Cranial & Spinal Nerves
The central nervous system communicates with the body by means of nerves that exit the CNS from the brain (cranial nerves) and spinal cord (spinal nerves.) These nerves, together with aggregations of cell bodies located outside the CNS, constitute the peripheral nervous system.
The peripheral nervous system (PNS) consists of nerves (collection of axons) and their associated ganglia (collection of cell bodies). The CNS cannot function without the PNS. Thus, this following section serves to complete the explanation of the CNS; it also introduces concepts concerning the PNS.
There are twelve pairs of cranial nerves. Two of these pairs arise from neuron cell bodies located in the forebrain and hindbrain. The cranial nerves are designated by Roman numerals and by names. The Roman numerals refer to the order in which the nerves are positioned from the front of the brain to the back. The names indicate structures innervated by these nerves (e.g. facial) or the principal function of the nerves (e.g. occulomotor). A summary of the cranial nerves is presented in table 8.4.
Most cranial nerves are classified as mixed nerves. This term indicates that the nerve contains both sensory and motor fibres. Those cranial nerves associated with the special senses (e.g. olfactory, optic), however, consist of sensory fibres only. The cell bodies of these sensory neurons are not located in the brain, but instead are found in ganglia near the sensory organ.
There are thirty-one pairs of spinal nerves. These nerves are grouped into eight cervical, twelve thoracic, five lumbar, five sacral, and one coccygeal according to the region of the vertebral column from which they arise .
Each spinal nerve is a mixed nerve composed of sensory and motor fibres. The fibres are packaged together in the nerve, but separate near the attachment of the nerve to the spinal cord. This produces two "roots" to each nerve. The dorsal root is composed of sensory fibres, and the ventral root is composed of motor fibres. the dorsal contains an enlargement called the dorsal root ganglion, where the cell bodies of the sensory neurons are located. The motor neuron is a somatic motor neuron that innervates skeletal muscles; its cell body is not located in a ganglion, but instead is with the gray matter of the spinal cord. Some autonomic motor neurons (which innervate involuntary effectors), however, have their cell bodies in ganglia outside the spinal cord.
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