CUTANEOUS STIMULATION FAILS TO ALTER MOTOR UNIT RECRUITMENT IN THE DECEREBRATE CAT

1. An attempt was made to repeat the observation that cutaneous input to the cat medial gastrocnemius (MG) muscle sometimes had the differen tial effect of inhibiting motoneurons with slow axonal conduction velo city while simultaneously exciting others with fast conduction velocit y. Dual microelectrode recording from intact ventral root filaments wa s used to study the effects of cutaneous inputs on recruitment order a nd on firing frequency of physiologically characterized MG motor units in decerebrate cats. Motor responses to pinch of the skin over the la teral surface of the ankle as well as electrical stimulation of the ca udal cutaneous sural (CCS) nerve were contrasted with the responses to static muscle stretch as well as muscle vibration. 2. In contrast to the prediction, recruitment order in pairwise tests was the same for s kin pinch or CCS stimulation as it was for MG stretch or vibration in all 32 tested pairs of motor units. This sample included seven pairs c omprising one slow-twitch (S) and one fast-twitch motor unit, where th e predicted reversal of recruitment should have been most apparent. Re gardless of the source of excitation, recruitment of motor units of th e MG was consistent with Henneman's size principle in approximately 90 % of trials. 3. Skin pinch increased the firing rate of 30 of 32 indiv idual motor units previously activated by stretch or vibration, includ ing 7 slow-twitch units. In the remaining two units, skin pinch transi ently (100-400 ms) slowed the firing of an S unit in 11 of 13 vibratio n + pinch trials. The other unit (type unknown) showed one or two reta rded spikes in each of four vibration + pinch trials. In three S units , including the lone inhibitable unit and two others that were only ex cited by skin pinch, there was a significant positive rank correlation between change in unit firing frequency and change in soleus integrat ed electromyographic activity. 4. In our hands there is little evidenc e for differential control of fast and slow-twitch motor units; the ph enomenon described previously was much more the exception than the rul e.