* Corresponding author. E-mail address: mike.neubig@phs.ulaval.ca (M. Neubig) Neurocomputing 26 } 27 (1999) 215 }221 Low threshold calcium ¹-current I< curve geometry is alterable through the distribution of ¹-channels in thalamic relay neurons Mike Neubig*, Alain Destexhe Neurophysiology Lab, University Laval, School of Medicine, Laval, Quebec, Canada Que. G1K-7P4, Canada Abstract Low-voltage-activated calcium T-channels enable low-threshold calcium spiking and asso- ciated bursting in thalamocortical relay neurons. One measure of this "ring pattern is the geometry of the ¹-current's I< curve, of which the current/voltage (I<) peak is a fundamental index. Using reconstructed neurons, we show here that: (1) in cells with equivalent I< peak currents, physiologically constrained distributions can shift the peak's voltage by 4.8 mV } which is just below the resolution at which I< data is usually gathered and corresponds to a change in the single-spike threshold from 5.4 to 29.3 pA; (2) in neurons with equivalent I< peak voltages, distribution can shift the peak current by 1.4 nA } which is on the order of 30% of maximal ¹-current in relay cells. Further, we show that these shifts are qualitatively di!erent than those induced by eight other factors. We conclude that channel distribution is a primary factor shaping I< curve geometry.  1999 Elsevier Science B.V. All rights reserved. Keywords: ¹-Channel distribution; I< curve; Thalamic relay neurons; Bursting 1. Introduction A plethora of experiments have established that burst behaviour of thalamocortical relay (TC) neurons is essential to several roles that these neurons play in brain function. During the waking state, bursts may encode particular qualities of sensory information in the midst of gross tonic relay [4]. During non-REM sleep, bursting occurs rhythmically as the predominant TC behaviour [3]. The electrophysiology of TC bursting is also well understood. Experiments on slice preparations have shown that bursts in thalamocortical neurons, as in many other 0925-2312/99/$ } see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 2 3 1 2 ( 9 9 ) 0 0 0 7 4 - 0