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153-398: The rectum ( pl. : rectums or recta ) is the final straight portion of the large intestine in humans and some other mammals , and the gut in others. The adult human rectum is about 12 centimetres (4.7 in) long, and begins at the rectosigmoid junction (the end of the sigmoid colon ) at the level of the third sacral vertebra or the sacral promontory depending upon what definition

306-537: 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

459-442: A laparotomy . In terms of diameter, the cecum is the widest, averaging slightly less than 9 cm in healthy individuals, and the transverse colon averages less than 6 cm in diameter. The descending and sigmoid colon are slightly smaller, with the sigmoid colon averaging 4–5 cm (1.6–2.0 in) in diameter. Diameters larger than certain thresholds for each colonic section can be diagnostic for megacolon . The cecum

612-452: 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

765-425: A reflex expulsion of the contents of the rectum occurs. Expulsion occurs through contractions of the muscles of the rectum. The urge to voluntarily defecate occurs after the rectal pressure increases to beyond 18 mmHg; and reflex expulsion at 55 mmHg. In voluntary defecation, in addition to contraction of the rectal muscles and relaxation of the external anal sphincter, abdominal muscle contraction, and relaxation of

918-399: 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

1071-408: 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

1224-401: 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

1377-400: 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

1530-416: A central hole down the length of the tube (the crypt lumen ). Four tissue sections are shown here, two cut across the long axes of the crypts and two cut parallel to the long axes. In these images the cells have been stained by immunohistochemistry to show a brown-orange color if the cells produce a mitochondrial protein called cytochrome c oxidase subunit I (CCOI). The nuclei of the cells (located at

1683-564: 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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1836-432: 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

1989-402: 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

2142-484: 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

2295-417: A complete covering of peritoneum , so they are fixed in location. Intraperitoneal organs are completely surrounded by peritoneum and are therefore mobile. Of the colon, the ascending colon, descending colon and rectum are retroperitoneal, while the cecum, appendix, transverse colon and sigmoid colon are intraperitoneal. This is important as it affects which organs can be easily accessed during surgery, such as

2448-478: 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

2601-556: 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

2754-487: 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

2907-449: 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

3060-495: A higher concentration of goblet cells in the rectal mucosa than other parts of the gastrointestinal tract. The lining of the rectum changes sharply at the line where the rectum meets the anus . Here, the lining changes from the column-shaped cells of the rectum to multiple layers of flat cells . The rectum acts as a temporary storage site for feces. The rectum receives fecal material from the descending colon , transmitted through regular muscle contractions called peristalsis . As

3213-497: 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

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3366-482: 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,

3519-414: 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 ,

3672-551: 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

3825-530: A process called gastrulation. Gastrulation occurs early in human development. The gastrointestinal tract is derived from these layers. One variation on the normal anatomy of the colon occurs when extra loops form, resulting in a colon that is up to five metres longer than normal. This condition, referred to as redundant colon , typically has no direct major health consequences, though rarely volvulus occurs, resulting in obstruction and requiring immediate medical attention. A significant indirect health consequence

3978-565: 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

4131-410: A small number of crypts reported a range of 1,500 to 4,900 cells per colonic crypt. Cells are produced at the crypt base and migrate upward along the crypt axis before being shed into the colonic lumen days later. There are 5 to 6 stem cells at the bases of the crypts. As estimated from the image in panel A, there are about 100 colonic crypts per square millimeter of the colonic epithelium. Since

4284-430: 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 ,

4437-436: 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 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

4590-501: 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

4743-410: 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

Rectum - Misplaced Pages Continue

4896-442: Is a single artery that is a continuation of the inferior mesenteric artery , when it crosses the pelvic brim . It enters the mesorectum at the level of S3, and then splits into two branches, which run at the lateral back part of the rectum, and then the sides of the rectum. These then end in branches in the submucosa, which join with ( anastamose ) with branches of the middle and inferior rectal arteries. The microanatomy of

5049-407: 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

5202-509: Is about 12 cm long. The cecum – the first part of the large intestine The taenia coli run the length of the large intestine. Because the taenia coli are shorter than the large bowel itself, the colon becomes sacculated , forming the haustra of the colon which are the shelf-like intraluminal projections. Arterial supply to the colon comes from branches of the superior mesenteric artery (SMA) and inferior mesenteric artery (IMA). Flow between these two systems communicates via

