The stretch reflex refers to the intrinsic reflex in which stretching of the muscle results in contraction of the muscle to either maintain or change muscle length. The stretch reflex is built on a monosynaptic reflex arc and is measured by the muscle spindles, which protect the muscle from overstretching. A medical professional tests the stretch reflex using the patellar tendon reflex, which in turn is also an intrinsic reflex that is triggered by a light blow to the patellar tendon, resulting in a contraction of the hamstring extensor muscles, which in turn results in extension of the knee joint. The stretch reflex then occurs shortly after the blow, causing the lower leg to advance.
What is the stretch reflex?
The stretch reflex is the intrinsic reflex in which stretching the muscle causes it to contract. The brain receives all information about the position, movement and posture of the body through proprioceptors. These are located in tendons, joints, muscles and ligaments and respond respectively to stretching, deformation and pressure. In this way, signals are transmitted that lead to decisions to rapidly change the position of the body if necessary. The brain then sends appropriate transmissions and commands back to the muscles and the feedback loop closes. In this way, all positions of the muscles are changed, corrected and adjusted. This happens mainly in the muscle spindles. They are located in the skeletal muscles and consist of muscle fibers. These in turn are surrounded by fine nerve fibers that register the changes in length by stretching. To be able to stretch the leg, the quadriceps femoris muscle, a skeletal muscle composed of four muscle heads in the thigh, is used.
Function and task
First of all, a stretch reflex serves to enable a person to walk and move upright. Secondly, it is responsible for the correct position of the limbs, which must return to their necessary starting position during target motor movements. The stretching state of the muscles can be influenced in this process. This occurs through contractions, which play an essential role in actively controlled movement sequences. Meanwhile, the proprioceptors in the joints and muscles convey information about the position, posture and movement of the body. In this way, it is possible that even if the muscles change, a stretching stimulus takes place and the muscle spindles ensure that even disturbances in the movement sequence can be corrected immediately. This can be the case, for example, when twisting your ankle. In skeletal muscles, Golgi tendon organs are located not parallel to the muscle fibers, as is the case with muscle spindles, but one behind the other. Mechanosensitive fibers lie in the connective tissues of the joints and also provide information that responds to changes in direction, velocity, and angle. During a stretch reflex, the excitation is transmitted via fibers to the spinal cord, where the information is simultaneously evaluated. From there, it is transmitted to the alpha-motoneurons, which leads to the contraction of the muscles in which muscle spindles are located. More precisely, this transmission is immediately answered with a reflex, even before the interneurons transmit the information further to the brain. At the same time, the fibers of the muscle spindle are connected to the contracted muscle. This is done via an inhibitory interneuron. As soon as the stretch and muscle tension becomes stronger, it is minimized again via tendon organs and their sensory fibers. Tendon organs are connected via alpha motoneurons and interneurons. The reflex operating over them is called monosynaptic in excitation transmission. In a monosynaptic stretch reflex, stretching of the muscle fibers is registered by the muscle spindles and an action potential is triggered in the nerve fibers, which is transmitted to the spinal cord. This results in increased activity of the alpha-motoneurons and contraction of the muscle. In this context, the Golgi tendon organs function as tension meters. Thus, stimuli are basically responded to quickly. The less the muscle fibers of an alpha motoneuron are innervated, the better the movement is tuned. This is the case, for example, in the finger or eye muscles.
Diseases and disorders
The patellar reflex as a stretch reflex is performed by the physician on a seated patient using a small reflex hammer.The patient loosely crosses one leg over the other while a light blow is applied below the kneecap on the patellar tendon. The leg then swings upward as the tendon and the core sac region of the muscle fiber are stretched. The dynamic stretch is transmitted monosynaptically to the alpha motoneurons via Ia afferents and the contraction starts immediately after the stretch. This allows the physician to check how strong the intrinsic reflex occurs and the condition of the muscles and nerves. The reflex is triggered several times, the other leg is also tested and finally the reflex response is compared. If the reflex is too weak, the doctor applies the so-called Jendrassik hand grip. This involves the patient bending his arms in front of his upper body and clasping his hands. The physician prompts to pull the hands apart forcefully and hold the position while the reflex is tested on the leg. A weakened reflex response may indicate neuropathy. This refers to diseases of the peripheral nerves that are not traumatic in origin. The damage may affect single nerves or distributed several. The disease is then differentiated into either mono- or polyneuropathy. An increased reflex response may be a pyramidal tract sign, by which is meant neurological symptoms that result from damage to the pyramidal tract and lead to abnormal reflexes. If no reflex occurs at all, there is a lumbar disc herniation or peripheral nerve injury.
