Stretch reflex

Abstract: Doubt about the role of stretch reflexes in movement and posture control has remained in part because the questions of reflex “usefulness” and the postural “set” have not been adequately considered in the design of experimental paradigms. The intent of this study was to discover the stabilizing role of stretch reflexes acting upon the ankle musculature while human subjects performed stance tasks requiring several different postural “sets”. Task specific differences of reflex function were investigated by experiments in which the role of stretch reflexes to stabilize sway during stance could be altered to be useful, of no use, or inappropriate. Because the system has available a number of alternate inputs to posture (e.g., vestibular and visual), stretch reflex responses were in themselves not necessary to prevent a loss of balance. Nevertheless, 5 out of 12 subjects in this study used long-latency (120 msec) stretch reflexes to help reduce postural sway. Following an unexpected change in the usefulness of stretch reflexes, the 5 subjects progressively altered reflex gain during the succeeding 3–5 trials. Adaptive changes in gain were always in the sense to reduce sway, and therefore could be attenuating or facilitating the reflex response. Comparing subjects using the reflex with those not doing so, stretch reflex control resulted in less swaying when the task conditions were unchanging. However, the 5 subjects using reflex controls oftentimes swayed more during the first 3–5 trials after a change, when inappropriate responses were elicited. Four patients with clinically diagnosed cerebellar deficits were studied briefly. Among the stance tasks, their performance was similar to normal in some and significantly poorer in others. Their most significant deficit appeared to be the inability to adapt long-latency reflex gain following changes in the stance task. The study concludes with a discussion of the role of stretch reflexes within a hierarchy of controls ranging from muscle stiffness up to centrally initiated responses.

Abstract: We studied stretch reflexes of soleus muscles of intercollicularly decerebrated cats using a new technique for estimating the component of a stretch reflex that results from the purely mechanical p...

Abstract: EMG responses evoked in hand muscles by transcranial stimulation over the motor cortex were conditioned by a single motor threshold electrical stimulus to the median nerve at the wrist in a total of ten healthy subjects and in five patients who had electrodes implanted chronically into the cervical epidural space. The median nerve stimulus suppressed responses evoked by transcranial magnetic stimulation (TMS) in relaxed or active muscle. The minimum interval between the stimuli at which this occurred was 19 ms. A similar effect was seen if electrical stimulation was applied to the digital nerves of the first two fingers. Median or digital nerve stimulation could suppress the responses evoked in active muscle by transcranial electrical stimulation over the motor cortex, but the effect was much less than with magnetic stimulation. During contraction without TMS, both types of conditioning stimuli evoked a cutaneomuscular reflex that began with a short period of inhibition. This started about 5 ms after the inhibition of responses evoked by TMS. Recordings in the patients showed that median nerve stimulation reduced the size and number of descending corticospinal volleys evoked by magnetic stimulation. We conclude that mixed or cutaneous input from the hand can suppress the excitability of the motor cortex at short latency. This suppression may contribute to the initial inhibition of the cutaneomuscular reflex. Reduced spinal excitability in this period could account for the mild inhibition of responses to electrical brain stimulation. Several groups have used transcranial magnetic stimulation (TMS) to test how the excitability of the motor cortex is affected by afferent input. Much of the initial work was concerned with testing the concept of excitatory transcortical reflexes. These reflexes are readily obtained in hand muscles after electrical or natural stimulation of cutaneous and/or muscle afferents and have a variety of names, such as LLR II/III, E2, V2, M2 and long-latency stretch reflex (Caccia et al. 1973; Marsden et al. 1976; Jenner & Stephens, 1982; Deuschl et al. 1985). Data from neurological patients very strongly suggests that many of these responses are produced by activity in a transcortical reflex pathway that operates in parallel with spinal systems (Marsden et al. 1977a,b; Jenner & Stephens, 1982; Noth et al. 1985). Experiments with transcranial stimulation gave results that were consistent with this idea. They showed that stimuli capable of eliciting long latency reflexes also increased the excitability of the motor cortex to transcranial magnetic stimulation with a time course consistent with traffic in a transcortical loop (Day et al. 1991). In contrast with these reports, some studies have indicated that peripheral input can suppress the excitability of motor cortex. In a short note, Delwaide & Olivier (1990) reported that stimulation of the median nerve at the wrist could profoundly suppress EMG responses evoked in relaxed hand muscles by transcranial magnetic stimulation of the cortex 18–21 ms later. Similar effects could be seen after stimulation of the cutaneous nerves of the index finger. Since H-reflexes in forearm muscles were unaffected Delwaide & Olivier (1990) suggested that the effect occurred at the cortical rather than the spinal level. Maertens de Noordhout et al. (1992) investigated the sequence of excitatory and inhibitory reflexes (Caccia et al. 1973) in the first dorsal interosseous muscle evoked by electrical stimulation of digital nerves. They used transcranial magnetic and electrical stimulation to show that motor cortical excitability was reduced by electrical stimulation of the digital nerves at a time corresponding to the transition between the initial inhibition and subsequent facilitation of the cutaneomuscular reflex. Palmer & Ashby (1992) reported the same result. Most recently, Bertolasi et al. (1998) found that stimulation of probable muscle afferents in the median nerve could suppress the excitability of cortical projections to forearm extensor muscles whilst radial stimulation suppressed the excitability of cortical projections to forearm flexor muscles. Stimulation of cutaneous afferents in digital nerves failed to have any effect. They suggested that the effect from muscle afferents was a cortical analogue of spinal reciprocal inhibition. The purpose of the present experiments was to extend the original observations of Delwaide & Olivier (1990). They confirm the presence of this early, striking period of inhibition, and show that it is a cortical phenomenon. We also speculate that it is related to and may even be responsible for the initial period of inhibition evident in cutaneo-muscular reflexes of the hand.