Summary
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1.
This report extends the work of Lundberg and Winsbury (1960a and 1960b) in emphasizing the usefulness of brief muscle stretch for selective activation of the primary endings of muscle spindles.
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In 36 experiments on anesthetized cats a distribution of rhomboid stretch (20 msec duration) thresholds was obtained for activation of 393 stretch receptors from de-efferented soleus muscles maintained at comparable degrees of initial tension by setting length to give a peak of active tension during an isometric twitch. These receptors included 153 Ia afferents with conduction velocities greater than 75 m/sec, 127 Ib afferents and 97 group II afferents with conduction velocities below 65 m/sec. A pull of 60 μ or less activated 95% of the Ia fibers in contrast to 41% of the group II fibers and only 3% of the Ib fibers. At threshold the mean latency (normalized) between the onset of stretch and impulse arrival at the spinal cord (recording electrodes) was 5.4 msec for the Ia fibers (S.D.±1.7 msec), 10.6 (± 3.9) msec for the group II fibers, and 9.1 (± 3.3) msec for the Ib afferents. In all cases only one impulse was initiated by the test stretch and no significant differences were encountered in the responses of receptors located in proximal, middle and distal portions of the muscle.
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In 7 experiments soleus nerve volleys were recorded at nerve-muscle and spinal cord-dorsal root levels during similar stretching procedures. A stretch of 5 μ amplitude was sufficient to generate a group I volley. As stretch amplitude was raised to 60 μ this volley grew progressively in size and decreased in duration as well as in latency from stretch onset to volley formation. A second volley, 1.5 to 2.0 msec after the first, developed with a 30 μ stretch. At 60 μ stretch its size was approximately 30–50% that of the first volley. Conduction velocity for the first volley was 90–110 m/sec and was faster than the second volley by 5–10 m/sec. Our data on isolated afferents suggest that the first volley is of Ia origin while the second volley represents predominantly a second discharge from some Ia afferents plus a minor group II input. Significant Ib contribution is excluded since only 4 of the 127 Ib fibers responded within this stretch range and then at latencies beyond formation of either volley.
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Stuart, D.G., Mosher, C.G., Gerlach, R.L. et al. Selective activation of Ia afferents by transient muscle stretch. Exp Brain Res 10, 477–487 (1970). https://doi.org/10.1007/BF00234264
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DOI: https://doi.org/10.1007/BF00234264