Page 373 - Atlas of Histology with Functional Correlations

SECTION 2 Peripheral Nervous System

(PNS)

The PNS consists of neurons, supportive cells, nerves, and axons that are located
outside of the CNS (Fig. 9.21). These include cranial nerves from the brain and

spinal nerves from the spinal cord along with their associated ganglia. Ganglia
(singular, ganglion) are small accumulations of neurons and their supportive
glial cells surrounded by a connective tissue capsule. The nerves of the PNS
contain both sensory and motor axons. These axons transmit information
between the peripheral organs and the CNS. The neurons of the peripheral

nerves are located either within the CNS or outside of the CNS in different
ganglia. Nerves, arteries, veins, and lymphatic vessels that course together form
the neurovascular bundle.

CONNECTIVE TISSUE LAYERS IN THE

PERIPHERAL NERVOUS SYSTEM

A peripheral nerve is composed of axons of various sizes surrounded by layers
of connective tissue that partition the nerve into several nerve (axon) bundles or

fascicles. The outermost connective tissue layer is the strong fibrous sheath, the
epineurium that binds all fascicles together. It consists of dense irregular
connective tissue that completely surrounds the peripheral nerve. A thinner
connective tissue layer, the perineurium, extends into the nerve, subdivides, and

surrounds one or more nerve fascicles. The cells in the perineum are joined
together by tight junctions, and the perineum serves as a selective metabolically
active diffusion barrier that forms the blood–nerve barrier. This barrier restricts
passage to many macromolecules and functions in maintaining the proper

internal microenvironment and protection of the axons. Within each fascicle are
individual axons and their supporting cells, the Schwann cells. Each myelinated
axon or a cluster of unmyelinated axons associated with a Schwann cell is
surrounded by a loose vascular connective tissue layer of thin reticular fibers,

called the endoneurium.

Supplemental micrographic images are available at

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