Quantitative analysis of firing properties of pyramidal neurons from layer 5 of rat sensorimotor cortex

Quantitative aspects of repetitive firing evoked by injected current steps and ramps were studied in layer 5 pyramidal neurons in brain slices of rat sensorimotor cortex to answer the following questions. Do the tonic firing properties of burst-firing and regular-spiking (nonbursting) neurons differ significantly? Does burst firing denote a discrete class of neurons or represent a continuum of firing properties? Is firing rate during the burst of action potentials related to stimulus amplitude? What aspect of the stimulus might the initial firing rate code? How stable are a neuron's firing properties over time? All recorded neurons fired tonically to a long-lasting current above a minimum value, and the tonic firing properties of most neurons were quite similar irrespective of their initial response to a current step. Only a group of high-resistance neurons had significantly different tonic firing properties. When slow current ramps (rising between 0.5 and approximately 20 nA/s) were applied, the relation between firing rate and current during the ramp was very similar to the relation between tonic firing rate and current obtained from long-lasting current steps. Low-resistance cells exhibited three distinct initial responses to a current step: fast adaptation, high-threshold bursts, and low-threshold bursts, observed in 54, 28, and 10% of recorded cells, respectively. High-resistance cells exhibited a distinctive slow adaptation of firing rate. Slowly adapting, fast-adapting (FA), and high-threshold burster (HTB) neurons exhibited no adaptation near the minimum current that evoked repetitive firing (I(o)). FA and HTB cells exhibited two-spike adaptation to a fina tonic firing rate during currents up to 1.6 times I(o). Only a higher current (2.1 times I(o)) evoked a burst in HTB cells, whereas a burst was evoked at I(o) in the low-threshold burster cells. In most cells analyzed, the initial firing rate, whatever its nature, increased monotonically with current step amplitude. The response to fast current ramps indicated that firing rate during adaptation or bursting may code rate of change of current. Repeated measurements during long-duration impalements indicated that both transient and tonic firing properties are stable over time. We discuss how the different tonic firing properties of large and small pyramidal neurons could be more important functionally than the different transient responses (burst/nonburst) of the large neurons. We conclude that the large neurons would perform a better linear transduction of time-varying synaptic current that reaches their somata. We compare the responses evoked by somatically injected current with those evoked by dendritic glutamate iontophoresis in previous studies.