Motoneuron Firing Frequency Research Articles

The effect of activation history on tension production by individual muscle units

Isometric responses of individual gastrocnemius muscle units of both fast (F) and slow (S) twitch type were studied during repetitive intracellular stimulation of the innervating motoneurons in anesthetized cats. Paired stimuli usually produced maximum responses (measured in terms of tension-time area) with interpulse intervals between 5 and 10 msec. However, longer pulse trains produced maximum tension-time area per pulse with relatively long interpulse intervals (referred to as the optimum interval). For each unit, stimulus intervals around the optimum produced maximum modulation of tension during unfused tetani. Post-activation potentiation produced shifts in the optimum toward longer intervals. The optimum intervals for type F units were shorter (mean 49.4 msec) than those for type S units (mean 86.9 msec), but there was no difference between the groups when the mean intervals were expressed in ratio with the twitch contraction time, T c (F units:1.42 ×T c; S units: 1.31 ×T c). The range of optimum intervals for all gastrocnemius muscle units corresponded to motoneuron firing frequencies of 7–40 pulses/sec, well within the frequency ranges observed for gastrocnemius motoneurons firing under natural synaptic drive. Insertion of a single short interval (5–10 msec) at the onset of a relatively low frequency stimulus train produced a prolonged ‘catch-like’ enhancement of tension output, particularly in type S units. The duration of the catch-like effect depended on the interval between pulses in the underlying basic train, and was maximum for intervals between 1 and 2 ×T c. The catch-like effect was also seen in type F units but its duration was curtailed by another process called the ‘sag’ property. The ‘sag’ property occurred in type F units over a wide range of stimulus intervals (usually 0.5 –3 ×T c) but was present in type S units only with relatively long intervals (> 2 ×T c). The ‘sag’ property appears to represent a disenhancement process intrinsic to muscle unit fibers activated in unfused tetani.

Hydrostatic pressure changes in the abdomen of the hermit crabPagurus pollicarus during movement

1. The hermit crabPagurus pollicarus maintains an internal pressure of from 4 to 10 g/cm2 in its abdomen; this increases by two to 3 g/cm2 during lift of the shell and walking. 2. Characteristic pressure changes during flexion and extension are described; in addition, the response of pressure and motoneuron firing frequency is correlated during these movements. 3. The motoneurons of the ventral superficial muscles reponds to pressure changes in the abdomen with a long latency high threshold rapidly adapting response; it is concluded that this is due to stimulation of touch receptors in the skin. 4. There is no relationship between abdominal load and internal pressure. In addition, the slopes and intercepts of load-abdominal extension curves were not significantly different from each other over a range of imposed internal pressures ranging from zero to 24 g/cm2. It is concluded that pressure does not generate lift in the absence of changes in muscle tension.