Natural cutaneous stimulation induces late and long-lasting facilitation of extensor motoneurons in the cat

An investigation was made of the effects of physiological cutaneous stimulation on the excitability of extensor motoneurons in spinal unanesthetized cats. The time course of changes in the monosynaptic reflex (MSR) amplitude of the soleus (Sol) and gastrocnemius medialis (GM) and lateralis (GL) was studied after conditioning stimulation with air jets (delivered to different regions of the skin of the ipsilateral hind limb), pinpricks, or stretching of the skin of the heel induced by passive rotation of the tibio-tarsal joint. Low-intensity electrical stimulation of the sural or saphenous nerves was also employed in order to condition the MSRs of the triceps surae muscles. Hair bending, skin indentation or stretching, as well as electrical nerve stimulation, can induce a similar biphasic excitability cycle of the extensor MSRs, characterized by an early inhibition followed by a late facilitatory period (LFP). The LFP started approximately 20 ms after the arrival of the cutaneous afferent volley, and lasted about 80 ms. Conditioned MSRs could attain values corresponding to 200% or more of controls. The receptive field of the LFP evoked by the air jet proved to be as large as the whole leg and foot skin surface. No significant differences were found in the extent of the late facilitation in the MSRs of Sol, GM and GL, conditioned by electrical stimulation. The LFP was also present, after conditioning stimulation of the same types as above, in intact (and spinal) chloralose-anesthetized cats.