METHODS

Data were collected from 37 experiments on adult cats maintained in an areflexic state by sodium pentobarbital. End-tidal CO, was continuously sampled from a cannula in the trachea (typically found to vary between 4.5 and 5.5%) and blood pressure was measured from a cannula in the carotid artery (the mean typically ranged from 110 to 130 mm Hg). The environmental temperature was kept at approximately 30 C and rectal temperature was automatically maintained at 37.5 C by external heat. The nerve (the middle branch of the posterior femoral cutaneous nerve except for one experiment on the medial branch of the superficial plantar nerve) was exposed for 34 cm with care taken to preserve its blood supply and was protected from drying in a pool of warm mineral oil formed from skin flaps. Recordings were made with fine platinum-iridium electrodes from thin filaments of the nerve prepared by dissection under microscopic control. The nerve retained its connection to the skin while being suspended by a pair of stimulating electrodes between the recording site and the margin of the pool. During search for unitary activity, the nerve was stimulated once every 2 set by a shock supramaximal for the “C” component of the compound action potential. Filaments from the nerve were progressively divided until the discharge of a single element conducting at C velocity (under 2.2 m/set (lo)), was uniquely prominent in the record and could be distinguished from all other activity on the basis of amplitude and configuration. Variation of the