Nerve tissue consists of two principal types of cells, neurons and supporting cells
The neuron, or nerve cell, is the functional unit of the nervous system; it consists of a cell body, containing the nucleus, and several processes of varying length. Nerve cells are specialized to receive stimuli from other cells and to conduct electrical impulses to other parts of the system via their processes. They are arranged as an integrated communications network, with several neurons in a chain-like fashion typically involved in sending impulses from one part of the system to another. Specialized contacts between neurons that provide for transmission of information from one neuron to the next are called synapses.
Supporting cells are nonconducting cells that are in intimate-apposition to the neurons. In the CNS they are called neuroglia or, simply, glia. In the PNS they are called Schwann cells and satellite cells. Schwann cells surround the processes of nerve cells and isolate them from adjacent cells and extracellular matrix. Within ganglia the supporting cells are called satellite cells. They surround the nerve cell bodies, the part of the cell that contains the nucleus, and are analogous to the Schwann cells. Supporting cells provide
• Physical support (protection) for delicate neuronal processes
• Electrical insulation for nerve cell bodies and processes
• Metabolic exchange pathways between the vascular system and the neurons of the nervous system
In addition to neurons and supporting cells, an extensive vasculature is present in both the CNS and the PNS. The blood vessels are separated from the nerve tissue by the basal laminae and variable amounts of connective tissue, depending on vessel size. The boundary between blood vessels and nerve tissue in the CNS excludes many substances that normally leave blood vessels to enter other tissues. This selective restriction of blood-borne substances in the CNS is called the blood-brain barrier, which is discussed on page 313.
The nervous system allows rapid response to external stimuli
The nervous system evolved from the simple neuroeffec-tor system of invertebrate animals. In primitive nervous systems, only simple receptor-effector reflex loops exist to respond to external stimuli. In higher animals and humans, the SNS retains the ability to respond to stimuli from the external environment through the action of effector cells (such as skeletal muscle), but the neuronal responses are infinitely more varied. They range from simple reflexes that require only the spinal cord to complex operations of the brain, including memory and learning.
The autonomic part of the nervous system regulates the function of internal organs
The specific effectors in the internal organs that respond to the information carried by autonomic neurons include
• Smooth muscle, whose contraction modifies the diameter or shape of tubular or hollow viscera such as the blood vessels, gut, gallbladder, and urinary bladder.
• Cardiac conducting cells (Purkinje fibers) located within the conductive system of the heart. The inherent frequency of Purkinje fiber depolarization regulates the rate of cardiac muscle contraction and can be modified.
• Glandular epithelium, in which the synthesis, composition, and release of secretions can be modified.
The regulation of the function of internal organs involves close cooperation between the nervous system and the endocrine system. Neurons in several parts of the brain and other sites behave as secretory cells and are referred to as neuroendocrine tissue. The varied roles of neurosecretions in regulating the functions of the endocrine, digestive, respiratory, urinary, and reproductive systems are described in subsequent chapters.
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