Proceedings of The Physiological Society

University of Oxford (2011) Proc Physiol Soc 23, C70

Oral Communications

Effects of Area 3a stimulation on muscle activity in the awake monkey

C. L. Witham1, S. N. Baker1

1. Institute of Neuroscience, Newcastle Univeristy, Newcastle, Tyne and Wear, United Kingdom.

Rathelot and Strick (2006) reported that ~15% of corticomotoneuronal cells in macaques originate in area 3a instead of M1. They proposed that these cells contacted gamma motoneurons based on the lack of overt stimulation effects in area 3a (Widener and Cheney, 1997). In an ongoing study we are examining the effects of intra-cortical microstimulation (ICMS) in area 3a, area 3b and M1 in awake macaques. Two monkeys were trained to allow neck and arm restraint. These monkeys were implanted under general anaesthesia (3.0-5.0% sevoflurane and 0.025mg/kg/hr alfentanil) and aseptic conditions with a headpiece, to allow head fixation, and a recording chamber placed over the central sulcus. In the same surgery patch electrodes were placed on the surface of 4 forearm muscles to record electromyographic (EMG) activity. In daily experiments electrodes were inserted into sensorimotor cortex using a microdrive. Different areas were identified based on location and receptive field testing. In some sessions surface EMG electrodes were used to record first dorsal interosseus (1DI) activity. The effects of single pulse and multiple pulse ICMS (5-70 µA , 100 ms pulse width) on resting EMG were examined. Preliminary results suggest that Area 3a sites are capable of generating overt twitches with multiple pulse ICMS. In the small number of sites so far examined, the ICMS effect closely corresponded to the receptive field of the cells at that site (for example ICMS at a site which responded to tapping the belly of the 1DI muscle produced twitches of 1DI at 20 µA). M1 sites close by generally had lower threshold ICMS effects but diffuse cutaneous receptive fields on the hand. Area 3b sites had clear focal cutaneous receptive fields on the digits but no clear ICMS effects. The latencies between the stimulus and the EMG response were similar for both M1 and Area 3a. These results suggest that, contrary to earlier studies, Area 3a is capable of activating extrafusal muscle fibres. Since the latencies of the area 3a and M1 responses were similar, it is plausible that the area 3a effect is mediated via corticomotoneuronal cells in area 3a. These could contact either alpha or beta motoneurons.

Where applicable, experiments conform with Society ethical requirements