5355-445: 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

5508-570: 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

5661-514: 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

5814-431: 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

5967-409: 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

6120-513: Is derived from the Latin intestinum rectum 'straight gut', a calque of Ancient Greek ἀπευθυσμένον ἔντερον, derived from ἀπευθύνειν, to make straight , and ἔντερον, gut , attested in the writings of Greek physician Galen . During his anatomic investigations on animal corpses, Galen observed the rectum to be straight instead of curved as in humans. The expressions ἀπευθυσμένον ἔντερον and intestinum rectum are therefore not appropriate descriptions of

6273-440: 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 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

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6426-416: Is joined to the end of the small intestine at the cecum, via the ileocecal valve . It then continues as the colon ascending the abdomen , across the width of the abdominal cavity as the transverse colon , and then descending to the rectum and its endpoint at the anal canal . Overall, in humans, the large intestine is about 1.5 metres (5 ft) long, which is about one-fifth of the whole length of

6579-412: Is largely indigestible carbohydrate in either soluble or insoluble form). As the chyme moves through the large intestine, most of the remaining water is removed, while the chyme is mixed with mucus and bacteria (known as gut flora ), and becomes feces. The ascending colon receives fecal material as a liquid. The muscles of the colon then move the watery waste material forward and slowly absorb all

6732-461: Is made available, thus permitting a higher portion of plant material in the diet. The large intestine produces no digestive enzymes — chemical digestion is completed in the small intestine before the chyme reaches the large intestine. The pH in the colon varies between 5.5 and 7 (slightly acidic to neutral). Water absorption at the colon typically proceeds against a transmucosal osmotic pressure gradient . The standing gradient osmosis

6885-469: 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

7038-399: 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

7191-448: Is stool in the rectum, and whether there is an obstruction. When symptoms such as weight loss, bleeding through the rectum, or pain are present, additional investigations such as a CT scan may be ordered. If constipation persists despite simple treatments, testing may also include anal manometry to measure pressures in the anus and rectum, electrophysiological studies, and magnetic resonance proctography. In general however, constipation

7344-435: Is stored before its release via the anal canal . An ampulla (from Latin bottle ) is a cavity, or the dilated end of a duct, shaped like a Roman ampulla . The rectum joins with the sigmoid colon at the level of S3 , and joins with the anal canal as it passes through the pelvic floor muscles. Unlike other portions of the colon, the rectum does not have distinct taeniae coli . The taeniae blend with one another in

7497-490: Is that use of a standard adult colonoscope is difficult and in some cases impossible when a redundant colon is present, though specialized variants on the instrument (including the pediatric variant) are useful in overcoming this problem. The wall of the large intestine is lined with simple columnar epithelium with invaginations . The invaginations are called the intestinal glands or colonic crypts. The colon crypts are shaped like microscopic thick walled test tubes with

7650-431: 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

7803-399: Is the first section of the large intestine and is involved in digestion, while the appendix which develops embryologically from it, is not involved in digestion and is considered to be part of the gut-associated lymphoid tissue . The function of the appendix is uncertain, but some sources believe that it has a role in housing a sample of the gut microbiota , and is able to help to repopulate

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7956-405: Is the reabsorption of water against the osmotic gradient in the intestines. Cells occupying the intestinal lining pump sodium ions into the intercellular space, raising the osmolarity of the intercellular fluid. This hypertonic fluid creates an osmotic pressure that drives water into the lateral intercellular spaces by osmosis via tight junctions and adjacent cells, which then in turn moves across

8109-433: Is treated by improving factors such as hydration, exercise, and dietary fibre. Laxatives may be used. Constipation that persists may require enemas or suppositories. Sometimes, use of the fingers or hand (manual evacuation) is required. Although peristalsis in the colon delivers material to the rectum, laxatives such as bisacodyl or senna that induce peristalsis in the large bowel do not appear to initiate peristalsis in

8262-422: Is used. Its diameter is similar to that of the sigmoid colon at its commencement, but it is dilated near its termination, forming the rectal ampulla . It terminates at the level of the anorectal ring (the level of the puborectalis sling) or the dentate line , again depending upon which definition is used. In humans, the rectum is followed by the anal canal , which is about 4 centimetres (1.6 in) long, before

8415-406: The appendix via Gerlach's valve . In ruminants , the ascending colon is known as the spiral colon . Taking into account all ages and sexes, colon cancer occurs here most often (41%). The transverse colon is the part of the colon from the hepatic flexure , also known as the right colic, (the turn of the colon by the liver ) to the splenic flexure also known as the left colic, (the turn of

