In Greek mythology , Priapus ( / p r aɪ ˈ eɪ p ə s / ; Ancient Greek : Πρίαπος , Príapos ) is a minor rustic fertility god , protector of livestock , fruit plants, gardens, and male genitalia . Priapus is marked by his oversized, permanent erection , which gave rise to the medical term priapism . He became a popular figure in Roman erotic art and Latin literature , and is the subject of the often humorously obscene collection of verse called the Priapeia .
123-456: See text Priapulida ( priapulid worms , from Gr. πριάπος, priāpos ' Priapus ' + Lat. -ul- , diminutive), sometimes referred to as penis worms , is a phylum of unsegmented marine worms . The name of the phylum relates to the Greek god of fertility, because their general shape and their extensible spiny introvert (eversible) proboscis may resemble the shape of a human penis . They live in
246-413: A basiepidermal configuration with a connection with the ectoderm , forming part of the body wall. There are no specialized sense organs , but there are sensory nerve endings in the body, especially on the proboscis. The priapulids are gonochoristic , having two separate sexes (i.e. male and female). Their male and female organs are closely associated with the excretory protonephridia . They comprise
369-537: A connectome including its synapses. Every neuron and its cellular lineage has been recorded and most, if not all, of the neural connections are known. In this species, the nervous system is sexually dimorphic ; the nervous systems of the two sexes, males and female hermaphrodites , have different numbers of neurons and groups of neurons that perform sex-specific functions. In C. elegans , males have exactly 383 neurons, while hermaphrodites have exactly 302 neurons. Arthropods , such as insects and crustaceans , have
492-457: A nerve net , a diffuse network of isolated cells. In bilaterian animals, which make up the great majority of existing species, the nervous system has a common structure that originated early in the Ediacaran period, over 550 million years ago. The nervous system contains two main categories or types of cells: neurons and glial cells . The nervous system is defined by the presence of
615-399: A sensory input and ends with a motor output, passing through a sequence of neurons connected in series . This can be shown in the "withdrawal reflex" causing a hand to jerk back after a hot stove is touched. The circuit begins with sensory receptors in the skin that are activated by harmful levels of heat: a special type of molecular structure embedded in the membrane causes heat to change
738-408: A Mauthner cell are so powerful that a single action potential gives rise to a major behavioral response: within milliseconds the fish curves its body into a C-shape , then straightens, thereby propelling itself rapidly forward. Functionally this is a fast escape response, triggered most easily by a strong sound wave or pressure wave impinging on the lateral line organ of the fish. Mauthner cells are not
861-754: A basal lineage within Ecdysozoa, leading to their classification as "living fossils". Priapulid-like fossils are known at least as far back as the Middle Cambrian . They were likely major predators of the Cambrian period. However, crown-group priapulids cannot be recognized until the Carboniferous. 22 extant species of priapulid worms are known, half of them being of meiobenthic size. Priapulids are cylindrical worm-like animals, ranging from 0.2–0.3 to 39 centimetres (0.08–0.12 to 15.35 in) long, with
984-405: A blood-space or hemocoel . There are no vascular or respiratory systems, but the body cavity does contain phagocytic amoebocytes and cells containing the respiratory pigment haemerythrin . The alimentary canal is straight, consisting of an eversible pharynx , an intestine, and a short rectum. The pharynx is muscular and lined by teeth. Three of the five extant families have gone through
1107-401: A capability for neurons to exchange signals with each other. Networks formed by interconnected groups of neurons are capable of a wide variety of functions, including feature detection, pattern generation and timing, and there are seen to be countless types of information processing possible. Warren McCulloch and Walter Pitts showed in 1943 that even artificial neural networks formed from
1230-400: A cell can send signals to other cells. One is by releasing chemicals called hormones into the internal circulation, so that they can diffuse to distant sites. In contrast to this "broadcast" mode of signaling, the nervous system provides "point-to-point" signals—neurons project their axons to specific target areas and make synaptic connections with specific target cells. Thus, neural signaling
1353-564: A central nervous system. In most jellyfish the nerve net is spread more or less evenly across the body; in comb jellies it is concentrated near the mouth. The nerve nets consist of sensory neurons, which pick up chemical, tactile, and visual signals; motor neurons, which can activate contractions of the body wall; and intermediate neurons, which detect patterns of activity in the sensory neurons and, in response, send signals to groups of motor neurons. In some cases groups of intermediate neurons are clustered into discrete ganglia . The development of
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#17327810988071476-432: A chemically gated ion channel is activated, it forms a passage that allows specific types of ions to flow across the membrane. Depending on the type of ion, the effect on the target cell may be excitatory or inhibitory. When a second messenger system is activated, it starts a cascade of molecular interactions inside the target cell, which may ultimately produce a wide variety of complex effects, such as increasing or decreasing
1599-402: A command neuron has, however, become controversial, because of studies showing that some neurons that initially appeared to fit the description were really only capable of evoking a response in a limited set of circumstances. At the most basic level, the function of the nervous system is to send signals from one cell to others, or from one part of the body to others. There are multiple ways that
1722-489: A common wormlike ancestor that appear as fossils beginning in the Ediacaran period, 550–600 million years ago. The fundamental bilaterian body form is a tube with a hollow gut cavity running from mouth to 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". Even mammals, including humans, show the segmented bilaterian body plan at
1845-478: A corresponding temporally structured stimulus, is called a central pattern generator . Internal pattern generation operates on a wide range of time scales, from milliseconds to hours or longer. One of the most important types of temporal pattern is circadian rhythmicity —that is, rhythmicity with a period of approximately 24 hours. All animals that have been studied show circadian fluctuations in neural activity, which control circadian alternations in behavior such as
