To analyze further the sensory mechanisms for triggering the lordosis reflex, single-unit (n = 345) activity was recorded extracellularly from spinal segments L5-S1 of urethan-anesthetized female rats. Unit responses to pressure on the skin (necessary and sufficient for evoking lordosis) and other mechanical stimuli were studied. 2. Units were classified according to their responses to the battery of mechanical stimuli: 16% of the units responded only to pressure. The majority of these pressure-responsive units were excited, while a few were inhibited or responded differently, depending on the site stimulated; 52% did not respond to pressure, but responded to brushing, muscle-joint, and/or visceral stimulation, or did not respond at all. The remaining 32% responded to pressure plus other forms of stimulation. 3. Units responding only to the movement of individual types of hair tended to be located in the dorsal horn, more dorsal than units responding only to pressure (found primarily in the intermediate gray). Units responding to subdermal stimulation were usually found at greater depths. Segmental and somatotopic distributions of spinal units observed were very similar to those reported for cat (3) and monkey (5). 4. Compared to primary sensory units (28), spinal units had higher resting firing rates, more complicated responses to a given stimulus, a wider variety of unit types, and much larger receptive fields. These comparisons tend to indicate convergence of primary sensory neurons onto individual spinal units. 5. The range of pressure thresholds of pressure-responsive units is comparable to the range effective for triggering lordosis. We postulate that excitation of units responding only to pressure is centrally involved in triggering the lordosis reflex. Those units not responding to pressure are probably irrelevant for this behavior. Presently undetermined are the roles of units with complex or mixed-response types.
