A Comparison between Autonomic and Somatic Motor Outflow to Vestibular Stimulation

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Abstract
The comparison showed a clearcut difference in threshold values between the two systems. The threshold of the parasympathetic system is about three times lower than that of the somatic system. The motor outflow of the former system does not show the same effect of temporal summation during higher frequency vestibular stimulation as does the latter. A strong tonic inhibitory control is exerted upon the vestibular nuclei by various central sources but no comparable control acts upon the vagal nuclear complex. However, the parasympathetic activity evoked by vestibular stimulation is markedly influenced by different phasic feedback mechanisms narrowing the limits of the motor outflow. Vestibular stimulation brings about a reflex increase in respiration, and in the competition for access to the vagal nuclear complex between impulses reaching it from the respiratory center and those evoked by vestibular stimulation, the former dominate. Widespread somato-visceral effects elicited throughout the body in response to vestibular stimulation lead to a subsequent activation of different sensory receptor systems (e.g. proprioceptors of the muscles, joints and tendons and Pacinian corpuscles of the intestines). Streams of afferent impulses, funneled through various input leads and ascending connections, can exert some degree of phasic control upon the parasympathetic motor outflow in response to vestibular stimulation. The interaction between this feedback mechanism and the vestibulo-vagal reflex discharge has been analyzed by stimulation of dorsal roots and splanchnic nerves.

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