Clinical Applications of the Stretch Reflex
Almost every time a clinician performs a physical exam-ination on a patient, he or she elicits multiple stretch reflexes. The purpose is to determine how much back-ground excitation, or “tone,” the brain is sending to the spinal cord. This reflex is elicited as follows.
Knee Jerk and Other Muscle Jerks. Clinically, a method usedto determine the sensitivity of the stretch reflexes is to elicit the knee jerk and other muscle jerks.The knee jerk can be elicited by simply striking the patellar tendon with a reflex hammer; this instantaneously stretches the quadriceps muscle and excites a dynamic stretch reflex that causes the lower leg to “jerk” forward. The upper part of Figure 54–6 shows a myogram from the quadri-ceps muscle recorded during a knee jerk.
Similar reflexes can be obtained from almost any muscle of the body either by striking the tendon of the muscle or by striking the belly of the muscle itself. In other words, sudden stretch of muscle spindles is all that is required to elicit a dynamic stretch reflex.
The muscle jerks are used by neurologists to assess the degree of facilitation of spinal cord centers. When large numbers of facilitatory impulses are being trans-mitted from the upper regions of the central nervous system into the cord, the muscle jerks are greatly exag-gerated. Conversely, if the facilitatory impulses are depressed or abrogated, the muscle jerks are consider-ably weakened or absent. These reflexes are used most frequently in determining the presence or absence of muscle spasticity caused by lesions in the motor areas of the brain or diseases that excite the bulboreticular facilitatory area of the brain stem. Ordinarily, large lesions in the motor areas of the cerebral cortex but notin the lower motor control areas (especially lesions caused by strokes or brain tumors) cause greatly exag-gerated muscle jerks in the muscles on the opposite side of the body.
Clonus—Oscillation of Muscle Jerks. Under some condi-tions, the muscle jerks can oscillate, a phenomenon called clonus (see lower myogram, Figure 54–6). Oscil-lation can be explained particularly well in relation to ankle clonus, as follows.
If a person standing on the tip ends of the feet sud-denly drops his or her body downward and stretches the gastrocnemius muscles, stretch reflex impulses are transmitted from the muscle spindles into the spinal cord. These impulses reflexively excite the stretched muscle, which lifts the body up again. After a fraction of a second, the reflex contraction of the muscle dies out and the body falls again, thus stretching the spindles a second time. Again, a dynamic stretch reflex lifts the body, but this too dies out after a fraction of a second, and the body falls once more to begin a new cycle. In this way, the stretch reflex of the gastrocnemius muscle continues to oscillate, often for long periods; this is clonus.
Clonus ordinarily occurs only when the stretch reflex is highly sensitized by facilitatory impulses from the brain. For instance, in a decerebrate animal, in which the stretch reflexes are highly facilitated, clonus develops readily. To determine the degree of facilitation of the spinal cord, neurologists test patients for clonus by sud-denly stretching a muscle and applying a steady stretch-ing force to it. If clonus occurs, the degree of facilitation is certain to be high.