| Summary: | Ongoing research studying the underlying mechanism and prevalence of Persistent Inward Currents (PICs) has posed a challenge to the conventional view that the firing rate of a motor unit is proportional to the amount of synaptic input it receives. Near the time of human motor unit recruitment during isometric muscle contraction, a sudden steep rise in firing rate is observed, which is suggested to result from rapid triggering of PICs, an intrinsic property of motor neurons. In evaluating the time constants associated with these steep rises in firing rate across different contractile speeds, it has been found that the time course of the steep rise is dependent on the time course of muscle contraction. This evidence suggests that the mechanism underlying this steep rise in firing rate is not PIC-associated, but rather depends on an alternate, unknown mechanism.
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