QUANTITATIVE-ANALYSIS OF FIRING PROPERTIES OF PYRAMIDAL NEURONS FROM LAYER-5 OF RAT SENSORIMOTOR CORTEX

Quantitative aspects of repetitive firing evoked by injected current s teps and ramps were studied in layer 5 pyramidal neurons in brain slic es of rat sensorimotor cortex to answer the following questions. Do th e tonic firing properties of burst-firing and regular-spiking (nonburs ting) neurons differ significantly? Does burst firing denote a discret e class of neurons or represent a continuum of firing properties? Is f iring rate during the burst of action potentials related to stimulus a mplitude? What aspect of the stimulus might the initial firing rate co de? How stable are a neuron's firing properties over lime? All recorde d neurons fired tonically to a long-lasting current above a minimum va lue, and the tonic firing properties of most neurons were quite simila r irrespective of their initial response to a current step. Only a gro up of high-resistance neurons had significantly different tonic firing properties. When slow current ramps (rising between 0.5 and approxima te to 20 nA/s) were applied, the relation between filing rate and curr ent during the ramp was very similar to the relation between tonic fir ing rate and current obtained from long-tasting current steps. Low-res istance 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-re sistance 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 adaptati on to a final tonic firing rate during currents up to 1.6 times I-o. O nly a higher current (2.1 times I-o) evoked a burst in HTB cells, wher eas a burst was evoked at I-o in the low-threshold burster cells. In m ost cells analyzed, the initial firing rate, whatever its nature, incr eased monotonically with current step amplitude. The response to fast current ramps indicated that firing rare during adaptation or bursting may code rate of change of current. Repeated measurements during long -duration impalements indicated that both transient and tonic firing p roperties are stable over time. We discuss how the different tonic fir ing properties of large and small pyramidal neurons could be more impo rtant functionally than the different transient responses (burst/nonbu rst) of the large neurons. We conclude that the large neurons would pe rform a better Linear transduction of time-varying synaptic current th at reaches their somata. We compare the responses evoked by somaticall y injected current with those evoked by dendritic glutamate iontophore sis in previous studies.