Milk ejection usually is the only way in which milk in the alveolar lumen can be expressed from the gland. The process of milk ejection can occur under many conditions. Milk ejection can occur under water - as for whales, porpoises, sea-cows, sea otters, hippopotamus. Milk ejection also can occur while in flight as for some bat species.

Most of the discussion below relates to milk ejection in cattle being milked by machine, however, the underlying principles are valid for other species being suckled by the offspring.

The streak canal must be opened to remove milk. This can occur by:

A small amount of milk may be obtained from the teat cistern, the gland cistern, and the large ducts of a cow by passive milk removal in the absence of milk ejection. However, to get milk from the alveoli requires an active process called the milk ejection reflex.

Milk Ejection Reflex

The milk ejection reflex actually is a neuroendocrine reflex. The reflex has an afferent pathway (neural) and an efferent pathway (hormonal, blood-borne).

Afferent Pathway:

The greatest amount of innervation in the mammary gland is in the teats, where there are pressure sensitive receptors in the dermis. Mechanical stimulation of the teats activates pressure sensitive receptors in the dermis where the pressure is transformed into nerve impulses that travel via the spinothalamic nerve tract to the brain. These nerves synapse in the paraventricular nucleus and in the supraoptic nucleus in the hypothalamus. When the cell bodies of the oxytocin-containing neurons are stimulated by these impulses originating in the teat, an action potential moves down the oxytocin-containing neurons from the cell body in the hypothalamus down the axon to the neuron ending in the posterior pituitary. This causes release of oxytocin and neurophysin into the blood. The efferent pathway starts at this point.

Efferent Pathway:

The efferent pathway begins with the release of oxytocin into the blood. The oxytocin then travels to the mammary gland via the blood, binds to oxytocin receptors on the myoepithelial cells, causing the myoepithelial cells to contract, and resulting in increased intra-lumenal (intramammary) pressure and ejection of milk from the alveolar lumen.

Oxytocin receptors are associated with the myoepithelial cells, not the smooth muscle of the mammary gland. In mice these receptors increase through-out gestation, but are fairly constant through lactation. (see Soloff, 1982, J. Dairy Sci. 65:326)

Although this seems straight-forward, the biological mechanisms involved are complex.
[See J. Dairy Sci. 1983 66:2251]

For example:

• Manual stimulation of the teat or nipple is not required for oxytocin release or milk ejection. Oxytocin can be released by conditioned visual and auditory cues, such as the sights and sounds of the milking parlor (occurs in ~38% of cows), resulting in milk letdown.
• Oxytocin is not always measurably elevated in blood during milk letdown. Milk yields of mammary glands transplanted to the neck region in goats is near normal despite having eliminated oxytocin release due to direct udder stimulation and innervation.
• No measurable increase in oxytocin in the blood after stimulation is seen in 38% of goats and 32% of cows studied.