Depression of muscle and cutaneous afferent-evoked monosynaptic field potentials during fictive locomotion in the cat

1. Monosynaptic extracellular field potentials evoked by electrical stimula tion of ipsilateral hindlimb nerves carrying muscle group I, II and cutaneo us afferents were examined during fictive locomotion. Fifty-eight field pot entials were recorded in the dorsal and intermediate laminae throughout the mid-lumbar to first sacral segments and fictive locomotion was evoked by m esencephalic locomotor region (MLR) stimulation in paralysed decerebrate ca ts. 2. The majority (96%) of group I, II and cutaneous-evoked field potentials were decreased during fictive locomotion. Group I, cutaneous and dorsal gro up If potentials were reduced on average to about 80% of control values. Gr oup II field potentials recorded in the intermediate laminae were reduced t o a mean of 49 % of control values. Cyclic variations in field potential am plitude between the flexion and extension phases were observed in 24 of 45 cases analysed. Of those 24 field potentials, the two groupI and four cutan eous field potentials were smaller during the flexion phase. All eleven gro up Il and the remaining seven cutaneous fields were smaller during extensio n. In all but two cases, these cyclic variations were smaller than the toni c depression upon which they were superimposed. 3. In 7/9 group II field potentials examined, reductions (on average to 85% of control) began with the onset of MLR stimulation that produced tonic ac tivity in the motor nerves before the onset of rhythmic alternating, locomo tor discharges. In six of the seven cases the field potential depression in creased with the establishment of fictive locomotion. This observation and the cyclic modulation of field potentials during fictive locomotion suggest s that the depression was strongly linked to the operation of the spinal lo comotor circuitry. 4. Depression of the monosynaptic components of the field potentials sugges ts a reduction in synaptic transmission from primary afferents to first-ord er spinal interneurones during fictive locomotion, accordingly, the larger depression of intermediate group II field potentials may indicate a prefere ntial reduction in transmission from group II afferents to interneurones lo cated in intermediate spinal laminae. 5. Flexion reflexes evoked by group II and cutaneous afferents were also de pressed during MLR-evoked fictive locomotion. The possibility that this dep ression results from a reduction in transmission from primary afferents, an d in particular from group II afferents, ending on interneurones in the int ermediate laminae is discussed.