Pressure Stimulation of Peripheral Nerves

Clinical experience suggests that pressure upon peripheral nerves or nerve roots is accompanied by pain, as well as by alterations in sensation and other neurological changes. The radicular pains caused by neoplasms of the spinal canal, the root pains arising from herniations of the intervertebral discs and the pain caused by pressure on the brachial plexus of cervical ribs or thickened scalenus muscles are common examples. Pain occurs early and persists throughout the course of these conditions and has been attributed to the stimulating effect of the steady pressure against the nerve. On the other hand, experimental physiological studies have repeatedly demonstrated that nerve fibers are relatively inexcitable to steady or even slowly changing stimuli. The classical studies on electrical stimulation of nerve emphasize that stimulation ordinarily occurs only on the make and break of a circuit, whereas constant currents do not stimulate at all. It is obvious, therefore, that there is a discrepancy between the clinical and experimental views. This may mean that the effects observed clinically are more than the effects of simple pressure stimulation or that the classical notions of stimulation have tended to minimize certain properties of nerve which might allow for repetitive response to steady, unchanging stimuli. Recent experiments have indeed shown that under suitable conditions constant electric currents may evoke a repetitive response (Erlanger and Blair, 1 and Rosenblueth 2 ). Moreover, very long sustained activity can be caused by steady chemical stimulation (Brink and Bronk, 3 Lehman 4 ). The experiments described in the present paper were undertaken to determine whether steady pressure applied to an isolated nerve preparation can similarly give rise to a continuous series of nerve impulses.