Peripheral Nervous System

We all know that the nervous system in our body controls all the activities performed by it. This control system comprises of two divisions; central nervous system or CNS, and peripheral nervous system or PNS. The central nervous system is contained within the brain and spinal cord.

It comprises of the nerve cells and fibers within these two structures. On the other hand, the peripheral nervous system is a division of nervous system present outside the brain and spinal cord. It comprises of nerve cells and nerve fibers located outside the brain and spinal cord.

The peripheral nervous system acts as a communication pathway between the central nervous system and the peripheral body parts. All the peripheral body parts are controlled by the higher centers of the brain via this division of the nervous system. 

In this article, we will have a detailed discussion about the peripheral nervous system and its divisions. We will briefly study the important functions performed by the peripheral nervous system and some of its pathologies.

Divisions

The peripheral nervous system comprises of two subdivisions;

  • Somatic Nervous System
  • Autonomic Nervous System

In the remaining article, we will discuss the peripheral nervous system in terms of its subdivisions. We will study the organization, location, functions, and pathologies of these divisions of the peripheral nervous system.

Somatic Nervous System

It is a subdivision of the peripheral nervous system that is mainly responsible for carrying out motor activities of the body. It performs all the activities of the body that are under the control of our will.

Anatomical Organization

The somatic nervous system comprises of sensory and somatic nerves. Some nerves perform both functions i.e. they are somatosensory nerves.

The sensory, somatosensory and sensory nerves are arranged into two major groups. These include;

  • Cranial nerves
  • Spinal Nerves

Cranial Nerves

The somatosensory function of the somatic system is performed by the cranial nerves in the head and neck region.

There are 12 pairs of cranial nerves in the human body. All these arise directly from the brain or the brain stem. Some of the cranial nerves are sensory only like Olfactory nerve (CN 1) or Optic nerves (CN2). Some cranial nerves have only somatic or motor functions like Trochlear nerve (CN 4) and Abducent nerve (CN 6) etc. The majority of the cranial nerves have both somatic as well as sensory functions such as Trigeminal nerve (CN 5) and Facial nerve (CN 7), etc.

Spinal Nerves

In the other regions of the body, the somatosensory functions are performed by the spinal nerves.

There are 31 pairs of spinal nerves in the human body. In contrary to the cranial nerves, all the spinal nerves are somatosensory in nature.

The spinal nerves arise directly from the spinal cord. These spinal nerves join together to form plexus before continuing further and supplying the peripheral body parts.

The 31 pairs of spinal nerves are distributed as follows;

  • Cervical nerves (C1 to C8)
  • Thoracic nerves (T1 to T12)
  • Lumbar nerves (L1 to L5)
  • Sacral  nerves (S1 to S5)
  • Coccygeal nerve (Co1)

These spinal nerves form three plexus in the body. These are;

  • Cervical Plexus (C1to C4)
  • Brachial Plexus (C5 to T1)
  • Lumbosacral Plexus  (L1 to Co1)

Autonomic Nervous System

It is a subdivision of the peripheral nervous system that is responsible for regulating the activities of the body that are not under the control of our will. All the physiological functions of the body are controlled by the higher centers of the brain through the autonomic division of the peripheral nervous system.

Divisions

The autonomic nervous system is further divided into two subdivisions. These are;

  • Sympathetic Nervous System
  • Parasympathetic Nervous System

Anatomical Organization

The autonomic nervous system is organized in the form of ganglia and nerve fibers. The nerve fibers are of two types;

  • Preganglionic Nerve Fibers
  • Postganglionic Nerve Fibers

The two subdivisions of the autonomic nervous system have slightly different anatomical organization.

The sympathetic nervous system comprises of ganglia located in the form of a chain along with the spinal cord. These are called the paravertebral ganglia. The preganglionic fibers in the case of sympathetic nervous system arise from the intermediolateral horn of the spinal cord in the thoracic and lumbar regions. Thus, sympathetic system have lumbosacral outflow.

