Nerves (Part One): The Structure Behind How We Move – By John Macy
Tying the Body’s Systems Together
We have covered the main individual structures that enable us to move – bones, ligaments, muscles, and tendons in previous posts. Now it is time to start tying all the parts together so a person can actually generate intentional movement. There are two major areas that tie things together, the nervous system that provides command and control and the fascial web which provides overall structural integrity. This post and the next will cover the nervous system before we step into the fascial web.
There are two major systems that connect everything:
“There are two major areas that tie things together – the nervous system that provides command and control and the fascial web which provides overall structural integrity.”
The System That Controls Movement
In this post I will cover the physical structure of the nervous system.
To create and control our movement in space we need a system to collect data on where our parts are in space and how they are moving, to send messages to the muscles, and to make decisions on what to do next.
That system is made of neurons (individual nerve cells) and nerves (strings of individual neurons bundled side by side).
The nervous system system is divided into the central nervous system, made of the brain and spinal cord, and the peripheral nervous system which is everything not brain and spinal cord.
I am writing mainly about the peripheral system today because that is what most people are familiar with but the ideas apply to the central system, too. There are some differences in the in the micro anatomy between the systems but they really don’t matter for our discussion.
“To create and control our movement in space we need a system to collect data… send messages to the muscles… and make decisions on what to do next.”
Sensory Nerves, Motor Nerves, and Neurons
Generally speaking, we call the nerves carrying signals towards the brain sensory nerves and call those carrying signals away from the brain motor nerves.
Individual nerve cells (neurons) have three main parts. At one end are dendrites that receive signals coming to the neuron, next is the cell body with the nucleus and, lastly, is the axon which carries electrical signals away from the cell body to the next neuron.
Usually the axon protrudes far from the cell body. Very far at times.
For example, there are a number of neuron cell bodies in your big toe whose axons go all the way to the spinal cord in the middle of your low back. For many people that is over 40 inches long.
“There are a number of neuron cell bodies in your big toe whose axons go all the way to the spinal cord in the middle of your low back.”
Bundles of Nerves in the Body
Most people are familiar with some of the larger nerves in the system like the sciatic nerve in the buttock made up of many of the axons going up and down the back of the leg.
You probably have discovered the ulnar nerve that goes behind your elbow that yelps if you hit your “funny bone”.
All the neurons in one part of the body, such as the tip of your index finger, come together and follow the same physical pathway so they get bundled together forming a nerve.
They are progressively bundled as you go up towards the spine and brain, making larger diameter nerves as you go from the ends of the body into the spine and brain.
The same bundles are also used for neurons coming out from the central nervous system to the distal body parts.
The Road System Analogy
The structure of the peripheral nervous system (outside the spinal cord) is much like a road system.
Traffic goes in both directions over the “same road” but actually cars going one way are using a different area of the road than those going the other way.
In a road system, the driveway at an individual home is small but connects to progressively larger roadways as more cars are going the same place.
So too with nerves.
Individual neurons from each small region of the body are headed to the spine and along the way they join with more neurons, that become larger in diameter as more neurons come together that are going to the same place -the spinal cord.
And the reverse is true.
Messages coming from the brain to the limbs come down the spinal cord and then spread out over smaller nerves as they send signals to individual muscles.
“The structure of the peripheral nervous system is much like a road system.”
Mapping Nerves in the Body
The result of this design is that we can map which nerves are connected to which body parts or regions.
While there is some variance from person to person, overall the nerves follow about the same routes in every body.
Because nerves are actual, physical, structures they can get stretched, compressed, or irritated along their route and may cause problems like carpal tunnel syndrome, foot drop, and changes in sensation in areas of the body.
For instance, the nerves that exit your spine at the bottom of the neck are connected to the side of your arm your little finger sits on.
This means if you are having pain along the side of your hand one of the possible causes could be pressure on the nerve up at the neck.
Nerve Injury & Disease
There is also a number of diseases that can affect nerves.
Things like multiple sclerosis, shingles, diabetes.
Often nerves are damaged by an overactive immune response triggered by various germs, particularly a variety of viruses.
Unfortunately, nerves regenerate very slowly making recovery from disease or injury generally a slow process measured in months not days or weeks.
“Unfortunately, nerves regenerate very slowly making recovery from disease or injury generally a slow process measured in months not days or weeks.”
The Nervous System’s Flexibility
Overall, however, the nervous system does a great job of handling the information traffic we need to handle our body as it moves through space to do what we ask of it.
It has adaptability and flexibility, and the structure of the system lets us often have several ways to do the same movement through space, just as the road system allows for detours and rerouting to get to the same destination.
Next: How Nerves Work
So that is a quick tour of the structure.
Next time I will delve into how the nerves work and explain how we use the nervous system to create signals to send on the nerves to control our movement.