1968-522: A deuterostomic development. Because the group is so ancient, it is assumed the deuterostome condition which appears to be ancestral for bilaterians have been maintained. Stem-group priapulids are known from the Middle Cambrian Burgess Shale , where their soft-part anatomy is preserved, often in conjunction with their gut contents – allowing a reconstruction of their diets. In addition, isolated microfossils (corresponding to
2091-556: A greatly simplified mathematical abstraction of a neuron are capable of universal computation . Historically, for many years the predominant view of the function of the nervous system was as a stimulus-response associator. In this conception, neural processing begins with stimuli that activate sensory neurons, producing signals that propagate through chains of connections in the spinal cord and brain, giving rise eventually to activation of motor neurons and thereby to muscle contraction, i.e., to overt responses. Descartes believed that all of
2214-487: A group of proteins that cluster together to form a structure resembling a postsynaptic density (the signal-receiving part of a synapse). However, the function of this structure is currently unclear. Although sponge cells do not show synaptic transmission, they do communicate with each other via calcium waves and other impulses, which mediate some simple actions such as whole-body contraction. Jellyfish , comb jellies , and related animals have diffuse nerve nets rather than
2337-449: A hierarchy of processing stages. At each stage, important information is extracted from the signal ensemble and unimportant information is discarded. By the end of the process, input signals representing "points of light" have been transformed into a neural representation of objects in the surrounding world and their properties. The most sophisticated sensory processing occurs inside the brain, but complex feature extraction also takes place in
2460-497: A hundred known neurotransmitters, and many of them have multiple types of receptors. Many synapses use more than one neurotransmitter—a common arrangement is for a synapse to use one fast-acting small-molecule neurotransmitter such as glutamate or GABA , along with one or more peptide neurotransmitters that play slower-acting modulatory roles. Molecular neuroscientists generally divide receptors into two broad groups: chemically gated ion channels and second messenger systems . When
2583-411: A median anterior mouth quite devoid of any armature or tentacles. The body is divided into a main trunk or abdomen and a somewhat swollen proboscis region ornamented with longitudinal ridges. The body is ringed and often has circles of spines, which are continued into the slightly protrusible pharynx. Family Priapulidae have species with a tail or a pair of caudal appendages. A slender tail or tail filament
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#17327810988072706-482: A microscope. The author Michael Nikoletseas wrote: "It is difficult to believe that until approximately year 1900 it was not known that neurons are the basic units of the brain ( Santiago Ramón y Cajal ). Equally surprising is the fact that the concept of chemical transmission in the brain was not known until around 1930 ( Henry Hallett Dale and Otto Loewi ). We began to understand the basic electrical phenomenon that neurons use in order to communicate among themselves,
2829-414: A nervous system made up of a series of ganglia , connected by a ventral nerve cord made up of two parallel connectives running along the length of the belly . Typically, each body segment has one ganglion on each side, though some ganglia are fused to form the brain and other large ganglia. The head segment contains the brain, also known as the supraesophageal ganglion . In the insect nervous system ,
2952-551: A neuron, many types of neurons are capable, even in isolation, of generating rhythmic sequences of action potentials, or rhythmic alternations between high-rate bursting and quiescence. When neurons that are intrinsically rhythmic are connected to each other by excitatory or inhibitory synapses, the resulting networks are capable of a wide variety of dynamical behaviors, including attractor dynamics, periodicity, and even chaos . A network of neurons that uses its internal structure to generate temporally structured output, without requiring
3075-412: A pair of branching tufts, each of which opens to the exterior on one side of the anus. The tips of these tufts enclose a flame-cell like those found in flatworms and other animals, and these probably function as excretory organs. As the animals mature, diverticula arise on the tubes of these organs, which develop either spermatozoa or ova. These sex cells pass out through the ducts. The perigenital area of
3198-416: A patron god for merchant sailors in ancient Greece and Rome is that of a protector and navigational aide. Recent shipwreck evidence contains apotropaic items carried on board by mariners in the forms of a terracotta phallus, wooden Priapus figure, and bronze sheath from a military ram. Coinciding with the use of wooden Priapic markers erected in areas of dangerous passage or particular landing areas for sailors,
3321-565: A protoplasmic protrusion that can extend to distant parts of the body and make thousands of synaptic contacts; axons typically extend throughout the body in bundles called nerves. Even in the nervous system of a single species such as humans, hundreds of different types of neurons exist, with a wide variety of morphologies and functions. These include sensory neurons that transmute physical stimuli such as light and sound into neural signals, and motor neurons that transmute neural signals into activation of muscles or glands; however in many species
3444-425: A significant miniaturization and become detritivores (Tubiluchidae and Meiopriapulidae) and filter feeders (Chaetostephanidae). The two remaining families Priapulidae and Halicryptidae are larger carnivores that feed on other animals, although some species also consume detritus as larvae. The shape of the teeth reflect these different lifestyles, and seem to be adapted mainly towards grasping prey or raking detritus from
3567-501: A son of Dionysus and Aphrodite. In later antiquity, his worship meant little more than a cult of sophisticated pornography. Outside his "home" region in Asia Minor, Priapus was regarded as something of a joke by urban dwellers. However, he played a more important role in the countryside, where he was seen as a guardian deity. He was regarded as the patron god of sailors and fishermen and others in need of good luck, and his presence
3690-430: A special type of cell—the neuron (sometimes called "neurone" or "nerve cell"). Neurons can be distinguished from other cells in a number of ways, but their most fundamental property is that they communicate with other cells via synapses , which are membrane-to-membrane junctions containing molecular machinery that allows rapid transmission of signals, either electrical or chemical. Many types of neuron possess an axon ,
