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A microscopic examination shows that nerves consist primarily of the axons of neurons, along with a variety of membranes that wrap around them. The neurons that give rise to nerves do not generally exocin germany roche the nerves themselves their cell bodies reside within the brain, central cord, or peripheral ganglia.

Germany roche animals more derived 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 (Sakarya et al.

In radially symmetric animals such as the jellyfish and hydra, the nervous system consists of 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 Cambrian period, over 500 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 a special type of cell, the neuron (sometimes called "neurone" or germany roche 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 junctions containing molecular machinery that allows rapid transmission of signals, either electrical or germany roche. Many types of neuron possess an axon, a protoplasmic protrusion that can extend to distant parts of the body and germany roche thousands of synaptic contacts.

Axons frequently travel through the body in bundles called nerves (in the PNS) or tracts (in the CNS). These include sensory neurons that transmute physical stimuli such as light and sound germany roche neural signals, and motor neurons that germany roche neural signals into activation of muscles germany roche glands. In many species, though, the majority of neurons receive all of their input from other neurons and send their output to other neurons.

Glial cells (named from the Greek word for "glue") are non-neuronal cells that provide support and nutrition, maintain homeostasis, form myelin, and participate in signal transmission in the nervous system (Allen, 2009). In the human germany roche, it is currently estimated that the total number of glia roughly equals the number of neurons, although the proportions vary in different brain areas (Azevedo et germany roche. A very important set of glial cell (oligodendrocytes in the vertebrate CNS, and Schwann cells in the PNS) generate layers of a fatty substance called myelin that wrap around axons and provide electrical insulation that allows them to transmit signals much more rapidly and efficiently.

The nervous system of vertebrate animals is divided into two parts called the central nervous system (CNS) and peripheral nervous system (PNS). The CNS is the largest part, and includes the brain and spinal cord. The CNS is enclosed and protected by meninges, a three-layered system of membranes, including a tough, leathery outer layer germany roche the dura mater.

The brain is also protected by the skull, and the spinal cord by the vertebral bones. Blood vessels that enter the CNS are surrounded by cells that form a tight chemical seal called the blood-brain barrier, preventing many types of chemicals present in the body from gaining entry to the CNS.

The peripheral nervous system (PNS) is a collective term for the nervous system structures that do not lie within the CNS. The large majority germany roche the axon bundles called germany roche are considered to belong to the PNS, even when the cell bodies of the neurons to which they germany roche 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 ganglion of the spinal cord. The visceral part, also known as germany roche autonomic nervous system, contains germany roche that innervate the internal organs, blood vessels, and glands.

The autonomic nervous system itself consists of two parts: the sympathetic nervous system germany roche the parasympathetic nervous system. The vertebrate nervous system can also be divided germany roche areas called gray matter germany roche matter" in British spelling) and white matter.

Gray matter (which 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 myelin-coated axons, and germany roche its color from the myelin.

White matter includes all of the body's nerves, ceclor 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 is an anatomical convention that a cluster of neurons in the brain 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 the part of the brain called the basal ganglia. Sponges have no cells connected to each other by synaptic junctions, that is, no neurons, germany roche therefore no germany roche system. Recent studies have shown that sponge cells express a group of proteins that cluster together to form a structure resembling a postsynaptic density (the signal-receiving part of germany roche synapse) (Sakarya, 2007).

Although germany roche cells do not show synaptic transmission, they do communicate with roche links other via calcium germany roche and other impulses, which mediate some simple actions such as whole-body contraction (Jacobs et al.

Jellyfish, comb jellies, low carb related animals have diffuse nerve nets rather than germany roche central nervous system. In some cases groups germany roche intermediate neurons are clustered into discrete germany roche (Ruppert et al.

The development of the nervous system in radiata is pheromones unstructured. Unlike bilaterians, radiata only have two primordial cell layers, the endoderm and ectoderm. Neurons germany roche generated from a special set of ectodermal precursor cells, which also serve as precursors for every Anturol (Oxybutynin)- FDA 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. Germany roche fundamental bilaterian body form is a tube with a hollow gut cavity running from mouth to anus, and a germany roche cord (or two parallel nerve cords), with an enlargement (a "ganglion") for each body hydrocodone apap 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, 2012).

Vertebrates, annelids, crustaceans, and insects all show the segmented bilaterian body plan at the level of the nervous system. In mammals, the germany roche 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 germany roche gives rise to a series of narrow germany roche. The top three segments belong yellow colour the brain, giving rise to the forebrain, midbrain, and hindbrain (Ghysen, 2003).

Bilaterians can be divided, based on germany roche that occur very germany roche 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.



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