8568-430: The basement membrane and into the capillaries , while more sodium ions are pumped again into the intercellular fluid. Although water travels down an osmotic gradient in each individual step, overall, water usually travels against the osmotic gradient due to the pumping of sodium ions into the intercellular fluid. This allows the large intestine to absorb water despite the blood in capillaries being hypotonic compared to

8721-400: The cecum (including the appendix ), colon (the longest part), rectum , and anal canal . The four sections of the colon are: the ascending colon , transverse colon , descending colon , and sigmoid colon . These sections turn at the colic flexures . The parts of the colon are either intraperitoneal or behind it in the retroperitoneum . Retroperitoneal organs, in general, do not have

8874-416: The distal gut , as it is further along the gastrointestinal tract than the proximal gut. Gut flora are very dense in this region. The sigmoid colon is the part of the large intestine after the descending colon and before the rectum. The name sigmoid means S-shaped (see sigmoid ; cf. sigmoid sinus ). The walls of the sigmoid colon are muscular and contract to increase the pressure inside the colon, causing

9027-409: 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

9180-438: 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

9333-407: The marginal artery of the colon that runs parallel to the colon for its entire length. Historically, a structure variously identified as the arc of Riolan or meandering mesenteric artery (of Moskowitz) was thought to connect the proximal SMA to the proximal IMA. This variably present structure would be important if either vessel were occluded. However, at least one review of the literature questions

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9486-406: 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

9639-407: 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

9792-484: The stool to move into the rectum. The sigmoid colon is supplied with blood from several branches (usually between 2 and 6) of the sigmoid arteries , a branch of the IMA. The IMA terminates as the superior rectal artery . Sigmoidoscopy is a common diagnostic technique used to examine the sigmoid colon. The rectum is the last section of the large intestine. It holds the formed feces awaiting elimination via defecation. It

9945-668: 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

10098-485: 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

10251-416: The aid of a microscope to determine if they are precancerous or not. It takes 15 years or fewer for a polyp to turn cancerous. Colonoscopy is similar to sigmoidoscopy —the difference being related to which parts of the colon each can examine. A colonoscopy allows an examination of the entire colon (1200–1500 mm in length). A sigmoidoscopy allows an examination of the distal portion (about 600 mm) of

10404-418: The anal canal above the pectinate line drain to the internal ileocolic nodes. The anal canal below the pectinate line drains into the superficial inguinal nodes. The pectinate line only roughly marks this transition. Sympathetic supply: superior & inferior mesenteric ganglia; parasympathetic supply: vagus & sacral plexus (S2-S4) The endoderm, mesoderm and ectoderm are germ layers that develop in

10557-402: The appendix an undetermined role in immunity. However, the appendix is known to be important in fetal life as it contains endocrine cells that release biogenic amines and peptide hormones important for homeostasis during early growth and development. By the time the chyme has reached this tube, most nutrients and 90% of the water have been absorbed by the body. Indeed, as demonstrated by

10710-509: The average length of the human colon is 160.5 cm and the average inner circumference of the colon is 6.2 cm, the inner surface epithelial area of the human colon has an average area of about 995 cm , which includes 9,950,000 (close to 10 million) crypts. In the four tissue sections shown here, many of the intestinal glands have cells with a mitochondrial DNA mutation in the CCOI gene and appear mostly white, with their main color being

10863-501: 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

11016-415: 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 ,

11169-502: 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 ,

11322-402: 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

11475-495: The blue-gray staining of the nuclei. As seen in panel B, a portion of the stem cells of three crypts appear to have a mutation in CCOI , so that 40% to 50% of the cells arising from those stem cells form a white segment in the cross cut area. Overall, the percent of crypts deficient for CCOI is less than 1% before age 40, but then increases linearly with age. Colonic crypts deficient for CCOI in women reaches, on average, 18% in women and 23% in men by 80–84 years of age. Crypts of

11628-428: 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

11781-416: 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, 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

11934-417: 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

12087-476: 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

12240-430: 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

12393-403: 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

12546-420: 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

12699-469: 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

12852-436: 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,

13005-409: 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 ,

13158-424: 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

13311-483: The colon are Bacillota and Bacteroidota . The ratio between the two seems to vary widely as reported by the Human Microbiome Project. Bacteroides are implicated in the initiation of colitis and colon cancer . Bifidobacteria are also abundant, and are often described as 'friendly bacteria'. A mucus layer protects the large intestine from attacks from colonic commensal bacteria . Following are

