Video: Peripheral nervous system

Have you ever wondered how you're able to feel this kind of pain or how you're able to quickly move your hand away without thinking? How does it all happen so fast? The answer to each of these questions is nerves – and we're not talking about the butterflies-in-the-stomach kind. We are, of course, talking about the bundles of fibers that transmit electrical signals throughout the body, acting as messengers in the communication network that is the peripheral nervous system. Let's get straight to it.

In this tutorial, we will be covering the major anatomical components of the peripheral nervous system, namely, nerves, ganglia, and nerve plexuses. But before we begin, let's briefly introduce the peripheral nervous system.

This system includes all the nerves, nerve plexuses, and ganglia outside of the brain and spinal cord. It connects our central nervous system to the rest of the body. It's divided into two key parts – the somatic nervous system which is primarily responsible for input and output related to skeletal muscles, bones, joints, and skin; and we also have the visceral nervous system which is related to the innervation of smooth muscle, cardiac muscle, and glandular cells.

Now that we are familiar with the peripheral nervous system, let's look at one of its major anatomical components – nerves. To properly understand what a nerve is, we must start by looking at the cellular level. The basic functional units of the nervous system are called neurons. These are cells of the nervous system which are responsible for carrying information throughout the body, and incredibly, there are around 86 billion of these in our bodies which is on the same order as the number of stars in the Milky Way.

Each neuron contains a large cell body, also known as the soma. This is essentially the core of the cell containing the nucleus and other specialized organelles. Often emerging from the cell body are numerous small wrench-like extensions called dendrites and these structures are able to receive information being passed on by other neurons. This singular, long extension you can see here is known as the axon, which is the main conducting part of the neuron. That is, it can quickly relay the information received by the dendrites onto the next target which is usually another neuron.

Axons can vary in length from just a few millimeters to over a meter. At the far end of the axon, we find the axon terminals or terminal boutons which will meet with the dendrites of the subsequent neurons or its target structure at the synaptic cleft, where the impulse can be relayed onward.

Now that we understand what a neuron is, we want to understand what a nerve is. Are they the same thing? The two terms are often used interchangeably which can be confusing and also quite nerve-wracking. The two are in fact different, namely, that neurons are just one cell type of the nervous system whereas a nerve is the term used to describe a structure composed of many bundles of axons, commonly referred to as nerve fibers, in the peripheral nervous system.

So, how are these nerve fibers or axons organized together to form what we see with our eyes as nerves? This is achieved with the help of three layers of connective tissue. First is the endoneurium, a connective tissue sheath that surrounds individual axons. Groups of axons are then bundled together into fascicles which are encased in another layer of connective tissue called perineurium. Blood vessels which provide oxygen and nutrients to the axons are located between the fascicles. Finally, the outermost and strongest layer, the epineurium, surrounds groups of fascicles giving us the overall structure that we see as a nerve.

Not all nerves in our body serve the same functions. We can, however, categorize nerves into a few different types according to how they transmit information. Nerve fibers that carry information from the periphery towards the central nervous system are called afferent or sensory. Specifically, those carrying information from the skin, skeletal muscle, and joints are called somatic sensory while those carrying information from the viscera or organs such as the liver or kidneys are aptly named visceral sensory.

Efferent or motor nerve fibers are the opposite to sensory. They carry information from the central nervous system to the periphery. Similarly, they can be divided into two types. There are somatic motor which provide innervation to skeletal muscles and are responsible for their contraction and there are visceral motor, also known as autonomic, which innervates smooth muscle, cardiac muscle, and glands causing different types of actions from those organs.

The final type are called mixed nerves, and as the name suggests, they contain a mixture of both sensory and motor fibers, meaning, they carry information both towards and away from the central nervous system. The majority of the peripheral nerves in the human body are mixed nerves.

Another major way in which nerves can be categorized is according to their origin in the body. There are two such groupings – cranial nerves and spinal nerves. Cranial nerves are those found exiting the skull, a.k.a. the cranium, while spinal nerves are found exiting the spine, a.k.a. the vertebral column. Combined, there are 43 paired cranial and spinal nerves in the human body.

Cranial nerves, the yellow-colored structure we can see in this image, make up 12 of these pairs. As can be seen in this inferior view of the brain, most of them emerge from the brain with the exception of the 11th, the accessory nerve, which arises from the superior part of the spinal cord. Each pair has a unique name alongside its given name which is typically written in Roman numerals from I to XII and ordered from top to bottom. They mainly innervate structures in the head and neck and cranial nerves can be purely sensory, purely motor, or mixed.

