull through a relatively permeable part . This nerve transmits to the neural circuitry of the olfactory bulb from where information

10944-450: The neural tube , a hollow cord of cells with a fluid-filled ventricle at the center. At the front end, the ventricles and cord swell to form three vesicles that are the precursors of the prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain). At the next stage, the forebrain splits into two vesicles called the telencephalon (which will contain the cerebral cortex, basal ganglia, and related structures) and

11115-541: The neuroimmune system in the brain. Mast cells in the central nervous system are present in a number of structures including the meninges; they mediate neuroimmune responses in inflammatory conditions and help to maintain the blood–brain barrier, particularly in brain regions where the barrier is absent. Mast cells serve the same general functions in the body and central nervous system, such as effecting or regulating allergic responses, innate and adaptive immunity , autoimmunity , and inflammation . Mast cells serve as

11286-401: The olfactory tubercle whereas the dorsal striatum consists of the caudate nucleus and the putamen . The putamen and the globus pallidus lie separated from the lateral ventricles and thalamus by the internal capsule , whereas the caudate nucleus stretches around and abuts the lateral ventricles on their outer sides. At the deepest part of the lateral sulcus between the insular cortex and

11457-428: The pineal gland , area postrema , and some areas of the hypothalamus . There is a similar blood–cerebrospinal fluid barrier , which serves the same purpose as the blood–brain barrier, but facilitates the transport of different substances into the brain due to the distinct structural characteristics between the two barrier systems. At the beginning of the third week of development , the embryonic ectoderm forms

11628-419: The prefrontal cortex , which is responsible for higher-level cognitive functioning; and Broca’s area , which is essential for language production. The motor system of the brain is responsible for the generation and control of movement. Generated movements pass from the brain through nerves to motor neurons in the body, which control the action of muscles . The corticospinal tract carries movements from

11799-404: The sensory , motor , and association regions. Although the left and right hemispheres are broadly similar in shape and function, some functions are associated with one side , such as language in the left and visual-spatial ability in the right. The hemispheres are connected by commissural nerve tracts , the largest being the corpus callosum . The cerebrum is connected by the brainstem to

11970-423: The skull from the brain. Cerebral arteries pierce the outer two layers of the meninges, the dura and arachnoid mater , into the subarachnoid space and perfuse the brain parenchyma via arterioles perforating into the innermost layer of the meninges, the pia mater . The endothelial cells in the cerebral blood vessel walls are joined tightly to one another, forming the blood–brain barrier , which blocks

12141-420: The spinal cord , with most connecting to interneurons , in turn connecting to lower motor neurons within the grey matter that then transmit the impulse to move to muscles themselves. The cerebellum and basal ganglia , play a role in fine, complex and coordinated muscle movements. Connections between the cortex and the basal ganglia control muscle tone, posture and movement initiation, and are referred to as

12312-417: The stomach , intestines , gallbladder , bladder , and rectum . In the thoracic cavity , the heart is a hollow, muscular organ. Splanchnology is the study of the viscera. The term "visceral" is contrasted with the term " parietal ", meaning "of or relating to the wall of a body part, organ or cavity ". The two terms are often used in describing a membrane or piece of connective tissue, referring to

12483-463: The thalamus , the epithalamus , the pineal gland , the hypothalamus , the pituitary gland , and the subthalamus ; the limbic structures , including the amygdalae and the hippocampi , the claustrum , the various nuclei of the basal ganglia , the basal forebrain structures, and three circumventricular organs . Brain structures that are not on the midplane exist in pairs; for example, there are two hippocampi and two amygdalae. The cells of

12654-497: The thalamus . Primary sensory areas include the visual cortex of the occipital lobe , the auditory cortex in parts of the temporal lobe and insular cortex , and the somatosensory cortex in the parietal lobe . The remaining parts of the cortex are called the association areas . These areas receive input from the sensory areas and lower parts of the brain and are involved in the complex cognitive processes of perception , thought , and decision-making . The main functions of

12825-431: The ventricles where the cerebrospinal fluid is produced and circulated. Below the corpus callosum is the septum pellucidum , a membrane that separates the lateral ventricles . Beneath the lateral ventricles is the thalamus and to the front and below is the hypothalamus . The hypothalamus leads on to the pituitary gland . At the back of the thalamus is the brainstem. The basal ganglia , also called basal nuclei, are

12996-424: The ventrobasal complex in the thalamus where they connect with third-order neurons which send fibres up to the sensory cortex. The spinothalamic tract carries information about pain, temperature, and gross touch. The pathway fibres travel up the spinal cord and connect with second-order neurons in the reticular formation of the brainstem for pain and temperature, and also terminate at the ventrobasal complex of

13167-461: The 20th century, organ transplants began to take place as scientists knew more about the anatomy of organs. These came later in time as procedures were often dangerous and difficult. Both the source and method of obtaining the organ to transplant are major ethical issues to consider, and because organs as resources for transplant are always more limited than demand for them, various notions of justice, including distributive justice , are developed in

