Peripheral Nervous System. The Story of a Front Kick. Part E.

Welcome to week 5! If you’re just joining us (go back and catch up, it won’t take long), we will be talking about the neural pathways involved in a karate kick. Before we dive into that though, we are covering the basics of the nervous system and how it works so we can look at this karate kick (and all the other fun stuff we are going to cover) with a little more understanding.

Fig 1. The nervous system. Everything outside of the central nervous system makes up the peripheral nervous system.

Title: Elementary anatomy, physiology and hygiene for higher grammar grades Year: 1900 (1900s) Authors: Hall, Winfield Scott, b. 1861

This is the last installment of our neuroanatomy primer!! So far we covered the structure and function of a neuron, the central nervous system (CNS)- brain and spinal cord, and this week, we are going to cover the…peripheral nervous system!

The peripheral nervous system (PNS) is the collection of nerves running to and from your spinal cord and brain (with some exceptions that we will talk about later). Remember this handsome fella in fig 1 from a few weeks back? All the white lines outside of his brain and spinal cord are part of the peripheral nervous system.

Fig 2. Cross-section of a spinal nerve showing the inner layers around the bundles of axons called the perineurim, the outer layer, called the epineurim, and the blood vessels.

Adapted from OpenStax College - Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013.

CC BY 3.0

The central and peripheral nervous system share many of the same features. As in the central nervous system, the peripheral nervous system has neurons and neuroglial (support cells), analogous to but slightly different compared to the neuroglial cells of CNS. Most neurons of the PNS are insulated with myelin, like the CNS. Unlike the tracts of axons through the CNS, the bundles of axons that make up peripheral nerves, not only have myelin insulating the axon, but also have progressive layers of insulation (fig 2).

Now that we know a wee bit about the wee bits, lets look PNS on a larger scale. The closest part of the PNS to the CNS are the nerve roots coming off the spinal cord (fig 3). (The cranial nerves are a bit different and probably best saved for a later discussion). Each root only contains either the efferent motor neurons, carrying signals out to the muscles, or afferent sensory neurons, transmitting signals the opposite direction. Still within the bony protection of the spinal canal, the sensory nerve root joins with its corresponding motor nerve root to form a spinal nerve.

Fig 3. A spinal nerve is formed when a motor roots (the ventral root) join with its correponding sensory root (dorsal root). This immediately splits into a dorsal and ventral rami. They also split off into the white and gray rami which are part of the autonomic nervous system. The cell bodies (soma) for the sensory nerves are located outside the spinal cord in the dorsal root ganglion, where as the cell bodies for the motor nerves are located within the gray matter of the spinal cord.

By Mysid (original by Tristanb) - Vectorized in CorelDraw by Mysid on an existing image at en-wiki by Tristanb., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=1420508

Because spinal nerves carry both motor and sensory information they are called mixed nerves. There are 31 pairs in total, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and one sad lonely coccygeal pair. They are numbered based on the vertebra above the opening through which the spinal nerve exits the spinal canal called the intervertebral foramina (fig 4), except the cervical spinal nerves. Because of the anatomy of the cervical spine, those are named based on the vertebra they exit above, except for the eighth pair, which exit below the 7th (and last) cervical vertebra.

Fig 4. Lateral view of the spinal column. The arrows are pointing to the intervertebral (or neural) foramen, which are the opening through which the spinal nerves travel to and from the spinal cord.

Public Domain, https://commons.wikimedia.org/w/index.php?curid=1601710

But like the weekend, spinal nerves end before they have seemly begun. After exiting the intervertebral foramina they immediately split into two major divisions, the dorsal and ventral rami (fig 3). The dorsal (or posterior) rami, innervates the muscles and skin of the back. The ventral (or anterior) rami are the larger of the two divisions, and they wrap around to the front. For most of the thoracic region, things stay simple. The nerves are called intercostal nerves and each innervate a strip of skin, intercostal muscles (between the ribs) and abdominal muscles.

Innervation to the skin and muscles from spinal nerves follows a fairly regular pattern, though it can get a little more complicated in the arms and legs. For the most part, parts of the body closer to the head are innervated by higher spinal nerves and moves distally. So the neck is innervated by some of the very first nerves off the cervical spine like C3 and C4, the upper chest is innervated by the lower cervical spinal nerves, etc, all the way down into the low back and sacrum. A sensory dermatome map (fig 5) shows how spinal nerves provide sensory input from the skin. Each level is innervated by a single pair of spinal nerves. The spinal nerves from part of the cervical and thoracic cord follow a relatively horizontal striped pattern across the chest, abdomen, and back, but the pattern gets a little stretched out in the arms and legs. A myotome map, which shows innervation to the muscles, follows a similar pattern. Unfortunately, our myotome model refused our minumum clothing requirement of a leaf, so we are not able to post the image.

Our shy dermatome map model. C represents the cervical spinal levels, so C5= 5th cervical spinal nerve. D represents the thoracic, and in most current literature has been changed to T. L represents lumbar, and S represents sacral. The ribs are labeled I-XI, and the 12th rib is not shown.

Adapted from- http://www.springerlink.com/content/h25t2658333v02mp/?p=81668d0e1acc45c785d3f06396a40075&pi=0, Public Domain, https://commons.wikimedia.org/w/index.php?curid=2164109

Cervical, thoracic, and lumbar spinal nerves and plexuses The brachial plexus is formed from the cervical and the first thoracic spinal nerve the lubosacral plexus is formed from the lumbar and sacral spinal nerves). Adapted from Andrewmeyerson - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=49411614

In addition to the myotomes and dermatomes getting stretched out over our limbs, the spinal nerves off of the cervical, thoracic, and lumbar cord don’t go straight to their targets like they do off the thoracic cord. Our limbs, especially our arms and hands, are capable of complicated movements, much more that the stabilization provided by our core, and their innervation gets a little more complicated as well. The ventral rami of the spinal nerves come together and split several times, in what is called a plexus. Each time those nerves split or come together they are renamed. Plexuses have roots, trunks, and branches, no . We have several, including the brachial plexus which innervates part of the anterior chest wall, arm, and shoulder, and the lumbosacral, which innervates the leg, lower back, and buttocks (fig 6,7).

Once past the tangle of the plexuses (seems like the plural should be plexi, but I looked it up), the peripheral nerves continue on, to innervate muscles, skin, bones, joints, blood vessels (though this is the autonomic system), tendons. Just about every single part of the body has efferent nerves running to it, transmitting information from the brain, and afferent nerves from it sending information to the brain. Our brain is constantly monitoring this sensory feed back and making adjustments, mostly without conscious awareness, which we will talk about more over the next few weeks. Thank-you, and please send questions or comments to frontal.lobe@duramatters.com.

Fig 7. Diagram of the brachial plexus. It is formed by C5-T1 spinal nerves. Each time it splits or comes together again the nerves take on a different name, The cords, which are the last major part that is still considered brachial plexus, will become the median, ulnar, axillary, and radial nerves, which will all branch again on their way to innervate the arm and hand. There are many other nerves that come off the brachial plexus at different points as well. In the first few days of medical school, we started gross dissections of the brachial plexus. I was not yet comfortable in my new role as a medical student, and so many years later, it still brings back vivid memories of formaldehyde and angst.

By Gray - Gray's Anatomy, Public Domain, https://commons.wikimedia.org/w/index.php?curid=4245589