13464-463: The colon by the spleen ). The transverse colon hangs off the stomach , attached to it by a large fold of peritoneum called the greater omentum . On the posterior side, the transverse colon is connected to the posterior abdominal wall by a mesentery known as the transverse mesocolon . The transverse colon is encased in peritoneum , and is therefore mobile (unlike the parts of the colon immediately before and after it). The proximal two-thirds of

13617-485: The colon can reproduce by fission, as seen in panel C, where a crypt is fissioning to form two crypts, and in panel B where at least one crypt appears to be fissioning. Most crypts deficient in CCOI are in clusters of crypts (clones of crypts) with two or more CCOI-deficient crypts adjacent to each other (see panel D). About 150 of the many thousands of protein coding genes expressed in the large intestine, some are specific to

13770-430: The colon does not play a major role in absorption of foods and nutrients. About 1.5 litres or 45 ounces of water arrives in the colon each day. The colon is the longest part of the large intestine and its average length in the adult human is 65 inches or 166 cm (range of 80 to 313 cm) for males, and 61 inches or 155 cm (range of 80 to 214 cm) for females. In mammals , the large intestine consists of

13923-464: The colon with microbiota if depleted during the course of an immune reaction. The appendix has also been shown to have a high concentration of lymphatic cells. The ascending colon is the first of four main sections of the large intestine. It is connected to the small intestine by a section of bowel called the cecum. The ascending colon runs upwards through the abdominal cavity toward the transverse colon for approximately eight inches (20 cm). One of

14076-463: The colon, which may be sufficient because benefits to cancer survival of colonoscopy have been limited to the detection of lesions in the distal portion of the colon. Nervous system In biology , 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

14229-427: The commonality of ileostomy procedures, it is possible for many people to live without large portions of their large intestine, or even without it completely. At this point only some electrolytes like sodium , magnesium , and chloride are left as well as indigestible parts of ingested food (e.g., a large part of ingested amylose , starch which has been shielded from digestion heretofore, and dietary fiber , which

14382-408: 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

14535-408: 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

14688-560: The excess water, causing the stools to gradually solidify as they move along into the descending colon . The bacteria break down some of the fiber for their own nourishment and create acetate , propionate , and butyrate as waste products, which in turn are used by the cell lining of the colon for nourishment. No protein is made available. In humans, perhaps 10% of the undigested carbohydrate thus becomes available, though this may vary with diet; in other animals, including other apes and primates, who have proportionally larger colons, more

14841-471: The existence of this vessel, with some experts calling for the abolition of these terms from future medical literature. Venous drainage usually mirrors colonic arterial supply, with the inferior mesenteric vein draining into the splenic vein , and the superior mesenteric vein joining the splenic vein to form the hepatic portal vein that then enters the liver . Middle rectal veins are an exception, delivering blood to inferior vena cava and bypassing

14994-469: The fecal odor is due to indoles , metabolized from the amino acid tryptophan. The normal flora is also essential in the development of certain tissues, including the cecum and lymphatics . They are also involved in the production of cross-reactive antibodies. These are antibodies produced by the immune system against the normal flora, that are also effective against related pathogens, thereby preventing infection or invasion. The two most prevalent phyla of

15147-422: The fluid within the intestinal lumen. The large intestine houses over 700 species of bacteria that perform a variety of functions, as well as fungi , protozoa , and archaea . Species diversity varies by geography and diet. The microbes in a human distal gut often number in the vicinity of 100 trillion, and can weigh around 200 grams (0.44 pounds). This mass of mostly symbiotic microbes has recently been called

15300-422: 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

15453-456: The gastrointestinal tract terminates at the anal verge . The word rectum comes from the Latin rēctum intestīnum , meaning straight intestine . The human rectum is a part of the lower gastrointestinal tract . The rectum is a continuation of the sigmoid colon , and connects to the anus . The rectum follows the shape of the sacrum and ends in an expanded section called an ampulla where feces

15606-468: 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

15759-415: The human gastrointestinal tract. The colon of the large intestine is the last part of the digestive system . It has a segmented appearance due to a series of saccules called haustra . It extracts water and salt from solid wastes before they are eliminated from the body and is the site in which the fermentation of unabsorbed material by the gut microbiota occurs. Unlike the small intestine ,