13338-441: The acquisition of new functional properties by these tissues, and novel interactions of distinct tissue types. The study of plant organs is covered in plant morphology . Organs of plants can be divided into vegetative and reproductive. Vegetative plant organs include roots , stems , and leaves . The reproductive organs are variable. In flowering plants , they are represented by the flower , seed and fruit . In conifers ,

13509-402: The animal is actively engaged in a task, called beta and gamma waves . During an epileptic seizure , the brain's inhibitory control mechanisms fail to function and electrical activity rises to pathological levels, producing EEG traces that show large wave and spike patterns not seen in a healthy brain. Relating these population-level patterns to the computational functions of individual neurons

13680-476: The animal. The tegmentum receives incoming sensory information and forwards motor responses to and from the forebrain. The isthmus connects the hindbrain with midbrain. The forebrain region is particularly well developed, is further divided into diencephalon and telencephalon. Diencephalon is related to regulation of eye and body movement in response to visual stimuli, sensory information, circadian rhythms , olfactory input, and autonomic nervous system .Telencephalon

13851-402: The back end of the nerve cord, known as a "tail brain". There are a few types of existing bilaterians that lack a recognizable brain, including echinoderms and tunicates . It has not been definitively established whether the existence of these brainless species indicates that the earliest bilaterians lacked a brain, or whether their ancestors evolved in a way that led to the disappearance of

14022-506: The body of the animal. Arthropods have a central brain, the supraesophageal ganglion , with three divisions and large optical lobes behind each eye for visual processing. Cephalopods such as the octopus and squid have the largest brains of any invertebrates. There are several invertebrate species whose brains have been studied intensively because they have properties that make them convenient for experimental work: The first vertebrates appeared over 500 million years ago ( Mya ) during

14193-416: The body. Neuroanatomists usually divide the vertebrate brain into six main subregions: the telencephalon (the cerebral hemispheres ), diencephalon ( thalamus and hypothalamus ), mesencephalon (midbrain), cerebellum , pons and medulla oblongata , with the midbrain, pons and medulla often collectively called the brainstem . Each of these areas has a complex internal structure. Some parts, such as

14364-415: The brain include neurons and supportive glial cells . There are more than 86 billion neurons in the brain, and a more or less equal number of other cells. Brain activity is made possible by the interconnections of neurons and their release of neurotransmitters in response to nerve impulses . Neurons connect to form neural pathways , neural circuits , and elaborate network systems . The whole circuitry

14535-586: The brain and spinal cord constitute the central nervous system in all vertebrates. In humans , the cerebral cortex contains approximately 14–16 billion neurons, and the estimated number of neurons in the cerebellum is 55–70 billion. Each neuron is connected by synapses to several thousand other neurons, typically communicating with one another via cytoplasmic processes known as dendrites and axons . Axons are usually myelinated and carry trains of rapid micro-electric signal pulses called action potentials to target specific recipient cells in other areas of

14706-498: The brain appears as three vesicular swellings at the front end of the neural tube ; these swellings eventually become the forebrain ( prosencephalon ), midbrain ( mesencephalon ) and hindbrain ( rhombencephalon ), respectively. At the earliest stages of brain development, the three areas are roughly equal in size. In many aquatic / semiaquatic vertebrates such as fish and amphibians, the three parts remain similar in size in adults , but in terrestrial tetrapods such as mammals,

14877-507: The brain but are not as ubiquitously distributed as glutamate and GABA. As a side effect of the electrochemical processes used by neurons for signaling, brain tissue generates electric fields when it is active. When large numbers of neurons show synchronized activity, the electric fields that they generate can be large enough to detect outside the skull, using electroencephalography (EEG) or magnetoencephalography (MEG). EEG recordings, along with recordings made from electrodes implanted inside

15048-402: The brain divides into repeating segments called neuromeres . In the hindbrain these are known as rhombomeres . A characteristic of the brain is the cortical folding known as gyrification . For just over five months of prenatal development the cortex is smooth. By the gestational age of 24 weeks, the wrinkled morphology showing the fissures that begin to mark out the lobes of the brain

15219-418: The brain in reptiles and mammals, with shared neuronal clusters enlightening brain evolution. Conserved transcription factors elucidate that evolution acted in different areas of the brain by either retaining similar morphology and function, or diversifying it. Anatomically, the reptilian brain has less subdivisions than the mammalian brain, however it has numerous conserved aspects including the organization of

15390-433: The brain include the regulation, or rhythmic control of the heart rate and rate of breathing , and maintaining homeostasis . Blood pressure and heart rate are influenced by the vasomotor centre of the medulla, which causes arteries and veins to be somewhat constricted at rest. It does this by influencing the sympathetic and parasympathetic nervous systems via the vagus nerve . Information about blood pressure

