Chapter 9: Motor Sensory Link

The Reflexes

A successful examination of reflexes requires both the patient and the examiner to be comfortable. The muscle is placed in a comfortable position under slight stretch. The examiner places his or her finger on the major tendon of the muscle and lets the hammer freely fall. The hammer should do most of the work.

The examiner notes three major components of a reflex:

1. Its threshold or how easy it is to elicit
2. The speed in which the antagonist muscle is brought into play (too quickly with spasticity and too late with cerebellar disease)
3. Pathologic spread; reflexes should stay at the segmental level

The gamma loop governs the length of intrafusal fibers of the nuclear bag. These are motor cells in the spinal cord that receive both excitatory and inhibitory influences and determine tone. The nuclear bag is in parallel with the long axis of somatic muscles, and the ease with which the intrafusal fibers fire the annulospiral endings that depolarize the anterior horn cell determines the threshold of the reflex. Thus, actively firing gamma neurons in the spinal cord cause the intrafusal fibers to shorten, increase the afferent barrage from annulospiral endings and partially depolarize anterior horn cells. Thus, when the volley of afferent synapses is excitatory to these anterior horn cells, they fire very easily (a low threshold). If there is poor inhibition in the spinal cord because of degeneration of the corticospinal tracts above the segment that is tested, or lack of supranuclear inhibition of these gamma neurons, the reflex will not only fire too easily, but will spread to other segments. The engagement of antagonist muscles stops the reflex. This is done by afferent information generated by “flower spray" endings in association with cerebellar and other modulatory influences. A normal reflex, an example being the knee-jerk, should elicit 2.5 swings of the leg before it stops. If the threshold to fire the reflex is too low (i.e., it can be elicited by a slight tap of the finger or a flick of the finger) and the antagonist muscles are engaged too easily, the patient has a spastic reflex. The agonist muscle cannot fully complete its action prior to the antagonist muscle being called into play. The tone will be increased and often Babinski's and Hoffman's signs will be elicited. The most common cause of these findings are lesions of the corticospinal pathways.