Preferential suppression of transmission and candidate neurones mediating reflex actions from muscle group II afferents during fictive motor activity
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This thesis examined two aspects of information processing by the feline spinal cord during centrally-evoked motor activity: 1) the modification of transmission from different sensory afferents and 2) the neuronal elements of reflex pathways from group II muscle afferents during fictive motor behaviours (i.e motoneuron activity under neuromuscular blockade). Fictive locomotion was evoked by electrical stimulation in the midbrain and fictive scratch was triggered by stimulation of the skin covering the ears following curare application to cervical dorsal roots in decerebrate in vivo feline preparations. Both monosynaptic and longer latency components of muscle and cutaneous afferent-evoked field potentials were reduced in amplitude during fictive locomotion and scratch, but field potentials evoked by muscle group II afferents were suppressed more than those evoked by cutaneous and group I muscle afferents recorded at the same spinal locations. The novel finding, that field potentials evoked at the same spinal locations by muscle and cutaneous afferents are suprressed differently, suggests that there is a preferential and non-uniform control of transmission from muscle and cutaneous fibres during motor activity. Extracellular recordings from neurons within the lumbar spinal segments showed that suppression of group II afferent input during fictive motor activity results in a powerful reduction of the activation of neurons with input from muscle group II afferents in 93% of the examined neurons after short trains of stimuli were delivered to peripheral nerves. However, more neurons remained recruitable by group II intensity stimulation if train duration was sufficiently long with only 33% showing a reduction in sensory-evoked firing. The majority of the neurons that remained responsive to muscle group II afferent input during fictive locomotion had axonal projections to supralumbar, or supraspinal areas and showed spontaneous, often rhythmic, firing activity. Overall, the studies presented in this thesis provide insights into the mechanisms by which the mammalian spinal cord processes sensory information and on how sensory input is able to control motor activity in spite of suppressive control provided by the nervous system.