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, The ultimate function of the nervous system is to control the body, especially its movement in the environment.
It does this by extracting information from the environment using sensory receptors, sending signals that encode this information into 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 perceptual capabilities such as vision, complex social interactions, rapid coordination of organ systems, and integrated processing of concurrent signals.
This is small in comparison to your overall body weight, but according to the Smithsonian Institute, your brain gets 20 percent of your oxygen supply and blood flow. A special barrier called the blood-brain barrier prevents harmful substances in the blood from entering your brain. Since the first neurotransmitter was discovered in , more than substances have been implicated in signal transmission between nerves.
A couple that you may be familiar with are dopamine and serotonin. Researchers are hard at work to develop ways to repair damage to the nervous system. This is accomplished using a device that sends electrical signals to your vagus nerve.
This, in turn, sends signals to specific parts of the brain. Vagus nerve stimulation can help to lower the number of seizures in people with some types of epilepsy. Its effectiveness is being assessed for conditions like headaches and rheumatoid arthritis as well.
A study in mice used imaging to visualize nerve cells surrounding fat tissue. Researchers found that stimulating these nerves also stimulated the breakdown of fat tissue. Additional research is needed, but this could have implications for conditions like obesity. The system is able to collect information on applied pressure and convert it into electric impulses that can be integrated on a transistor.
This transistor then releases electrical impulses in patterns consistent with those produced by neurons. This vast system of nerves works together as a communication network. Sensory nerves deliver information from your body and your environment to the CNS. Meanwhile, the CNS integrates and processes this information in order to send messages on how to respond via motor nerves.
The 12 cranial nerves are pairs of nerves that start in different parts of your brain. They control everything from your facial expression to…. If you have a pinched nerve in the neck, doing exercises can help. These seven stretches relieve mild pain by decompressing the nerve and loosening….
A nerve conduction velocity NCV test is used to assess nerve damage and dysfunction. The axons are bundled together into groups called fascicles. Each fascicle is wrapped in a layer of connective tissue called the perineurium. Finally, the entire nerve is wrapped in a layer of connective tissue called the epineurium.
See the following illustrations of these structures. The endoneurium consists of an inner sleeve of material called the glycocalyx and a mesh of collagen. Nerves are bundled along with blood vessels, which provide essential nutrients and energy to the enclosed, and metabolically demanding, neurons. Within the endoneurium, individual nerve fibers are surrounded by a liquid called the endoneurial fluid.
The endoneurium has properties analogous to the blood—brain barrier. It prevents certain molecules from crossing from the blood into the endoneurial fluid. In this respect, endoneurial fluid is similar to cerebrospinal fluid in the central nervous system. During nerve irritation or injury, the amount of endoneurial fluid may increase at the site of damage. This increase in fluid can be visualized using magnetic resonance neurography to diagnose nerve damage.
An illustration of a cross-section of a nerve highlighting the epineurium and perineurium. Individual axons can also be seen as tiny circles within each perineurium. A nerve conveys information in the form of electrochemical impulses known as nerve impulses or action potentials carried by the individual neurons that make up the nerve.
The impulses travel from one neuron to another by crossing a synapse, and the message is converted from electrical to chemical and then back to electrical.
Neurologists usually diagnose disorders of the nerves by a physical examination, including the testing of reflexes, walking and other directed movements, muscle weakness, proprioception, and the sense of touch. This initial exam can be followed with tests such as nerve conduction study, electromyography, or computed tomography. Myelination speeds up the movement of APs in the axon by reducing the number of APs that must form for a signal to reach the end of an axon.
The myelination process begins speeding up nerve conduction in fetal development and continues into early adulthood.
Myelinated axons appear white due to the presence of lipids and form the white matter of the inner brain and outer spinal cord. White matter is specialized for carrying information quickly through the brain and spinal cord. The gray matter of the brain and spinal cord are the unmyelinated integration centers where information is processed. Reflexes are fast, involuntary responses to stimuli. Reflexes are integrated in the gray matter of the spinal cord or in the brain stem.
Reflexes allow the body to respond to stimuli very quickly by sending responses to effectors before the nerve signals reach the conscious parts of the brain. This explains why people will often pull their hands away from a hot object before they realize they are in pain.
All sensory receptors can be classified by their structure and by the type of stimulus that they detect. Structurally, there are 3 classes of sensory receptors: free nerve endings, encapsulated nerve endings, and specialized cells. Free nerve endings are simply free dendrites at the end of a neuron that extend into a tissue. Pain, heat, and cold are all sensed through free nerve endings.
An encapsulated nerve ending is a free nerve ending wrapped in a round capsule of connective tissue. When the capsule is deformed by touch or pressure, the neuron is stimulated to send signals to the CNS.
Specialized cells detect stimuli from the 5 special senses: vision, hearing, balance, smell, and taste. Each of the special senses has its own unique sensory cells—such as rods and cones in the retina to detect light for the sense of vision. Functionally, there are 6 major classes of receptors: mechanoreceptors, nociceptors, photoreceptors, chemoreceptors, osmoreceptors, and thermoreceptors. By: Tim Taylor.
0コメント