Peripheral Nervous System (PNS)

Peripheral nerves include motor (efferent) fibers and sensory (afferent) fibers.

• Cranial and spinal (dorsal root) ganglia contain the nerve cell bodies, supporting satellite cells, nerve fibers of the afferent somatic and autonomic neurons leading to and from them and Schwann cells. They function as sensory, and are devoid of dendrites, nerve endings, or synapses. Note: Cell bodies of some neurons of the PNS are located in the CNS.
• Autonomic ganglia are paravertebral, prevertebral, and terminal. They are motor in function and contain the efferent neurons that belong to the postsynaptic motor neurons of the autonomic nervous system. Neurons are multipolar, and the ganglia contain dendrites and synapses.

Nerve endings are where synapses happen.

In the somatic nervous system (SNS), single somatic efferent (motor) neurons conveys impulses from the CNS to effector organs. Cell bodies lie in the CNS, located in either the brain, brain stem, or spinal cord. Their axons leave the CNS and travel in peripheral nerves to innervate skeletal muscle.

In the autonomic nervous system (ANS), a chain of two efferent neurons connect the CNS to smooth muscle, cardiac muscle, and glands. Preganglionic (presynaptic) neurons have cell bodies in the CNS. Their axons travel in the peripheral nerves to synapse with postganglionic (postsynaptic) neurons in autonomic ganglia.

Connective Tissue Components of the Peripheral Nerves

Connective tissue organized into three components:

• Endoneurium contains a layer of reticular fibers and is associated with individual nerve fibers. It is obvious only in TEM and is longitudinally situated (parallel with the nerve fibers). Functionally, they bind nerve fibers into bundles or fascicles. Most of the collagen is likely made by Schwann cells.
• Perineurium is a unique connective tissue that surrounds the nerve fascicle. It is five to six cell layers thick, made of squamous cells. Each layer exhibits an external lamina on both sides with collagen between layers and no fibroblasts. The cells are contractile with numerous actin microfilaments. There are tight junctions between the cells of the same layer, creating a semi-permeable blood-nerve barrier. They serve an important protective role.
• Epineurium is the outermost tissue of the peripheral nerve. Dense connective tissue binds the fascicles together. Adipose tissue is often seen in larger nerve sheaths. It carries blood vessels supplying the nerves (the vessels branch and penetrate the perineurium). It is absent in small nerves.