An ipsilateral reflex is a reflex in which the motor repose occurs on the same side of the body as the sensory stimulus. A contralateral reflex is one in which the motor response takes place on the opposite (contralateral) side of the body from the side of the sensory stimulus.
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An example of an ipsilateral reflex is the knee jerk (patellar tendon) reflex. The knee jerk reflex is monosynaptic reflex. The crossed-extensor reflex is a contralateral reflex. It is also polysynaptic. If one steps on a sharp object-- a tack, a nail, or a glass splinter-- with one's left foot (L), the pain receptors will immediately transmit pain stimuli to the spinal cord by way of sensory neurons. In the spinal cord, these sensory neurons will synapse with interneurons which will send both inhibitory and excitatory impulses to the injured limb, by way of motor neurons . The inhibitory impulses will relax/inhibit the quadriceps femoris and the excitatory impulses will cause the hamstrings to flex, thus lifting the injured limb (L), and removing it from the noxious stimulus. At the same time, pain stimuli from some sensory neurons will cross to the other side of the spinal cord and synapse with inhibitory and excitatory interneurons; some of these interneurons will send inhibitory impulses to the hamstrings which will relax, and excitatory impulses to the quadriceps femoris which will contract and stiffen the limb(leg, thigh) on this side (R) The contralateral reflex (which will involve other muscles up and down the uninjured side) serves to prepare the contralateral limb to receive the full weight of he body, and to adjust the position of the pelvis as the center of gravity shifts; this prevents one from becoming unbalanced and falling over.