Faculty of Medicine Memorial Minute: Elwood Henneman

Memorial Minute on the Life of Elwood Henneman (1915-1995)

[for presentation to the Faculty of Medicine, Harvard Medical School]

In 1955, at the time Elwood Henneman was invited to Harvard, there was no department of Neurobiology and indeed the word had not yet been coined. Pre-clinical instruction in functional aspects of the nervous system was the responsibility of departments of Physiology and the department at Harvard under the leadership of Walter B. Cannon excelled in almost all aspects of what we now term Neurobiology. Cannon himself was renowned for his research on the autonomic nervous system, Alexander Forbes was a leader in electrophysiology, Hallowell Davis in special senses and Philip Bard in the central nervous system. After Eugene Landis succeeded Cannon in 1944, the focus of the department shifted to the cardiovascular system and for the next ten years neurophysiology was poorly represented. Elwood Henneman was recruited to remedy this situation and he came with the strong support of Philip Bard who had left Harvard to become Chairman of the department of Physiology at the Johns Hopkins.

Elwood graduated from Harvard College in 1937 and went on to medical school at McGill University in Montreal with an internship in Neurology at the Royal Victoria Hospital and Neurological Institute. This Institute, under the leadership of Wilder Penfield, was a center for research on the localization of function in the cerebral cortex and it was therefore natural for Dr. Henneman to seek postdoctoral training in central nervous system physiology in Professor Bard's department at the Johns Hopkins. After serving in the Navy as neurosurgeon in the Pacific Theater during WW II he returned to Bard's department; in collaboration with Vernon Mountcastle he undertook an investigation of localization of electrical potentials in the brain evoked by sensory afferents. In experiments on cats and monkeys he and Mountcastle showed that the tactile surface of the body is represented in the ventro-lateral thalamus by a three dimensional "figurine" of the body. Illustrations from this now "classical" work are reproduced in modern textbooks of physiology, psychology and neurology. In this period, also, Henneman mapped the reciprocal projections linking the cerebral and cerebellar cortices; the chapter on the cerebellum he wrote for Bard's textbook of Medical Physiology became standard fare for medical students.

After completing postdoctoral studies at Hopkins, Dr. Henneman spent two years at the Illinois Neuropsychiatric Institute in Chicago and another two years with David Lloyd at the Rockefeller Institute in New York. In Chicago he made an important discovery in neuropharmacology that immediately established his reputation world-wide. He found that mephenesin, a drug that causes flaccid paralysis of skeletal muscle, acted by inhibiting interneurones, thus blocking excitatory impulses to spinal motor neurones. Henneman and his colleagues also noted the tranquilizing effects of the drug in normal cats; this was first use of the word in pharmacology. Subsequent more clinically useful drugs like meprobamate (Miltown) were called tranquilizers. In Lloyd's laboratory at the Rockefeller, Dr. Henneman mastered the latest technical advances in electrophysiology and armed with this sophisticated technical knowledge he returned to Hopkins as Assistant Professor of Physiology.

Two years later, when Dr. Henneman came to Harvard, he was uniquely prepared to organize an entire course in neurophysiology, covering everything from the revolutionary new Hodgkin-Huxley theory of excitation and conduction in peripheral nerve to the physiology of special senses and localization of function in the neo-cortex. This he did almost single-handed via a series of stimulating lectures supplemented by well planned laboratory experiments utilizing the latest available electronic equipment. He introduced 150 students each year to the complexities of electronic amplifiers and oscilloscopes without losing sight of the essential neurophysiology. The students were able to record and analyze evoked potentials from the sensory cortex of anesthetized cats, thereby sharing the excitement of important experiments that had only recently been published.

At the same time, Henneman began the series of original investigations, which were destined to become a permanent part of neurophysiology, namely elucidation of the functional organization of the neuromuscular system in terms of the dimensional, electrical and metabolic properties of its individual components. The generalizations revealed by these studies are regarded by many neurophysiologists as the most important advance in the field of motor control since the work of Sir Charles Sherrington. These generalizations, each based on Henneman's work at Harvard, may be summarized very briefly as follows:-

1) Each afferent nerve fiber from a muscle stretch receptor divides within the spinal cord to innervate more than 90% of the motoneurons that innervate that muscle.

