In the spinal or decerebrate animal, almost any type of cutaneous sensory stimulus from a limb is likely to cause the flexor muscles of the limb to contract, thereby with drawing the limb from the stimulating object. This reflex is called the flexor reflex.
In its classic form, the flexor reflex is elicited most powerfully by stimulation of pain endings, such as by a pinprick, heat, or a wound, for which reason it is also called a nociceptive reflex, or simply a pain reflex. Stimulation of touch receptors can also elicit a weaker and less prolonged flexor reflex.
If some part of the body other than one of the limbs is painfully stimulated, that part will similarly be withdrawn from the stimulus, but the reflex may not be confined to flexor muscles, even though it is basically the same type of reflex. Therefore, the many patterns of these reflexes in the different areas of the body are called withdrawal reflexes.
Neuronal Mechanism of the Flexor Reflex. The lefthand portion of Figure 1 shows the neuronal path ways for the flexor reflex. In this instance, a painful stimulus is applied to the hand; as a result, the flexor muscles of the upper arm become excited, thus withdrawing the hand from the painful stimulus.
Fig1. Flexor reflex, crossed extensor reflex, and reciprocal inhibition.
The pathways for eliciting the flexor reflex do not pass directly to the anterior motor neurons but instead pass first into the spinal cord interneuron pool of neurons and only secondarily to the motor neurons. The shortest possible circuit is a three- or four-neuron pathway; however, most of the signals of the reflex traverse many more neurons and involve the following basic types of circuits: (1) diverging circuits to spread the reflex to the necessary muscles for withdrawal; (2) circuits to inhibit the antagonist muscles, called reciprocal inhibition circuits; and (3) circuits to cause afterdischarge that lasts many fractions of a second after the stimulus is over.
Figure 2 shows a typical myogram from a flexor muscle during a flexor reflex. Within a few milliseconds after a pain nerve begins to be stimulated, the flexor response appears. Then, in the next few seconds, the reflex begins to fatigue, which is characteristic of essentially all complex integrative reflexes of the spinal cord. Finally, after the stimulus is over, the contraction of the muscle returns toward the baseline, but because of after discharge, it takes many milliseconds for this contraction to occur. The duration of afterdischarge depends on the intensity of the sensory stimulus that elicited the reflex; a weak tactile stimulus causes almost no afterdischarge, but after a strong pain stimulus, the afterdischarge may last for a second or more.
Fig2. Myogram of the flexor reflex showing rapid onset of the reflex, an interval of fatigue, and, finally, afterdischarge after the input stimulus is over.
The afterdischarge that occurs in the flexor reflex almost certainly results from both types of repetitive dis charge circuits discussed in Chapter 47. Electrophysio logical studies indicate that immediate afterdischarge, lasting for about 6 to 8 milliseconds, results from repeti tive firing of the excited interneurons themselves. Also, prolonged afterdischarge occurs after strong pain stimuli, almost certainly resulting from recurrent pathways that initiate oscillation in reverberating interneuron circuits. These, in turn, transmit impulses to the anterior motor neurons, sometimes for several seconds after the incoming sensory signal is over.
Thus, the flexor reflex is appropriately organized to withdraw a pained or otherwise irritated part of the body from a stimulus. Further, because of afterdischarge, the reflex can hold the irritated part away from the stimulus for 0.1 to 3 seconds after the irritation is over. During this time, other reflexes and actions of the central nervous system can move the entire body away from the painful stimulus.
Pattern of Withdrawal During Flexor Reflex. The pattern of withdrawal that results when the flexor reflex is elicited depends on which sensory nerve is stimulated. Thus, a pain stimulus on the inward side of the arm elicits not only contraction of the flexor muscles of the arm but also contraction of abductor muscles to pull the arm outward. In other words, the integrative centers of the cord cause the muscles to contract that can most effectively remove the pained part of the body away from the object causing the pain. Although this principle applies to any part of the body, it is especially applicable to the limbs because of their highly developed flexor reflexes.
