The HPA Axis

Lesson objective:

We learn in this section about the fast and unconscious chain and its role in how the brain and body respond to stress.

Let’s dig a bit deeper into the stress response. 

We learned that the amygdala receives and interprets sensory information. And after receiving initial inputs, the amygdala sends signals to multiple parts of the brain to coordinate appropriate action.

When it interprets an incoming signal as threatening, stress-inducing, or in any way requiring a quick, defensive response, the amygdala first sends signals to one of its neighbors, the hypothalamus.  

The hypothalamus plays a large role in the regulation of bodily and emotional activity. If the signal received by the hypothalamus indicates stress or threat, specific neurons in the hypothalamus begin to secrete hormones that stimulate the nervous system.

The hormones released in the hypothalamus then stimulate the pituitary gland, which then secretes a hormone that activates the adrenal cortex, on top of your kidneys.  

The adrenal cortex–which is the last stop on the communication cascade– produces a variety of hormones, including adrenaline, norepinephrine, and cortisol, which serve multiple functions in the body. 

 We can envision this process as a cascade of signals. 

This rapid and unconscious cascade of information enables your body to physically respond to stressors or threats. The entire communication cascade from the amygdala to the hypothalamus to the adrenal cortex and back is called the HPA Axis: or the hypothalamic- pituitary-adrenal axis. 

The HPA axis is the cascaded process of communication from the sensory world, to the amygdala and hypothalamus, the pituitary gland, and the adrenal cortex, and back.

 Each “stop” on the communication chain sends unique signals, releases unique hormones, each of which help us confront stress. 

For example, when cortisol is released from the adrenal cortex back to the hypothalamus, it indicates that the body has begun to respond to the stress or threat. When this happens, the hypothalamus and pituitary gland can slow down their stress signals, and the cycle is completed. 

Many bodily systems have receptors for the various stress hormones that are released by the HPA Axis, and organs and systems with receptors receive unique signals and instructions from hormones, activating various bodily processes. This means that stress hormones like cortisol regulate a variety of body functions including blood sugar, the immune system, digestion, and memory formation–all below our conscious control or awareness. Cortisol, among other hormones, facilitates allostatic changes across systems. 

The stress response process in the HPA axis is designed to help us confront and respond to increased demands on our systems–to confront any form of stress–whether it be caused by something good, or something bad; whether it be caused by something physically stressful like exercise or running away from danger, or something mentally stressful like a scary film, or negative memories.

The HPA axis is just one component of how our brains and bodies respond to stress. The human body is immensely complex, and dozens of different systems and organs play a role in our regulation, in our constantly changing communication, and in turning things on or off, on high or low.