Back36Spinal ReflexesKeir PearsonJames GordonDURING NORMAL MOVEMENTS the central nervous system uses information from a vast array of sensory receptors to ensure the generation of the correct pattern of muscle activity. Sensory information from muscles, joints, and skin, for example, is essential for regulating movement. Without this somatosensory input, gross movements tend to be imprecise, while tasks requiring fine coordination in the hands, such as fastening buttons, are impossible (see Chapter 33).Charles Sherrington was among the first to recognize the importance of sensory information in regulating movements. In an influential monograph published in 1906 he proposed that simple reflexes, stereotyped movements elicited by the activation of receptors in skin or muscle, are the basic units for movement. He further proposed that complex sequences of movements can be produced by combining simple reflexes. This view has been the guiding principle in motor physiology for much of this century. Only relatively recently has it been modified by the recognition that many coordinated movements can be produced in the absence of sensory information. For example, in a variety of species locomotor patterns can be initiated and maintained in the absence of patterned sensory input (Chapter 37). Nevertheless, the notion that reflexes play an important role in the patterning of motor activity is beyond doubt. The contemporary view is that reflexes are integrated with centrally generated motor commands to produce adaptive movements.Figure 36-1 Reflex responses are often complex and can change depending on the task.A. Perturbation of one arm causes an excitatory reflex response in the contralateral elbow extensor muscle when the contralateral limb is used to prevent the body from moving forward, but the same stimulus produces an inhibitory response in the muscle (reduced EMG) when the contralateral hand holds a filled cup. (Adapted from Marsden et al. 1981.)B. Loading the thumb during a rhythmic sequence of finger-to-thumb movements produces a reflex response (shaded blue area) in the muscle moving the finger as well as the loaded thumb muscle. The additional movement of the finger ensures that the pinching movement remains accurate. (Adapted from Cole et al. 1984.)P.714In this chapter we consider the principles underlying the organization and function of reflexes, focusing on spinal reflexes. The sensory stimuli for spinal reflexes arise from receptors in muscles, joints, and skin and the neural circuitry responsible for the motor response is entirely contained within the spinal cord. Reflexes have been viewed traditionally as automatic, stereotyped movements in response to stimulation of peripheral receptors. This view arose primarily from early studies on reduced animal preparations in which reflexes were examined under a set of standard conditions. However, as investigators extended their studies to measure reflexes during normal behavior, our concept of reflexes changed substantially. We now know that under normal conditions reflexes can be modified to adapt to the task. This flexibility allows reflexes to be smoothly incorporated into complex movements initiated by central commands.Reflexes Are Highly Adaptable and Control Movements in a Purposeful MannerA good example of the adaptability of reflexes is seen when the muscles of the wrist of one arm are stretched while a subject is kneeling or standing. The muscles that are stretched contract, but muscles in other limbs also contract to prevent a loss of balance. Interestingly, the reflex response of the elbow extensor of the opposite arm depends on the task being performed by that arm. If the arm is used to stabilize the body (by holding the edge of a table), a large excitatory response is evoked in the elbow extensor muscles to resist the forward sway of the body. If the arm is holding an unsteady object, such as a cup of tea, a reflex inhibition of the elbow extensors prevents movement of the cup (Figure 36-1A).Another example of adaptability in reflexes is the reflex of finger and thumb flexor muscles in response to P.715stretching the thumb muscles. If flexion of the thumb is resisted while a subject is attempting to touch the tip of the finger rhythmically to the tip of the thumb, a short-latency reflex response is produced in both the finger and thumb flexor muscles. The reflex in the finger flexor muscle produces a larger flexion movement of the finger to compensate for the reduced flexion of the thumb, thus ensuring the performance of the intended task (Figure 36-1B). If the subject is simply making rhythmic thumb movements, a reflex response is produced only in the thumb flexor muscle.A third example of the adaptable nature of reflexes involves a conditioned flexion-withdrawal reflex. Flexion withdrawal can be associated with an auditory tone by classical conditioning techniques (Chapter 62). Subjects are asked to place an index finger, palmar surface down, on an electrode. A mild electrical shock is then paired with the auditory tone. As one might expect, after only a very few such pairings the auditory tone alone will elicit the withdrawal reflex. What exactly has been conditioned? Is it the contraction of a fixed group of muscles or a behavioral act that withdraws the finger from the noxious stimulus? This question can be answered by having the subjects turn their hands over after conditioning is complete, so that now the dorsal surface of the finger is in contact with the electrode. Most subjects will withdraw their fingers from the electrode when the tone is played, even though this means that the opposite muscles now contract. Thus, the conditioned reflex in response to the tone is not only a stereotyped set of muscle contractions but also an appropriate behavior.Three important principles are illustrated by these examples. First, transmission in reflex pathways is set according to the motor task. The state of the reflex pathways for any task is referred to as functional set. Exactly how functional set is established for most motor tasks is largely unknown (the unraveling of the underlying mechanisms constitutes one of the challenging and exciting areas of contemporary research on motor systems). Second, sensory input from a localized source generally produces reflex responses in many muscles, some of which may be distant from the stimulus. These multiple responses are coordinated to achieve an intended goal. Third,
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