QUANTITATIVE ANALYSIS OF FIRING PROPERTIES OF PYRAMIDAL NEURONS FROM LAYER
5 OF RAT SENSORIMOTOR CORTEX.
Peter Schwindt, Jennifer A. O'Brien and Wayne Crill
Department of Physiology & Biophysics, University of Washington School of
Medicine, Box 357290, Seattle, WA 98195-7290.
APStracts 4:0041N, 1997.
ABSTRACT
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 proper-
ties? 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 sig-
nificantly different tonic firing properties. When slow current ramps
(rising between 0.5 and ÷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 (FA), high-
threshold bursts (HTB) and low-threshold bursts (LTB) observed in 54%,
28% and 10% of recorded cells, respectively. High-resistance cells
exhibited a distinctive slow adaptation (SA) of firing rate. SA, FA
and HTB neurons exhibited no adaptation near the minimum current that
evoked repetitive firing (Io). FA and HTB cells exhibited 2-spike
adaptation to a final tonic firing rate during currents up to 1.6 Io.
Only a higher current (2.1 Io) evoked a burst in HTB cells, whereas a
burst was evoked at Io in the LTB 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 (dI/dt). 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 soma. We compare the responses
evoked by somatically-injected current to those evoked by dendritic
glutamate iontophoresis in previous studies.
Received 26 June 1996; accepted in final form 14 January 1997.
APS Manuscript Number J503-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 5 February 1997