The regularity of muscle spindle discharge in man.

Abstract

SUMMARY1. The variability of discharge of thirty-nine muscle spindle afferents from the pretibial muscles of normal human subjects was determined for spike train sequences recorded with the ankle joint fixed in 250 plantar flexion, during further stretch and during graded voluntary contractions of the receptor-bearing muscle.2. In non-contracting muscles with the ankle joint in 250 plantar flexion, a sustained discharge was maintained by twenty-four of the thirty-nine endings. The mean discharge frequency for the active endings was 11-1 Hz (range 4-8-22-1 Hz), the mean coefficient of variation 0-073 (range 0*021-0 183). With further stretch, the discharge of endings maintaining frequencies below 10-12 Hz became more regular. For endings maintaining higher frequencies, changes in the coefficient of variation were small and occurred in either direction. All secondary endings maintained a highly regular discharge, but, at these frequencies, there was no statistically significant difference in the variability of primary and secondary endings.3. It is considered that these findings are comparable to those of Matthews & Stein (1969) for de-efferented feline spindle endings, and support the view that there is no functionally effective background fusimotor drive to non-contracting muscles of normal human subjects. 4. A voluntary contraction sufficient to accelerate a spindle ending invariably decreased the regularity of its afferent discharge. During voluntary contractions, coefficients of variation up to 0*345 were recorded. However, coefficients as low as 01 were not uncommon, and thus the absence of fusimotor drive cannot necessarily be inferred from a regular afferent discharge pattern.5. With contractions of different strength, the increase in the coefficient of variation did not parallel the increase in discharge frequency. It is concluded that not all fusimotor influences acting on a spindle ending are translated into variability, and that measurements of the variability of discharge do not accurately reflect the level of fusimotor drive.6. The discharge frequency of some spindle endings decreased slightly in some contractions and this was accompanied by an increase in the variability of discharge. It is suggested that contracting extrafusal muscle fibres can modulate the discharge pattern of spindle endings and contribute to the variability of discharge during a voluntary contraction.

Microneurographically Recorded Ia Discharge from the Tibial Nerve Mainly Transmits the Angular Velocity of the Ankle Joint in Humans

“…The spontaneous discharge from the calf muscles had a frequency of 8.2 ± 3.3 imp/s, which was compatible with a previously reported discharge frequency [21]. The coefficient of variation of discharge patterns was the same as that reported by Burke et al [8,11,12,21] and Nordh et al [22].…”

“…The spontaneous discharges of 13 Ia afferents of the triceps surae in a resting position (0°extension of knee joint, and resting position of ankle joint) had an average frequency of 8.2 ± 3.3 imp/s (mean ± SD, range: 4.5-15.7 imp/s), and the coefficients of variation (SD/mean) for these units were 0.047 ± 0.066 (range: 0.004-0.167) [21]. The spontaneous discharges of one II afferent in a resting position showed an average frequency of 9.0 ± 0.3 imp/s, and the coefficient of variation for these units was 0.004.…”

“…To our knowledge, this is the first study to analyze the properties of Ia discharge from the tibial nerve innervating the calf muscles. Previous studies focused on the Ia discharge from the radial nerve innervating the extensor pollicis longus [7] or the peroneal nerve innervating the tibialis anterior [21], while the present study investigated the Ia discharges from the tibial nerve innervating the calf muscles, which moves the ankle joint plantar flexion direction, a single joint movement. Furthermore, the investigation of the soleus muscle afferent is important for the survey of myatrophy, the soleus is comprised of slow-twitch fibers that are readily susceptible to atrophy.…”

“…Our study investigated the relationships not only to the static discharge, but also to the kinesthesic discharge. Burke et al observed that Ia discharge from the tibialis anterior was enhanced during dorsiflection, but their description was not quantitative, nor did it refer to the relationship to the ankle joint angle [12,21]. Vallbo et al relationship between the angle of the metacarpo-pharangeal joint and Ia discharge from the muscle spindle during finger movement, although the finger position was not dynamic but static [23].…”

Modulation of Excitability in the Cerebral Cortex Projecting to Upper Extremity Muscles by Rotational Positioning of the Forearm

“…Reduction of muscle length usually causes a decrease in afferent activities from the muscle spindle (Burke et al 1979) and thus a decrease in excitability of the α -motor neurons of that muscle. MEP amplitudes of BB, however, increased during supination in spite of a decrease in muscle length.…”