Discharge from total populations of Ia and II afferent fibers in the cat's deefferented medial gastrocnemius muscle during static stretch

Effects of high-frequency (500-600 Hz), antidromic stimulation on primary and secondary spindle afferents

The aftereffects of antidromic stimulation at 500 or 600 Hz on the discharge of isolated spindle Ia and II, and tendon organ Ib afferent fibers from the cat medial gastrocnemius muscle were examined to see what proprioceptive disturbances to expect when such "high-frequency," tension-attenuating stimulation is used to modulate contractions in orthotic applications. Three phases of poststimulation depression in spindle discharge were recognized: a complete pause in firing, a rapidly accelerating return of discharge, and a final more gradual approach to the control rate. When steady prestimulation discharge rates of Ia and II endings were equated through adjustment of muscle length, no obvious difference in effect on duration of the pause or position sensitivity was detected. Sensitivity to dynamic change in muscle length was also depressed, but responses returned earlier than when the muscle was held at a steady length. The dynamic responses of tendon organs were similarly depressed.

Joint angle signaling by muscle spindle receptors

Nerve impulses were recorded from sensory fibers supplying the tibialis anterior and soleus muscles of anesthetized cats as the ankle joint was moved from one end of the flexion-extension axis to the other and back again in steps of 6-7 degrees. The rate of movement from one position to the next was 40 deg/s and each position was held for 16-18 s. Plots were made of receptor discharge frequency as a function of ankle joint angle during joint movement (dynamic input-output (I-O) functions) as well as 2 and 15 s after movement terminated (2 and 15 s static I-O functions). Only receptors with a sustained (5s) static response within the physiological range were studied. A total of 229 tibialis anterior receptors met this criterion, of which 11 were identified as tendon organs. One hundred and five soleus receptors were studied, of which 6 were tendon organs. Thus tendon organ activity accounted for only a small part of the muscle afferent signal under passive conditions. The spindle receptors in soleus and tibialis anterior divided the ankle flexion-extension range about equally between them, those in soleus signaling over the flexion half of the range and those in tibialis anterior over the extension half. At angles where the receptors in a particular muscle did not signal joint angle, the tendon of the muscle was observed to be slack. Thus the total muscle afferent discharge in a relaxed animal is high at one end of the range, declines progressively as the ankle is displaced to an intermediate position, and then increases again as the joint moves toward the opposite end of the range. The spindle receptors within an individual muscle were recruited rather early as the muscle came under tension so that over most of a muscle's signaling range joint angle could have been coded by changes in receptor discharge frequency but not by which spindle receptors were active. To evaluate the information signaled by individual muscle spindle receptors, the following measurements were made from plots of impulse frequency vs joint angle: dynamic response, defined as the frequency difference between the dynamic and 2 s static I-O functions during muscle lengthening; adaptation, defined as the frequency difference between the 2 and 15 s static I-O functions during muscle lengthening.(ABSTRACT TRUNCATED AT 400 WORDS)