Page 378 - Atlas of Histology with Functional Correlations

The supportive cells in the PNS are the Schwann cells. Their main

function is to surround and form the insulating, lipid-rich myelin sheaths
around the larger axons. The myelin sheaths protect axons and maintain
proper ionic environment for impulse conduction and propagation. Each
Schwann cell myelinates a portion of a single axon. However, a single
Schwann cell can surround its cytoplasm with numerous unmyelinated axons.

The function of Schwann cells in the PNS is similar to that of the
oligodendrocytes in the CNS, except that the cytoplasmic processes of a
single oligodendrocyte can myelinate numerous axons. Myelin sheaths are

not continuous, solid sheets along the entire axon; rather, they exhibit small
nodal gaps called nodes of Ranvier that are located between the myelin
sheaths produced by the myelinating cells. The length of the axon covered by
the myelin sheath of one Schwann cell is called the internode or internodal
segment. The size of the internodes varies with the size of the axon. The

node of Ranvier measures between 1 and 2 μm, whereas the internodes can
be a few millimeters, depending on the size of the axon. At the nodes of
Ranvier, the axons are not insulated by myelin sheaths. As a result, these

nodes significantly accelerate the conduction of nerve impulses (action
potentials) along the axons. In large, myelinated axons, the nerve impulse or
action potential jumps from node to node, resulting in a more efficient and
faster conduction of the impulse. This type of fast impulse propagation along
the myelinated axons is called saltatory conduction.

Small unmyelinated axons conduct nerve impulses at a much slower rate
than larger, myelinated axons. In unmyelinated axons, even though they are
surrounded by the cytoplasm of the Schwann cell, the impulse travels along
the entire length of the axon; as a result, conduction efficiency of the impulse

and velocity is reduced. Thus, the larger, myelinated axons have the highest
velocity of impulse conduction. Also, the rate of impulse conduction depends
directly on the axon size and the myelin sheath.

The satellite cells are small, flat cells that surround the neurons of PNS
ganglia. Ganglia are collections of neurons that are located outside of the

CNS. Peripheral ganglia are located parallel to the vertebral column near the
junction of the dorsal and ventral roots of the spinal nerves and near visceral
organs. Satellite cells provide structural support for the neurons, insulate

them, and regulate the exchange of different metabolic substances between
the neurons and the interstitial fluid.

377