On the other hand, in the case of parasympathetic nervous system, the preganglionic fibers arise from three sources;

  • Tectum of Midbrain
  • Cranial Nerve Nuclei
  • Intermediolateral horn of spinal cord in Sacral region

Thus, the parasympathetic system is said to have a craniosacral outflow. The parasympathetic system has four ganglia in the head and neck region. The ganglia in other regions of the body are present in nerve plexus.

Physiology

As we have studied the subdivisions of the peripheral nervous system and their anatomical organization, let us now move forward and have a look at the functions performed by these subdivisions.

Somatic Nervous System

This subdivision of the peripheral nervous system performs two functions. These are;

  • Sensory Perception and Conduction
  • Somatic Control of the Body

Sensory Perception and Conduction

This is the major function performed by the spinal nerves. All the sensations such as pain, temperature, touch, etc. are perceived by the sensory nerve endings of the spinal or some cranial nerves, or the specialized receptors connected to these nerve endings. The nerve impulse generated by different stimuli are then carried by spinal nerves to the spinal cord and by cranial nerves to the brainstem. These nerve impulses than reach the somatosensory area of the brain via different ascending tracts.

Somatic Control

Somatic control means controlling the motor activities of the peripheral body parts. This is the second major function performed by the somatic division of the peripheral nervous system. You move your hand, your foot or rotate your head, all due to the somatic control of the peripheral nervous system.

When a person wishes to move his body parts, the motor areas of the brain send signals to that body part via spinal or cranial nerves. These nerves innervate the muscles and cause them to contract. In this way, the movement occurs.

Autonomic Nervous System

This division of the peripheral nervous system is extremely important. The higher centers of the brain control and regulate all the physiological activities of the body via this system. Here, we will only mention the important body functions regulated via the autonomic nervous system. The detail of these functions will be covered in a separate article on our website.

The important functions regulated by the autonomic nervous system are;

  • Heart Rate
  • Respiratory Rate
  • Process of Digestion
  • Process of Urination
  • Process of Defecation
  • Blood Pressure of the Body
  • Sexual Responses

Pathology

Any injury or lesion of the peripheral nervous system will result in loss of the functions performed by it. The loss of functionality depends on the degree of the injury and the place of the injury. The various ways through which the peripheral nervous system may be injured and the resulting effects on the body are mentioned below.

  • Nerve Injury, a single spinal or cranial nerve may be injured due to trauma at the site where the nerve is located. For example, trauma to the lateral side of the knee causes injury to the peroneal nerve. The nerve can also be damaged by a disease process. For example, peripheral neuropathy caused by diabetes. When a nerve is injured, it causes loss of all the functions performed by that nerve. Injury to peroneal nerve results in foot drop, etc.
  • Spinal Cord Pathologies, various pathological processes that damage the spinal cord such as hemisection of the cord, central cord syndrome etc. directly damages the somatic as well as the autonomic divisions of the peripheral nervous system. All the functions of the peripheral nervous system below the site of injury are completely lost.

Summary

The peripheral nervous system consists of the neurons and nerve fibers located outside the CNS. It basically originates from the CNS and then supplies the peripheral parts of the body.

The peripheral nervous system has two subdivisions;

  • Somatic nervous system
  • Autonomic nervous system

The somatic nervous system comprises of the spinal nerves and the cranial nerves.

There are 12 pairs of cranial nerves and 31 pairs of spinal nerves in the human body.

These nerves carry sensory as well as motor fibers to and from the peripheral body parts.

The somatic nervous system is responsible for two major functions;

  • Conduction of sensory impulses from the peripheral body parts to the CNS
  • Providing the motor impulses generated in the CNS to the muscles located in the peripheral parts of the body

The autonomic nervous system is further subdivided into a sympathetic system and a parasympathetic system.

Both these divisions of the autonomic system are organized into ganglia having preganglionic and postganglionic fibers.

The autonomic division of the peripheral nervous system controls all the physiological activities of the body.

The peripheral system may be damaged due to an injury or a disease process.

The involvement of a single nerve results in loss of only those functions which are specific to that particular nerve.

The involvement of the spinal cord results in the loss of all the functions performed by the peripheral nervous system.

References

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