Cutaneous responsiveness of rat single motor units activated by natural stimulation

Recordings of withdrawal reflexes have been used extensively to study sensory-motor integration and processing of nociceptive information in the spinal cord. We describe here a new technique for the manufacture of improved EMG electrodes that permit the characterisation of the physiological properties of single motor units as well as the easy location of the muscles studied. Individual motor units from three rat hind-limb muscles: peroneus longus, tibialis cranialis and extensor digitorum longus, were activated by thermal and mechanical stimulation applied to their cutaneous receptive fields, which were located mainly on the 4th and 5th toes. Thresholds for thermal and mechanical (Von Frey hairs) stimulation were similar in the three muscles studied, with a value of 44 +/- 1 degrees C and 100 mN (median), respectively. However, when a mechanical pincher with a stimulus area of 14 mm2 was used, the values seen were similar for peroneus longus and tibialis cranialis (342 +/- 23 and 330 +/- 71 mN, respectively, mean +/- S.E.M.) but lower for extensor digitorum longus (220 +/- 37 mN, mean +/- S.E.M.). The firing rate of the single motor units was similar for all types of stimulation at threshold intensity, and showed a linear relationship with stimulus intensity, except for units of the tibialis cranialis, which showed a greater degree of adaptation.