A Partial Account of Its Role in Maintaining
The human body is a complex creation. After thousands of years of study, humankind has yet to discover all its mysteries. However, thanks to the joint efforts of researchers throughout the ages, much has been learned about how our bodies function. For instance, we now have a more in depth understanding of how the relationship between the Central Nervous System (CNS — brain and spinal chord) and the Peripheral Nervous System (PNS –cranial and spinal nerves) facilitates maintenance of musculoskeletal integrity.
The purpose of this composition is to explore one aspect of this relationship, and its significance to bodyworkers. Specifically, it will focus on the following:
1. How the PNS and CNS work synergistically to monitor and produce changes in muscle length, thereby, among other things, maintaining proper body posture, providing position (spatial) sense and preventing injury.
2. The importance of this information in the client-therapist relationship.
To this end, a summary of the basic functions and structure of neurons — sensory (afferent) and motor (efferent) — muscle spindle fibers, Golgi tendon organs and reflex arcs follows.
Neurons are highly specialized cells which transmit impulses (messages) throughout the body. All neurons have a cell body and nerve fibers (also known as nerve processes). The nerve fibers which receive impulses from other neurons and transmit the information to the cell body are called dendrites, whereas the fibers which conduct impulses away from the cell body (by releasing neurotransmitters called acetycholine) to other neurons are called axons. Most cell bodies have one or more dendrites, and all have only one axon. Clusters of cell bodies which reside in the CNS are called nuclei, while clusters found in the PNS are known as ganglia. Bundles of spinal nerve processes found throughout the body in the PNS are known as nerves, while those residing in the CNS (spinal cord) are called tracts. Neurons associated with the spinal cord are divided into two categories: Motor and sensory.
Motor neurons are found in nerves and in tracts; their cell bodies reside in the CNS (spinal cord). Their function is to receive impulses from the sensory neurons and transmit messages away from the CNS via the PNS to effector organs (e.g., muscles) throughout the body. In the muscle, the impulses transmitted by the motor neurons stimulate the muscle to contract or relax.
Sensory neurons also are found in nerves and tracts. However, their cell bodies reside in ganglia just outside of the spinal cord. They conduct impulses from sensory organs in muscles and tendons to the CNS. Some sensory neuron fibers found in muscles and tendons are known as proprioceptive fibers. They monitor the stretch or contraction of a muscle, and the tension within a tendon, and constantly transmit this information to the CNS for processing. They are the spatial monitors of the body; their feedback enables the body to be aware of the posture or position of a limb even when the eyes are closed.
The sensory organs where the propriocetive neuron fibers terminate are known as muscle spindles and Golgi tendon organs.
Muscle spindles are found in the bellies of muscles, where a sensory nerve fiber ending, known as a primary or “anulospiral” ending, wraps around a group of 8 to 14 intrafusal muscle fibers. Their primary function is to continuously monitor and measure the length of the muscle at rest and when stretched, and the speed at which the muscle length changes. The following statement succinctly illustrates the spindle’s continuous monitoring activity:
…anterior and posterior trunk muscles continuously contract and relax, preventing our swaying from a vertical position. The spindles play their part here, since a slight pull to one side tilts the body but simultaneously muscles on the other side are stretched a little, which stretches the spindle muscle fibers and thereby leads to the discharge of sensory impulses to the spindle. The nerve fiber travels to the spinal cord and synapses with a motor nerve of the same muscle, and the muscle contracts to maintain the erect posture.[i]
Golgi tendon organs lie in the collagen fibers of the tendon, in the area where the tendon and muscle fibers merge. They function similarly to the muscle spindles in that they also measure changes in the muscle. However, whereas the muscle spindle is active even while the muscle is at rest, the Golgi tendon organ only becomes activated when the tendon contracts. Furthermore, Golgi tendon organs are not concerned with changes in muscle length, but in the increased tension of the muscle as a result of a change in its length. Golgi tendon organs are “high-threshold, slowly adapting receptors and apparently serve, at least in part, to prevent excessive stresses at joints by reflex inhibition of the adjacent muscles.”[ii] If the strain on the muscle and tendon becomes excessive, the Golgi tendon organ sends an impulse via afferent neurons to the CNS, where they synapse with motor neuron fibers of that same muscle. The efferent neurons instantly transmit an impulse to the muscle, causing it to relax, thereby preventing injury. This feedback loop, as well as that initiated by muscle spindles, is known as a reflex arc or neuropathway.
The following quote by author Deane Juhan further describes a reflex arc:
(A) “sensory-to-motor synapse in the spinal cord …( is) the most direct linkage we have between local sensory events and local motor response. Activity in specific muscle cells creates a local sensory impulse which directly effects the subsequent activity of the same muscle cells. Thus a reflex arc constitutes a feedback loop which both keeps my muscles themselves constantly informed as to what they are up to, and constantly modifies their efforts.”[iii]
The above is a brief synopsis of the functional and structural classifications of neurons, muscle spindles, Golgi tendon organs and reflex arcs. It is helpful for Massage Therapists to have a basic understanding of this aspect of the CNS and PNS, particularly when working with soft tissue injuries. Understanding how an injury affects the neuropathway helps a Therapist be more effective in providing the client not only with appropriate treatment but also with valuable education. For example, if a client has neck pain resulting from a car accident, the therapist would be able to explain that the pain might be in part due to overshortened muscles. When the accident occurred, the neck was likely tossed back and forth very quickly. Therefore, the Golgi tendon organ, which responds to increased muscle tension, did not have enough time to send a message to the muscles to lengthen. The muscle spindle (which measures the length and the speed at which the muscle length changes) then responded in a protective manner by sending an impulse to the muscles to contract–overshorten– rather than allow the muscles to extend. The muscles were overshortened and remain in a constant state of contraction because, due to the trauma, the neuropathway was changed. The proprioceptors “remember” the trauma; their perception is that they need to remain in a state of contraction in order to prevent injury. Therefore, the proprioceptors need to be retrained to return to their normal state. This may be accomplished through Proprioceptive Neuromuscular Facilitation, a type of assisted therapeutic exercise.
[i] Ben Pansky, Dynamic Anatomy and Physiology (New York: MacMillan Publishing, 1975), pp. 189-190.
[ii] Williams, Warwick, Dyson, Bannister, Gray’s Anatomy, 37th edition (New York: Churchill Livinstone), p. 915.
[iii] Deane Juhan, Job’s Body (a Handbook for Bodywork) (New York: Station Hill Press, Inc., 1987), p. 194.