3813-415: A spirit of fertility and growth, though he was perennially frustrated by his impotence. In a ribald anecdote told by Ovid, he attempted to rape the goddess Hestia but was thwarted by an ass , whose braying caused him to lose his erection at the critical moment and woke Hestia. The episode gave him a lasting hatred of asses and a willingness to see them killed in his honour. The emblem of his lustful nature
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3936-454: A symbol of health and fertility. The 13th century Lanercost Chronicle , a history of northern England and Scotland, records a "lay Cistercian brother" erecting a statue of Priapus ( simulacrum Priapi statuere ) in an attempt to end an outbreak of cattle disease. In the 1980s, D. F. Cassidy founded the St. Priapus Church as a modern church centred on worship of the phallus . Priapus' role as
4059-507: A three-layered system of membranes, including a tough, leathery outer layer called the dura mater . The brain is also protected by the skull, and the spinal cord by the vertebrae . The peripheral nervous system (PNS) is a collective term for the nervous system structures that do not lie within the CNS. The large majority of the axon bundles called nerves are considered to belong to the PNS, even when
4182-410: A variant form of LTP that is conditioned on an extra input coming from a reward-signalling pathway that uses dopamine as neurotransmitter. All these forms of synaptic modifiability, taken collectively, give rise to neural plasticity , that is, to a capability for the nervous system to adapt itself to variations in the environment. The basic neuronal function of sending signals to other cells includes
4305-430: A woman steals from me, or a man, or a boy, let the first give me her cunt, the second his head, the third his buttocks. per medios ibit pueros mediasque puellas mentula; barbatis non-nisi summa petet. My dick will go through the middle of boys and the middle of girls, but with bearded men it will aim only for the top. A number of Roman paintings of Priapus have survived. One of the most famous images of Priapus
4428-407: Is a special type of identified neuron, defined as a neuron that is capable of driving a specific behavior individually. Such neurons appear most commonly in the fast escape systems of various species—the squid giant axon and squid giant synapse , used for pioneering experiments in neurophysiology because of their enormous size, both participate in the fast escape circuit of the squid. The concept of
4551-541: Is activated when organisms are in a relaxed state. The enteric nervous system functions to control the gastrointestinal system . Nerves that exit from the brain are called cranial nerves while those exiting from the spinal cord are called spinal nerves . The nervous system consists of nervous tissue which, at a cellular level, is defined by the presence of a special type of cell, called the neuron . Neurons have special structures that allow them to send signals rapidly and precisely to other cells. They send these signals in
4674-404: Is also found in family Tubiluchidae. Appendages are absent in the remaining families. The body has a chitinous cuticle that is moulted as the animal grows. Members of the family Chaetostephanidae also secretes a gelatinous tube, open in both ends, which they live in. There is a wide body-cavity, which has no connection with the renal or reproductive organs, so it is not a coelom ; it is probably
4797-445: Is an abuse of terminology—it is the receptors that are excitatory and inhibitory, not the neurons—but it is commonly seen even in scholarly publications. One very important subset of synapses are capable of forming memory traces by means of long-lasting activity-dependent changes in synaptic strength. The best-known form of neural memory is a process called long-term potentiation (abbreviated LTP), which operates at synapses that use
4920-570: Is an anatomical convention that a cluster of neurons in the brain or spinal cord is called a nucleus , whereas a cluster of neurons in the periphery is called a ganglion . There are, however, a few exceptions to this rule, notably including the part of the forebrain called the basal ganglia . Sponges have no cells connected to each other by synaptic junctions , that is, no neurons, and therefore no nervous system. They do, however, have homologs of many genes that play key roles in synaptic function. Recent studies have shown that sponge cells express
5043-514: Is called identified if it has properties that distinguish it from every other neuron in the same animal—properties such as location, neurotransmitter, gene expression pattern, and connectivity—and if every individual organism belonging to the same species has one and only one neuron with the same set of properties. In vertebrate nervous systems very few neurons are "identified" in this sense—in humans, there are believed to be none—but in simpler nervous systems, some or all neurons may be thus unique. In
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5166-431: Is called presynaptic, and the cell that receives signals is called postsynaptic. Both the presynaptic and postsynaptic areas are full of molecular machinery that carries out the signalling process. The presynaptic area contains large numbers of tiny spherical vessels called synaptic vesicles , packed with neurotransmitter chemicals. When the presynaptic terminal is electrically stimulated, an array of molecules embedded in
5289-409: Is capable of a much higher level of specificity than hormonal signaling. It is also much faster: the fastest nerve signals travel at speeds that exceed 100 meters per second. At a more integrative level, the primary function of the nervous system is to control the body. It does this by extracting information from the environment using sensory receptors, sending signals that encode this information into
5412-579: Is identical with that of priapulids, indicating that their mouth is an original trait inherited from the last common ancestor of both priapulids and arthropods, even if modern arthropods no longer possess it. Among Ecdysozoa, their nearest relatives are Kinorhyncha and Loricifera , with which they constitute the Scalidophora clade named after the spines covering the introvert ( scalids ). They feed on slow-moving invertebrates, such as polychaete worms. Some analyses suggest that Priapulida may represent
5535-469: Is only gray in preserved tissue, and is better described as pink or light brown in living tissue) contains a high proportion of cell bodies of neurons. White matter is composed mainly of myelinated axons, and takes its color from the myelin. White matter includes all of the nerves, and much of the interior of the brain and spinal cord. Gray matter is found in clusters of neurons in the brain and spinal cord, and in cortical layers that line their surfaces. There
5658-399: Is processed by the brain. In insects, many neurons have cell bodies that are positioned at the edge of the brain and are electrically passive—the cell bodies serve only to provide metabolic support and do not participate in signalling. A protoplasmic fiber runs from the cell body and branches profusely, with some parts transmitting signals and other parts receiving signals. Thus, most parts of
