Recent evidence does indicate that the milking or suckling stimulus causes a marked increase in the blood oxytocin concentrations in mammals. The amount of oxytocin in the bloodstream disappears quite rapidly after it has been released. This is brought about either by the destruction of the oxytocin in the body or by the elimination of the oxytocin from the blood. The kidneys, liver, and mammary gland are mainly involved in removing the oxytocin from the bloodstream. The contractile tissue within the mammary gland is the myoepithelial cell.
Myoepithelial cells surround the alveoli, the interlobular ducts, and the cisterns.
The myoepithelial cells contract in response to oxytocin. The contraction is accompanied by an increase in the diameter of the ducts in order to produce a high rate of milk flow to the exterior of the gland. The preponderance of evidence indicates that the myoepithelial cells are under direct hormonal control, and the motor innervation of the cells plays no part in normal milk ejection. The smooth muscles are too sparsely located throughout the mammary gland to be important in the normal milk-ejection process. The milk-ejection process is inhibited by the release of adrenalin, which can be brought about by fright and emotional disturbances.