Excitatory and inhibitory inputs to hypoglossal motoneurons and adjacent reticular formation neurons in cats

Abstract

To elucidate some of the brain stem mechanisms involved in tongue motility, extracellular microelectrode recordings were made from single neurons in the region of the hypoglossal nucleus in 10 decerebrate and 23 anesthetized (chloralose) adult cats. The antidromic response characteristics and the synaptically evoked responses of 71 motoneurons that supplied tongue protrusive (P) or retrusive (R) muscles were documented. Protrusive motoneurons could be synaptically excited by temporomandibular joint (TMJ), glossopharyngeal (IX), and/or superior laryngeal (SLN) nerve stimuli, whereas R motoneurons could be activated by lingual and/or IX nerve stimulation. Conditioning effects revealed that the inhibition of the antidromic responses was shorter in duration than the inhibitory effects noted when synaptically evoked responses were conditioned. Conditioning stimuli delivered to the lingual, TMJ, IX, and SLN nerves were most effective in inhibiting the synaptically evoked responses of P and R motoneurons for conditioning-test intervals of as much as 400 ms. Those conditioning stimuli which also could synaptically activate a motoneuron tended to facilitate the cell's synaptically evoked responses at conditioning-test intervals of about 10 ms, whereas conditioning stimuli which did not synaptically activate the cell resulted in only the long-lasting inhibition.