Anatomy of the Peripheral Nervous System: Functions, Development, Disorders and Evaluation

The nervous system is a complex network of nerves that send messages from the brain and spinal cord to various body parts.

The human nervous system is complex, with many interactive units constantly changing to reflect human behavior and activity. Today we will focus on the peripheral nervous system, including its function and parts.

General description of the peripheral nervous system

There is much to learn about the nervous system and its pathologies. We know that two nervous systems in the human body are related to each other.

The first is the central nervous system, including the brain and spinal cord. The second nervous system, the peripheral nervous system, contains all the body nerves outside the spinal cord and the brain.

These two systems communicate to ensure that our body parts, like our fingers, can send signals to the central nervous system for processing in our brain.

Can you see the difference between the two nervous systems?

The peripheral nervous system includes all the nerves from the skin, muscles, and organs to the spinal cord and the brain.


However, the central nervous system is composed of the brain and spinal cord.

Anatomy and Parts

The peripheral nervous system consists of 12 pairs of cranial nerves and 31 pairs of spinal nerves.

Some of these nerve pairs are exclusively sensory cells, like cells that detect information such as smell and vision.

Others are exclusively motor cells, such as eyeballs and the ear. In addition, there are pairs of nerves with sensory and motor cells, such as those related to taste and some aspects of swallowing.

The sensory cells carry messages to the central nervous system; the motor cells carry the signal from the central nervous system to the internal organs, the muscles, and the glands of the periphery or the outer edges of the body.

Both types of cells travel together to the spinal cord but then separate into two areas; one area is called the posterior sensory root, and the other is called the anterior sensory root.

Motor nerve cells are somatic or autonomic. Somatic nerve cells carry messages from the outer areas of the body that have to do with the senses.

That seems simple compared to autonomous cells because autonomic nerve cells are divided into three separate divisions: parasympathetic, sympathetic, and enteric.

These divisions are named for the functions in which they are involved throughout the body. The parasympathetic division is engaged with decelerating bodily functions, while the sympathetic division increases bodily functions.

The enteric division is involved with all functions in the gastrointestinal areas, such as the pancreas and the gallbladder.

The function of the peripheral nervous system

The primary role of the peripheral nervous system is to connect the central nervous system to the organs, extremities, and skin to allow complex movements and behaviors.

The sensory cells carry messages to the central nervous system; an example would be heat or cold (known as a stimulus) that is felt on the skin of the fingers.

The sensory receptors in the skin bring heat or cold stimuli to the central nervous system. After the central nervous system is processed, somatic motor cells carry the signal to the skeleton and sensory organs, such as the skin.

These somatic cells are sometimes voluntary because the person controls most of these areas. The individual knows the responses sent from the central nervous system, so the individual is aware of the reaction.

Autonomic motor cells control muscles like somatic motor cells, but these muscles are involuntary.

A couple of examples would be smooth muscles in the liver or salivary glands in the mouth. Therefore, somatic cells take the central nervous system signals to muscles and involuntary glands.


The primary cell layers differentiate before forming the nervous system in the embryo. The innermost layer, the endoderm, gives rise to the gastrointestinal tract, the lungs, and the liver.

The mesoderm gives rise to the muscle, the connective tissues, and the vascular system. The third and most external layer, the ectoderm, formed by columnar epithelium, gives rise to the nervous system and the skin.

During the third week of development, the ectoderm on the dorsal surface of the embryo between the primitive node and the oropharyngeal membrane thickens to form the neural plate.

The plate, which is pear-shaped and cranially wider, develops a longitudinal neural groove. The groove is now deepened so that it is limited on both sides by neuronal folds.

With further development, the neuronal folds fuse, converting the neural groove into a neural tube.

Fusion begins at approximately the midpoint along the groove and extends cranially and caudally. The tube cavity remains in communication with the amniotic cavity at the earliest stage through the anterior and posterior neuropores.


The disorders can result from damage or dysfunction of the cell body, myelin sheath, axons, muscle, or neuromuscular junction.

The disorders can be genetic or acquired (toxic, metabolic, traumatic, infectious, or inflammatory).

Peripheral neuropathies can affect a nerve (mononeuropathy), several discrete nerves (multiple mononeuropathy or multiple mononeuritis), or numerous diffuse nerves (polyneuropathy).

Some conditions involve a plexus (plexopathy) or a nerve root (radiculopathy). More than one site may be affected; For example, in the most common variant of Guillain-Barré syndrome, multiple segments of the cranial nerves, usually the facial nerves, may be affected.


Clinical evaluation usually begins with history. The focus should remain on the type of symptom, onset, progression, location, and information about possible causes (e.g., family history, toxic exposures, and past medical disorders).

The physical and neurological examination should further define the type of deficit (e.g., motor deficit, type of sensory deficit, combination).

Sensation (using puncture and light touch for tiny fibers and vibration for large fibers), motive power, and deep tendon reflexes are evaluated.

The cranial nerve and autonomic function are also evaluated. It is observed if the motor weakness is proportional to the degree of atrophy and the type and distribution of the reflex anomalies.

