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The Nervous System:
A Partial Account of Its Role in Maintaining
Musculoskeletal
Integrity
by
Maria I. Martos
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.
  
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