5781-664: Is that from the House of the Vettii in Pompeii . A fresco depicts the god weighing his phallus against a large bag of coins. In nearby Herculaneum , an excavated snack bar has a painting of Priapus behind the bar, apparently as a good-luck symbol for the customers. The medical condition priapism derives its name from Priapus, alluding to the god's permanently engorged penis. [REDACTED] The dictionary definition of Priapus at Wiktionary Nervous system In biology ,
5904-431: Is the ability to extract biologically relevant information from combinations of sensory signals. In the visual system , for example, sensory receptors in the retina of the eye are only individually capable of detecting "points of light" in the outside world. Second-level visual neurons receive input from groups of primary receptors, higher-level neurons receive input from groups of second-level neurons, and so on, forming
6027-410: The central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord . The PNS consists mainly of nerves , which are enclosed bundles of the long fibers, or axons , that connect the CNS to every other part of the body. Nerves that transmit signals from the brain are called motor nerves (efferent), while those nerves that transmit information from
6150-409: The human brain , it is estimated that the total number of glia roughly equals the number of neurons, although the proportions vary in different brain areas. Among the most important functions of glial cells are to support neurons and hold them in place; to supply nutrients to neurons; to insulate neurons electrically; to destroy pathogens and remove dead neurons; and to provide guidance cues directing
6273-438: The insect brain have passive cell bodies arranged around the periphery, while the neural signal processing takes place in a tangle of protoplasmic fibers called neuropil , in the interior. The cephalic molluscs have two pairs of main nerve cords organized around a number of paired ganglia, the visceral cords serving the internal organs and the pedal ones serving the foot. Most pairs of corresponding ganglia on both sides of
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#17327810988076396-470: The nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes that impact the body, then works in tandem with the endocrine system to respond to such events. Nervous tissue first arose in wormlike organisms about 550 to 600 million years ago. In vertebrates, it consists of two main parts,
6519-406: The primary somatosensory cortex and the inferior parietal cortex . The function of the mirror system is a subject of much speculation. Many researchers in cognitive neuroscience and cognitive psychology consider that this system provides the physiological mechanism for the perception/action coupling (see the common coding theory ). They argue that mirror neurons may be important for understanding
6642-407: The radially symmetric organisms ctenophores (comb jellies) and cnidarians (which include anemones , hydras , corals and jellyfish ) consist of a diffuse nerve net . All other animal species, with the exception of a few types of worm , have a nervous system containing a brain, a central cord (or two cords running in parallel), and nerves radiating from the brain and central cord. The size of
6765-1040: The Carboniferous. Priapulida [REDACTED] Kinorhyncha [REDACTED] Nematoda [REDACTED] Nematomorpha [REDACTED] Onychophora [REDACTED] Tardigrada [REDACTED] Arthropoda [REDACTED] † Archaeopriapulida [paraphyletic] † Ancalagonida Adrianov & Malakhov 1995 † Markuelia Valkov 1983 † Palaeoscolecida Conway Morris & Robinson 1986 Kinorhyncha Reinhard 1887 (Spiny crown worms; Mud dragons) † Paratubiluchus Han et al. 2004 Meiopriapulus Morse 1981 Tubiluchus van der Land 1968 Maccabeus Por 1973 Halicryptus von Siebold 1849 † Priapulites Schram 1973 † Xiaoheiqingella Hu 2002 Priapulopsis Koren & Danielssen 1875 Acanthopriapulus van der Land 1970 Priapulus Lamarck 1816 There are 22 known extant species: Phylum Priapulida Théel 1906 Stem-group † Scalidophora Stem-group † Palaeoscolecida Priapus Priapus
6888-668: The action potential, in the 1950s ( Alan Lloyd Hodgkin , Andrew Huxley and John Eccles ). It was in the 1960s that we became aware of how basic neuronal networks code stimuli and thus basic concepts are possible ( David H. Hubel and Torsten Wiesel ). The molecular revolution swept across US universities in the 1980s. It was in the 1990s that molecular mechanisms of behavioral phenomena became widely known ( Eric Richard Kandel )." A microscopic examination shows that nerves consist primarily of axons, along with different membranes that wrap around them and segregate them into fascicles . The neurons that give rise to nerves do not lie entirely within
7011-485: The actions of other people, and for learning new skills by imitation. Some researchers also speculate that mirror systems may simulate observed actions, and thus contribute to theory of mind skills, while others relate mirror neurons to language abilities. However, to date, no widely accepted neural or computational models have been put forward to describe how mirror neuron activity supports cognitive functions such as imitation. There are neuroscientists who caution that
7134-501: The axons of neurons to their targets. A very important type of glial cell ( oligodendrocytes in the central nervous system, and Schwann cells in the peripheral nervous system) generates layers of a fatty substance called myelin that wraps around axons and provides electrical insulation which allows them to transmit action potentials much more rapidly and efficiently. Recent findings indicate that glial cells, such as microglia and astrocytes, serve as important resident immune cells within
7257-415: The behavior of the other, as though the observer were itself acting. Such neurons have been directly observed in primate species. Birds have been shown to have imitative resonance behaviors and neurological evidence suggests the presence of some form of mirroring system. In humans, brain activity consistent with that of mirror neurons has been found in the premotor cortex , the supplementary motor area ,
7380-509: The behaviors of animals, and most of the behaviors of humans, could be explained in terms of stimulus-response circuits, although he also believed that higher cognitive functions such as language were not capable of being explained mechanistically. Charles Sherrington , in his influential 1906 book The Integrative Action of the Nervous System , developed the concept of stimulus-response mechanisms in much more detail, and behaviorism ,
7503-402: The best known identified neurons are the gigantic Mauthner cells of fish. Every fish has two Mauthner cells, in the bottom part of the brainstem, one on the left side and one on the right. Each Mauthner cell has an axon that crosses over, innervating neurons at the same brain level and then travelling down through the spinal cord, making numerous connections as it goes. The synapses generated by