The remaining 31 pairs of nerves are known as spinal nerves. There are 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. Most of them are mixed nerves, meaning, they contain both sensory and motor neurons. The spinal nerves originate from the spinal cord and innervate structures below the head. They can be seen exiting specific segments of the spinal cord through the union of posterior and anterior roots. Preceding the roots are several nerve rootlets emerging from posterior and anterior portions of the spinal cord.

The cell bodies of the motor neurons of spinal nerves are located in the anterior horn of the spinal cord gray matter with their axons or fibers traveling through the anterior rootlets, roots, and out to the peripheral target organs. The sensory fibers, on the other hand, return sensory information from the trunk and limbs to the central nervous system via the posterior root with their cell bodies located within the spinal ganglion which is this swollen section of the posterior root you can see highlighted here which is also commonly known as the dorsal root ganglion.

And this brings us to the topic of ganglia. The ganglia, as we have just seen, are clusters of neuronal cell bodies located in the peripheral nervous system. Now, there are two main types of ganglia – the sensory ganglia which is any type of ganglion sending sensory impulses from the peripheral nervous system to either the spinal cord or brain and the autonomic motor ganglia which is any type of ganglion sending involuntary motor movements to the viscera.

Let's first take a closer look at the sensory ganglia. These ganglia specifically contain the cell bodies of sensory neurons and receive sensory information from the periphery before transmitting that information to either the spinal cord or the brain. There are two types of sensory ganglia – the sensory spinal ganglia, or dorsal root ganglia as they are known, and the sensory ganglia of the cranial nerves.

As we have seen, the spinal ganglion is a cluster of cell bodies found on the posterior root of a spinal nerve which carries sensory information relating to pain, touch, and temperature from the periphery to the spinal cord. The sensory cranial nerve ganglia can carry somatic, visceral, and special sensory information and transmit information from the head and neck periphery to the brain. There are eight such ganglia; for example, the geniculate ganglion of the facial nerve related to the sensation of taste or the cochlear ganglion of the vestibulocochlear nerve related to the acoustic pathway.

The second main type of ganglia found in the peripheral nervous system are the autonomic ganglia which are relay points along the autonomic pathways between the central nervous system and target organs where pre- and postganglionic autonomic neuron synapse, and these can be further divided into the sympathetic ganglia which include the prevertebral and paravertebral ganglia and the parasympathetic ganglia which include four cranial nerve ganglia; for example, the ciliary ganglion or the submandibular ganglion as well as terminal or intramural ganglia that lie close to or within their respective organs.

So now that you have covered nerves and ganglia, let's look at the final major component of the peripheral nervous system – a nerve plexus.

Now the term plexus derives from the Latin word for 'braid', and when looking at this image of a typical nerve plexus, you may be able to appreciate its association with an intricately woven network of intersecting nerves that collectively make up a nerve plexus.

There are two main types of nerve plexuses in the body – the spinal nerve plexuses which are formed by the merging of the anterior rami of the spinal nerves and the autonomic plexuses which can be mainly found in the thorax, abdomen, and pelvis where they are responsible for regulating the activity of visceral organs. In this tutorial, we will only mention the spinal nerve plexuses in further detail.

One important point to note regarding spinal nerve plexuses of which there are four in total is that not all 31 pairs of spinal nerves contribute to one. First there is the cervical plexus which is formed from the anterior rami of spinal nerves C1 to C4 and also receives small contributions from spinal nerve C5. Next is the brachial plexus which is formed by the anterior rami of spinal nerves C5 to T1.

Further down is the lumbosacral plexus which consists of a superior portion referred to as the lumbar plexus formed from the anterior rami of spinal nerves L1 to L4. And there is an inferior portion referred to as the sacral plexus formed from the anterior rami of spinal nerves L4 to S4. It should be noted that some textbooks describe these as two entirely separate plexuses, and for that reason, they will mention a total of five spinal nerve plexuses. Finally, the coccygeal plexus is formed from the anterior rami of spinal nerves S4 to S5 and the coccygeal nerve.

And that concludes our tutorial on the major components of the peripheral nervous system. To revise this content, check out our quiz and other learning materials in our study unit on this topic.

Until next time.