Polysynaptic reflex paths branch in a complex manner. The number of synapses in each of their branches varies. Because of the synaptic delay at each synapse, activity in the branches with fewer synapses reaches the motor neurons first, followed by activity in the longer pathways. This causes prolonged bombardment of the motor neurons from a single stimulus and consequently prolonged responses. Furthermore, some of the branch pathways turn back on themselves, permitting activity to reverberate until it becomes unable to cause a propagated transsynaptic response and dies out. Such reverberating circuits are common in the brain and spinal cord.
The withdrawal reflex is a typical polysynaptic reflex that occurs in response to a noxious stimulus to the skin or sub cutaneous tissues and muscle. The response is flexor muscle contraction and inhibition of extensor muscles, so that the body part stimulated is flexed and withdrawn from the stimulus. When a strong stimulus is applied to a limb, the response includes not only flexion and withdrawal of that limb but also extension of the opposite limb. This crossed extensor response is properly part of the withdrawal reflex. Strong stimuli can generate activity in the interneuron pool that spreads to all four extremities. This spread of excitatory impulses up and down the spinal cord to more and more motor neurons is called irradiation of the stimulus, and the increase in the number of active motor units is called recruitment of motor units.
IMPORTANCE OF THE WITHDRAWAL REFLEX
Flexor responses can be produced by innocuous stimulation of the skin or by stretch of the muscle, but strong flexor responses with withdrawal are initiated only by stimuli that are noxious or at least potentially harmful (ie, nociceptive stimuli). The withdrawal reflex serves a protective function as flexion of the stimulated limb gets it away from the source of irritation, and extension of the other limb supports the body. The pattern assumed by all four extremities puts one in position to escape from the offending stimulus. Withdrawal reflexes are prepotent; that is, they preempt the spinal pathways from any other reflex activity taking place at the moment.
Many of the characteristics of polysynaptic reflexes can be demonstrated by studying the withdrawal reflex. A weak noxious stimulus to one foot evokes a minimal flexion response; stronger stimuli produce greater and greater flexion as the stimulus irradiates to more and more of the motor neuron pool supplying the muscles of the limb. Stronger stimuli also cause a more prolonged response. A weak stimulus causes one quick flexion movement; a strong stimulus causes prolonged flexion and sometimes a series of flexion movements. This prolonged response is due to prolonged, repeated firing of the motor neurons. The repeated firing is called after-discharge and is due to continued bombardment of motor neurons by impulses arriving by complicated and circuitous polysynaptic paths.
As the strength of a noxious stimulus is increased, the reaction time is shortened. Spatial and temporal facilitation occurs at synapses in the polysynaptic pathway. Stronger stimuli produce more action potentials per second in the active branches and cause more branches to become active; summation of the EPSPs to the threshold level for action potential generation occurs more rapidly.
FRACTIONATION & OCCLUSION
Another characteristic of the withdrawal response is the fact that supramaximal stimulation of any of the sensory nerves from a limb never produces as strong a contraction of the flexor muscles as that elicited by direct electrical stimulation of the muscles themselves. This indicates that the afferent inputs fractionate the motor neuron pool; that is, each input goes to only part of the motor neuron pool for the flexors of that particular extremity. On the other hand, if all the sensory inputs are dissected out and stimulated one after the other, the sum of the tension developed by stimulation of each is greater than that produced by direct electrical stimulation of the muscle or stimulation of all inputs at once. This indicates that the various afferent inputs share some of the motor neurons and that occlusion occurs when all inputs are stimulated at once.