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#17327810988077626-428: The body are linked by commissures (relatively large bundles of nerves). The ganglia above the gut are the cerebral, the pleural, and the visceral, which are located above the esophagus (gullet). The pedal ganglia, which control the foot, are below the esophagus and their commissure and connectives to the cerebral and pleural ganglia surround the esophagus in a circumesophageal nerve ring or nerve collar . A neuron
7749-410: The body to the CNS are called sensory nerves (afferent). The PNS is divided into two separate subsystems, the somatic and autonomic , nervous systems. The autonomic nervous system is further subdivided into the sympathetic , parasympathetic and enteric nervous systems. The sympathetic nervous system is activated in cases of emergencies to mobilize energy, while the parasympathetic nervous system
7872-417: The body, while all vertebrates have spinal cords that run along the dorsal midline. Worms are the simplest bilaterian animals, and reveal the basic structure of the bilaterian nervous system in the most straightforward way. As an example, earthworms have dual nerve cords running along the length of the body and merging at the tail and the mouth. These nerve cords are connected by transverse nerves like
7995-476: The brain is anatomically divided into the protocerebrum , deutocerebrum , and tritocerebrum . Immediately behind the brain is the subesophageal ganglion , which is composed of three pairs of fused ganglia. It controls the mouthparts , the salivary glands and certain muscles . Many arthropods have well-developed sensory organs, including compound eyes for vision and antennae for olfaction and pheromone sensation. The sensory information from these organs
8118-430: The brain to the spinal cord that are capable of enhancing or inhibiting the reflex. Although the simplest reflexes may be mediated by circuits lying entirely within the spinal cord, more complex responses rely on signal processing in the brain. For example, when an object in the periphery of the visual field moves, and a person looks toward it many stages of signal processing are initiated. The initial sensory response, in
8241-403: The brain. One target is a set of spinal interneurons that project to motor neurons controlling the arm muscles. The interneurons excite the motor neurons, and if the excitation is strong enough, some of the motor neurons generate action potentials, which travel down their axons to the point where they make excitatory synaptic contacts with muscle cells. The excitatory signals induce contraction of
8364-420: The cell bodies of the neurons to which they belong reside within the brain or spinal cord. The PNS is divided into somatic and visceral parts. The somatic part consists of the nerves that innervate the skin, joints, and muscles. The cell bodies of somatic sensory neurons lie in dorsal root ganglia of the spinal cord. The visceral part, also known as the autonomic nervous system, contains neurons that innervate
8487-436: The central nervous system, processing the information to determine an appropriate response, and sending output signals to muscles or glands to activate the response. The evolution of a complex nervous system has made it possible for various animal species to have advanced perception abilities such as vision, complex social interactions, rapid coordination of organ systems, and integrated processing of concurrent signals. In humans,
8610-409: The central nervous system. The nervous system of vertebrates (including humans) is divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS is the major division, and consists of the brain and the spinal cord . The spinal canal contains the spinal cord, while the cranial cavity contains the brain. The CNS is enclosed and protected by the meninges ,
8733-424: The claims being made for the role of mirror neurons are not supported by adequate research. In vertebrates, landmarks of embryonic neural development include the birth and differentiation of neurons from stem cell precursors, the migration of immature neurons from their birthplaces in the embryo to their final positions, outgrowth of axons from neurons and guidance of the motile growth cone through
8856-407: The density of its larvae can be as high as 58,000 per square meter (5,390 per square foot). Together with Echiura and Sipuncula , they were once placed in the taxon Gephyrea , but consistent morphological and molecular evidence supports their belonging to Ecdysozoa , which also includes arthropods and nematodes . Fossil findings show that the mouth design of the stem -arthropod Pambdelurion
8979-590: The donkey, which was put by Dionysus among the stars. The first extant mention of Priapus is in the eponymous comedy Priapus , written in the 4th century BC by Xenarchus . Originally worshipped by Greek colonists in Lampsacus in Asia Minor , the cult of Priapus spread to mainland Greece and eventually to Italy during the 3rd century BC. Lucian ( De saltatione ) tells that in Bithynia Priapus
9102-408: The electrical field across the membrane. If the change in electrical potential is large enough to pass the given threshold, it evokes an action potential, which is transmitted along the axon of the receptor cell, into the spinal cord. There the axon makes excitatory synaptic contacts with other cells, some of which project (send axonal output) to the same region of the spinal cord, others projecting into
9225-408: The embryo towards postsynaptic partners, the generation of synapses between these axons and their postsynaptic partners, and finally the lifelong changes in synapses which are thought to underlie learning and memory. All bilaterian animals at an early stage of development form a gastrula , which is polarized, with one end called the animal pole and the other the vegetal pole . The gastrula has
9348-503: The form of electrochemical impulses traveling along thin fibers called axons , which can be directly transmitted to neighboring cells through electrical synapses or cause chemicals called neurotransmitters to be released at chemical synapses . A cell that receives a synaptic signal from a neuron may be excited , inhibited , or otherwise modulated . The connections between neurons can form neural pathways , neural circuits , and larger networks that generate an organism's perception of
9471-422: The form of electrochemical waves called action potentials , which produce cell-to-cell signals at points where axon terminals make synaptic contact with other cells. Synapses may be electrical or chemical. Electrical synapses make direct electrical connections between neurons, but chemical synapses are much more common, and much more diverse in function. At a chemical synapse, the cell that sends signals
9594-513: The function of Priapus is much more extensive than previously thought. Although Priapus is commonly associated with the failed attempts of rape against the nymphs Lotis and Vesta in Ovid's comedy Fasti and the rather flippant treatment of the deity in urban settings, Priapus' protection traits can be traced back to the importance placed on the phallus in ancient times (particularly his association with fertility and garden protection). In Greece,
