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Peripheral Nervous System: Definition, Function & Parts

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Lesson Transcript
Instructor: Nadine James

Nadine has taught nursing for 12 years and has a PhD in Nursing research

The human nervous system is very complex with many interactive units that are changing constantly to reflect human behavior and activity. This lesson focuses on the peripheral nervous system, including its function and parts.

Overview of Peripheral Nervous System

There is a lot to learn about the nervous system and the pathologies associated with it. What we do know is that there are two nervous systems in the human body, and they relate to each other. The first is the central nervous system, which includes the brain and the spinal cord. The second nervous system, known as the peripheral nervous system, contains all the nerves in the body that lie outside of the spinal cord and brain. These two systems communicate with each other to make sure our body parts, such as our fingers, can send signals to the central nervous system for processing in our brains.

Nervous system diagram

Can you see the difference in the two nervous systems? The peripheral nervous system includes all the nerves that go from the skin, muscle, and organs to the spinal cord and, eventually, the brain. However, the central nervous system is composed of the brain and spinal cord.

Parts of Peripheral Nervous System

The peripheral nervous system consists of 12 pairs of cranial nerves and 31 pairs of spinal nerves. Some of those nerve pairs are exclusively sensory cells, such as cells that detect information like smell and vision. Others are exclusively motor cells, like the eyeballs and hearing. Also, there are nerve pairs that have both sensory and motor cells, such as those involved in taste and some aspects of swallowing.

Sensory cells carry messages to the central nervous system. Motor cells carry the signal from the central nervous system to the internal organs, muscles, and glands in the periphery or the outer edges of the body. Both types of cells travel together to the spinal cord, but then they separate into two areas. One area is called the posterior sensory root, and the other is called the anterior sensory root.

The motor nerve cells are either somatic or autonomic. The somatic nerve cells carry messages from the outer areas of the body having to do with the senses. It is like a passageway from the environment to the central nervous system. That seems simple compared to the autonomic cells because the autonomic nerve cells are divided into three separate divisions called the parasympathetic, the sympathetic, and the enteric divisions.

These divisions are named by the functions that they are involved in throughout the body. This will be discussed later in detail in the lesson, but as a sneak preview, here is a brief description for each division. The parasympathetic division is involved with slowing body functions, while the sympathetic division increases body functions. The enteric division is involved with all the functions in the gastrointestinal areas, such as the pancreas and the gallbladder.

Function of Peripheral Nervous System

The primary role of the peripheral nervous system is to connect the central nervous system to the organs, limbs, and skin to allow for complex movements and behaviors. Let's talk about the sensory somatic system first. Sensory cells carry messages to the central nervous system. An example would be heat or cold (known as a stimulus) felt on the skin of the fingers. The sensory receptors in the skin carry the heat or cold stimulus to the central nervous system. After being processed by the central nervous system, the somatic motor cells take the signal to the skeleton and sensory organs like the skin. These somatic cells are sometimes called voluntary because the person has control over most of these areas. The responses sent from the central nervous system are known by the individual, so the individual is conscious of the response.

Similar to the somatic motor cells, the autonomic motor cells control muscles, but these muscles are involuntary. A couple of examples would be the smooth muscles in the liver or the salivary glands in the mouth. Therefore, the somatic motor cells take signals from the central nervous system to involuntary muscles and glands. These muscles and glands are also known as effectors because this is where the responses from the central nervous system are translated into action or become effective.

The autonomic nerve cells are appropriately named because they are involved in regulating automatic body functions, such as breathing and heartbeat. Body functions, such as slowing of the heart rate, are controlled in the parasympathetic division. The sympathetic division does the opposite of the parasympathetic division: It increases body functions like heart rate.

Finally, the enteric division has nerve fibers that supply the viscera of the body or the gastrointestinal system. The enteric division is sometimes known as the 'brain of the gut.' This is misleading, though, because the enteric division involves the entire gastrointestinal tract, not just the stomach.

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