High-threshold Units Research Articles

Double discharges in human motoneurons

Potentials of motor units from the trapezius and rectus femoris muscles were recorded with selective needle electrodes during weak and moderately strong voluntary isometric contraction. The sequence of interspike intervals was analyzed. Double discharges (interspike interval not exceeding 20 msec) were found most commonly during recruiting of the motor units, but also at its end. Intervals between double discharges arising while the motor units were firing at a mean rate of 10–18 spikes/sec were outside the limits of statistical scatter of the remaining intervals. Double discharges were recorded chiefly in high-threshold units. The mean interval between double discharges recorded from the trapezius muscle was significantly smaller and the double discharges appeared considerably more often than in the rectus femoris muscle. Comparison of the results of these experiments with those obtained by other workers showed correlation between the mean duration of the interval between the double discharges and the duration of delayed depolarization of the motoneuron; this fact probably plays an important role in the creation of double discharges.

The orderly recruitment of human motor units during voluntary isometric contractions.

1. The contractile properties of human motor units from the first dorsal interosseus muscle of the hand were studied during voluntary isometric contractions using recently developed techniques.2. The twitch tensions produced by motor units varied widely from about 0.1-10 g. The twitch tension of a motor unit varied nearly linearly as a function of the level of voluntary force at which it was recruited over the entire range of forces studied (0-2 kg).3. The number of additional motor units recruited during a given increment in force declined sharply at high levels of voluntary force. This suggests that even though the high threshold units generate more tension, the contribution of recruitment to increases in voluntary force declines at higher force levels.4. Contraction times for these motor units varied from 30 to 100 msec. Over 80% had contraction times less than 70 msec, and might be classed as fast twitch motor units. The larger motor units, which were recruited at higher threshold forces, tended to have shorter contraction times than the smaller units.

Nonlinear dynamic properties of a somatomotor reflex system. A model study.

AbstractThe time course of polysynaptic reflex responses is known to exhibit two prominent non‐linearities, one dependent on the amplitude of the stimulus and the other dependent on the direction of change of the stimulus. Both nonlinearities were demonstrated in the open‐loop responses of the acoustic middle ear reflex and simulated in a piecewise linear model. Their significance for stability of the closed‐loop feed‐back system was investigated. In this model the source of nonlinearity was a pool of “reflex units” with various thresholds and time constants. The mode! responses were studied in the time domain and the frequency domain. The amplitude dependent nonlinearity could be simulated when low threshold reflex units were made slow and tonic and high threshold units fast and tonic or when some of the high threshold units were phasic. A decrease in transport delay at increased input step amplitude was seen under these conditions but also when all units mere made equal. Both types of nonlinearities were found to improve the stability of the model system without decreasing its bandwidth (a measure of the speed of action). The nonlinearities were suggested to be important in systems with high values of transport delay.

Cutaneous afferent fibre collaterals in the dorsal columns of the cat.

Microelectrode recordings were made from cutaneous afferent fibre collaterals in the lumbar dorsal columns of cats, which were antidromically excited from the dorsal column at C2. The following types of cutaneous afferent unit have axon collaterals which ascend the dorsal columns to the dorsal column nuclei; hair follicle units Types G (36.4%) and T (31.8%), slowly-adapting units Types I (8.3%) and II (5.3%) pad mechanoreceptor units (8.3%) and a few high threshold units. The conduction velocities of dorsal column collaterals were measured. All collaterals had slower velocities than corresponding peripheral nerve fibres and the degree of slowing was determined by the class of afferent unit, with the pad units slowing the least and Type T hair follicle units the most. The differential slowing was confirmed by recording compound action potentials from various parts of dorsal column-peripheral nerve pathways.

Mesencephalic reticular influence on electrical activities of the pontine reticular formation.

Some characteristics of the electrical activities of the pontine reticular formation (PRF), recorded with gross- and micro-electrodes following stimulation of either the sciatic or the superficial radial nerve, were studied under the effects of stimulation of the mesencephalic reticular formation (MRF). The experimental animals were exclusively unanesthetized cats, immobilized with succinylcholine chloride or Faxedil.1. The peripherally evoked PRF potentials were composed of the initial positivity and the following negativity, this potential form being independent of the site of stimulation. The latency and the duration of the initial positive component were 10-20 msec and 10-30 msec, respectively, while the negative component had a duration of 40-75 msec. The MRF-induced, PRF potential was triphasic having the initial positivity. The threshold intensity of stimulus for eliciting the PRF potential was dependent on stimulating sites of the MRF.2. The peripherally evoked PRF unit response was classified into two types. The first type unit response occurred with an initial burst of spikes without being followed by after-discharges, whereas in the second type an initialburst of spikes continued to regular discharges with gradually decreasing frequencies. The mode of activation of the PRF unit was not essentially different between the MRF and the peripheral nerves. For a given stimulating point of the MRF, two types of the PRF unit were distinguished according to the threshold of activation (low-threshold and high-threshold). The discharge pattern of a given PRF unit to MRF stimulation was not fixed but variable depending on the site of stimulation.3. There are two types of spontaneous discharges in the PRF, the one with an irregular pattern of less than 10c/s and the other with a relatively regular pattern at 30-60 c/s. The firing rates of the PRF unit were mostly increased by MRF arousal stimulation.4. A single shock applied to the MRF with strengths sufficient for eliciting the PRF potential suppressed the peripherally induced, PRF potential for 10-40 msec.5. The peripherally induced, PRF potentials were suppressed during and after repetitive stimulation of the MRF, this being parallel to EEG arousal.6. Repetitive stimulation of the MRF increased the peripherally evoked, PRF unit response. The facilitatory effect was found more frequently in the high-threshold units than in the low-threshold ones, whereas this relation was reversed with the inhibition.7. It was suggested that the descending connection from the MRF is not diffuse, but rather specific.