Axial electromyogram and intervertebral length gauge responses during lordosis behavior in rats

Electromyogram activity from transversospinalis (TS) and lateral longissimus (LL) muscles was recorded concomitant with resistance changes of an intervertebral length gauge during manually elicited lordosis performance. Subjects were estrogen-treated ovariectomized rats. The EMG activity was evoked by manual stimulation of the cutaneous fields contacted by the male rat during mounting. During bilateral palpation of flanks at 2/s, time-locked bursts of EMG activity or more generalized EMG activation was recorded. The EMG response was accompanied by continuous and cumulative length gauge shortening, reflecting progressive rump elevation. The earliest recorded length gauge response had a latency of 10 ms from the first flank stimulus in a sequence of repetitive palpations. Perineal pressure recruited motor units in both the TS and LL which fired at high frequency. The latencies of EMG responses from the TS tended to be briefer than those from the LL; 24/35 TS responses were within 50 ms of the onset of perineal pressure. During some lordoses, the EMG response in both muscles was substantially briefer than the duration of the pressure stimulus and of the length gauge response. Activation of axial muscles during postural adjustments, standing, turning, and other behaviors is also reported. These data represent the first analysis of axial EMG and vertebral length gauge recordings during an endocrine-dependent mammalian behavior.