15561-427: The brain is folded into ridges ( gyri ) and grooves ( sulci ), many of which are named, usually according to their position, such as the frontal gyrus of the frontal lobe or the central sulcus separating the central regions of the hemispheres. There are many small variations in the secondary and tertiary folds. The outer part of the cerebrum is the cerebral cortex , made up of grey matter arranged in layers. It

15732-445: The brain or distant parts of the body. The prefrontal cortex , which controls executive functions , is particularly well developed in humans. Physiologically , brains exert centralized control over a body's other organs. They act on the rest of the body both by generating patterns of muscle activity and by driving the secretion of chemicals called hormones . This centralized control allows rapid and coordinated responses to changes in

15903-510: The brain to the spinal cord. The most obvious difference between the brains of mammals and other vertebrates is their size. On average, a mammal has a brain roughly twice as large as that of a bird of the same body size, and ten times as large as that of a reptile of the same body size. Size, however, is not the only difference: there are also substantial differences in shape. The hindbrain and midbrain of mammals are generally similar to those of other vertebrates, but dramatic differences appear in

16074-463: The brain varies greatly between species, and identifying common features is often difficult. Nevertheless, there are a number of principles of brain architecture that apply across a wide range of species. Some aspects of brain structure are common to almost the entire range of animal species; others distinguish "advanced" brains from more primitive ones, or distinguish vertebrates from invertebrates. The simplest way to gain information about brain anatomy

16245-401: The brain, axons initially "overgrow", and then are "pruned" by mechanisms that depend on neural activity. In the projection from the eye to the midbrain, for example, the structure in the adult contains a very precise mapping, connecting each point on the surface of the retina to a corresponding point in a midbrain layer. In the first stages of development, each axon from the retina is guided to

16416-437: The brain, often areas dedicated to a particular function. Serotonin , for example—the primary target of many antidepressant drugs and many dietary aids—comes exclusively from a small brainstem area called the raphe nuclei . Norepinephrine , which is involved in arousal, comes exclusively from a nearby small area called the locus coeruleus . Other neurotransmitters such as acetylcholine and dopamine have multiple sources in

16587-486: The brain, through the spinal cord , to the torso and limbs. The cranial nerves carry movements related to the eyes, mouth and face. Gross movement – such as locomotion and the movement of arms and legs – is generated in the motor cortex , divided into three parts: the primary motor cortex , found in the precentral gyrus and has sections dedicated to the movement of different body parts. These movements are supported and regulated by two other areas, lying anterior to

16758-450: The brain. The medical history of people with brain injury has provided insight into the function of each part of the brain. Neuroscience research has expanded considerably, and research is ongoing. In culture, the philosophy of mind has for centuries attempted to address the question of the nature of consciousness and the mind–body problem . The pseudoscience of phrenology attempted to localise personality attributes to regions of

16929-430: The brains of animals such as rats, show that the brain of a living animal is constantly active, even during sleep. Each part of the brain shows a mixture of rhythmic and nonrhythmic activity, which may vary according to behavioral state. In mammals, the cerebral cortex tends to show large slow delta waves during sleep, faster alpha waves when the animal is awake but inattentive, and chaotic-looking irregular activity when

17100-435: The brainstem. The brainstem also contains many cranial nerve nuclei and nuclei of peripheral nerves , as well as nuclei involved in the regulation of many essential processes including breathing , control of eye movements and balance. The reticular formation , a network of nuclei of ill-defined formation, is present within and along the length of the brainstem. Many nerve tracts , which transmit information to and from

17271-410: The cavernous sinus and superior and inferior petrosal sinuses. The sigmoid drains into the large internal jugular veins . The larger arteries throughout the brain supply blood to smaller capillaries . These smallest of blood vessels in the brain, are lined with cells joined by tight junctions and so fluids do not seep in or leak out to the same degree as they do in other capillaries; this creates

17442-464: The cerebral blood vessels. The pathway drains interstitial fluid from the tissue of the brain. The internal carotid arteries supply oxygenated blood to the front of the brain and the vertebral arteries supply blood to the back of the brain. These two circulations join in the circle of Willis , a ring of connected arteries that lies in the interpeduncular cistern between the midbrain and pons. The internal carotid arteries are branches of

17613-421: The cerebral cortex is the basement membrane of the pia mater called the glia limitans and is an important part of the blood–brain barrier . In 2023 a fourth meningeal membrane has been proposed known as the subarachnoid lymphatic-like membrane . The living brain is very soft, having a gel-like consistency similar to soft tofu. The cortical layers of neurons constitute much of the cerebral grey matter , while

17784-411: The cerebral cortex to the rest of the body, pass through the brainstem. The human brain is primarily composed of neurons , glial cells , neural stem cells , and blood vessels . Types of neuron include interneurons , pyramidal cells including Betz cells , motor neurons ( upper and lower motor neurons ), and cerebellar Purkinje cells . Betz cells are the largest cells (by size of cell body) in

