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Nervous System of the Rat
The nervous system along with the endocrine system serves to integrate and coordinate the various body components. The nerve cell or neuron is the basic unit of the nervous system. A neuron consists of a cell body plus its processes. The cell body is made up of a nucleus with a prominent nucleolus, cytoplasm, and cell membrane.
In a typical neuron, the cytoplasm is drawn out into short processes called dendrites and a single long process called an axon. There are many variations on this basic pattern. In situations where the axon is long, it may have several branches known as collaterals. The terminus of the axon usually branches into numerous fine ramifications. Neurons are anatomically and functionally related to each other and to other cells by their processes which are in contact with other nerve cells, or with epithelial, muscular or gland cells. The junctions between neurons are called synapses. Nerve impulses bridge synapses via chemical mediators that are unidirectional in their activity.
The axons of many vertebrate nerve cells are often wrapped in a sheath of myelin, which is not part of the neuron, but is laid down by special cells known as Schwann cells. A single axon will have derived its myelin sheath from several Schwann cells arranged serially along the axon. A narrow constriction marks the junction between the myelin sheath laid down by adjacent Schwann cells. These constrictions are known as nodes of Ranvier. The conduction of an impulse down a myelinated axon occurs in a jumping fashion with the depolarization moving from node of Ranvier to node of Ranvier. For this reason, conduction in myelinated nerves is faster than non-myelinated nerves.
The nerve cells found in any multicellular animal are remarkably similar in their functional properties with the major differences being due to the anatomical organizations of these nerve cells. The simplest nervous system, as seen in the cnidarians (such as Hydra), consists of a diffuse network of nerve cells and little or no concentration of nervous tissues. Among the more complicated multicellular animals there is a marked tendency for the cell bodies of neurons to be grouped into masses known as ganglia. Ganglia tend to be lined up along one or more central nerve cords. The most anterior of these ganglia tends to form a "brain". Associated with the accumulation of nervous tissue in the anterior region is an accompanying proliferation of sense organs (eyes, antennae, chemoreceptors, ears, etc.) in this same area. The term cephalization is used to describe the overall evolutionary process and it seems to be definitely linked with bilateral symmetry
Vertebrates possess a dorsal hollow nerve cord with a hollow expanded brain at the anterior end. These two components of the nervous system comprise what is known as the central nervous system (CNS). All nervous connectins outside of the CNS are referred to as the peripheral nervous system. Remember that these are anatomical subdivisions of convenience and that functionally both systems are integrated. The CNS is protected by the skull and the vertebral column. Paired spinal nerves join the dorsal nerve cord at regular intervals (one pair associated with each vertebra).
