Muscle Fibers and Nerve Excitability
I have spent a great deal of time dissecting cadavers this year. This has been an amazing opportunity to learn in person about human anatomy and physiology and is deeply informative for my continuing work with clients seeking to overcome pain. In examining these bodies, generously donated to UCSF/SFSU, I have spent a great deal of time isolating muscles as well as bony landmarks and nerve bundles. A muscle cell, technically called a muscle fiber, is composed of interconnected proteins which contract and release. The first part of my revelation was that each of these fibers is the full length of the muscle of which it is part. This means that a fiber (remember, that means a muscle cell) which makes up a small part of the Rectus Femoris (the outermost of the quadriceps muscles, it runs from the pelvis down to the knee cap) also runs the full length from the pelvis to the knee. My second breakthrough was in connecting this fact to a similar detail about nerve cells. A nerve cell is called a neuron and the aspect of the cell responsible for transmitting electrical impulses from the body of the cell to the outputting ends is called the axon. Note the axon of the neuron below, covered in a myelin sheath.
When I bump my toe everything happens so fast that it is nearly impossible to tell what is going on. The sensory neurons in my toe send a signal to my spinal cord or my brain for processing, which then facilitates either a reflex or a processed reaction to the stimulus. Perhaps, I withdraw my toe and cradle it in pain. The signal, as it travels in both directions, is traveling from neuron to neuron or along the axon of many neurons, from extremity to the central nervous system (CNS, see following post for further discussion of this system) and back out again. Some of the axons responsible for conducting the impulse to and from the toe are the length of the distance from toe to CNS! Once the signal reaches the injured extremity it excites muscles fibers which contract (too late) to bring the toe out of harm’s way. In these contractions, remember, fibers the length of the muscle are contracting.
Given two facts – that muscle fibers run the length of a muscle and that axons may run the distance between an extremity and the central nervous system – we can begin to understand why we can experience pain in parts of the body distant from a specific injury. Neurons begin to respond when other neurons in their vicinity are excited. Thus a wave of signals traveling away from the CNS may excite offshoots and facilitate muscle contraction in an area not directly impacted by the original stimulus. As part of the healing process, this interconnectivity may be utilized by subtly adjusting areas peripheral to the site of injury.
I was recently working with a client, a professional dancer, who suffered injury to his ankle some years ago. Since that time his career has been successful but he reports always having noticed less mobility at the site of injury. He had seen physical therapists and massage practitioners about the issue with little or no success. He reported that these practitioners had spent considerable time working directly on his foot and ankle and wondered aloud why I was dedicating so much attention elsewhere on his body. But consider: if muscle fibers systematically run the length of a muscle and the axons of nerves may run from an extremity to the CNS, what impact might working elsewhere i.e. on the same leg have on the point of injury? Muscle cells that directly connect to the area may be as far away as the knee. Neurons that directly relate the the area may end as far as the upper spine or head. Conceivably – just given these two facts – we could have worked on his head and seen results in his foot. Certainly, my clients saw results!
That muscle fibers and neurons can be the lengths discussed should not be taken to completely explain interconnection throughout the body. How neurons communicate is a very active field of research. How axons come to be a certain length is not thoroughly understood. Nor should the story of my client be an incentive to start poking at a friend’s head in hopes of provoking a response in her foot. It probably will only serve to get you a good swift kick. Of course, none of this changes the two tenants of the discussion.
Next time you stub your toe, consider: where did your responses originate?