Input-output relations of large neurons from layer V of cat sensorimotor cortex were studied in an in vitro slice preparation using steps and ramps of intracellularly injected current. Depolarization attained during the interspike interval (ISI) was compared to the voltage levels required to activate a previously described (29) persistent sodium current [I(NaP)]. I(NaP) was studied using a single-electrode voltage clamp in the same cells tested for firing behavior. Following an injected current step, firing rate declined smoothly to a steady level with a time course that was approximately exponential in most cells (τ, 9-43 ms). In most cells, the relation between firing rate and injected current (f-I relation) consisted of two linear segments, both for adapted, steady firing and for early intervals during adaptation. The slope of the steeper, initial (or sole) linear segment of the f-I curve averaged 26.2 Hz/nA during steady firing and was steeper when plotted for early interspike intervals. The variation of the depolarization at which spike initiation occurred (firing level) and the membrane potential between rhythmic spikes was examined during adaptation and steady firing. In most cells, firing level rose rapidly during a rhythmic train to a steady value. The steady firing level attained remained unchanged over a wide range of steady firing rates. Nevertheless, the mean depolarization during the interspike interval (V̄) increased approximately linearly with steady firing rate. Even at the slowest firing rates, V̄ is sufficient to activate I(NaP). The use of injected current ramps demonstrated that neocortical cells were sensitive to rate of change of stimulus current (dI/dt) as well as its amplitude (I). The use of ramps followed by steady currents demonstrated that the repetitive response lagged behind changes in stimulus parameters and did not reach a steady state even during slow ramps; i.e., the response depended on time as well as on I and dI/dt. Instantaneous firing rate during the ramp increased linearly with time for a wide range of ramp slopes (dI/dt). The instantaneous firing rate of early interspike intervals was also linearly related to ramp slope for small ramp slopes. In spite of these linear relationships, quantitative analysis indicated that firing rate during ramp stimulation cannot, in general, be described by a simple linear combination of separate amplitude- and rate-dependent terms. The repetitive firing properties of the in vitro neurons are compared to those of in vivo neocortical neurons and other cell types. The main features of firing properties in vitro are similar to those measured in vivo, but some differences are noted. The firing properties of neocortical neurons resemble those of cat spinal motoneurons in that both neuron types exhibit rhythmic spike trains instead of the burst mode of firing seen in several other types of central mammalian neurons, but the quantitative relations between injected current input and spike output are quite different. As in spinal motoneurons, a persistent, subthreshold, inward, ionic current plays a role in governing firing behavior.
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