Effects of Local and Remote Muscle Pain on Stretch ReflexActivities in the Elbow Joint Flexors and Extensors of Unanesthetized Cats |
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Authors: | А N Tal’nov |
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Institution: | (1) Laboratory for Neuromuscular Control, Department of Biomechanical Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands;(2) Department of Rehabilitation Medicine, Leiden University Medical Center, P.O. Box 9600, 2300 RC Leiden, The Netherlands |
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Abstract: | In experiments on unanesthetized cats, we compared the effects of experimentally induced pain in the m. biceps brachii or in the neck muscles on EMG activity of the flexors and extensors of the elbow joint (mm. biceps et triceps brachii, respectively) evoked by a passive extension-flexion of the above joint. Muscle pain was induced by injections of 0.5 ml
of a hypertonic (7%) NaCl solution into the above-mentioned muscles. In the case of pain in the biceps, i.e., in the muscle
directly involved in realization of the reflex, we observed an increase in the amplitude and significant shortening of the
latency of EMG responses of this muscle. The amplitude of a short-latency (supposedly monosynaptic) component of the biceps
reflex (М1 response) increased by 65%, while an increment of the latter (supposedly polysynaptic) М2 component was 117%. When
pain was induced in anatomically remote neck muscles, the stretch reflex in the biceps was considerably suppressed. The maximum
amplitudes of the М1 and М2 components decreased by 25 and 30%, respectively, but the latencies of these components decreased
significantly, similarly to what was observed in the case of induction of experimental pain in the biceps. Under both conditions
of experimental pain, changes in the parameters of EMG responses of the forearm extensor (m. triceps brachii) demonstrated similarity with those of the biceps responses. The maximum effect of pain induction was observed within the
first 5 min after injections of the hypertonic solution; full recovery of the stretch reflex parameters was observed on the
20th to 30th min. We conclude that the effects of pain induction on the reflex under study are not generalized. They depend
on the site of such induction with respect to the muscle where the stretch reflex is elicited. Unidirectional effects of both
types of pain on the antagonist muscles allow us to suppose that modulation of the reflex reactions upon pain induction is
mediated by influences from the supraspinal CNS structures. Induction of pain in the biceps increased the amplitude of EMG
manifestations of the stretch reflex, while such induction in the neck muscles decreased such responses; nonetheless, in both
cases the latency of the reflexes decreased. This fact allows us to believe that the sensitivity of muscle spindles increased
under both conditions of the pain influence. |
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