A part of the peripheral nervous system called the autonomic nervous system controls many of the body processes we almost never need to think about, like breathing, digestion, sweating, and shivering. The autonomic nervous system has two parts: the sympathetic nervous system and the parasympathetic nervous system. The sympathetic nervous system prepares the body for sudden stress, like if you witness a robbery.
When something frightening happens, the sympathetic nervous system makes the heart beat faster so that it sends blood quickly to the different body parts that might need it. It also causes the adrenal glands at the top of the kidneys to release adrenaline, a hormone that helps give extra power to the muscles for a quick getaway. This process is known as the body's "fight or flight" response. The parasympathetic nervous system does the exact opposite: It prepares the body for rest.
It also helps the digestive tract move along so our bodies can efficiently take in nutrients from the food we eat. Sight probably tells us more about the world than any other sense.
Light entering the eye forms an upside-down image on the retina. The retina transforms the light into nerve signals for the brain. The brain then turns the image right-side up and tells us what we are seeing. Every sound we hear is the result of sound waves entering our ears and making our eardrums vibrate. These vibrations then move along the tiny bones of the middle ear and turned into nerve signals. The cortex processes these signals, telling us what we're hearing.
The tongue contains small groups of sensory cells called taste buds that react to chemicals in foods. Taste buds react to sweet, sour, salty, bitter, and savory. The taste buds send messages to the areas in the cortex responsible for processing taste. Olfactory cells in the mucous membranes lining each nostril react to chemicals we breathe in and send messages along specific nerves to the brain. The skin contains millions of sensory receptors that gather information related to touch, pressure, temperature, and pain and send it to the brain for processing and reaction.
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Skip to main content. Brain and nerves. Home Brain and nerves. Nervous system. Actions for this page Listen Print. Summary Read the full fact sheet. On this page. Neurones are the building blocks Central nervous system The peripheral nervous system Problems of the nervous system Where to get help Things to remember.
Neurones are the building blocks The basic building block of the nervous system is a nerve cell, or neurone. Central nervous system The brain and the spinal cord make up the central nervous system. The spinal cord The spinal cord is connected to the brain and runs the length of the body. The peripheral nervous system Nerves connect the brain and spinal cord to the peripheral nervous system, which is what nerve tissue outside of the central nervous system is called.
The autonomic nervous system The autonomic nervous system is part of the peripheral nervous system. In some cases groups of intermediate neurons are clustered into discrete ganglia Ruppert et al. The development of the nervous system in radiata is relatively unstructured. Unlike bilaterians, radiata only have two primordial cell layers, the 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 Sanes et al.
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 a common wormlike ancestor that appeared during the Cambrian period, — million years ago Balavoine, The fundamental bilaterian body form is a tube with a hollow gut cavity running from mouth to anus, and a nerve cord or two parallel nerve cords , with an enlargement a "ganglion" for each body segment, with an especially large ganglion at the front, called the "brain".
It has not been definitively established whether the generic form of the bilaterian central nervous system is inherited from the so-called "Urbilaterian" — the last common ancestor of all existing bilaterians — or whether separate lines have evolved similar structures in parallel Northcutt, On one hand, the presence of a shared set of genetic markers, as well as a tripartite brain structure shared by widely separated species Hirth, , suggest common derivation; on the other hand, the fact that some modern types of bilaterians such as echinoderms lack a central nerve cord, while many lack recognizably tripartite brains, suggest that this might have been the primitive state Northcutt, Vertebrates, annelids, crustaceans, and insects all show the segmented bilaterian body plan at the level of the nervous system.
In mammals, 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 Ghysen, Bilaterians can be divided, based on events that occur very early in embryonic development, into two groups superphyla called protostomes and deuterostomes Erwin et al.
Deuterostomes include vertebrates as well as echinoderms, hemichordates mainly acorn worms , and Xenoturbellidans Bourlat et al. Protostomes, the more diverse group, include arthropods, molluscs, and numerous types 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 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 the body, while all vertebrates have spinal cords that run along the dorsal midline Lichtneckert and Reichert, 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 to each other by transverse nerves resembling the rungs of a ladder. These transverse nerves help coordinate movement of the two sides of the animal. Two ganglia at the head end function as a simple brain. Photoreceptors in the animal's eyespots provide sensory information on light and dark Adey, WR.
The nervous system of one particular type of nematode, the tiny roundworm Caenorhabditis elegans , has been mapped out down to the synaptic level. This has been possible because in this species, every individual worm ignoring mutations and sex differences has an identical set of neurons, with the same locations and chemical features, and the same connections to other cells.
Every neuron and its cellular lineage has been recorded and most, if not all, of the neural connections are mapped. The nervous system of C. Males have exactly neurons, while hermaphrodites have exactly neurons Hobert, , an unusual feature called eutely. Arthropods, such as insects and crustaceans, have a nervous system made up of a series of ganglia, connected by a pair of ventral nerve cords running along the length of the abdomen Chapman, Most body segments have one ganglion on each side, but some 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, 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 is processed by the brain. In arthropods, most 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, called the primary neurite, runs from the cell body and branches profusely, with some parts transmitting signals and other parts receiving signals.
Thus, most parts of 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 Chapman, There are, however, important exceptions to this rule, including the mushroom bodies, which play a central role in learning and memory.
A neuron is called identified if it has properties that distinguish it from every other neuron in the same animal — 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 Hoyle and Wiersma, 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.
As mentioned above, in the roundworm Caenorhabditis Elegans every neuron in the body is uniquely identifiable, with the same location and the same connections in every individual worm.
The brains of many molluscs and insects also contain substantial numbers of identified neurons Hoyle and Wiersma, In vertebrates, the best known identified neurons are the gigantic Mauthner cells of fish Stein, Every fish has two Mauthner cells, located 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 traveling down through the spinal cord, making numerous connections as it goes. The synapses generated by 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 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 all by itself, 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 is a special type of identified neuron, defined as a neuron that is capable of driving a specific behavior individually Stein, , p.
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 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 Simmons and Young,
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