Julius Bernstein

Julius Bernstein (18 December 1839 – 6 February 1917) was a German physiologist born in Berlin. His father was Aron Bernstein (1812—1884), a founder of the Reform Judaism Congregation in Berlin 1845; his son was the mathematician Felix Bernstein (1878—1956).

He studied medicine at the University of Breslau under Rudolf Heidenhain (1834-1897), and at the University of Berlin with Emil Du Bois-Reymond (1818-1896). He received his medical degree at Berlin in 1862, and two years later began work in the physiological institute at the University of Heidelberg as an assistant to Hermann von Helmholtz (1821-1894). In 1872 he succeeded Friedrich Goltz (1834-1902) as professor of physiology at the University of Halle, where in 1881 he founded an institute of physiology.

Bernstein's work was concentrated in the fields of neurobiology and biophysics. He is largely recognized for his "membrane hypothesis" in regards to the origin of the "resting potential" and the "action potential" in the nerve. Bernstein (1902, 1912) correctly proposed that excitable cells are surrounded by a membrane selectively permeable to K⁺ions at rest and that during excitation the membrane permeability to other ions increases. His "membrane hypothesis" explained the resting potential of nerve and muscle as a diffusion potential set up by the tendency of positively charged ions to diffuse from their high concentration in cytoplasm to their low concentration in the extracellular solution while other ions are held back. During excitation, the internal negativity would be lost transiently as other ions are allowed to diffuse across the membrane, effectively short-circuiting the K⁺ diffusion potential. In the English-language literature, the words "membrane breakdown" were used to describe Bernstein's view of excitation. (From Ion Channels of Excitable Membranes, Third Edition, by Bertil Hille).

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