Lesson objective:

In this lesson, we learn about passive responses to stress, in addition to the role of the periaqueductal grey in freeze response.

For many, it may be easy to understand the “fight” and the “flight” responses to stress.

Most of us have experienced these phenomena at some point in our lives. We have felt adrenaline spike and course through us as we ran, or fought, or generally been scared, dealing with whatever the stressful or threatening situation may be. 

It may be a bit harder to understand the “freeze” response, though. 

If our brains prioritize survival, why then would it allow us to freeze when confronted with stress? Freezing, or being totally passive in the face of stress jeopardizes survival. If we freeze, a threat or stress may overtake us. At first glance, it seems a bit illogical. 

 

Let’s look at an example. We’ll turn to the animal kingdom.  

Some species have developed special tactics to avoid attack or being eaten. In some cases, a small animal simply cannot fight off an attack from a predator. It needs another way out. 

Certain species of mammals and even birds have complicated mechanisms that allow them to fake death by freezing when there is a predator nearby.

 They lie frozen, not moving, not blinking, and often not even breathing, waiting silently and motionless until the threat passes. In these cases, the risk of fighting or fleeing is too great, so the only remaining option is to freeze, in the hope that the predator passes by, no longer interested in their possible meal.

The freeze response in humans functions in a similar way. Deep in the brainstem we have a small region called the periaqueductal grey, or the PAG. the PAG, sits on the highway of communication from the amygdala to the rest of the brain and body. In general, it is known that the PAG helps coordinate multiple parts of defensive responses to stress, both active and passive.

The PAG is divided into two sides and is connected with the amygdala, the cerebellum, and the medulla, all of which help coordinate voluntary and involuntary movements in the body. 

 

Each side of the PAG is connected to a different “side” of the autonomic nervous system. One side coordinates with the sympathetic nervous system, and the other coordinates with the parasympathetic nervous system.

In other words, the PAG has one foot on the gas and one foot on the brakes.

When we sense a threat or stress, the amygdala and hypothalamus initiate the stress response. As part of that response, the amygdala signals the PAG, which begins to coordinate with the “gas” and the “brakes.”

It uses each of its sides to signal to the “gas,” or sympathetic nervous system, as well as to the “brakes,” or parasympathetic nervous system, ensuring that our organs and muscles have enough oxygen, blood, and energy to confront the stress. 

In some cases, the brain may interpret a stressor as inescapable. Overwhelming. Too much to handle. 

In these rare cases, PAG sends much stronger signals to the parasympathetic system. In other words, when the brain recognizes that a threat is inescapable or too great to confront, the parasympathetic response overrides any sympathetic nervous system response. 

Since the parasympathetic nervous system is responsible for relaxation functions, this leads to an immediate freezing, or complete passivity to the point of immobility. 

You can think of this outcome as a forced override of the normal sympathetic stress response, where the PAG force freezes the body to wait, frozen, until the threat of stress disappears. 

In this way, humans also have a way to “fake death,” in times of crisis. And it is critical to note that this form of stress response is also completely normal, and really quite adaptive when you think about it. Again, the brain prioritizes survival, and it sometimes pursues that priority with innovative strategies, including the freeze response.