Henneman received his bachelor's degree from
Harvard College in Cambridge, Massachusetts in 1937. In 1943 he finished his medical studies at
McGill University in Montreal. During a research fellowship at
Johns Hopkins University in Baltimore, Maryland, Henneman and colleague,
Vernon Mountcastle, showed that tactile information about the extremities is represented in an orderly map in the ventrolateral
thalamus of the cat and monkey. Further research positions followed, including at the Royal Victorian Hospital and at the Illinois Neuropsychiatric Institute (NPI) in Chicago. At NPI, Henneman discovered that the drug
Mephenesin (Myensin) inhibits interneurons in the spinal cord and thus causes muscle relaxation. This discovery helped lead to the development of muscle relaxant drugs. Of greater impact for the scientific community was Henneman's work describing the physiology of
motor neurons, the neurons that control contraction of the muscles. In 1957, Henneman published experimental results that showed that motor neurons that project to the same muscle are recruited on the basis of their size.
Henneman's Size Principle describes this relationship. In 1971, Henneman became chair of the Department of Physiology at Harvard Medical School, a position he held until his retirement in 1984. ==Awards and honors==