Reflex effects from high threshold neck muscle afferents on hind limb extensor gamma motoneurones in the cat

The supra-segmental control of hind limb gamma motoneurones from neck muscle receptors has been studied in decerebrated and in spinal cats. Stretch of individual dorsal neck muscles was not an adequate stimulus for evoking long spinal reflexes to gamma motoneurones of gastrocnemius-soleus (GS) muscles unless the stretch was maximal or excessive. Pressure applied to the neck muscles, or intramuscular injections of KCl solution (0.1 ml, 5%), did affect the discharge of GS gamma motoneurones. Excitation was more evident than inhibition. We conclude that the long spinal reflex effects originate from high threshold mechanoreceptors, or nociceptors, rather than muscle spindles.