2 - Minute neuroscience - Hpa axis

Welcome to 2 minute neuroscience, where I simplistically explain neuroscience topics in 2 minutes or less. In this installment I will discuss the HPA axis. The hypothalamic-pituitary-adrenal, or HPA, axis is best known for its role in our body’s natural reaction to stress. The HPA axis includes a group of hormone-secreting glands from the nervous and endocrine systems. The hypothalamus is a small neuroendocrine structure situated just above the brainstem that controls the release of hormones from the pituitary gland, a hormone-secreting gland that sits just below the hypothalamus. The pituitary gland can release hormones into the bloodstream that travel down to the kidneys and influence the secretion of hormones from endocrine glands called the adrenal glands, which sit on top of the kidneys. The primary function of the HPA axis is to regulate the stress response. When we experience something stressful, the hypothalamus releases a hormone called corticotropin-releasing hormone (or CRH). CRH signals the pituitary gland to secrete a hormone called adrenocorticotropic hormone, or ACTH into the bloodstream. ACTH travels down to the adrenal glands where it prompts the release of different hormones from different parts of the adrenal glands. It causes the secretion of a class of steroid hormones called glucocorticoids from the cortex, or outer layer, of the adrenal glands. One of these glucocorticoids is cortisol, which plays an important role in the stress response. ACTH also triggers the secretion of epinephrine and norepinephrine from the adrenal medulla, which is the center of the adrenal glands. Epinephrine and norepinephrine are substances that act as hormones and neurotransmitters; when released from the adrenal glands they act as hormones. The release of cortisol causes a number of changes that help the body to deal with stress. For example, it helps to mobilize energy like glucose so the body has enough energy to cope with a prolonged stressor. Norepinephrine and epinephrine act to keep the body vigilant and able to deal with a stressor by promoting alertness, and by increasing blood flow to skeletal muscles to ensure they are ready to act. When cortisol levels in the blood get high, this is sensed by receptors in areas of the brain like the hypothalamus, which leads to the shutting off of the stress response through what is known as a negative feedback mechanism.