A stretch reflex, also referred to as a deep tendon reflex or myotatic stretch reflex, is a physical response to the extension of a muscle. In essence, it is a process that causes a contraction, thereby stopping a stretch. This mechanism, which occurs in two major stages, serves several beneficial purposes, one of which is to prevent injury. Medical professionals hope that it holds the key to treating a number of brain and spinal cord conditions.
This mechanism a protective response. If people did not have it, overextending a muscle and getting hurt would be extremely easy. Individuals also would not be able to resist loads, meaning it would be extremely difficult to carry anything. It is what allows the body to maintain or change skeletal posture and positioning, as well.
The Basic Process
Whenever stretching occurs, muscle spindles, which are sensory receptors, detect the change in the length of the tissue. They send signals through sensory neurons to the spinal cord, and interneurons, which essentially are conductors or directors for nerve impulses, direct the signal to motor neurons. Contraction occurs as a result, pulling back the extent of the stretch.
During this process, the opposing muscle — for example, the hamstrings when the stretch happens in the quadriceps — also receives an impulse that inhibits it from tightening. This secondary response, called reciprocal inhibition, makes it easier for the primary muscle to contract, which reduces the likelihood of an injury from both muscles trying to tighten at the same time. The reflex, therefore, always involves a muscle pair.
Experts sometimes separate this response into two stages: dynamic and static. The dynamic stage is the initial reaction, which lasts only a few moments, but which is very powerful in preventing injury. The static stage occurs for the duration of the stretch.
Alpha, Gamma and Beta Neurons
Muscles contain both extrafusal tissue, which does the real work, and intrafusal tissue, which contains the spindles and which provides sensory information. Alpha motor neurons innervate the extrafusal tissue, causing contractions, and gamma motor neurons stimulate the intrafusal fibers within the spindles. Beta neurons are responsible for exciting both types of tissue.
Around the spindles are afferent axon endings. As a stretch lengthens the intrafusal fibers, ion channels in the axon endings open, and the number of electrical impulses moving away from the gamma motor neurons, or action potential, goes up. Subsequently, the alpha motor neurons fire efferent or return signals to the muscle more rapidly, and the contraction intensifies. Put another way, the stretch initiates a circuit of nerve impulses.
When a contraction occurs, the intrafusal fibers shorten, releasing tension on the sensory axons. When alpha motor neurons fire, however, gamma motor neurons are stimulated at the same time. As a result of this co-activation, both the extrafusal and intrafusal fibers contract simultaneously, and the slack in the spindles decreases, maintaining stretch sensitivity.
Stretching is always present to some degree, so even though the amount of action potential might change, the circuit of nerve impulses that occurs during this reflex is continuous. Medical professionals refer to the subsequent constant tension as muscle tone. Some people describe it as the resistance to stretch because of its relationship to the stretch reflex.
Using Stretching Effectively
Improper stretching techniques, such as extending too fast or bouncing, can elicit a stronger stretch reflex and result in injury. For this reason, it is critical to be sensitive to this defense mechanism during everyday movement and exercise. Many experts recommend extending just to the point of highest tension, breathing slowly and trying to let the reflex relax a little. They also recommend using fluid, natural movements in preparation for more strenuous work, because they require a continuous adjustment in tension that makes it very difficult to trigger intense contractions.
Medical professionals test the stretch reflex during routine physical examinations. To do this, they gently tap specific areas of the body, most commonly the area just below the knee cap — this is one reason why people refer to the response as a "knee jerk reaction." Problems during the test might indicate issues with the central nervous system.
This reaction, unlike many other processes in the body, doesn't rely on the brain. In fact, it happens too fast for the brain to process properly. Researchers hope that, by studying it, they can figure out how to bypass the brain, using the central nervous system's reflex capabilities alone to control movement. Such an advancement would be a drastic step in the treatment of conditions where communications between the spinal cord and brain are limited or stopped.