15912-461: The increased acidity resulting from the formation of these fatty acids. These bacteria also produce large amounts of vitamins , especially vitamin K and biotin (a B vitamin ), for absorption into the blood. Although this source of vitamins, in general, provides only a small part of the daily requirement, it makes a significant contribution when dietary vitamin intake is low. An individual who depends on absorption of vitamins formed by bacteria in

16065-464: 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

16218-450: The large intestine may become vitamin-deficient if treated with antibiotics that inhibit the vitamin producing species of bacteria as well as the intended disease-causing bacteria. Other bacterial products include gas ( flatus ), which is a mixture of nitrogen and carbon dioxide , with small amounts of the gases hydrogen , methane , and hydrogen sulfide . Bacterial fermentation of undigested polysaccharides produces these. Some of

16371-404: The latest human organ to be "discovered" or in other words, the "forgotten organ". The large intestine absorbs some of the products formed by the bacteria inhabiting this region. Undigested polysaccharides (fiber) are metabolized to short-chain fatty acids by bacteria in the large intestine and absorbed by passive diffusion . The bicarbonate that the large intestine secretes helps to neutralize

16524-734: 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,

16677-434: The liver. Lymphatic drainage from the ascending colon and proximal two-thirds of the transverse colon is to the ileocolic lymph nodes and the superior mesenteric lymph nodes , which drain into the cisterna chyli . The lymph from the distal one-third of the transverse colon , the descending colon , the sigmoid colon, and the upper rectum drain into the inferior mesenteric and colic lymph nodes. The lower rectum to

16830-400: 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 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

16983-422: The main functions of the colon is to remove the water and other key nutrients from waste material and recycle it. As the waste material exits the small intestine through the ileocecal valve , it will move into the cecum and then to the ascending colon where this process of extraction starts. The waste material is pumped upwards toward the transverse colon by peristalsis . The ascending colon is sometimes attached to

17136-470: The medication working, such as leakage. One cause of constipation is faecal impaction in the rectum, in which a dry, hard stool forms. Constipation is most commonly due to dietary and lifestyle factors such as inadequate hydration , immobility, and lack of dietary fibre, although there are many potential causes. Such causes may include obstruction because of narrowing, local disease (such as Crohn's disease, fissures or haemorrhoids), or diseases affecting

17289-485: 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

17442-414: 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

17595-530: The most common diseases or disorders of the colon: Colonoscopy is the endoscopic examination of the large intestine and the distal part of the small bowel with a CCD camera or a fiber optic camera on a flexible tube passed through the anus . It can provide a visual diagnosis (e.g. ulceration , polyps ) and grants the opportunity for biopsy or removal of suspected colorectal cancer lesions. Colonoscopy can remove polyps as small as one millimetre or less. Once polyps are removed, they can be studied with

17748-504: The mucous membrane in different regions and include CEACAM7 . The large intestine absorbs water and any remaining absorbable nutrients from the food before sending the indigestible matter to the rectum. The colon absorbs vitamins that are created by the colonic bacteria, such as thiamine , riboflavin , and vitamin K (especially important as the daily ingestion of vitamin K is not normally enough to maintain adequate blood coagulation ). It also compacts feces, and stores fecal matter in

17901-410: 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

18054-426: 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

18207-403: 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

18360-402: 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"

18513-496: 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

18666-531: 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

18819-435: 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

18972-524: The neurological control of the bowel, or slow bowel transit time, including spinal cord injury and multiple sclerosis ; use of medications such as opioids , and conditions such as diabetes mellitus , as well as severe illness. High calcium levels and low thyroid activity may also cause constipation. Testing may be carried out to investigate the cause. This may include blood tests such as biochemistry , calcium levels, thyroid function tests . A digital rectal examination may be performed to see if there

19125-486: 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

19278-450: 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

19431-474: 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

19584-438: 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

19737-401: The outer edges of the cells lining the walls of the crypts) are stained blue-gray with haematoxylin . As seen in panels C and D, crypts are about 75 to about 110 cells long. Baker et al. found that the average crypt circumference is 23 cells. Thus, by the images shown here, there are an average of about 1,725 to 2,530 cells per colonic crypt. Nooteboom et al. measuring the number of cells in

19890-399: The part of the colon from the splenic flexure to the beginning of the sigmoid colon. One function of the descending colon in the digestive system is to store feces that will be emptied into the rectum. It is retroperitoneal in two-thirds of humans. In the other third, it has a (usually short) mesentery. The arterial supply comes via the left colic artery . The descending colon is also called

