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FUNCTIONAL CLASSIFICATION OF NEURONS

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The neuron, or nerve cell, is the basic functional unit of the nervous system. A neuron includes its cell body and processes (axons and dendrites). Long neuronal processes are frequently referred to as fibers. Neurons are generally classified as either afferent or efferent:

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  • Afferent, or sensory, neurons receive input from peripheral structures and transmit it to the spinal cord and/or brain.

  • Efferent, or motor, neurons transmit impulses from the brain and/or spinal cord to effectors (skeletal muscle, cardiac muscle, smooth muscle, glands) throughout the body.

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ANATOMICAL DIVISIONS OF THE NERVOUS SYSTEM

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The nervous system has two anatomical divisions:

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  1. The central nervous system (CNS) includes the brain and spinal cord.

  2. The peripheral nervous system (PNS) consists of spinal nerves, their roots, and branches; cranial nerves (CN) and their branches; and components of the autonomic nervous system (ANS).

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Collections of nerve cell bodies in the CNS form nuclei, whereas those in the PNS form ganglia. Ganglia and nuclei contain either motor or sensory neurons. Bundles of axons in the CNS are called tracts. Similar neuronal processes collected in the PNS form nerves. Nerves are categorized based on their CNS origin:

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  • Spinal nerves are attached to the spinal cord. They transmit both motor and sensory impulses and are, thus, considered mixed nerves.

  • Most CN are attached to the brain. Some CN are either motor or sensory only, while others are mixed.

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Spinal Nerves

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The spinal cord is composed of segments, as indicated by the 31 pairs of spinal nerves. Each segment has numerous dorsal (posterior) and ventral (anterior) rootlets that arise from the respective surfaces of the spinal cord (Fig. 1.1). Dorsal rootlets contain neuronal processes that conduct afferent impulses to the spinal cord, whereas the ventral rootlets conduct efferent impulses from the spinal cord. Respective rootlets from each segment unite to form dorsal and ventral roots:

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  • The dorsal root contains the central processes of sensory neuronal cell bodies that are located in the dorsal root ganglion (DRG). The DRG is also called a spinal ganglion. The peripheral processes of these neurons are located in the spinal nerve, its rami, and their branches. These processes end at or form receptors.

  • The ventral root contains motor fibers. Their neuronal cell bodies are found in the gray matter of the spinal cord: ventral horn if the axons innervate skeletal muscle; lateral horn if the axons supply smooth muscle, cardiac muscle, or glands.

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Figure 1.1

Somatic components of a spinal nerve.

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The dorsal and ventral roots join to form a short, mixed spinal nerve. Almost immediately after its formation, the spinal nerve divides into ...

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