Activity Patterns of Motoneurons in the Spinal Dogfish in Relation to Changing Fictive Locomotion

Rhythmic motoneuronal activity was recorded from segmental motor nerves of moving (spinal swimming) and paralysed (fictive swimming) spinal dogfish (Scyliorhinus canicula), and, in the paralysed preparation, microelectrode recordings were made from spinal cord motoneurons. The motoneurons could be divided into two groups, according to their activity patterns. Group I (n = 31) were inactive during fictive swimming and did not respond to gentle tactile stimulation; when recorded from intracellularly they showed stable to weakly oscillating (< 1 mV) membrane potentials. Group II (n = 15) fired bursts of action potentials in phase with the motor nerve activity, which were superimposed upon larger (up to 17 mV) depolarizations, and responded to gentle tactile stimulation. Two of these cells discharged also in the interburst interval of the nerve activity. Decreases in cycle period of the fictive swimming (i.e. increases in locomotor frequency) were instantaneously accompanied by increases in the amplitude of the rectified and integrated motor nerve signal, which represents peak activity of group II motoneurons, and decreases in the duration of the motor burst. Similar instantaneous changes were seen in the firing frequency and burst duration of individual group II motoneurons. The conformity between unit and population behaviour with changing speed of fictive swimming, and the close correspondence observed between the form of the excitatory postsynaptic potentials recorded from individual motoneurons and the form of the integrated neurogram, suggest that the group II motoneurons receive a common excitatory drive. Re- and decruitment of motoneurons were virtually absent during these changes of speed. During unstimulated spinal swimming, regular left-right alternating EMG activity is recorded from the red but not from the white part of the myotome. The ratio of group I to group II motoneurons (31: 15) recorded in this study agrees with the previously reported proportion of axons in the spinal motor nerve that project to the white and red muscle fibres, respectively. We suggest, therefore, that group II motoneurons innervate the red and superficial muscle fibres and group I the white fibres. The different activity patterns of the two motoneuronal groups in the spinal fish probably reflect the different ways the red and white muscle systems are used during locomotion.