20043-428: 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

20196-475: The puborectalis muscle occurs. This acts to make the angle between the rectum and anus straighter, and facilitate defecation. For the diagnosis of certain ailments, a rectal exam may be done. These include faecal impaction , prostatic cancer and benign prostatic hypertrophy in men, faecal incontinence , and internal haemorrhoids . Forms of medical imaging used to examine the rectum include CT scans and MRI scans. An ultrasound probe may be inserted into

20349-783: 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

20502-413: The rectal walls expand due to the materials filling it from within, stretch receptors from the nervous system located in the rectal walls stimulate the desire to pass feces, a process called defecation . An internal and external anal sphincter , and resting contraction of the puborectalis , prevent leakage of feces ( fecal incontinence ). As the rectum becomes more distended, the sphincters relax and

20655-499: The rectum ( Latin : per rectum ). By their definitions, suppositories are inserted, and enemas are injected into the rectum. Medications might be given via the rectum to relieve constipation, to treat conditions near the rectum, such as fissures or haemorrhoids, or to give medications that are systemically active when taking them by mouth is not possible. People do not tend to like medications administered by this route because of both cultural issues, discomfort, and issues that may affect

20808-464: The rectum in humans. Apeuthysmenon is the Latinization of ἀπευθυσμένον and euthyenteron has a similar meaning (εὐθύς 'straight). Much of the knowledge of the anatomy of the rectum comes from detailed descriptions provided by Andreas Vesalius in 1543. Large intestine In humans, the large intestine begins in the right iliac region of the pelvis , just at or below the waist , where it

20961-402: The rectum to view nearby structures such as the prostate. Colonoscopy and sigmoidoscopy are forms of endoscopy that use a guided camera to directly view the rectum. The instruments may have the ability to take biopsies if needed, for diagnosis of diseases such as cancer . A proctoscope is another instrument that is used to visualise the rectum. Body temperature can also be taken in

21114-408: The rectum until it can be discharged via the anus in defecation . The large intestine also secretes K+ and Cl-. Chloride secretion increases in cystic fibrosis. Recycling of various nutrients takes place in the colon. Examples include fermentation of carbohydrates, short chain fatty acids, and urea cycling. The appendix contains a small amount of mucosa-associated lymphoid tissue which gives

21267-699: The rectum. Rectal temperature can be taken by inserting a medical thermometer not more than 25 mm (0.98 in) into the rectum via the anus . A mercury thermometer should be inserted for 3 to 5 minutes; a digital thermometer should remain inserted until it beeps. Normal rectal temperature generally ranges from 36 to 38 °C (97 to 100 °F) and is about 0.5 °C (32.9 °F) above oral (mouth) temperature and about 1 °C (34 °F) above axilla (armpit) temperature. Availability of less invasive temperature-taking methods including tympanic (ear) and forehead thermometers has facilitated reduced use of this method. Some medications are also administered via

21420-489: The rectum. They induce a sensation of rectal fullness and contraction that frequently leads to defecation, but without the distinct waves of activity characteristic of peristalsis. Other diseases of the rectum include: Due to the proximity of the anterior wall of the rectum to the vagina in females or to the prostate in males, and the shared nerves thereof, the rectum is an erogenous zone and its stimulation or penetration can result in sexual arousal . English rectum

21573-646: 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

21726-510: 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,

21879-399: 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

22032-713: 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,

22185-400: 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

22338-401: The sigmoid colon five centimeters above the rectum, becoming a singular longitudinal muscle that surrounds the rectum on all sides for its entire length. The blood supply of the rectum changes between the top and bottom portions. The top two thirds is supplied by the superior rectal artery . The lower third is supplied by the middle and inferior rectal arteries . The superior rectal artery

22491-531: 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

22644-498: 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

22797-399: 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

22950-597: 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 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

23103-399: The transverse colon is perfused by the middle colic artery , a branch of the superior mesenteric artery (SMA), while the latter third is supplied by branches of the inferior mesenteric artery (IMA). The "watershed" area between these two blood supplies, which represents the embryologic division between the midgut and hindgut , is an area sensitive to ischemia . The descending colon is

23256-419: 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

23409-495: The wall of the rectum is similar to the rest of the gastrointestinal tract ; namely, that it possesses a mucosa with a lining of a single layer of column-shaped cells with mucus-secreting goblet cells interspersed, resting on a lamina propria , with a layer of smooth muscle called muscularis mucosa . This sits on an underlying submucosa of connective tissue, surrounded by a muscularis propria of two bands of muscle, an inner circular band and an outer longitudinal one. There are

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