The mechanosensitivity of the receptors in the nonacoustic part (vestibular end organs) of the guinea pig’s inner ear was studied by microelectrode recordings. These receptors are directionally sensitive displacement detectors. Shearing force, i.e., the component of mechanical force, acting tangentially on the surface of the sensory cell layer, was proven to be the effective stimulus. Its characteristic curve is a sinusoidal function representing the information input. This holds for the statolith apparatus (utricle and saccule as organs of equilibrium) when tilting of the head occurs in one or the other direction. An almost identically sinusoidal characteristic curve was obtained by measuring d-c potentials (receptor potentials) inside the sensory cell layer of the macula utriculi. The significant fact is the bidirectional response to tilting: inclination in one direction produces depolarization, and in the opposite direction gives hyperpolarization. Corresponding results were obtained from the macular nerve twigs. Most of the units show reasonably constant resting discharge; tilting in one direction increases, and in the opposite direction decreases, the nerve action-potential frequency. This impulse frequency represents the information flow to the central nervous system. In the other part of the vestibular organs, the semicircular canals, the mechanical behavior of the coupled system cupula-endolymph can be described in terms of a highly damped torsion pendulum. The time course of mechanical stimulus, and that of nervous excitation at the different levels of the peripheral and central systems, was studied, during deflexions of the cupula, by recording the potential changes inside the sensory cell layer of the cristae ampullares. When a sudden mechanical event acts as stimulus, the central systems get a very true image of its time course.

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