The Esophageal Groove Diverts the Flow of Ingested Milk Past the Forestomach and into the Abomasum
For proper rumen development in the suckling animal, it is important for milk to be diverted away from the developing rumen. This is accomplished by the actions of the reticular groove (also called the esophageal groove).
This structure is a gutterlike invagination traversing the wall of the reticulum from the cardia to the reticulo-omasal orifice. When stimulated, muscles of the groove contract, causing it to shorten and twist. The twisting action causes the lips of the groove to close together, forming a nearly complete tube from the cardia to the omasal canal. When the groove is contracted, milk entering the cardia is directed into the omasum, with 10% or less entering the rumen. Milk quickly traverses the omasum and enters the abomasum.Reticular groove closure is a reflex action, with efferent impulses arriving from the brainstem through the vagus nerve. Afferent stimuli arise centrally and from the pharynx. Anticipation of suckling invokes central stimulation of reticular groove closure, which may be considered a cephalic phase. Fluid, especially sodium-containing fluid in the pharynx, stimulates afferent fibers that reinforce the cephalic phase of groove closure. The posture of the calf or lamb when suckling does not appear to have much influence on reticular groove function, but rapid drinking from an open pail, in contrast to suckling from a nipple, frequently results in inefficient groove function and spillage of milk into the rumen. Milk in the rumen results in the formation of improper fermentation patterns.
The reticular groove has its primary function in suckling animals, and the activity of the groove reflex appears to diminish after weaning and with advancing age. However, the groove reflex is stimulated by antidiuretic hormone (ADH; sec Chapter 43), indicating that it may have some physiological function in adult life. ADH is secreted by the posterior pituitary in response to dehydration or increases in plasma osmolality. ADH is associated with thirst, and because it stimulates the reticular groove, a large portion of the water drunk by water-deprived animals may bypass the rumen. This may be a functional mechanism to ensure that water arrives quickly at the site of most rapid absorption, the small intestine.