The Milk-Ejection Reflex: A Neuroendocrine Theme in Biology, Myth and Art

THE MILK-EJECTIONREFLEX: A NEUROENDOCRINE THEME IN BIOLOGY, MYTH AND ART* S. J. FOLLEYt I am deeply and sincerely appreciative ofthe honour accorded to me by the Committee ofthe Society for Endocrinology in inviting me to give the Dale Lecture for 1969 coupled with the award ofthe Henry Dale Medal. This is not least because of the great eminence and distinction of my predecessors in whose footsteps I feel most unworthy to follow. Unlike them I have not had the privilege ofparticipating in the making of discoveries offundamental importance which have added lustre to the history ofendocrinology. Most ofmy research has been in the field oflactation which, because it is related to many branches ofendocrinology, might, ifI attempted to survey the whole, proveto be a topic ofdiffuse and, paradoxically , rather specialized interest. As the subject ofmy lecture I have, therefore , decided to concentrate on an aspect oflactational research which has been pursued in our laboratory for some years and which is offairly general interest, namely the milk-ejection reflex. This topic will enable me to introduce a feature novel to this lecture series, that is to conclude with a briefsurvey ofthe treatment ofthe milk-ejection reflex in mythology and art. The subject I have chosen is, moreover, closely connected with one of the discoveries made by Sir Henry Dale early in this century and which forms one of his many claims to have his name associated with the Society 's Medal. Thiswashis discovery [1] ofthe action ofposteriorpituitary * The Sir Henry Dale Lecture, London, May 1969. Permission to reprint has been given by the Journal ofEndocrinology. Perspectives is grateful to the Cambridge University Press for the use ofits plates. The work described in this lecture has been supported by grants from the Rockefeller Foundation, the Pig Industries Development Authority, the Wellcome Trust, and the U.S. Public Health Service (grant no. HD 00724). Professor Folley diedJune 29, 1970. t The National Institute for Research in Dairying, University of Reading, Shinfield, Reading, England. 476 S.J. Folley · Milk-Ejection Reflex Perspectives in Biology and Medicine · Summer 1970 extracts in causing contraction ofunsttiated uterine muscle, which he subsequently studied in greater detail [2]. This finding ultimately gave rise to an immense amount of research, which culminated in the award of the Nobel Prize for Chemistry to du Vigneaud in 1955 for his work on the isolation, chemical structureand finallythe synthesis oftheneurohypophysial hormones oxytocin and vasopressin. One ofthe initial consequences following Dale's original discovery was the observation by Ott & Scott [3] ofwhat was at first described as the 'galactogogue' action of posterior pituitary extract in lactating animals, which isbasicto the theme ofthis lecture. This action ofposterior pituitary extract was quickly confirmed and studied by various workers over the years and for a time it was believed, as its original name implied, that the effect was due to a temporary increase in the rate ofmilk secretion. However , in due course it was realized that this was not so and that the action of posterior pituitary extract or, as we now know, its neurohypophysial hormone component, oxytocin, on the lactating mammary gland can more properly be described as galactokinetic in nature. This means that a contractile process within the alveolar tissue ofthe mammary gland is evoked and hence a sudden rise in intramammary pressure is caused so that ifthe teat has been cannulated a stream of milk is forced through the cannula. Turning now to the milk-ejection reflex, it has long been known that when the suckling or milking stimulus is applied to die teat or nipple of the lactating mammary gland a sudden increase in intramammary pressure occurs after a latent period ofsome 40-50 sec. The rise in intramammary pressure is a reflex response to stimulation ofsensory nerve endings with whichthe skin ofthe teat is richly endowed and is due to the reflex contraction ofa contractile effector tissue surrounding the alveoli forcing the milk present there, and also thatcontained in the finer ducts, down into dielarge sinuses or, in domestic ruminants, the gland cisterns from which it can be readily withdrawn by the suckling, hand milker or milking machine. It was once believed that the milk-ejection reflex was a purely neural reflex...