A rapid stretch of a voluntarily contracting muscle evokes electromyographic (EMG) responses at various latencies, as described by Loo, K. & McCloskey, D. (1985). Response latency measures the time delay between a perturbation and response, the response is typically greater in a stretch than a jolt as found by Lee and Tatton (1975) who also proposed that there are typically 3 responses to a muscle stretch namely M1, M2 and M3. The first, M1, represents the short latency involuntary monosynaptic spinal stretch reflex involving primary afferents. M2 represents the delayed response corresponding to the, perhaps transcortical, long loop reflex response and M3 represents the latency for a voluntary response mediated by the cerebellum. These 3 distinct responses can be displayed graphically, as shown in figure 1. From analyzing figure 1 we can see that, using the terminology introduced by Lee & Tatton (1985), M1 is the response seen approximately 45-60ms after the perturbation, M2 represents the increase in EMG activity 60-90ms after the perturbation and the increase in EMG activity between a latency of 90-110ms is termed M3. Any response with a latency of greater than 110ms is a voluntary response and is not considered to be a reflex response. These findings have been widely accepted and are frequently cited in later studies, for example in the study by Thilmann, A. F., Schwartz, M., Topper, R. Fellows, S.J. and Noth, J. (1991). Suminski, A.J., Rao, S.M., Mosier, K.M. and Scheidt, R.A. (2007) made a similar discovery finding short latency responses arising from monosynaptic reflexes, consistent with the latency of the M1 response. Petersen, N., Christensen, L., Morita, H., Sinkjor, T. and Nielsen, J. (1998) showed that ankle dorsiflexors typically show an M3 response. More interestingly, this paper also claims that the M2 response in the upper limb seems to correspond with the M3 response in the lower limbs.
According to Corden, D.M., Lippold, O.C.J., Buchanan, K. and Norrington, C. (2000), the second component of the stretch reflex response, M2, was first discovered by Hammond (1955) who believes that the long latency is due to the long loop reflex travelling the extra distance to the cortex. Hammond (1956) studied the EMG response in the bicep muscle and found that the earliest voluntary muscle activation in response to mechanical taps occur after 90-100ms which contradicts with the later findings from Lee & Tatton (1975) who claim voluntary response represents latencies greater than 110ms. This raises the possibility that the long loop reflex may have voluntary input. There have been many studies carried out investigating if the long-loop reflex is mediated by transcortical pathways. Logically, one would expect reflexes to be a hard-wired response and voluntary movement to have variation in responses. However, Evarts and Fromm (1981) provides evidence suggesting variability in their study of the wrist position. They concluded that the long loop reflex gives a pathway for the motor cortex to initiate closed loop feedback control to the flexors and extensors of the wrist. It can be argued that long latency responses fit both voluntary and reflex criteria. Arthur Prochazka, for one example, took particular interest in ambiguity for the correct definition of a reflex. For instance, is it regarded as a response which happens too quickly for the brain to notice, in which case, the M2 response would not be classed as a reflex since it is of long latency, or can it be defined as an involuntary response, in which case, the M2 response would be classed as a reflex since it occurs below the time threshold for it to be a voluntary action and occurs without any conscious awareness of the movement.
If the long loop reflex goes via the motor cortex, it could be influenced voluntarily. A notable study by Loo & McCloskey (1985) proposed that long loop reflexes are variable. This report studied the EMG responses of the flexor pollicis longus when a stretch was applied to the thumb-tip. The subject was required to initially have the muscle in a fixed, contracting state generating a constant force to give a baseline EMG to compare any results found against. Their results showed that, in the isometric holding task, all participants of the study could significantly alter the long latency responses to a stretch with some subjects recording up to 95% reduction in EMG activity when instructed to let go as opposed to resist. This indicates that motor set has an influence on the long loop reflex. Although the results for the isometric tracking, isotonic tracking and weight lifting tasks were less convincing, they still showed the ability to decrease EMG activity when told to let go not resist, contradicting previously claimed results from Marsden et al (1976) which suggested that prior instructions had no influence on EMG responses. When the thumb was anaesthetised, there was no evidence of abolishment of the long latency EMG response, contrary to what was noted by Marsden, Merton &Morton (1971). However, Loo & McCloskey (1985) found there was a significant linear correlation between the percentage increase in perceived heaviness and the percentage reduction in long latency reflex. This study provides us with defining results, however, not all subjects performed all tests and not all results were significant so there still remains room for debate. Long loop reflexes were found to be abolished or depressed by lesions to the pathways to and from the cerebral cortex, again, giving the view that the long loop reflex does take a transcortical pathway. Matthews, P. B., Farmer, S. F. & Ingram, D. A. (1990) also concluded from their study on the localization of the stretch reflex of intrinsic hand muscles in a patient with mirror movements that long loop reflexes are mediated transcortically.
