Neural encoding of input transients investigated by intracellular injection of ramp currents in cat alpha-motoneurones

1. Input-output relations were analysed in spinal alpha-motoneurones during current transients reaching a steady level after a linear growth of different slopes. The motoneurone output considered in the analysis was the instantaneous frequency of the cell discharge.2. In all motoneurones firing frequency during the ramp exceeded that of the final steady level and it was related to the velocity of rise of the current. In the majority of motoneurones the instantaneous frequency grew during the ramp stimulus, as if it were dependent on current intensity as well as on its rate of rise. Only in a few cells was firing frequency constant over the first two interspike intervals during the ramp, as would be expected if this response depended solely on the rate of rise.3. Frequency-velocity (f/v) plots for different rates of rise of the injected current showed a linear relation for each interspike interval. Presence or absence of an intensity component was revealed in these plots by divergence or, respectively, overlapping of the f/v relations for the first and second intervals. Divergence was eliminated by subtraction of the estimated intensity component. The slope of the f/v relation for the first interval did not change significantly after subtraction of the intensity component and was taken as an index of the dynamic sensitivity of the motoneurones. The slope of the f/v relation varied greatly (from 47 to 330 impulses s(-1). (nA ms(-1))(-1)) in the population examined and was higher in motoneurones with a long-lasting afterhyperpolarization (a.h.p.) than in those where it was short-lasting.4. It is proposed that the ability of the motoneurones to encode both the steady level and the rate of change of input signals depends on the conductance changes responsible for the a.h.p. and their accumulation. A positive correlation was found between the size of the a.h.p. potassium current, estimated as a.h.p. peak voltage/cell input resistance, and the slope of the f/v relation for the first interval.

The dynamic response of cat alpha-motoneurones investigated by intracellular injection of sinusoidal currents

Sine-wave currents intracellularly injected into spinal alpha-motoneurones were found to modulate sinusoidally the regular rhythmic firing (carrier frequency) evoked by a current step. Cycle histograms of the instantaneous frequency could be accurately fitted by sinusoidal functions. Those functions were treated as the cell output. For a given modulation frequency between 2 and 14-18 Hz, the amplitude of the cell output was linearly related to the amplitude of the sine-wave current, all over a wide range of current intensities. The sensitivity (gain) and the phase relationships were estimated by varying the modulation frequency of a given sine-wave. When modulation frequency varied from 1-2 Hz to 14-18 Hz, there was a progressive increase of the gain and a phase advance. The experimental gain curve closely conformed to the response of an ideal linear transducer sensitive to both the intensity and the velocity of the input. The phase advance was instead less than that predicted by the model. No "carrier dependent" variations of gain and phase were detected. Differences among motoneurones regarded both the static gain and the "corner frequency" (a measure of the dynamic sensitivity). In 10 motoneurones, the corner frequency ranged between 5 and 10 Hz.

The dynamic response of cat gastrocnemius motor units investigated by ramp-current injection into their motoneurones

1. The isometric force developed by single motor units in response to injection of ramp-and-hold currents into their motoneurones was recorded from the common tendon of the gastrocnemius muscles of the cat. The average rate of rise of the force (force-slope) produced by the ramp-evoked discharge, was found to grow almost linearly with the rate of current injection (current-slope) up to a saturation value (maximal force-slope). 2. The slope of the function which links the force slope to the current-slope is the gain (dF/dI) of the motor unit under dynamic conditions. The value of the dynamic gain, measured in the linear region of growth, displays a large variability, i.e. for each nanoampere of current injected, the force developed is as much as 40 times larger in the strongest than in the weakest motor units. Such large gain differences, however, are drastically reduced if the force is expressed as a percentage of the maximal tetanic tension, Ft: per nanoampere injected, most of the units deliver from 1.0 to 3.0% of Ft. 3. The maximal force-slope which each unit could reach exhibits a large variability, ranging from 0.06 to 4.0 g ms-1. Like the dynamic gain, the maximal force-slope is positively related to Ft. 4. It was found that the dynamic sensitivity of the motoneurone, i.e. the increase of the firing rate per unitary increase of the current-slope, governs the fractional growth of the force-slope, whereas the motor unit contraction time determines the firing rate at which maximal force-slope is reached. Together, the two factors co-operate in defining, for each motor unit, the range of input-slopes within which the force-slope is regulated. 5. The motoneurones which supply the weak motor units, those with the lowest dynamic gain, have higher dynamic sensitivity and lower rheobase than those innervating the strong motor units. This suggests that weak motor units need less synaptic current both to be recruited and to reach the maximal speed of force development when their input is supraliminal.

Impulse coding of ramp currents intracellularly injected into pyramidal tract neurones

Relationships between the repetitive discharge and dynamic aspects of the input were analyzed in pyramidal tract neurones of the cat. Inputs were intracellularly injected currents reaching a steady level after ramps of different slopes (from 0.02 to 1.5 nA ms-1). Output was the instantaneous frequency of the discharge. During the transient phase, instantaneous frequency appeared to be related both to the velocity of rise and to the intensity of the stimulating current. The dynamic component of the cell response was estimated by subtraction of the intensity-bound component (derived from the steady-state response to current steps). After subtraction, the instantaneous frequency of interspike intervals following the first one became proportional to the ramp slope. The instantaneous frequency of the first interval also increased with the current slope, but at a lower rate than the frequency of other intervals. Moreover its dynamic component virtually stopped growing when the ramp slope exceeded 0.3-0.5 nA ms-1.

Properties of the spike afterhyperpolarization in pyramidal tract neurons

Features of the spike afterhyperpolarization (AHP) recorded intracellularly have been analyzed in fast pyramidal tract neurons of cats. Cell input conductance increases during the AHP, possibly because of a change in potassium conductance, as suggested by an AHP equilibrium potential 10--15 mV negative to the resting membrane potential. When more spikes are evoked in succession, AHPs following the first one are strongly reduced in amplitude. The effect is virtually maximal (30--50% of the control) after a single spike and fades out by 200-400 ms after the last spike. At short interspike intervals the initial time course of the depression is hidden by summation occurring between consecutive AHPs.