The membrane properties and firing characteristics of rat jaw‐elevator motoneurones.

Abstract

SUMMARY1. We have determined the membrane and firing properties of fifty-six jawelevator motoneurones in rats that were anaesthetized with pentobarbitone, paralysed and artificially ventilated.2. Forty-two neurones were identified as masseter motoneurones and fourteen as masseter synergist motoneurones. The membrane potentials for the sample ranged from -60 to -86 (mean = -68; S.D. = 7-3; n = 56), and spike amplitudes from 50 to 95 mV. The duration of the after-hyperpolarization following antidromic spikes in masseter motoneurones ranged from 15 to 50 ms (mean = 30; S.D. .-12-8) and their amplitudes from 1-0 to 4-5 mV (mean = 2-7; S.D. = 2-2; n = 42).3. The mean input resistance for the total sample was 2-3 MQ (S.D. = 0-9; n = 56), membrane time constant 3-9 ms (S.D. = 0-9; n = 48) and rheobase 4-2 nA (S.D. = 2-6; n = 56). The distribution of these parameters was independent of membrane potential. We found no significant interrelationships between the membrane properties and one interpretation of this is that our sample may be drawn from a homogenous population of motoneurones. We also suggest that elevator motoneurones may have a lower Rm (specific membrane resistivity) value than cat hindlimb motoneurones because they have a similar range of input resistance values but only half the total surface area.4. Forty-six out of forty-nine neurones fired repetitively to a depolarizing current pulse at a mean threshold of 1-6 x rheobase. Current-frequency plots were constructed for thirteen neurones and all but one showed a primary and secondary range in the firing of the first interspike interval. The mean slope in the primary range was 31 impulses s-1 nA-1 and 77 impulses s-1 nA-1 for the secondary range. The mean minimal firing frequency for steady firing was 26 impulses s-1 and, in response to an increase of stimulation, the rate increased monotonically with a slope of 11 impulses s-1 nA-1.5. The dynamic sensitivity of twelve neurones was assessed from their response to ramp waveforms of current of constant amplitude but varying frequencies (0-2-2 Hz). Firing initially increased along a steep slope up to a frequency of between 40 and 60 impulses s-1 and then increased along a much shallower slope. Both the threshold for eliciting firing and the firing at the transition point of the two slopes remained constant with changes in ramp frequency. This suggests that within the frequency range studied firing is dependent primarily on the amplitude of injected current and not on its rate of change.MS 7810

Characteristics and Organization of Discharge Properties in Rat Hindlimb Motoneurons

“…We do not expect that this effect was large; previous work indicates that the speed of relatively slow ramps has little influence on discharge responses (Moore and Appenteng 1990). Although the rate of current injection affects discharge during injection of faster ramps, the tendency of discharge to increase with ramp speed (Baldissera et al 1982) would produce a direct correlation of f-I gain with ramp amplitude and input conductance, opposite to that found here.…”

“…Also, it should be noted that discharge characteristics consistent with a subprimary range were reported previously for rat jaw-elevator motoneurons. Moore and Appenteng (1990), using injected current ramps, also found steep increases in discharge rate following recruitment before transition to a region of smaller gain. This subprimary range was insensitive to the rate of current injection.…”

Craniofacial Pain and Motor Function: Pathogenesis, Clinical Correlates, and Implications

“…Such input has also been shown to come from both sides of the head and the upper neck (Sessle et al, 1986;Dallel et al, 1990). The fact that unilateral pain has bilateral jaw motor effects can be due to the projections of trigeminal interneurons to both ipsilateral and contralateral trigeminal motor nuclei (Moore and Appenteng, 1990;Donga and Lund, 1991).…”

Synaptic Control of Motoneuronal Excitability

“…In cat lumbosacral motoneurons, the secondary range starts when steady-state firing reaches ~50 spikes/s, with a slope 2-6 times that of the primary range (623). Many motoneurons lack the secondary range at steady state, with a linear F-I relationship over the entire firing range (573,859,870,919). A persistent inward current (I i , Ca 2+ mediated) may underlie the secondary range (1003,1114).…”

“…The postsynaptic response to activation of a single glutamatergic synapse has been characterized in detail at few motoneuron synapses (412,841,1035 (197,198) in having mechanisms for boosting the synaptic current at distal synapses to ensure equivalence in actions of synapses regardless of their location on the dendritic tree. Indeed, motoneurons may be geometrically arranged for optimal current transfer from dendrites to soma (859,862). components on rhythmic drive currents may play an important role in maximizing output for a given current input (960).…”