The long loop reflex, it has been suggested, has a slower onset due to the longer route the reflex has to take. A monosynaptic spinal reflex arc is clearly a shorter route than the long loop reflex which, as some evidence shows, could go via the cortex. In a previous study, Hammond (1954) suggests the main feasible explanations for the delayed M2 response could be due to the longer neural pathway it takes or that the neurones involved are slower conducting. Matthews (1984) discovered the same findings as he suggested in his paper that the M2 response is mediated by muscle spindle secondary endings which by nature are slower conducting afferents. Corna, S., Grasso, M., Nardone, A. and Schieppati, M. (1995) also concluded that M2 response in the ankle muscles is mediated by group II afferents. Marsden, C., Merton, P., and Morton, H., (1976) argued that the long loop reflex could not be altered by the motor set and hence concluded that the response was more likely to be a reflex response than voluntarily response. However, as pointed out by Loo & McCloskey (1985), the subjects of the experiment were in fact the researchers themselves, hence, the results may be bias because sub-consciously they are aware of the experiment and what is going to happen and already have a prediction of what they want to happen. Rothwell, Traub and Marsden (1980) also suggested that long loop reflexes are not variable. Gassel (1970) claims that long loop reflexes occur predominantly with stimulation of cutaneous nerves or dorsal roots. To this end, Marsden et al. (1978) studied the stretch reflex response in the human flexor pollicis longus, which when stimulated, results in flexion of the thumb. If this muscle is stabilized, for example, fixed in plasticine, then cutaneous nerve activity can be detected.
It is proposed that long loop reflexes going via the motor cortex, have become progressively more important in effective motor control of motor skills. There is an initial judgement of the required strength of the muscle contractions needed before any specific movement. Any error in the estimate will result in the activation of the muscle spindle receptors and will result in a corrective long loop reflex, which causes an appropriate change in the signals from the motor cortex, correcting the response of the movement. This happens with a latency of less than 50 msec. This is about 70msec for lower limbs. This corrective compensation is automatic and unconscious. The pathways for 1a receptors up to the motor cortex and hence participation in long loop reflexes have been recognized in mammals such as the cat (Landgren, 1984). Clarac, F. (2005) suggests that the long loop reflexes play an important role in the adaptation of flexors and extensors and hence are useful in posture and movement. He also suggests that they are involved in the mechanisms for anticipating movement, which supports the evidence of a transcortical route since there is input from the brain.
Shemmell, J., An, J.H. and Perreault, E.J. (2009) claim transcortical long-loop reflexes are useful in adding flexibility to the human stretch reflex allowing adaptation to a wider range of functional tasks. They also highlight in their report that reflex sensitivity is increased in unstable environments. This study also provides evidence supporting the transcortical route of the long loop reflex since, similar to the findings of Loo & McCloskey (1985), if the subject was given instruction prior to the perturbation, the long-loop reflex provides the ability to achieve the desired result even if this is contrary to the stabilizing response you would expect. Their study concludes that “stretch reflex modulation in tasks that require changes in limb stability is mediated by motor cortical pathways, and that these differ from pathways contributing to reflex modulation that depend on how the subject is instructed to react to an imposed perturbation.” The experiment went on to observe the effects of using transcranial magnetic stimulation to create a cortical silent period whereby the muscle stretch was timed so that the M2 response of the stretch reflex occurred during this silent phase. As a result of this, the idea that reflex sensitivity could be increased when in a stable environment was abolished. The reflex responses seen from altered task instruction was found to be not influenced by cortical silence. These results demonstrate that task-dependent changes in reflex function can be mediated through multiple neural pathways and that these pathways have task-specific roles. More recently, Petersen, N. et al. (1998) investigated the possibility of a transcortical pathway by applying stretch to ankle dorsiflexors and recording the EMG signals. In the introduction, Peterson et al. (1998) states that it is widely accepted, for muscles in the distal upper limb, for the long-loop reflex (M2) to be mediated by a transcortical reflex pathway. There is little evidence showing the same result in proximal and lower limb muscles. Thilmann et al. (1991) found that the M2 response showed no significant change in proximal and lower limb muscles after lesions of supraspinal pathways whereas the M2 responses disappeared in hand muscles after the same lesion.
A more clinical approach by Diener, H., Dichgans, J., Hulser, P.-J., Buettner, U.-W., Bacher, M. and Guschbauer, B. (1984) suggests the long loop reflex is useful in diagnosing multiple sclerosis. Their results showed that 69% of the patients who have multiple sclerosis have a significantly longer M3 latency response in the antagonistic anterior tibial muscle. This increased delay in M3 response suggests demyelination of the neurones and they concluded that their results support evidence that the long loop reflex is mediated by a transcortical pathway.
Clarac , F (2005) The History of Reflexes Part 2: From Sherrington to 2004, IBRO History of Neuroscience
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