Publication Summary and Abstract
Lepora N, Porrill J, Evinger LC, Yeo C and Dean P (2009), Recruitment in Retractor Bulbi muscle during eyeblink conditioning: EMG Analysis and Common-drive model, Journal of Neurophysiology.
To analyze properly the role of the cerebellum in classical conditioning of the eyeblink and nictitating membrane (NM) response, the control of conditioned response dynamics must be better understood. Previous studies have suggested that the control signal is linearly related to the conditioned response as a result of recruitment within the accessory abducens motoneuron pool, which acts to linearise retractor bulbi muscle and NM response mechanics (Lepora et al. 2007). Here we investigate possible recruitment mechanisms. Data came from simultaneous recordings of NM position and multi-unit electromyographic (EMG) activity from the retractor bulbi muscle of rabbits during eyeblink conditioning in which tone and periocular shock act as conditional and unconditional stimuli, respectively. Action potentials (spikes) were extracted and classified by amplitude. Firing rates of spikes with different amplitudes were analyzed with respect to NM response temporal profiles and total EMG spike firing-rate. Four main regularities were revealed and quantified: (i) spike amplitude increased with response amplitude; (ii) smaller spikes always appeared before larger spikes; (iii) subsequent firing rates co-varied for spikes of different amplitude, with smaller spikes always firing at higher rates than larger ones, (iv) firing-rate profiles were approximately Gaussian for all amplitudes. These regularities suggest that recruitment does take place in the retractor bulbi muscle during conditioned NM responses, and that all motoneurons receive the same command signal (common-drive hypothesis). To test this hypothesis, a model of the motoneuron pool was constructed in which motoneurons had a range of intrinsic thresholds distributed exponentially, with threshold linearly related to EMG spike amplitude. Each neuron received the same input signal as required by the common-drive assumption. This simple model reproduced the main features of the data, suggesting that conditioned NM responses are controlled by a common drive mechanism that enables simple commands to determine response topography in a linear fashion.