ROLE OF MULTIUNIT ACTIVITY IN RYTHMOGENESIS: INSIGHTS FROM DELETIONS

Abstract

The rhythmic activity of locomotion is most frequently modeled as a periodic oscillation coordinated by a spinal Central Pattern Generator (CPG) controlling reciprocal activation of flexor and extensor muscles. Expression of locomotion errors in the form of spontaneous deletions in the motor output has been critical in formulating models of CPG network structure governing locomotion in mammals (Lafreniere-Roula et al 2005, Duysens 2006). Deletions are defined as the disappearance of either antagonist or agonist muscles’ activity along with the simultaneous tonic/rhythmic activity of the corresponding agonist or antagonist muscles. The formulation of a two-layer model of the CPG (Rhythm Generator (RG) layer & Pattern Formation (PF) layer) by Rybak et al (2006) stems from observations of such deletions in the fictive locomotion of the decerebrated cat. The RG functions as a clock controlling the temporal activity of the PF layer which controls the firing pattern of motor neuron pools that activate muscles. The deletion episodes are said to be “resetting” if the EMG activity after the deletion does not return after an integer value of the pre-deletion average period. If the motoneuron activity returns in phase with the pre-deletion “clock”, the deletion period is considered to be “non-resetting”. Multiunit Activity (MUA) recorded from a spinalised air-stepping cat was analyzed against its corresponding EMG activity to investigate the role of MUA in rhythmogenicity, specifically whether or not MUA activity may represent the RG layer of the Central Pattern Generator (CPG) model. This hypothesis would predict that MUA activity should be disrupted in phase or amplitude when and only when deletions episodes are re-setting.. Alternatively, MUA activity may reflect PF layer activity. In this case MUA activity should be disrupted in phase or amplitude during each of the deletions episodes. MUA’s spatio-temporal characteristics were compared to that of the EMG activity during the deletion periods for analysis. From the analysis performed, there was a significant proportion (average more than 25%) of the MUA (collected from the lumbar region of the spinal cord of spinalized cat) that were disrupted in phase or amplitude during non-resetting deletions or undisrupted during resetting episodes, indicating that MUA activity is unlikely to represent the RG layer activity during . In addition, MUA oscillation during the period of deletions was unchanged (amplitude or phase) for more than 25% of the deletion episodes, ruling out the possibility that MUA represents the activity of the PF layer. So although MUA has been found to be highly synchronized throughout the lumbar extent during locomotor activity, it does not appear to act as a “clocking” mechanism for the locomotor rhythm.