Introduction

NERVES, NERVE FIBERS, AND GANGLIA

The nervous system is divided into the central nervous system which is composed of the brain and spinal cord and the peripheral nervous system (PNS) which includes all the nervous tissue outside of the central system. The peripheral system functions by keeping other tissues of the body in communication with the central sytem (CNS). The PNS includes the cranial nerves, spinal nerves and their roots and branches and certain portions of the autonomic system. The term peripheral nerve usually refers to the fibers arising from the spinal cord or the dorsal root ganglia. A nerve is a collection of fibers that conduct impulses to and/or from the central nervous system. Those impulses that travel toward or into the central system are sensory. Those that leave the central system are motor. Most peripheral nerves contain a mixture of motor and sensory fibers.

The nerve fiber is composed of an axon, a neurite and often its covering. The axons of the peripheral nervous system are covered by a sheath (neurilemma) composed of Schwann cells. The neurilemma is comparable to certain neuroglia of the central nervous system. Large diameter peripheral nerves have an additional sheath called myelin internal to the neurilemma, and are called myelinated nerves. Smaller peripheral nerves lacking the myelin are called non-myelinated nerves.

With the electron microscope the relationship between the myelin sheath and the neurilemmal sheath of Schwann cells has been elucidated. Myelin is part of the surface membrane of the Schwann cell that has been wrapped around the axon. The myelin and neurilemmal sheaths are interrupted at regular intervals. These interruptions are called nodes of Ranvier, and there is usually one Schwann cell between two successive nodes of Ranvier (an internode). In nonmyelinated nerves, the surface of the Schwann cell surrounds a number of axons

The nerve fiber within a peripheral nerve is bound to other nerve fibers by connective tissue which contains the blood supply to the nerves. External to the Schwann cell is a layer of connective tissue; this C.T. plus the Schwann cells is called the endoneurium. Small bundles of individual nerve fibers are surrounded by a connective tissue sheath called the perineurium which serves as a semi-permeable barrier. The whole nerve trunk is contained in the dense C.T. of the epineurium.

Ganglia are collections of nerve cell bodies (neurons) outside the CNS. Pseudo-unipolar cells of the dorsal root ganglia (also called sensory ganglia) are round and granular with centrally placed nuclei and prominent nucleoli. These are surrounded by a network of satellite cells. The neurons of the autonomic ganglia (sympathetic ganglia or parasympathetic ganglia) are multipolar and somewhat smaller than those of the dorsal root ganglia. The nuclei are located at the cell periphery. There is a discontinuous arrangement of satellite cells in the autonomic ganglia.