17955-548: The cerebral cortex were magnified so that its cell body became the size of a human body, its axon, equally magnified, would become a cable a few centimeters in diameter, extending more than a kilometer. These axons transmit signals in the form of electrochemical pulses called action potentials, which last less than a thousandth of a second and travel along the axon at speeds of 1–100 meters per second. Some neurons emit action potentials constantly, at rates of 10–100 per second, usually in irregular patterns; other neurons are quiet most of

18126-500: The cerebral cortex. The cerebellum of mammals contains a large portion (the neocerebellum ) dedicated to supporting the cerebral cortex, which has no counterpart in other vertebrates. In placental mammals , there is a wide nerve tract connecting the cerebral hemispheres called the corpus callosum . The brains of humans and other primates contain the same structures as the brains of other mammals, but are generally larger in proportion to body size. The encephalization quotient (EQ)

18297-404: The cerebrum and consists of the midbrain , pons and medulla . It lies in the back part of the skull , resting on the part of the base known as the clivus , and ends at the foramen magnum , a large opening in the occipital bone . The brainstem continues below this as the spinal cord , protected by the vertebral column . Ten of the twelve pairs of cranial nerves emerge directly from

18468-507: The coordination of motor control ( muscle activity and endocrine system ). While invertebrate brains arise from paired segmental ganglia (each of which is only responsible for the respective body segment ) of the ventral nerve cord , vertebrate brains develop axially from the midline dorsal nerve cord as a vesicular enlargement at the rostral end of the neural tube , with centralized control over all body segments. All vertebrate brains can be embryonically divided into three parts:

18639-556: The cortex in the 19th century. In science fiction, brain transplants are imagined in tales such as the 1942 Donovan's Brain . The adult human brain weighs on average about 1.2–1.4 kg (2.6–3.1 lb) which is about 2% of the total body weight, with a volume of around 1260  cm in men and 1130 cm in women. There is substantial individual variation, with the standard reference range for men being 1,180–1,620 g (2.60–3.57 lb) and for women 1,030–1,400 g (2.27–3.09 lb). The cerebrum , consisting of

18810-402: The crest and migrate in a craniocaudal (head to tail) wave inside the tube. Cells at the cephalic end give rise to the brain, and cells at the caudal end give rise to the spinal cord. The tube flexes as it grows, forming the crescent-shaped cerebral hemispheres at the head. The cerebral hemispheres first appear on day 32. Early in the fourth week, the cephalic part bends sharply forward in

18981-508: The darker-colored grey matter that marks areas with high densities of neuron cell bodies. Except for a few primitive organisms such as sponges (which have no nervous system) and cnidarians (which have a diffuse nervous system consisting of a nerve net ), all living multicellular animals are bilaterians , meaning animals with a bilaterally symmetric body plan (that is, left and right sides that are approximate mirror images of each other). All bilaterians are thought to have descended from

19152-443: The deeper subcortical regions of myelinated axons , make up the white matter . The white matter of the brain makes up about half of the total brain volume. The cerebrum is the largest part of the brain and is divided into nearly symmetrical left and right hemispheres by a deep groove, the longitudinal fissure . Asymmetry between the lobes is noted as a petalia . The hemispheres are connected by five commissures that span

19323-418: The developing brain, and apparently exist solely to guide development. In humans and many other mammals, new neurons are created mainly before birth, and the infant brain contains substantially more neurons than the adult brain. There are, however, a few areas where new neurons continue to be generated throughout life. The two areas for which adult neurogenesis is well established are the olfactory bulb, which

19494-448: The dura mater and the covering of the skull. Blood from the cerebellum and midbrain drains into the great cerebral vein . Blood from the medulla and pons of the brainstem have a variable pattern of drainage, either into the spinal veins or into adjacent cerebral veins. The blood in the deep part of the brain drains, through a venous plexus into the cavernous sinus at the front, and the superior and inferior petrosal sinuses at

19665-411: The early stages of neural development are similar across all species. As the embryo transforms from a round blob of cells into a wormlike structure, a narrow strip of ectoderm running along the midline of the back is induced to become the neural plate , the precursor of the nervous system. The neural plate folds inward to form the neural groove , and then the lips that line the groove merge to enclose

19836-401: The ectoderm) populate the lateral edges of the plate at the neural folds . In the fourth week—during the neurulation stage —the neural folds close to form the neural tube , bringing together the neural crest cells at the neural crest . The neural crest runs the length of the tube with cranial neural crest cells at the cephalic end and caudal neural crest cells at the tail. Cells detach from

20007-433: The edge of the neocortex, including the hippocampus and amygdala , are also much more extensively developed in mammals than in other vertebrates. The elaboration of the cerebral cortex carries with it changes to other brain areas. The superior colliculus , which plays a major role in visual control of behavior in most vertebrates, shrinks to a small size in mammals, and many of its functions are taken over by visual areas of

