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The peripheral nervous system includes the nerve roots, dorsal root ganglia, brachial and lumbosacral plexuses, and peripheral nerves. Nerve roots join to form plexuses (cervical plexus, brachial plexus, lumbosacral plexus), which give rise to peripheral nerves.

Individual peripheral nerves may be sensory, motor, autonomic, or mixed. Mixed nerves are two-way conduits: Efferent motor information travels from the spinal cord to the muscles and afferent sensory information travels in from the periphery to the spinal cord.

Efferent motor signals travel from the anterior horn cells (alpha motor neurons) into peripheral nerves by way of ventral roots. These are the lower motor neurons that are under the control of the corticospinal tracts (see Ch. 4).

Afferent sensory information travels from the peripheral nerves to dorsal root ganglia, and from dorsal root ganglia into the spinal cord by way of dorsal roots to enter the ascending sensory pathways (see Ch. 4).

Efferent peripheral nervous system sympathetic autonomic signals originate in the intermediolateral columns of the thoracic spinal cord, synapse in paraspinal ganglia, and then travel in postganglionic neurons to end organs. The parasympathetic control of the organs of the thorax and most of the organs of the abdomen comes from the vagus nerve (cranial nerve 10), with the exception of the final third of the gastrointestinal tract, the bladder, and the reproductive organs, which receive parasympathetic input from nerves arising from nerve roots from sacral levels 2 through 4. The central control of the autonomic peripheral nervous system comes from hypothalamic-brainstem pathways.

Peripheral nervous system pathology can affect the roots (radiculopathy), dorsal root ganglia (ganglionopathy; also known as sensory neuronopathy), alpha motor neurons (motor neuron disease), brachial or lumbosacral plexus (plexopathy), or one or more peripheral nerves (peripheral neuropathy).

Localization of focal or multifocal peripheral nervous system findings requires determining whether a pattern of weakness, sensory disturbance, and/or reflex diminution/absence can be explained by a problem with:

  • A single nerve (mononeuropathy)

  • A single root (radiculopathy)

  • Multiple individual nerves (mononeuropathy multiplex)

  • Multiple roots (polyradiculopathy)

  • The brachial or lumbosacral plexus (plexopathy)

If a peripheral nervous system process is generalized rather than focal, clinical symptoms and signs can help to determine which level(s) of the peripheral nervous system is/are affected (i.e., polyneuropathy, polyradiculopathy, polyradiculoneuropathy, or ganglionopathy). The main symptoms of each category of peripheral nervous system disorder are listed in Table 15–1 and discussed in more detail in their respective sections in this chapter. Localization guides differential diagnosis since different types of pathologic processes can selectively affect different levels of the peripheral nervous system. Common causes of pathology at each level of the peripheral nervous system are listed in Table 15–2...

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