Electromyography of male rat perineal musculature during copulatory behavior

Electromyogram (EMG) recordings were made from the bulbospongiosus (BS) and ischiocavernosus (IC) muscles of the awake male rat in order to evaluate the roles played by these muscles in the implementation of penile intromission during a copulatory encounter. Male rats with bipolar wire electrodes implanted chronically in the BS and IC muscles were mated with sexually receptive females. Concurrent videotape recordings established a relationship between EMG and components of male rat copulatory behavior, i.e. mount, intromission and ejaculation. On EMGs recorded from the IC muscle, the initial low-amplitude phase at the initiation of each behavioral component was distinguished from the late high-amplitude phase whenever the male accomplished mounting or intromission. In the BS muscle, the late high-amplitude phase was observed only when the male accomplished intromission. Power spectrum analysis by fast Fourier transformation revealed that in the IC muscle the two phases differed at an overall range of 100-1,000 Hz. In the BS muscle, however, the spectra of the two phases differed at a specific range of 400-600 Hz. Recruitment of additional motor units would culminate in such a characteristic rise in the power spectrum. The results suggest that somatosensory inputs contingent on the intromission cause such recruitment in the BS to regulate penile cups and other movements.