9717-521: The genus Tubiluchus exhibit sexual dimorphism. For the species Priapulus caudatus , the 80 μm egg undergoes a total and radial cleavage following a symmetrical and subequal pattern. Development is remarkably slow, with the first cleavage taking place 15 hours after fertilization, gastrulation after several days and hatching of the first 'lorica' larvae after 15 to 20 days. The species Meiopriapulus fijiensis have direct development. In current systematics, they are described as protostomes, despite having
9840-420: The great majority of neurons participate in the formation of centralized structures (the brain and ganglia) and they receive all of their input from other neurons and send their output to other neurons. Glial cells (named from the Greek for "glue") are non-neuronal cells that provide support and nutrition , maintain homeostasis , form myelin , and participate in signal transmission in the nervous system. In
9963-412: The hero Paris having the temerity to judge Aphrodite more beautiful than Hera. In another account, Hera's anger and curse were because the baby had been fathered by her husband Zeus. The other gods refused to allow him to live on Mount Olympus and threw him down to Earth, leaving him on a hillside. He was eventually found by shepherds and was brought up by them. Priapus joined Pan and the satyrs as
10086-464: The internal organs, blood vessels, and glands. The autonomic nervous system itself consists of two parts: the sympathetic nervous system and the parasympathetic nervous system . Some authors also include sensory neurons whose cell bodies lie in the periphery (for senses such as hearing) as part of the PNS; others, however, omit them. The vertebrate nervous system can also be divided into areas called gray matter and white matter . Gray matter (which
10209-734: The level of the nervous system. The spinal cord contains a series of segmental ganglia, each giving rise to motor and sensory nerves that innervate a portion of the body surface and underlying musculature. On the limbs, the layout of the innervation pattern is complex, but on the trunk it gives rise to a series of narrow bands. The top three segments belong to the brain, giving rise to the forebrain, midbrain, and hindbrain. Bilaterians can be divided, based on events that occur very early in embryonic development, into two groups ( superphyla ) called protostomes and deuterostomes . Deuterostomes include vertebrates as well as echinoderms , hemichordates (mainly acorn worms), and Xenoturbellidans . Protostomes,
10332-485: The membrane are activated, and cause the contents of the vesicles to be released into the narrow space between the presynaptic and postsynaptic membranes, called the synaptic cleft . The neurotransmitter then binds to receptors embedded in the postsynaptic membrane, causing them to enter an activated state. Depending on the type of receptor, the resulting effect on the postsynaptic cell may be excitatory, inhibitory, or modulatory in more complex ways. For example, release of
10455-414: The more diverse group, include arthropods , molluscs , and numerous phyla of "worms". There is a basic difference between the two groups in the placement of the nervous system within the body: protostomes possess a nerve cord on the ventral (usually bottom) side of the body, whereas in deuterostomes the nerve cord is on the dorsal (usually top) side. In fact, numerous aspects of the body are inverted between
10578-401: The mud and in comparatively shallow waters up to 90 metres (295 ft) deep. Some species show a remarkable tolerance for hydrogen sulfide , anoxia and low salinity . Halicryptus spinulosus appears to prefer brackish shallow waters. They can be quite abundant in some areas. In an Alaskan bay as many as 85 adult individuals of Priapulus caudatus per square meter has been recorded, while
10701-410: The muscle cells, which causes the joint angles in the arm to change, pulling the arm away. In reality, this straightforward schema is subject to numerous complications. Although for the simplest reflexes there are short neural paths from sensory neuron to motor neuron, there are also other nearby neurons that participate in the circuit and modulate the response. Furthermore, there are projections from
10824-426: The nerves themselves—their cell bodies reside within the brain, spinal cord , or peripheral ganglia . All animals more advanced than sponges have nervous systems. However, even sponges , unicellular animals, and non-animals such as slime molds have cell-to-cell signalling mechanisms that are precursors to those of neurons. In radially symmetric animals such as the jellyfish and hydra, the nervous system consists of
10947-403: The nervous system and looks for interventions that can prevent or treat them. In the peripheral nervous system, the most common problem is the failure of nerve conduction, which can be due to different causes including diabetic neuropathy and demyelinating disorders such as multiple sclerosis and amyotrophic lateral sclerosis . Neuroscience is the field of science that focuses on the study of
11070-402: The nervous system as well as many peripheral organs, but in mammals, all of these "tissue clocks" are kept in synchrony by signals that emanate from a master timekeeper in a tiny part of the brain called the suprachiasmatic nucleus . A mirror neuron is a neuron that fires both when an animal acts and when the animal observes the same action performed by another. Thus, the neuron "mirrors"
11193-496: The nervous system in radiata is relatively unstructured. Unlike bilaterians , radiata only have two primordial cell layers, endoderm and ectoderm . Neurons are generated from a special set of ectodermal precursor cells, which also serve as precursors for every other ectodermal cell type. The vast majority of existing animals are bilaterians , meaning animals with left and right sides that are approximate mirror images of each other. All bilateria are thought to have descended from
11316-536: The nervous system ranges from a few hundred cells in the simplest worms, to around 300 billion cells in African elephants . The central nervous system functions to send signals from one cell to others, or from one part of the body to others and to receive feedback. Malfunction of the nervous system can occur as a result of genetic defects, physical damage due to trauma or toxicity, infection, or simply senescence . The medical specialty of neurology studies disorders of
11439-435: The nervous system. The nervous system derives its name from nerves, which are cylindrical bundles of fibers (the axons of neurons ), that emanate from the brain and spinal cord , and branch repeatedly to innervate every part of the body. Nerves are large enough to have been recognized by the ancient Egyptians, Greeks, and Romans, but their internal structure was not understood until it became possible to examine them using
