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Voiceover: Let's talk now about how the mother's breast knows when it's time to secrete and make milk to feed her infant. You might guess that it has something to do with the fact that there will be a hungry little guy there. You're right. I'll try and draw him without making him look like an alien. They do open their mouths very, very wide. You'd be surprised, and it does have something to do with the fact that the baby begins to suckle. The baby does begin to suckle, and the actual suction of that act does help to drain milk out of the breast. Remember we had the mammary glands here that contained the milk and they were lined by those myoepithelial cells, and they drained toward the nipple via the lactiferous ducts. So, yes, it does have to do with the baby suckling, but it's actually a little bit more exciting than that. Let me tell you why it's so exciting. In the nipple itself, are these things called mechanoreceptors. They detect when the baby begins to suckle, and they send messages to the mother's brain via her spinal cord. These are ascending sensory messages that go toward the spinal cord and up the spinal cord to the mother's brain. They go to a part of the brain called the hypothalamus. The hypothalamus then does two really important things. Firstly, the hypothalamus sends an on message, sends a go message to another kind of neuron called an oxytocin neuron. As you can probably guess, an oxytocin neuron releases a hormone called oxytocin, and I'll just get my pen to start working again. There we go. Then I'll just finish writing the word neuron over here for you. The oxytocin neuron is secreting oxytocin, and it's secreting that hormone from the posterior part of another segment in the mother's brain. That segment is called the pituitary gland. The oxytocin is being released from the posterior pituitary in the mother. The function of that oxytocin hormone is to cause these myoepithelial cells to contract. Secondly, the hypothalamus has to do something else. It has to turn off a different kind of neuron. It's sending an off signal to a neuron called a prolactin inhibitory neuron. I'll write this one down for you. This is a prolactin inhibitory cell or neuron. The prolactin inhibitory cell's job all year round basically, is to tell the cells that make prolactin another kind of hormone to tell them to be off. This is the prolactin inhibitory neuron, and its message is to say, "You know what? "Don't make prolactin." I'll tell you what prolactin does in a minute. Obviously, this is our prolactin-making cell, so our prolactin cell, and what the hypothalamus does in response to this incoming information that hey there's a hungry baby out there, the hypothalamus is going to turn off the off signal. That is going to result in more prolactin being secreted, and what does prolactin do? Well, maybe you can guess from it's name, pro, meaning that it's sorry, I'm writing pro. We don't want to do that. Prolactin is basically pro lactin, or pro milk. It is going to cause something called lactogenesis, or the making of milk inside the mammary gland. Together these two things, these two hormones, prolactin, and I'll write it in for you here, prolactin which is being released from the anterior pituitary, the ant. pit., and also oxytocin that's being released from the posterior pituitary. Those two hormones are going to cause milk to be made via prolactin and milk to be ejected, or shot out of the lactiferous ducts via the action of oxytocin on the myoepithelial cells. Now here is the best part. You ready for this? This whole cascade of events can also be triggered by the sound of a baby's cry. It doesn't even have to be your baby. The sound of a baby's cry will be picked up by a mother, a lactating mother's ear, and that information will be sent via other sort of higher centers in the brain. I'll just write higher centers. Those are auditory centers and other higher parts of the brain. Those higher centers will send a message to the hypothalamus saying initiate this cascade of events and cause milk to be ejected even if a baby isn't suckling. This is kind of a back-up mechanism because it means that potentially any lactating mother in the vicinity could nourish any hungry child. This whole pathway, this amazing ability to communicate between the baby and the mother is called the let down reflex, the let down reflex.