Area postrema and the mesencephalic trigeminal nucleus

The ultrastructural morphology of the area postrema (AP) was investigated in the guinea pig: no peculiar features differentiate it from the AP of other vomiting and nonvomiting species. After discrete chronic lesion of the AP, electron micrographs showed clear signs of axonal terminal degeneration on some cells of the ipsilateral mesencephalic trigeminal nucleus (MTN). A direct AP-MTN connection was supported also by electrophysiological investigations. Electrical stimulation of the AP, using liminal or just supraliminal stimuli (4 to 10 μA),—to avoid a simultaneous antidromic activation of Probst's tract—elicited evoked potentials in the ipsilateral MTN with a latency as short as 0.2 to 0.3 ms in both the guinea pig and the rabbit. The same microstimulation of the AP inhibited about the 50% of the ipsilateral MTN units and decreased the excitability of the ipsilateral MTN in the curarized rabbit. In agreement with such findings, AP microstimulation inhibited the jaw movements elicited by MTN stimulation in the noncurarized rabbit. We conclude that the AP exerts an inhibitory influence on the afferent limb of the monosynaptic masseteric reflex.