The nervous system is by far one of the most complex systems in the human body, both physiologically and histologically speaking. It provides rapid and precise communication between different parts of the body via highly specialized nerve cells called neurons. These neurons form networks that function to process information and generate proper response signals to the continuous changes in the external and internal environment.
Anatomically, the nervous system can be divided into two main parts: the central nervous system (CNS), comprised of the brain and the spinal cord; and the peripheral nervous system (PNS), comprised of nerves and ganglia.
Peripheral nerves, which include cranial and spinal nerves, are anatomical structures containing any combination of afferent or efferent nerve fibers of either the somatic or autonomic nervous systems.
Afferent fibers carry impulses from the interior of the body and the environment to the CNS, while efferent fibers carry impulses from the CNS to the effector organs. Nerves formed by afferent fibers only are called sensory nerves, and those possessing efferent fibers only are called motor nerves. However, most nerves have both sensory and motor fibers, and are called mixed nerves.
Each nerve is composed of bundle(s) of nerve fibers (axons) termed fascicles. Within these fascicles, nerve fibers are sheathed by Schwann cells (neurolemmocytes), which may or may not be myelinated, depending on the nerve fiber’s diameter.
Nerve fibers of small diameter are usually unmyelinated, while those who are progressively thicker are sheathed by numerous concentric myelin sheath wrappings. These are known as myelinated fibers. Myelin layers are rich in lipid. They provide insulation and facilitate the formation of action potentials along the nerve fiber.
Between adjacent Schwann cells are small nodal gaps free of myelin sheath called nodes of Ranvier. They allow action potentials to pick up speed and propagate down the axon. This rapid conduction of action potentials from one node to another is termed ‘saltatory conduction’.
Connective tissue layers in peripheral nerves
Peripheral nerves that are formed of more than one fascicle are enclosed by a dense external layer of loose collagenous tissue called the epineurium. It binds the fascicles together, and is condensed peripherally to form a strong cylindrical sheath.
Each fascicle is enclosed by a specialized sleeve of connective tissue called the perineurium, which is formed by layers of flat epithelial like cells. These cells are joined at their junctions to form a barrier to the passage of most macromolecules, protecting and maintaining the internal microenvironment of the nerve fibers.
Within the fascicles, each nerve fiber, together with its investing Schwann cell, is covered by a sparse layer of loose connective tissue called the endoneurium.
The blood supply of the peripheral nerves is through numerous penetrating vessels from surrounding tissue and accompanying arteries. Large vessels run longitudinally within the epineurium, with capillary networks piercing the perineurium into the endoneurium.
Ganglia are ovoid structures that contain neuronal cell bodies and glial cells supported by connective tissue. They transmit nerve impulses and act as relay stations.The direction of the nerve impulses is the determinant of whether the ganglion is considered sensory or autonomic.
Sensory ganglia receive afferent impulses that go to the CNS. They are associated with both the cranial nerves and the dorsal roots of the spinal nerves. The neuronal cell bodies of the ganglia are associated with small thin glial cells called satellite cells.
Satellite cells are responsible for creating the microenvironment of the perikarya, the cell body of the neuron. They provide structural support, permit the production of membrane action potentials, and regulate metabolic exchanges.
Sensory ganglia neurons are pseudounipolar. They relay information from the ganglion’s nerve endings to the gray matter of the spinal cord by synapsing with local neurons.
Autonomic ganglia are small dilatations present in autonomic nerves. They affect the activity of smooth muscle, and the secretions of some glands. Moreover, they modulate the cardiac rhythm, as well as other involuntary activities that allow the body to maintain homeostasis. Autonomic ganglia can be found within certain organs, especially in the walls of the digestive tract.
Autonomic ganglia neurons are mostly multipolar, with a layer of satellite cells enveloping them. They have two neuron circuits, the first being the preganglionic fibers located in the CNS. The axons then synapse with the postganglionic fibers of the second multipolar neuron located in the peripheral ganglion system. Acetylcholine is the chemical mediator present in the synaptic vesicles of all preganglionic axons.