Why Do Chemotherapy Cause Peripheral Neuropathy?
As you already know, hair-loss and nausea are not the only side-effects of chemotherapy. These drugs also damage the peripheral nervous system.
A Brief Overview of the Nervous System
The nervous system is responsible for sending and receiving signals throughout the body. It is the system that responsible for purposeful movement, like closing your hand into a fist, or unconscious movements like breathing or stomach function. It is divided into two categories: the Central Nervous System and the Peripheral Nervous System.
The Central Nervous System is the brain, brain stem, and spinal cord. The Peripheral Nervous System connects to the central nervous system at the spine and reaches into your peripheries, like your hands and feet.
Nerve cells, or neurons, resemble a root ball with extended roots hanging off of them. The soma is the large portion that encloses the nucleus and the axon is a large branch that extends out into other tissue, sending and receiving electrical signals. The axon is insulated by a myelin sheath that ensures the signal is quickly relayed to the brain and it prevents signal distortion.
Unlike other cells, once a nerve cell has reached maturity, it becomes post-mitotic. This means it has completed its mitosis phase and will not enter it again. Because nerve cells do not go through mitosis, once a nerve cell is damaged, it is not replaced by a healthy cell.
Peripheral nerve cells come in two different varieties: sensory and motor. The whole point of sensory nerves is to communicate touch and feel between your external body and your brain. Motor nerves receive movement orders from your brain and cause the muscles to contract in the way your brain wanted.
Current research believes the area of your peripheral nervous system most affected by chemotherapy is the dorsal root ganglia. Let’s take a minute to describe what that is and why it’s important.
Now, imagine you are able to look down at yourself lying face-down on a table. You see your spine and the muscles of your back.
Inside your spine is the spinal cord. The spinal cord is made up of two different types of tissue: gray matter and white matter. If you were to hold a cross-section slice of your spinal cord, like a slice of bread, the center would look like a gray butterfly. That’s the gray matter. The tops of the wings are towards your back and the bottom of the wings are pointing towards your front.
The white matter is made up of myelinated axons connecting the nerve bodies (gray matter) to the nerve branches outside of the spinal column. The roots coming out of the top of the butterfly-shaped gray matter is the dorsal root ganglia. These nerve bundles house a convergence of sensory and motor nerves.
If the dorsal root ganglia, this nerve bundle coming out of your spine, are damaged, it can affect both motor and sensory nerves in your extremities.
In this case, gravity is not on our side. You see your hands and feet are the farthest points away from the main part of your body. So circulation can be very hard for the body to do if you are on chemotherapy. The medication typically gets trapped at the extremities and causes the decline of the nerve.
Chemotherapy acts on weaknesses in the cell cycle defense. Your nerves are very exposed to the effects of chemotherapy because not only are they not protected by the blood–brain-barrier, but they require lots of blood to function correctly. This means that there is usually a relatively large amount of chemotherapy penetrating the nervous system.
Once inside the cell, chemotherapy can damage the nervous system in a variety of ways. In the next chapter, I’ll discuss the most common chemotherapies and precisely how they damage the peripheral nervous system, but here are a few of the mechanisms:
Chemotherapy destroys the myelin sheath surrounding the axon, causing disrupted signaling
It can cause swelling in the dorsal root ganglia, affecting both your sensory and movement nerves
It can prevent the mitochondria, “factories” providing power to each cell, from functioning or functioning well
Main Takeaways: A side-effect of their cancer-killing properties, chemotherapy also cause damage to nerve cells, some of which are found in the dorsal root ganglia.