20178-402: The ethical analysis. This situation continues as long as transplantation relies upon organ donors rather than technological innovation, testing, and industrial manufacturing. The English word "organ" dates back to the twelfth century and refers to any musical instrument. By the late 14th century, the musical term's meaning had narrowed to refer specifically to the keyboard-based instrument . At

20349-419: The fibres from the temporal halves of the opposite retinas to form the optic tracts . The arrangements of the eyes' optics and the visual pathways mean vision from the left visual field is received by the right half of each retina, is processed by the right visual cortex, and vice versa. The optic tract fibres reach the brain at the lateral geniculate nucleus , and travel through the optic radiation to reach

20520-465: The forebrain becomes much larger than the other parts, the hindbrain develops a bulky dorsal extension known as the cerebellum , and the midbrain becomes very small as a result. The brains of vertebrates are made of very soft tissue. Living brain tissue is pinkish on the outside and mostly white on the inside, with subtle variations in color. Vertebrate brains are surrounded by a system of connective tissue membranes called meninges , which separate

20691-422: The forebrain, which is greatly enlarged and also altered in structure. The cerebral cortex is the part of the brain that most strongly distinguishes mammals. In non-mammalian vertebrates, the surface of the cerebrum is lined with a comparatively simple three-layered structure called the pallium . In mammals, the pallium evolves into a complex six-layered structure called neocortex or isocortex . Several areas at

20862-484: The frontal lobe are to control attention , abstract thinking, behaviour, problem-solving tasks, and physical reactions and personality. The occipital lobe is the smallest lobe; its main functions are visual reception, visual-spatial processing, movement, and colour recognition . There is a smaller occipital lobule in the lobe known as the cuneus . The temporal lobe controls auditory and visual memories , language , and some hearing and speech. The cerebrum contains

21033-430: The internal carotid arteries. Cerebral veins drain deoxygenated blood from the brain. The brain has two main networks of veins : an exterior or superficial network , on the surface of the cerebrum that has three branches, and an interior network . These two networks communicate via anastomosing (joining) veins. The veins of the brain drain into larger cavities of the dural venous sinuses usually situated between

21204-447: The left and right subclavian arteries . They travel upward through transverse foramina which are spaces in the cervical vertebrae . Each side enters the cranial cavity through the foramen magnum along the corresponding side of the medulla. They give off one of the three cerebellar branches . The vertebral arteries join in front of the middle part of the medulla to form the larger basilar artery , which sends multiple branches to supply

21375-468: The liver and heart evolved in the chordates about 550-500 million years ago, while the gut and brain are even more ancient, arising in the ancestor of vertebrates, insects, molluscs, and worms about 700–650 million years ago. Given the ancient origin of most vertebrate organs, researchers have looked for model systems, where organs have evolved more recently, and ideally have evolved multiple times independently. An outstanding model for this kind of research

21546-404: The longitudinal fissure, the largest of these is the corpus callosum . Each hemisphere is conventionally divided into four main lobes ; the frontal lobe , parietal lobe , temporal lobe , and occipital lobe , named according to the skull bones that overlie them. Each lobe is associated with one or two specialised functions though there is some functional overlap between them. The surface of

21717-399: The main effector cell through which pathogens can affect the biochemical signaling that takes place between the gastrointestinal tract and the central nervous system . Some 400 genes are shown to be brain-specific. In all neurons, ELAVL3 is expressed, and in pyramidal cells, NRGN and REEP2 are also expressed. GAD1 – essential for the biosynthesis of the neurotransmitter GABA –

21888-440: The map, leaving it finally in its precise adult form. Similar things happen in other brain areas: an initial synaptic matrix is generated as a result of genetically determined chemical guidance, but then gradually refined by activity-dependent mechanisms, partly driven by internal dynamics, partly by external sensory inputs. In some cases, as with the retina-midbrain system, activity patterns depend on mechanisms that operate only in

22059-435: The medulla and pons, and the two other anterior and superior cerebellar branches . Finally, the basilar artery divides into two posterior cerebral arteries . These travel outwards, around the superior cerebellar peduncles, and along the top of the cerebellar tentorium, where it sends branches to supply the temporal and occipital lobes. Each posterior cerebral artery sends a small posterior communicating artery to join with

22230-406: The membrane of the synapse's target cell (or cells), and thereby alter the electrical or chemical properties of the receptor molecules. With few exceptions, each neuron in the brain releases the same chemical neurotransmitter, or combination of neurotransmitters, at all the synaptic connections it makes with other neurons; this rule is known as Dale's principle . Thus, a neuron can be characterized by

22401-436: The membrane of the target cell. Synapses are the key functional elements of the brain. The essential function of the brain is cell-to-cell communication , and synapses are the points at which communication occurs. The human brain has been estimated to contain approximately 100 trillion synapses; even the brain of a fruit fly contains several million. The functions of these synapses are very diverse: some are excitatory (exciting