11562-486: The neurotransmitter acetylcholine at a synaptic contact between a motor neuron and a muscle cell induces rapid contraction of the muscle cell. The entire synaptic transmission process takes only a fraction of a millisecond, although the effects on the postsynaptic cell may last much longer (even indefinitely, in cases where the synaptic signal leads to the formation of a memory trace ). There are literally hundreds of different types of synapses. In fact, there are over
11685-454: The neurotransmitter glutamate acting on a special type of receptor known as the NMDA receptor . The NMDA receptor has an "associative" property: if the two cells involved in the synapse are both activated at approximately the same time, a channel opens that permits calcium to flow into the target cell. The calcium entry initiates a second messenger cascade that ultimately leads to an increase in
11808-561: The new gardens. A number of epigrams , apparently written as if to adorn shrines of Priapus, were collected in the Priapeia . In these, Priapus frequently threatens sexual assault against potential thieves: Percidere, puer, moneo; futuere, puella; barbatum furem tertia poena manet. I warn you, boy, you will be screwed; girl, you will be laid with; a third penalty awaits the bearded thief. Femina si furtum faciet mihi virve puerve, haec cunnum, caput hic praebeat, ille nates. If
11931-474: The number of glutamate receptors in the target cell, thereby increasing the effective strength of the synapse. This change in strength can last for weeks or longer. Since the discovery of LTP in 1973, many other types of synaptic memory traces have been found, involving increases or decreases in synaptic strength that are induced by varying conditions, and last for variable periods of time. The reward system , that reinforces desired behaviour for example, depends on
12054-438: The only identified neurons in fish—there are about 20 more types, including pairs of "Mauthner cell analogs" in each spinal segmental nucleus. Although a Mauthner cell is capable of bringing about an escape response individually, in the context of ordinary behavior other types of cells usually contribute to shaping the amplitude and direction of the response. Mauthner cells have been described as command neurons . A command neuron
12177-460: The other Olympians come to her rescue. Ovid's anecdote served to explain why donkeys were sacrificed to Priapus in the city of Lampsacus on the Hellespont, where he was worshipped among the offspring of Hermes . Once, a donkey that had been given human speech by Dionysus challenged Priapus to a contest about which between them had the better penis . Priapus won the contest, and then killed
12300-402: The party with "raucous braying". Lotis awoke and pushed Priapus away, but her only true escape was to be transformed into the lotus tree . To punish the donkey for spoiling his opportunity, Priapus bludgeoned it to death with his gargantuan phallus. When the same story is recounted later in the same book, Lotis is replaced with the virginal goddess Hestia , who avoids being changed into a tree as
12423-434: The phallus was thought of to have a mind of its own, animal-like, separate from the mind and control of the man. The phallus is also associated with "possession and territorial demarcation" in many cultures, attributing to Priapus' other role as a navigational deity. Priapus' iconic attribute was his priapism (permanently erect penis ); he probably absorbed some pre-existing ithyphallic deities as his cult developed. He
12546-428: The possibilities for generating intricate temporal patterns become far more extensive. A modern conception views the function of the nervous system partly in terms of stimulus-response chains, and partly in terms of intrinsically generated activity patterns—both types of activity interact with each other to generate the full repertoire of behavior. The simplest type of neural circuit is a reflex arc , which begins with
12669-783: The receptors that it activates. Because different targets can (and frequently do) use different types of receptors, it is possible for a neuron to have excitatory effects on one set of target cells, inhibitory effects on others, and complex modulatory effects on others still. Nevertheless, it happens that the two most widely used neurotransmitters, glutamate and GABA , each have largely consistent effects. Glutamate has several widely occurring types of receptors, but all of them are excitatory or modulatory. Similarly, GABA has several widely occurring receptor types, but all of them are inhibitory. Because of this consistency, glutamatergic cells are frequently referred to as "excitatory neurons", and GABAergic cells as "inhibitory neurons". Strictly speaking, this
12792-646: The retina of the eye, and the final motor response, in the oculomotor nuclei of the brainstem , are not all that different from those in a simple reflex, but the intermediate stages are completely different. Instead of a one or two step chain of processing, the visual signals pass through perhaps a dozen stages of integration, involving the thalamus , cerebral cortex , basal ganglia , superior colliculus , cerebellum , and several brainstem nuclei. These areas perform signal-processing functions that include feature detection , perceptual analysis, memory recall , decision-making , and motor planning . Feature detection
12915-510: The roundworm C. elegans , whose nervous system is the most thoroughly described of any animal's, every neuron in the body is uniquely identifiable, with the same location and the same connections in every individual worm. One notable consequence of this fact is that the form of the C. elegans nervous system is completely specified by the genome, with no experience-dependent plasticity. The brains of many molluscs and insects also contain substantial numbers of identified neurons. In vertebrates,
13038-399: The rungs of a ladder. These transverse nerves help coordinate the two sides of the animal. Two ganglia at the head (the " nerve ring ") end function similar to a simple brain . Photoreceptors on the animal's eyespots provide sensory information on light and dark. The nervous system of one very small roundworm, the nematode Caenorhabditis elegans , has been completely mapped out in
13161-713: The school of thought that dominated psychology through the middle of the 20th century, attempted to explain every aspect of human behavior in stimulus-response terms. However, experimental studies of electrophysiology , beginning in the early 20th century and reaching high productivity by the 1940s, showed that the nervous system contains many mechanisms for maintaining cell excitability and generating patterns of activity intrinsically, without requiring an external stimulus. Neurons were found to be capable of producing regular sequences of action potentials, or sequences of bursts, even in complete isolation. When intrinsically active neurons are connected to each other in complex circuits,
13284-455: The sediment into the mouth. The anus is terminal, although in Priapulus one or two hollow ventral diverticula of the body-wall stretch out behind it. The nervous system consists of a nerve ring around the pharynx and a prominent cord running the length of the body with ganglia and longitudinal and transversal neurites consistent with an orthogonal organisation. The nervous system retains
