Structure of a Nerve
The peripheral nervous system is a part of the nervous system that contains all nerves not included in the CNS. A nerve consists of a bundle of neuron fibers surrounded by protective connective tissue. Each fiber is wrapped in a fine tissue called the endoneurium. Fibers are arranged in bundles called fascicles, which are covered by a coarser tissue sheath – the perineurium. All fascicles are wrapped together by a tough tissue covering referred to as the epineurium.
Nerves are classified depending on the direction of transmitting signals. Mixed nerves, which include all spinal nerves, conduct both incoming sensory impulses and outgoing motor signals. Sensory or afferent nerves carry sensory information toward the CNS. In contrast, motor or efferent nerves conduct only impulses from the CNS to muscles and glands.
Cranial Nerves
There are 12 pairs of cranial nerves, most of which serve neck and head. Only the vagus nerve extends to the thorax and abdomen. All pairs of cranial nerves are numbered, and their names reflect their functions. For example, oculomotor cranial nerves control the movement of eyeballs and eyelids, the lens shape, and pupil size. Most cranial nerves belong to the mixed group; the three exceptions are olfactory, optic, and vestibulocochlear nerves, which are sensory nerves.
Spinal Nerves
The human body has 31 pairs of spinal nerves. Each spinal nerve is formed by a combination of ventral and dorsal roots of the spinal cord. Right after being formed, each nerve divides into ventral and dorsal rami. As a result, every single spinal nerve has a length of only about ½ inch. Spinal nerves and rami contain both motor and sensory fibers. Therefore, if a spinal nerve or its ramus is damaged, it leads to flaccid paralysis and lost sensation in the respective area.
Spinal Nerve Plexuses
Dorsal rami of spinal nerves serve the muscles and skin of the human back. The ventral rami of most spinal nerves form networks called plexuses. There are four nerve plexuses: cervical, brachial, lumbar, and sacral. The cervical plexus contains nerves C1-C5 and serves the diaphragm and skin and muscles of the neck and shoulders. The brachial plexus includes nerves C5-C8 and T1 and serves the deltoid muscle and skin of shoulder and muscles and skin of forearms, wrists, and hands. Nerves L1-L4 belong to the lumbar plexus, responsible for the lower abdomen, and muscles and skin of hips and thigs. The sacral plexus contains nerves L4-L5 and S1-S4 and serves the lower trunk, legs, and feet. Nerves T1-T12 do not form a plexus. They are called intercostal nerves and supply the muscles and skin of the lateral and anterior trunk and muscles between the ribs.
Autonomic Nervous System
The autonomic or involuntary nervous system (ANS) is part of the PNS that works automatically and controls certain body activities, such as blood pressure, pupil size, heart and breathing rate, stomach secretions, and other unconscious processes. It consists of neurons regulating glands, smooth muscles, and the cardiac muscle. The structure of the ANS differs from that of the somatic nervous system, controlling skeletal muscles. In the somatic system, cell bodies of motor neurons are located in the CNS, and their axons extend to the respective skeletal muscles. In the ANS, there are two motor neurons rather than one. The first motor neuron is located in the CNS, and its axon, called the preganglionic axon, extends to a ganglion outside of the CNS, where it synapses with the second motor neuron. The axon of this second neuron, called the postganglionic axon, extends right to the organ that it supplies. The ANS consists of two divisions: sympathetic and parasympathetic, which serve the same organs but result in opposite effects.
Sympathetic Division
The preganglionic neurons are located in the gray matter of the spinal cord in nerves T1-L2. Their axons exit the cord through the ventral root. Then, they enter the spinal nerve and go through a small communicating branch called ramus communicans. After that, they enter a sympathetic chain ganglion located on both sides of the vertebral column. In the ganglion, axons either synapse with the second neuron or go further to form the splanchnic nerves. These nerves travel to a collateral ganglion where they synapse with ganglionic neurons. Then, the postganglionic axon exits the collateral ganglion and extends to a visceral organ.
The sympathetic division is often called a “fight-or-flight” system because its function is to help the body quickly cope with situations that threaten homeostasis. The effects of this system are short-term and can be manifested in an increased heart rate, sweating, the dilation of vessels in skeletal muscles, etc. The sympathetic system also controls most blood vessels.
Parasympathetic Division
In the parasympathetic division, preganglionic neurons are located in cranial nerves III, VII, IX, and X and spinal nerves S2-S4. Preganglionic axons pass through cranial nerves and enter a terminal ganglion, where they synapse with the motor neuron. From there, postganglionic neurons extend to the organs they serve. In the sacral region, axons leaving the spinal cord form pelvic nerves that extend to the pelvic cavity and synapse with motor neurons in the terminal ganglia. Then, postganglionic axons extend to their organs.
The parasympathetic division works when the body is not threatened and rests. It is sometimes called a “resting-and-digesting system” because it ensures normal digestion, urination and defecation, and the conservation of body energy.
Developmental Aspects of the Nervous System
The nervous system is formed during early fetal development. Therefore, various adversities during pregnancy, such as maternal infections, smoking, radiation, and drugs, may lead to congenital impairments. The hypothalamus, responsible for temperature regulation, matures at later stages of fetal development. Thus, premature babies often cannot control their body heat loss. The nervous system matures during childhood by means of myelination. It reaches its developmental peak in young adulthood when the brain weight is at its maximum. As people age, their neurons become damaged and die, and their brain weight declines. The sympathetic system’s functioning also declines with age, which is manifested in orthostatic hypotension, arteriosclerosis, and hypertension.
Reference
Marieb, E. N. (2008). Essentials of human anatomy and physiology (9th ed.). Benjamin Cummings.