His family was Jewish and his parents both immigrated from the Kingdom of Württemberg, Germany and met in California during the Gold Rush.

Erlanger also had an interest in cardiology, specifically the way that excitation transferred from the atrium to the ventricle and researched with Arthur Hirschfelder.

This paper caught the attention of William Henry Howell, a physiology professor at Johns Hopkins School of Medicine.

[4] As part of this work, Erlanger was able to produce heart block in an animal model by clamping the bundle of His and tightening it.

[5] Together, they managed to amplify the action potential of a bullfrog sciatic nerve in 1922 and published the results in the American Journal of Physiology.

[2][6] It is uncertain why the pair had such a sudden shift in interest to neuroscience, as Erlanger was already widely respected in the cardiology field.

With this research, the pair discovered that the velocity of action potentials was directly proportional to the diameter of the nerve fiber.