13407-400: The sensitivity of the cell to stimuli, or even altering gene transcription . According to a rule called Dale's principle , which has only a few known exceptions, a neuron releases the same neurotransmitters at all of its synapses. This does not mean, though, that a neuron exerts the same effect on all of its targets, because the effect of a synapse depends not on the neurotransmitter, but on
13530-531: The sleep-wake cycle. Experimental studies dating from the 1990s have shown that circadian rhythms are generated by a "genetic clock" consisting of a special set of genes whose expression level rises and falls over the course of the day. Animals as diverse as insects and vertebrates share a similar genetic clock system. The circadian clock is influenced by light but continues to operate even when light levels are held constant and no other external time-of-day cues are available. The clock genes are expressed in many parts of
13653-498: The sophistication of the nervous system makes it possible to have language, abstract representation of concepts, transmission of culture, and many other features of human society that would not exist without the human brain. Most neurons send signals via their axons , although some types are capable of dendrite-to-dendrite communication. (In fact, the types of neurons called amacrine cells have no axons, and communicate only via their dendrites.) Neural signals propagate along an axon in
13776-399: The spinal cord and in peripheral sensory organs such as the retina. Although stimulus-response mechanisms are the easiest to understand, the nervous system is also capable of controlling the body in ways that do not require an external stimulus, by means of internally generated rhythms of activity. Because of the variety of voltage-sensitive ion channels that can be embedded in the membrane of
13899-419: The two groups, including the expression patterns of several genes that show dorsal-to-ventral gradients. Most anatomists now consider that the bodies of protostomes and deuterostomes are "flipped over" with respect to each other, a hypothesis that was first proposed by Geoffroy Saint-Hilaire for insects in comparison to vertebrates. Thus insects, for example, have nerve cords that run along the ventral midline of
14022-587: The various teeth and spines that line the pharynx and introvert) are widespread in Cambrian deposits, allowing the distribution of priapulids – and even individual species – to be tracked widely through Cambrian oceans. Trace fossils that are morphologically almost identical to modern priapulid burrows ( Treptichnus pedum ) officially mark the start of the Cambrian period, suggesting that priapulids, or at least close anatomical relatives, evolved around this time. Crown-group priapulid body fossils are first known from
14145-674: The world and determine its behavior. Along with neurons, the nervous system contains other specialized cells called glial cells (or simply glia), which provide structural and metabolic support. Many of the cells and vasculature channels within the nervous system make up the neurovascular unit , which regulates cerebral blood flow in order to rapidly satisfy the high energy demands of activated neurons. Nervous systems are found in most multicellular animals , but vary greatly in complexity. The only multicellular animals that have no nervous system at all are sponges , placozoans , and mesozoans , which have very simple body plans. The nervous systems of
14268-404: Was a trunk of fig, a useless piece of wood, when a carpenter, unsure whether he should make a bench or a Priapus, decided to make a god. So I am a god, of thieves and birds a very great scarer; for my right hand curbs thieves, as does the red pole which projects from my indecent groin; but as for the importunate birds, the reed fixed on my head terrifies them and forbids them to settle in
14391-500: Was accounted as a warlike god, a rustic tutor to the infant Ares , "who taught him dancing first and war only afterwards," Karl Kerenyi observed. Arnobius is aware of the importance accorded Priapus in this region near the Hellespont . Also, Pausanias notes: This god is worshipped where goats and sheep pasture or there are swarms of bees; but by the people of Lampsacus he is more revered than any other god, being called by them
14514-435: Was believed to avert the evil eye . Priapus does not appear to have had an organized cult and was mostly worshiped in gardens or homes, though there are attestations of temples dedicated to the god. His sacrificial animal was the ass , but agricultural offerings (such as fruit, flowers, vegetables and fish) were also very common. Long after the fall of Rome and the rise of Christianity , Priapus continued to be invoked as
14637-476: Was described in varying sources as the son of Aphrodite by Dionysus ; as the son of Dionysus and Chione ; as perhaps the father or son of Hermes ; or as the son of Zeus or Pan . According to legend, Hera cursed him with inconvenient impotence (he could not sustain an erection when the time came for sexual intercourse), ugliness and foul-mindedness while he was still in Aphrodite's womb, in revenge for
14760-578: Was his permanent erection and his large penis. Another myth states that he pursued the nymph Lotis until the gods took pity on her and turned her into a lotus plant. "Priapos" is a title given to Eros Phanes in the Orphic Hymn to Protogonos, the "firstborn" god of the Greeks who came from the Cosmic Egg . In The Orphic Hymn to Dionysus, Dionysus is given epithets similar to Protogonos and
14883-451: Was represented in a variety of ways, most commonly as a misshapen gnome-like figure with an enormous erect phallus. Statues of Priapus were common in ancient Greece and Rome, standing in gardens. The Athenians often conflated Priapus with Hermes , the god of boundaries, and depicted a hybrid deity with a winged helmet, sandals, and huge erection. Another attribute of Priapus was the sickle which he often carries in his right hand. This tool
15006-591: Was thought of as the incarnation of Protogonos, so he was considered both the father of fertility god Priapus and also the incarnation of the primordial Priapus. As well as the collection known as the Priapeia mentioned above, Priapus was a frequent figure in Latin erotic or mythological verse. In Ovid 's Fasti , the nymph Lotis fell into a drunken slumber at a feast, and Priapus seized this opportunity to advance upon her. With stealth he approached, and just before he could embrace her, Silenus 's donkey alerted
15129-430: Was used to threaten thieves, doubtless with castration: Horace ( Sat. 1.8.1–7) writes: Olim truncus eram ficulnus, inutile lignum, cum faber, incertus scamnum faceretne Priapum, maluit esse deum. deus inde ego, furum aviumque maxima formido; nam fures dextra coercet obscenoque ruber porrectus ab inguine palus; ast importunes volucres in vertice harundo terret fixa vetatque novis considere in hortis. Once I
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