Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, C69
Extracellular spike duration in macaque motor cortex pyramidal neurons is correlated with axonal conduction velocity
G. Vigneswaran1, A. Kraskov1, R. Lemon1
1. Sobell Dept, UCL Institute of Neurology, London, WC1N 3BG, United Kingdom.
A number of recent studies have suggested that it may be possible to distinguish extracellular recordings of cortical interneurons from pyramidal neurons on the basis of the relatively brief spike duration in interneurons (1, 2). If this suggestion is correct, it would provide an important means of identifying cell types in recordings from awake monkeys. We investigated the spike duration of one class of pyramidal neuron, namely corticospinal neurons physiologically identified via antidromic stimulation of the pyramidal tract using pairs of implanted electrodes in 4 awake macaque monkeys, prepared for recording under isoflurane anaesthesia (1.8-2.0% in 50:50 O2 + N2O) (3). We first analysed the distribution of antidromic latencies (ADLs) of 265 identified pyramidal tract neurons (PTNs)recorded in primary motor cortex (M1, 172 PTNs, 4 animals)and ventral premotor cortex (PMv, area F5, 93 PTNs, 2 animals). The distribution of M1 ADLs was skewed towards short ADLs (0.5-5.6ms, median 1.1ms) and significantly different from that of F5 ADLs, which was more symmetrical (median 2.5ms). We found a few M1 PTNs with longer ADLs (>5ms) and some F5 PTNs with ADLs>8ms, which equates to an axonal conduction velocity of <10m/s. These slow conducting PTNs are known to far outnumber large ones but are much less studied due to recording bias. We also investigated the distribution of the duration of extracellularly recorded spike shapes defined as the time between the trough to the next positive peak of the spike. The shortest and longest duration spikes had widths of 150 and 750µs, respectively. Interestingly, the distribution of F5 spike duration was bimodal, possibly suggesting different classes of PTNs in F5 (3). In general we saw no significant differences in the distribution of durations of identified PTNs vs. large spikes recorded from other unidentified neurons (n=97). Importantly, we found that there was a positive linear correlation between ADL and spike duration in both M1 (R2=0.53, p<0.001) and F5 (R2=0.52, p<0.001) (see Figure). Since the ADL reflects axonal conduction velocity, this result means that PTNs with the fastest axons (shortest ADLs) had the briefest spikes. Further, since it is known that PTN soma size is correlated with axon size and conduction velocity(4),it is likely that the largest pyramidal neurons (Betz cells in M1) have spikes with short durations (150-450µs) which overlap heavily with those recorded for putative interneurons (430±270µs (1)) measured in the same way as in this study. In summary, spike duration may not be a reliable indicator of cell type, but more likely reflects discharge properties shared between cortical interneurons and pyramidal neurons.
Where applicable, experiments conform with Society ethical requirements