22572-513: The midbrain; the middle pair connects to the medulla, and the inferior pair connects to the pons. The cerebellum consists of an inner medulla of white matter and an outer cortex of richly folded grey matter. The cerebellum's anterior and posterior lobes appear to play a role in the coordination and smoothing of complex motor movements, and the flocculonodular lobe in the maintenance of balance although debate exists as to its cognitive, behavioural and motor functions. The brainstem lies beneath

22743-447: The nervous system. The adult human brain is estimated to contain 86±8 billion neurons, with a roughly equal number (85±10 billion) of non-neuronal cells. Out of these neurons, 16 billion (19%) are located in the cerebral cortex, and 69 billion (80%) are in the cerebellum. Types of glial cell are astrocytes (including Bergmann glia ), oligodendrocytes , ependymal cells (including tanycytes ), radial glial cells , microglia , and

22914-493: The neurotransmitters that it releases. The great majority of psychoactive drugs exert their effects by altering specific neurotransmitter systems. This applies to drugs such as cannabinoids , nicotine , heroin , cocaine , alcohol , fluoxetine , chlorpromazine , and many others. The two neurotransmitters that are most widely found in the vertebrate brain are glutamate , which almost always exerts excitatory effects on target neurons, and gamma-aminobutyric acid (GABA), which

23085-467: The new generation of plants (see clonal colony ). Many societies have a system for organ donation , in which a living or deceased donor's organ are transplanted into a person with a failing organ. The transplantation of larger solid organs often requires immunosuppression to prevent organ rejection or graft-versus-host disease . There is considerable interest throughout the world in creating laboratory-grown or artificial organs . Beginning in

23256-403: The nucleus basalis, is considered to be the major cholinergic output of the central nervous system to the striatum and neocortex. The cerebellum is divided into an anterior lobe , a posterior lobe , and the flocculonodular lobe . The anterior and posterior lobes are connected in the middle by the vermis . Compared to the cerebral cortex, the cerebellum has a much thinner outer cortex that

23427-403: The opposing sides. Aristotle used the word frequently in his philosophy, both to describe the organs of plants or animals (e.g. the roots of a tree, the heart or liver of an animal) because, in ancient Greek, the word ' organon ' means 'tool', and Aristotle believed that the organs of the body were tools for us by means of which we can do things. For similar reasons, his logical works, taken as

23598-406: The opposing sides. The organ level of organisation in animals can be first detected in flatworms and the more derived phyla , i.e. the bilaterians . The less-advanced taxa (i.e. Placozoa , Porifera , Ctenophora and Cnidaria ) do not show consolidation of their tissues into organs. More complex animals are composed of different organs, which have evolved over time. For example,

23769-514: The organ that bears the reproductive structures is called a cone . In other divisions ( phyla ) of plants, the reproductive organs are called strobili , in Lycopodiophyta , or simply gametophores in mosses . Common organ system designations in plants include the differentiation of shoot and root. All parts of the plant above ground (in non- epiphytes ), including the functionally distinct leaf and flower organs, may be classified together as

23940-406: The pallium are associated with perception , learning , and cognition . Beneath the pallium are the two components of the subpallium, the striatum and pallidum . The subpallium connects different parts of the telencephalon and plays major roles in a number of critical behaviours. To the rear of the telencephalon are the thalamus , midbrain , and cerebellum . The hindbrain connects the rest of

24111-421: The parenchyma, the blood vessels that oxygenate and nourish it and carry away its metabolic wastes, and the connective tissues that provide a suitable place for it to be situated and anchored. The main tissues that make up an organ tend to have common embryologic origins, such as arising from the same germ layer . Organs exist in most multicellular organisms . In single-celled organisms such as members of

24282-428: The passage of many toxins and pathogens (though at the same time blocking antibodies and some drugs, thereby presenting special challenges in treatment of diseases of the brain). As a result of the osmotic restriction by the blood-brain barrier, the metabolites within the brain are cleared mostly by bulk flow of the cerebrospinal fluid within the glymphatic system instead of via venules like other parts of

24453-435: The primary motor cortex: the premotor area and the supplementary motor area . The hands and mouth have a much larger area dedicated to them than other body parts, allowing finer movement; this has been visualised in a motor homunculus . Impulses generated from the motor cortex travel along the corticospinal tract along the front of the medulla and cross over ( decussate ) at the medullary pyramids . These then travel down

24624-438: The qualities of mind , personality, and intelligence can be attributed to heredity or to upbringing . Although many details remain to be settled, neuroscience shows that both factors are important. Genes determine both the general form of the brain and how it reacts to experience, but experience is required to refine the matrix of synaptic connections, resulting in greatly increased complexity. The presence or absence of experience

