152. The Peripheral Nervous System and Cranial Nerves

  1. Introduction: the peripheral nervous system consists of the cranial, spinal, and visceral nerves as well as the cranial, spinal, and autonomic ganglia. The peripheral nervous system develops from a variety of sources
    1. ALL SENSORY CELLS, both somatic and visceral, of the peripheral nervous system are derived from neural crest cells, and the cell bodies of these cells are found outside the central nervous system
      1. All the peripheral sensory cells are at first bipolar (except for the cells in the spiral ganglion of the cochlear and in the vestibular ganglion of cranial nerve VIII). However, the 2 processes soon unite to form a single process and a unipolar type neuron
        1. The process has peripheral and central branches or processes
        2. The peripheral process terminates in an afferent ending; the central process enters the spinal cord or brain
        3. The sensory cells of the ganglion of the vestibulocochlear (VIII) cranial nerve remain bipolar
    2. THE CELL BODY OF EACH AFFERENT NEURON is invested by a capsule of satellite cells, also derived from the neural crest. The capsule is continuous with the neurilemmal sheath of Schwann cells (of neural crest origin) that surrounds the axon of the afferent neuron
      1. A layer of connective tissue, continuous with the endoneuria1 sheath of the nerve fiber and derived from mesenchyme, surrounds the satellite cells
  2. The cranial nerves
    1. THE NEURAL CREST CELLS in the region of the brain migrate to form sensory ganglia only in relation to the trigeminal (V), the facial (VII), the glossopharyngeal (IX), the vagus (X), and the accessory (XI) cranial nerves. These are the branchial mixed nerves
    2. THE PURELY SENSORY NERVES are the olfactory (I), the optic (II), and the vestibulocochlear (VIII) cranial nerves
      1. The neural crest cells in the brain region form the sensory ganglia for the vestibulocochlear (VIII) nerve
      2. The olfactory and optic nerves, however, are not typical sensory nerves since the olfactory bulbs and the eyes are outgrowths from the brain. Thus, these nerves are really tracts of the brain
    3. THE REMAINING CRANIAL NERVES, the oculomotor (III), the trochlear (IV), the abducens (VI), the accessory (XI), and the hypoglossal (XII) are motor nerves
      1. They all, however, contain sensory fibers of proprioception, except for cranial nerve XI. The cell bodies of these proprioceptive fibers do not form ganglia but remain scattered among the axons of the motor neurons as they leave the brainstem
    4. THE CRANIAL ROOT OF THE ACCESSORY (XI) NERVE is actually an isolated root of the vagus (X) cranial nerve that becomes enclosed with the fibers of the so-called spinal root of the accessory nerve. After traveling with the spinal root for a short time, the cranial root enters the foramen magnum and joins the vagus nerve and is enclosed with it
      1. The spinal portion of the accessory (XI) nerve is actually a cervical spinal motor nerve arising from the lateral surface of the upper 5 or 6 cervical segments of the spinal cor It innervates the sternocleidomastoid and trapezius muscles.
  3. Cells of the neural crest also differentiate into multipolar neurons of the autonomic ganglia: sympathetic ganglia, collateral or prevertebral ganglia (cardiac, celiac, mesenteric, etc.), and parasympathetic or terminal ganglia (Meissner's or submucosal). They also give rise to melanoblasts and cells of the adrenal medulla
    1. CELLS OF THE PARAGANGLIA are called chromaffin cells (also of neural crest origin)

the peripheral nervous system and cranial nerves: image #1