The stretch reflex is an important and commonly-used clinical tool to test the i

Question

The stretch reflex is an important and commonly-used clinical tool to test the integrity of the spinal cord and muscle. It is evoked by striking the tendon of a muscle with a reflex hammer. Under various pathological conditions one can find either reflexes that are stronger (hyper-reflexia) or weaker (hyporeflexia) than normal

a) Draw a diagram of the elements of the stretch reflex, be sure to label the elements in the diagram, and explain how the reflex is elicited.

b) Under what conditions do you get hyper-reflexia? How about hypo-reflexia?
c) If gamma motoneurons were hyper-excited, what would be the effect on the reflex? Explain why this

would occur.

Explanation / Answer

a)

b) Cause of hyper-reflexia

Hyporeflexia is the condition of below normal or absent reflexes (areflexia). It can be tested for by using a reflex hammer. It is the opposite of a condition calledhyperreflexia. Hyporeflexia is generally associated with a lower motor neuron deficit (at the alpha motor neurons from the spinal cord to muscle), whereas hyperreflexia is often attributed to upper motor neuron lesions (along the long motor tracts from the brain). The upper motor neurons are thought to be inhibitory of the reflex arc, which is formed by sensory neurons from intrafusal fibers of muscles, lower motor neurons (including alpha and gamma motor fibers) and appurtenant interneurons. Therefore, damage to lower motor neurons will subsequently lead to hyporeflexia and/or areflexia.

c) Although muscles can be in a relaxed state, muscles have a general resting level of tension. This is termed muscle tone and is maintained by the motor neurons innervating the muscle. Its purpose is to maintain posture and assist in quicker movements, since if muscles were completely loose, then more neuronal firing would need to take place.

The amount of tension in the muscles depends primarily on the resting level discharge of alpha motor neurons, mainly the Ia spindle afferents. Gamma motor neurons are also involved through their action on intrafusal muscle fibers. The intrafusal muscle fibers control the resting level of the Ia afferent pathway, which in turn creates a steady level of alpha neuron activity.

Muscle tone can also be due to tonic discharge of gamma motor neurons. The activation to these neurons are mostly from the descending fibers of the facilitatoryreticular formation. This leads to the stretching of muscle spindle, activation of alpha motor neurons and finally a partially contracted muscle. The cerebellum is the alpha-gamma motor neuron linkage. Therefore, with the cerebellum the muscle tension is maintained via alpha motor neurons as well as the gamma motor neurons.

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