During nursing or milking, which hormone is released to stimulate milk letdown?

Oxytocin is the hormone responsible for stimulating milk letdown during nursing or milking. It is produced in the hypothalamus and released by the posterior pituitary gland in response to sensory signals such as the sound of a calf or the sensation of suckling. When oxytocin is released, it causes the myoepithelial cells surrounding the mammary glands to contract, leading to the ejection of milk into the teat canal, making it available for the nursing offspring or milking process.

Prolactin, while important for milk production, does not play a direct role in the milk letdown reflex. Estrogen is involved in the development of breast tissue during pregnancy but is not primarily responsible for the letdown process. Luteinizing hormone (LH) is mainly involved in the regulation of reproductive processes, including ovulation, and does not have a role in milk letdown. Thus, oxytocin is specifically the hormone that enables the physical release of milk from the mammary glands when necessary.