The crossed extensor reflex involves a flexor reflex plus an extensor reflex The combined reflex actions comprise both serial and parallel processing. In serial processing a stimulus goes from neuron to neuron in a predictable manner to an expected target. Examples of serial processing are found in spinal reflexes ( withdrawal reflex, plantar reflex) and in the transmittal of some impulses straight through from receptor to brain. In parallel processing several sensory pathways are involved. The same stimulus triggers different neurons and impulses travel along uniquely different pathways. But the messages are coded in relation ship to one another to produce a complete picture.
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The Flexor withdrawal. If one steps on a sharp object ( a tack or a sliver of broken glass) with one's left foot, the injured lower limb will be flexed and withdrawn automatically. This mechanism of this action is an ipsilateral, polysynaptic spinal withdrawal reflex. Incoming afferent fibers synapse with interneurons that control the flexor muscle that effect the injured lower limb.withdrawal. At the same time impulses are transmitted by interneurons to inhibitor neurons in the injured limb to keep its extensor muscles from contracting. The crossed extensor reflex. The incoming sensory neurons from the site of injury of the left foot/leg also cross the spinal cord to synapse with interneurons on the contralateral side that control extensor muscles in the right . These interneurons on the right side will synapse with motor neurons for extensor muscles in the right leg. The contraction of these extensor muscles will stiffen the right limb and prepare it to support the additional weight shited it. Stimuli from the crossed sensory neuron will also send impulses via interneurons to motor neurons of contralateral thigh and hip, thigh and lower trunk muscles, These muscles will contract to varying degrees to facilitate the maintenance of balance as the center of gravity changes. Incoming sensory neurons from the left leg also synapse with neurons that carry sensations of pain to the brain. The withdrawal of the left leg is an example of serial processing. However, when the crossed extensor reflex is viewed as a whole we find that the sensory input from a single sources travels by several pathways to several targets to produce the the combined response to the piercing of the left foot. This is the result of parallel processing.