This spring break, I am studying for the MCAT exam I will be taking this coming May. In the midst of my studies yesterday, I came across an extremely interesting mechanism. When we go to the doctors office for our annual check up, the doctor hits our knee with a platic hammer. Almost always, if the doctor hits the correct area of the knee, our leg jerks up as an automatic response. This is known as the knee jerk reflex, which comes from another mechanism called the muscle stretch reflex.
The muscle stretch reflex is an automatic response our skeletal muscle goes through when it is stretched. Once the skeletal muscle is stretched, there is a reflex that wants to bring the muscle back to contraction. This is thought to hold a protective purpose. This is how the knee jerk reflex comes in. When the doctor hits your knee with the hammer, he hits your tendon which in turn causes your thigh muscle to stretch a little bit. Shown through the muscle strecth reflex, once that thigh muscle stretches even a little, the reflex causes the muscle to contract which in turn kicks your leg up.
There are two parts to the reflex: the afferent and the efferent parts. The afferent part of the reflex is the stimulus, the feeling of the tendon getting hit. Muscle spindle that are located in your thigh detect the stretch of the muscle. Somatosensory neurons get information from those muscle spindles, which then make an excitatory synapse with another neuron that are afferent and go to the spinal cord. The efferent part of the reflex is the response, the leg kick. Lower motor neurons are neurons that make synapses with skeletal muscle. These lower motor neurons make a synapse with the somatosensory neurons and receive a signal that goes all the way down their axon and makes a synapse with the skeletal muscle, in this case the thigh muscle which in turn produces the leg kick. The most interesting part of this whole process is that it does not involve the brain at all, only the spinal cord.