24795-413: The relationship between the muscular and skeletal systems . In the study of anatomy , viscera ( sg. : viscus ) refers to the internal organs of the abdominal , thoracic , and pelvic cavities . The abdominal organs may be classified as solid organs or hollow organs . The solid organs are the liver , pancreas , spleen , kidneys , and adrenal glands . The hollow organs of the abdomen are

24966-413: The rest of the body. Like in all chordates , the avian brain is contained within the skull bones of the head . The bird brain is divided into a number of sections, each with a different function. The cerebrum or telencephalon is divided into two hemispheres , and controls higher functions. The telencephalon is dominated by a large pallium , which corresponds to the mammalian cerebral cortex and

25137-411: The rest of the body. The brain is contained in, and protected by, the skull of the head . The cerebrum, the largest part of the human brain, consists of two cerebral hemispheres . Each hemisphere has an inner core composed of white matter , and an outer surface – the cerebral cortex – composed of grey matter . The cortex has an outer layer, the neocortex , and an inner allocortex . The neocortex

25308-433: The right general vicinity in the midbrain by chemical cues, but then branches very profusely and makes initial contact with a wide swath of midbrain neurons. The retina, before birth, contains special mechanisms that cause it to generate waves of activity that originate spontaneously at a random point and then propagate slowly across the retinal layer. These waves are useful because they cause neighboring neurons to be active at

25479-586: The same basic components are present in all vertebrate brains, some branches of vertebrate evolution have led to substantial distortions of brain geometry, especially in the forebrain area. The brain of a shark shows the basic components in a straightforward way, but in teleost fishes (the great majority of existing fish species), the forebrain has become "everted", like a sock turned inside out. In birds, there are also major changes in forebrain structure. These distortions can make it difficult to match brain components from one species with those of another species. Here

25650-489: The same time, a second meaning arose, in reference to a "body part adapted to a certain function". Plant organs are made from tissue composed of different types of tissue. The three tissue types are ground, vascular, and dermal. When three or more organs are present, it is called an organ system. The adjective visceral , also splanchnic , is used for anything pertaining to the internal organs. Historically, viscera of animals were examined by Roman pagan priests like

25821-400: The same time; that is, they produce a neural activity pattern that contains information about the spatial arrangement of the neurons. This information is exploited in the midbrain by a mechanism that causes synapses to weaken, and eventually vanish, if activity in an axon is not followed by activity of the target cell. The result of this sophisticated process is a gradual tuning and tightening of

25992-409: The shoot organ system. The vegetative organs are essential for maintaining the life of a plant. While there can be 11 organ systems in animals, there are far fewer in plants, where some perform the vital functions, such as photosynthesis , while the reproductive organs are essential in reproduction . However, if there is asexual vegetative reproduction , the vegetative organs are those that create

26163-416: The sides, and the inferior sagittal sinus at the back. Blood drains from the outer brain into the large superior sagittal sinus , which rests in the midline on top of the brain. Blood from here joins with blood from the straight sinus at the confluence of sinuses . Blood from here drains into the left and right transverse sinuses . These then drain into the sigmoid sinuses , which receive blood from

26334-524: The site of tumours , both benign and malignant ; these mostly originate from other sites in the body . The study of the anatomy of the brain is neuroanatomy , while the study of its function is neuroscience . Numerous techniques are used to study the brain. Specimens from other animals, which may be examined microscopically , have traditionally provided much information. Medical imaging technologies such as functional neuroimaging , and electroencephalography (EEG) recordings are important in studying

26505-419: The skin, the brain receives information about fine touch , pressure , pain , vibration and temperature . From the joints, the brain receives information about joint position . The sensory cortex is found just near the motor cortex, and, like the motor cortex, has areas related to sensation from different body parts. Sensation collected by a sensory receptor on the skin is changed to a nerve signal, that

26676-598: The smallest. Turtles have the largest diencephalon per body weight whereas crocodilians have the smallest. On the other hand, lizards have the largest mesencephalon. Yet their brains share several characteristics revealed by recent anatomical, molecular, and ontogenetic studies. Vertebrates share the highest levels of similarities during embryological development, controlled by conserved transcription factors and signaling centers , including gene expression, morphological and cell type differentiation. In fact, high levels of transcriptional factors can be found in all areas of

26847-498: The species diversity, reptiles have diverged in terms of external morphology, from limbless to tetrapod gliders to armored chelonians , reflecting adaptive radiation to a diverse array of environments. Morphological differences are reflected in the nervous system phenotype , such as: absence of lateral motor column neurons in snakes, which innervate limb muscles controlling limb movements; absence of motor neurons that innervate trunk muscles in tortoises; presence of innervation from

