Invited reviewStretch sensitive reflexes as an adaptive mechanism for maintaining limb stability
Introduction
Observations of stretch reflexes, rapid excitatory responses of a muscle following stretch, were reported as early as 1751 by Robert Whytt (Pearce, 1997). Since that time it has been revealed that the stretch reflex is a complex muscle reaction, with multiple excitatory responses occurring at different latencies following a muscle stretch (Hammond, 1955). In the human upper limb for example, stretching the biceps brachii produces a ‘short latency’ response in the same muscle beginning approximately 20 ms after the onset of stretch, and a ‘longer latency’ response beginning around 50 ms after stretch onset (Hammond, 1955, Marsden et al., 1972). Both short and longer latency responses are generally regarded as involuntary actions since they occur prior to the fastest voluntary reaction, which in the biceps brachii has been shown to begin 90–100 ms following an auditory or proprioceptive ‘go’ signal (Hammond, 1956).
Though usually considered to be involuntary, the behavior of both short and long-latency stretch reflexes can be modulated in a task dependent manner. This modulation has led to much debate regarding the functional role of these fundamental responses. There is strong evidence from decerebrate animal preparations that the shortest latency stretch reflexes, mediated by the spinal cord, serve to compensate for muscle nonlinearities and regulate muscle stiffness over a wide range of operating conditions (Nichols and Houk, 1976, Hoffer and Andreassen, 1981). In these studies, stretch reflexes generally augment the intrinsic properties of a muscle to oppose external perturbations of muscle length, thereby increasing stability of the musculoskeletal system. While there also are ample data supporting contributions of the short latency stretch reflex to stiffness regulation in humans, the role of longer latency stretch reflexes is less clear. Longer latency reflexes also have been reported to contribute to limb stiffness and stability, but counter examples have been provided in which these involuntary actions appear to destabilize limb posture (see review by Hasan, 2005). We propose that these apparently contradictory results arise largely from the fact that multiple pathways can contribute to perturbation-evoked muscle activity occurring in the period corresponding to the long-latency stretch reflex, and the attempt to ascribe a single functional role to these multiple pathways. This review summarizes literature supporting this proposal, and describes conditions under which the role of the long-latency stretch reflex is consistent with the regulation of limb stiffness and stability.
Section snippets
Pathways mediating the stretch reflex
Liddell and Sherrington (1924) first described the neural pathway mediating the stretch reflex in the decerebrate cat. They described a pathway with a single synapse in the spinal cord separating the Ia afferent fiber from the homonymous α-motoneuron. This monosynaptic pathway is considered to be a major contributor to the short latency component of the stretch reflex observed in human studies (Magladery et al., 1951, Burke et al., 1984).
In contrast to the short latency stretch reflex, there is
Modulation of short latency stretch reflexes
The traditional view of the short latency stretch reflex as stereotyped and unreceptive to adaptation based on changes in cognition, such as intention or learning, has been challenged by evidence that its amplitude can be altered according to anticipation of an expected stimulus or voluntary action. While there are many tasks in which the short latency stretch reflex displays limited flexibility, it has been shown that in some tasks the amplitude of both the short latency stretch reflex and the
Muscle stiffness
The reflex modulation described above strongly suggests that stretch reflexes can contribute to the regulation of limb stability. This is consistent with the numerous studies describing how stretch reflexes contribute the mechanical properties of a limb. During the maintenance of posture, these mechanical properties often are quantified in terms of stiffness, which is the steady state force generated in response to an imposed static displacement of limb posture. Houk was among the first to
Summary
In this review, we have argued that a fundamental role of the human stretch reflex is to regulate the mechanical properties of a limb and to adapt those properties in a task appropriate manner. Furthermore, we emphasize that the regulation of limb mechanics is but one important role of the perturbation-evoked muscle activity that can be observed in the time period often attributed to the long-latency stretch reflex. Our recent results demonstrate that there are at least two distinct neural
Acknowledgments
This work was supported by NIH Grant NS053813. The authors also would like to thank the anonymous reviewers for their constructive comments.
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