Physicians should suspect a peripheral nervous system disorder according to the pattern and type of neurological deficits, mainly if deficiencies are found in the territories of nerve roots, spinal nerves, plexuses, specific peripheral nerves, or a combination.

These disorders are also suspected in patients with mixed sensory and motor impairments, multiple foci, or a guide incompatible with a single anatomic site in the central nervous system.

Physicians should also suspect peripheral nervous system disorders in patients with generalized or diffuse Weakness but without sensory deficits; in these cases, diseases of the peripheral nervous system may be overlooked because they are not the most likely cause of such symptoms.

The clues that a disorder of the peripheral nervous system may be the cause of generalized Weakness include the following:

  • Patterns of generalized Weakness suggest a specific reason.
  • Symptoms and signs that are not weaknesses point to a particular disorder or a group of diseases.
  • Deficits in distribution suggest diffuse axonal disorders or polyneuropathy.
  • Fasciculations.
  • Hypotonia.
  • Muscle wear without hyperreflexia.
  • Weakness that is progressive, chronic, and inexplicable.

The clues that the cause may not be a peripheral nervous system disorder include signs of the upper motor neuron, including hyperreflexia and hypertonia.

The hyporeflexia is consistent with the deficits of the peripheral nervous system, but it is not specific.

Although many exceptions are possible, certain clinical clues can also suggest possible causes of peripheral nervous system deficits.

Neurological history and examination can reduce the chances of diagnosis and additional guidance with tests.

In general, nerve conduction studies are performed to help identify the level of involvement in the nerve, plexus, root, muscle, or neuromuscular junction.

In addition, it can sometimes help to distinguish demyelinating lesions from axonal ones.

Dermatomes of supply of nerve roots

With few exceptions, there is a complete overlap between the adjacent dermatomes; this means that the loss of a single nerve root rarely causes a significant loss of skin sensitivity.

The exception to this rule is found in small patches in the distal extremities, called “autonomous zones.”

In these regions, the individual nerve roots provide distinct and non-overlapping areas of skin. The “autonomous zones” represent only a tiny portion of any dermatome, and only a few nerve roots have such autonomous zones.

For example, the nerve root C5 may be the only supply to an area of ​​the lateral arm and the proximal part of the lateral forearm.

The C6 nerve root can provide some skin for the thumb and index finger. Injuries to the C7 nerve root can diminish the sensation on the medium and, sometimes, the index finger and a restricted area on the back of the hand.

C8 nerve root lesions can produce similar symptoms over the small digit, occasionally extending to the hypothenar area of ​​the hand.

In the lower limb, damage to the L4 nerve root can decrease the sensation on the medial part of the leg, while the L5 lesions affect sense on the part of the back of the foot and the big toe.

S1 nerve root lesions typically decrease sensation on the lateral side of the foot.

Damage to the peripheral nerves often produces a recognizable pattern of severe Weakness and (over time) atrophy.

Damage to the individual nerve roots usually does not produce the complete Weakness of the muscles since no nerve roots supply muscles. However, the Weakness is often detectable.

Examples of upper extremities include Weakness of the shoulder abductors and external rotators with C5 nerve root lesions, Weakness of the elbow flexors with C6 nerve root lesions, possible wrist weakness and extension of a finger with C7 nerve root lesions, and specific Weakness of the muscles of the intrinsic hand Lesions C8 and T1.

In the lower limb, there may be some weakness in the extension of the knee with lesions L3 or L4; There may be some difficulty with the extension of the big toe (and, to a lesser extent, the ankle) with L5 lesions and Weakness of the plantar flexion of the big toe with nerve S1 root damage.

The motor nerve fibers end in the myoneural junctions. These consist of a single motor axon terminal in a skeletal muscle fiber.

The myoneural junction includes a complex bulging of the muscular membrane, whose crests contain nicotinic acetylcholine receptors. A matrix in the synaptic cleft contains acetylcholinesterase, involved in the termination of the action of the neurotransmitter.

A motor neuron has connections with many muscle fibers through collateral branches of the axon.

This is called a “motor unit” and can range from a handful of muscle fibers to a motor neuron in excellent control muscles (such as the eye muscles) to several thousand (as in the gluteal muscles).

The autonomic nervous system consists of 2 main divisions:

  • Sympathetic nervous system.
  • Parasympathetic nervous system.

The sympathetic are mainly involved in the responses associated with the fight or flight, such as increased heart rate and blood pressure, as well as the constriction of blood vessels in the skin and its dilation in the muscles.

The parasympathetic nervous system participates in energy conservation and increases motility and gastrointestinal secretion. It also increases the contractility of the bladder.

Some areas in which blood vessels are under sympathetic and competitive parasympathetic control, such as in the nose or erectile tissues.

There are some areas where there is a competitive balance between sympathetic and parasympathetic, such as effects on heart rate.

For some functions, sympathizers and parasympathetic cooperate; An example is the parasympathetic nerves, which are necessary for the erection, and the sympathetic ones for the ejaculation.