Ayako Ishida

Recruitment threshold force and its changing type of motor units during voluntary contraction at various speeds in man

The motor unit discharges in human hand muscles were recorded during voluntary isometric contraction. Bipolar wire electrodes were inserted in the right adductor pollicis muscle (AP) and first dorsal interosseous muscle (FDI) for the recordings. Motor unit activities from these muscles were investigated when subjects exerted voluntary force to each target force at various speeds. On 27 motor units of AP and 28 motor units of FDI, ramp RTF and ballistic RTF were compared. Furthermore, RTF changes of motor units with the different force speeds were investigated. It is clear that the RTF of the motor units during ballistic contractions were lower than those during ramp contraction. The processes of motor unit RTF changes were classified into 3 types: RTF of type I motor units were relatively low and decreased slightly even when force speed increased widely; RTF of type II motor units were relatively high and decreased irregularly; and RTF of type III motor units, which appeared rarely, decreased their relatively high RTF with increases of force speed increment. It is suggested that the volitional outflow related with the force speed change to the spinal motor pool might cause the different RTF change effects in the various motor units.

CALCIUM UPTAKE BY AXON TERMINALS OF RAT NEUROHYPOPHYSIS AT REST AND DURING MEMBRANE DEPOLARIZATION

1) Uptake and localization of 45Ca were investigated in the isolated rat neurohypophysis by means of electron microscope autoradiography. The gland was incubated in 45Ca-Lockes solution in which 1/10-1/20 of CaCl2 was replaced by 45CaCl2. The emulsion film was developed by a special method to obtain extremely fine silver grains and the number of grains struck by β-particles from 45Ca was counted in photographs of various parts of the tissue.2) In the gland incubated in 56 mM K-45Ca-Lockes solution or stimulated by electrical pulses at 20Hz for 5min, 20-40 grains/mm2 were found in the fibers, whereas only 4-6 grains/mm2 were found in the control.3) In 23 fibers depolarized by excess K (678 grains) and 18 fibers stimulated electrically, 50-60% of the grains were found on the granule membrane or within it. Less than 5% of the grains were found on the nerve membrane and in the mitochondria.4) The conclusion is that 45Ca uptake of neurosecretory nerve fibers is increased by depolarization and that more than half the 45Ca entering a fiber moves to the membrane of the neurosecretory granules or into the granules themselves.

Variation of amount of muscle discharges during ballistic isometric voluntary contraction in man

The effect of force velocity on the relation between voluntary force exertion and amount of muscle discharge was investigated. Surface e.m.g. from the adductor pollicis muscle and voluntary force curve were recorded simultaneously. Examined forces were selected from 500 to 3500 g at the peak force (about 30% of MVC) with time to peak of force curve between 60 ms and 500 ms. The amount of discharge during slow ramp contraction increased with the force increment. In ballistic contractions there was a wide variation in both the amount and rate of discharge. It is suggested that the modes of motor unit activities are different between force exertion with time to peak of less than 150 ms and force exertion with time to peak of over 150 ms.

An analysis of frequency response of motor units during voluntary isometric contractions at various speeds

To ascertain the differences in discharge properties of a hand muscle between ramp and ballistic contractions, firing frequencies of 48 motor units of adductor pollicis muscle were analyzed during 3-kg isometric voluntary contractions at various force speeds. The force exertions were classified into 3 degrees: slow ramp, fast ramp, and ballistic contractions. The instantaneous frequencies (IFs) of motor units increased gradually when the force exertions were progressed to the target in slow and fast ramp contractions. The slopes of the linear regression equations between IFs and force levels were almost all constant during slow ramp contractions and were scattered slightly during fast ramp contractions. However, the slopes during ballistic contractions scattered widely. The y-intercepts of the same regression lines increased slightly with respect to the increment of force speed when slow and fast ramp contractions were performed. However, a remarkably wide variation of the y-intercepts was observed during ballistic contractions. Motor units could be fired at high frequencies at the beginning of discharge when ballistic contractions were performed as compared with slow and fast ramp contractions.