2) The susceptibility of each spinal motoneuron to excitation or inhibition is a function of its size. The smallest neurons are the most easily excited and least readily inhibited; large cells are the least susceptible to excitation and are most readily inhibited.

3) Motoneurons are brought into action by afferent input in order of their size and they are inhibited in reverse order. This "size principle" governs all systems projecting to motoneurons, including local segmental reflexes, long spinal reflex pathways and descending systems of brainstem and cortical origin.

4) Finally, the size, metabolism and functional properties of muscle fibers are precisely matched to the motoneurons that innervate them. Small motoneurons innervate small muscle fibers that contract slowly and contain the metabolic constituents required for sustained aerobic contraction. In contrast, large motoneurons innervate large muscle fibers that are adapted for rapid contraction utilizing anaerobic metabolism.

These generalizations provide a theory of motor control in which anatomy, histochemistry and electrophysiology are closely interwoven. The "Henneman size principle" is widely accepted by neurophysiologists and may be regarded as a major extension of Sherrington's theory of recruitment. At a time when most neuroscientists focused on cellular physiology, Henneman looked beyond individual units to seek understanding of how the individual units act together to achieve coordinated control of function in complex systems.

Henneman's publications and also his lectures, were distinguished by exceptional style and clarity of exposition. He was able to present new ideas about complex systems in relatively simple terms and his lectures to medical students were enlivened also with whimsical humor. The experimental basis of the "size principle" was described in 18 logically connected papers published in the J. Neurophysiology over a period of 25 years. Most of his experimental work was carried out by himself or in collaboration with one or two graduate or postdoctoral students. Although he published relatively few papers each one was generally very long and of exceptionally high quality; he never published under pressure or for non-scientific reasons. The editor-in chief of the J. Neurophysiology once told a member of this Memorial Minute committee that Henneman's papers were the most important papers he had received for publication in the journal.

When a separate department of Neurobiology was established in 196(5?) under the dynamic leadership of Steven Kuffler, Elwood was invited to join the new department but he chose instead to remain in the department of Physiology. Responsibility for teaching neurosciences was transferred to the new department of Neurobiology, leaving Elwood in an isolated position and delaying his already long overdue promotion to full professorship until 1969. In 1971, however, he was appointed chairman of the department of Physiology which he served faithfully until his formal retirement in 1984.

Elwood was something of a "lone wolf" in everything he did and few people came to know him well. Nevertheless, he enjoyed social gatherings, including his association with the Senior Common Room of Dunster House and evenings at the American Academy of Arts and Sciences in Cambridge. One of his junior colleagues at McGill University (now a professor at Harvard) described him as ".... a glamorous figure; he exuded optimism, his bow tie was always perfectly tied, his shirts were beautifully ironed, and his face was usually lit by a slightly mischievous grin." As an undergraduate he was a star on the Harvard tennis team and he was also an excellent downhill skier and wind-surfer. His strong physique enabled him to survive a series of major surgical operations, including installation of an artificial aortic valve, coronary bypass surgery and emergency repair of a dissecting aortic aneurysm. The latter operation was indeed spectacular because the aneurysm occurred in Amphitheater C while he was lecturing to the first year class. He diagnosed the problem himself from the referred pain and ordered students in the front row to alert surgery at the Brigham where he was rushed to the operating room. It was a very close call.

In 1950 Elwood married Karel Toll, a neurologist who became associated with the Mass General Hospital after they moved to Boston with their two young daughters. Elwood did much of his scholarly writing and preparation of lectures in the library of their attractive home on Belmont Hill. In addition to editorial work for several journals, his eight chapters on Neural Control of Movement in Bard-Mountastle's textbook of Medical Physiology required continual revision as this popular textbook went through successive editions.

In 1975 Karel and Elwood suffered the tragic loss of their elder daughter, Cyrena and in 1983 Karel herself died of ovarian cancer. Elwood continued to maintain their home in impeccable condition and he continued also to write scholarly reviews until his death from circulatory failure on February 22, 1996. He is survived by his daughter, Mrs. Abby Friedman of Rowe, Mass.

Respectfully submitted

Raoul Bott

Peter Dews

J. Allan Hobson

James C. Houk

Vernon B. Mountcastle, Johns Hopkins

John R. Pappenheimer