27018-699: The spinal cord and cranial nerve, as well as elaborated brain pattern of organization. Elaborated brains are characterized by migrated neuronal cell bodies away from the periventricular matrix, region of neuronal development, forming organized nuclear groups. Aside from reptiles and mammals , other vertebrates with elaborated brains include hagfish , galeomorph sharks , skates , rays , teleosts , and birds . Overall elaborated brains are subdivided in forebrain, midbrain, and hindbrain. The hindbrain coordinates and integrates sensory and motor inputs and outputs responsible for, but not limited to, walking, swimming, or flying. It contains input and output axons interconnecting

27189-418: The spinal cord, midbrain and forebrain transmitting information from the external and internal environments. The midbrain links sensory, motor, and integrative components received from the hindbrain, connecting it to the forebrain. The tectum, which includes the optic tectum and torus semicircularis, receives auditory, visual, and somatosensory inputs, forming integrated maps of the sensory and visual space around

27360-416: The spinal cord. It also fills some gaps in the subarachnoid space, known as subarachnoid cisterns . The four ventricles, two lateral , a third , and a fourth ventricle , all contain a choroid plexus that produces cerebrospinal fluid. The third ventricle lies in the midline and is connected to the lateral ventricles. A single duct , the cerebral aqueduct between the pons and the cerebellum, connects

27531-431: The spinal cord. The brainstem consists of the midbrain , the pons , and the medulla oblongata . The cerebellum is connected to the brainstem by three pairs of nerve tracts called cerebellar peduncles . Within the cerebrum is the ventricular system , consisting of four interconnected ventricles in which cerebrospinal fluid is produced and circulated. Underneath the cerebral cortex are several structures, including

27702-441: The striatum is a thin neuronal sheet called the claustrum . Below and in front of the striatum are a number of basal forebrain structures. These include the nucleus basalis , diagonal band of Broca , substantia innominata , and the medial septal nucleus . These structures are important in producing the neurotransmitter , acetylcholine , which is then distributed widely throughout the brain. The basal forebrain, in particular

27873-405: The subarachnoid space. It is constantly being regenerated and absorbed, and is replaced about once every 5–6 hours. A glymphatic system has been described as the lymphatic drainage system of the brain. The brain-wide glymphatic pathway includes drainage routes from the cerebrospinal fluid, and from the meningeal lymphatic vessels that are associated with the dural sinuses , and run alongside

28044-422: The target cell); others are inhibitory; others work by activating second messenger systems that change the internal chemistry of their target cells in complex ways. A large number of synapses are dynamically modifiable; that is, they are capable of changing strength in a way that is controlled by the patterns of signals that pass through them. It is widely believed that activity-dependent modification of synapses

28215-415: The thalamus for gross touch. Vision is generated by light that hits the retina of the eye. Photoreceptors in the retina transduce the sensory stimulus of light into an electrical nerve signal that is sent to the visual cortex in the occipital lobe. Visual signals leave the retinas through the optic nerves . Optic nerve fibres from the retinas' nasal halves cross to the opposite sides joining

28386-441: The third ventricle to the fourth ventricle. Three separate openings, the middle and two lateral apertures , drain the cerebrospinal fluid from the fourth ventricle to the cisterna magna , one of the major cisterns. From here, cerebrospinal fluid circulates around the brain and spinal cord in the subarachnoid space, between the arachnoid mater and pia mater. At any one time, there is about 150mL of cerebrospinal fluid – most within

28557-438: The time, but occasionally emit a burst of action potentials. Axons transmit signals to other neurons by means of specialized junctions called synapses . A single axon may make as many as several thousand synaptic connections with other cells. When an action potential, traveling along an axon, arrives at a synapse, it causes a chemical called a neurotransmitter to be released. The neurotransmitter binds to receptor molecules in

28728-426: The tissue to reach their ultimate locations. Once neurons have positioned themselves, their axons sprout and navigate through the brain, branching and extending as they go, until the tips reach their targets and form synaptic connections. In a number of parts of the nervous system, neurons and synapses are produced in excessive numbers during the early stages, and then the unneeded ones are pruned away. For vertebrates,

28899-411: The trigeminal nerve to pit organs responsible to infrared detection in snakes. Variation in size, weight, and shape of the brain can be found within reptiles. For instance, crocodilians have the largest brain volume to body weight proportion, followed by turtles, lizards, and snakes. Reptiles vary in the investment in different brain sections. Crocodilians have the largest telencephalon, while snakes have

29070-405: The two anterior cerebral arteries shortly after they emerge as branches. The internal carotid arteries continue forward as the middle cerebral arteries . They travel sideways along the sphenoid bone of the eye socket , then upwards through the insula cortex , where final branches arise. The middle cerebral arteries send branches along their length. The vertebral arteries emerge as branches of

29241-420: The visual cortex. Hearing and balance are both generated in the inner ear . Sound results in vibrations of the ossicles which continue finally to the hearing organ , and change in balance results in movement of liquids within the inner ear . This creates a nerve signal that passes through the vestibulocochlear nerve . From here, it passes through to the cochlear nuclei , the superior olivary nucleus ,

#214785