Segmental organization of rat lateral longissimus, a muscle involved in lordosis behavior: EMG and muscle nerve recordings.

Following stimulation of lumbar cutaneous and high threshold muscle afferents in urethane-anesthetized ovariectomized rats, polysynaptic multi-unit responses were recorded from muscle nerves to lateral longissimus (LL) at a mean latency of 9.5 ms. These responses could be reciprocally evoked between segments with no apparent rostro-caudal asymmetry. Polysynaptic responses habituated to repeated stimulation within 0.5-1.5 min but could be evoked again after several minutes rest. In estradiol-treated rats, 60% of the 9.5 ms responses were followed by late discharges (greater than 40 ms) as compared to 24% in untreated control animals (P = 0.05). Following stimulation to the L3 and L4 motor nerves to lateral longissimus, EMG responses were recorded from overlapping fields of innervation. The conduction velocity of the fast muscle fibers was determined to be 7 m/s. The short length of most of the LL muscle fibers and the intersegmental recruitment of LL motoneurons by cutaneous afferents is compatible with the integrated action of axial muscles in the performance of lordosis behavior.