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It also activates when the head and face is submerged in any form of cold water.
It triggers a cascade of changes in the body, including a slowing of heart rate and increased peripheral vasoconstriction. These changes help to reduce oxygen consumption and direct blood flow to vital organs like the brain and heart, ensuring their continued function during a period of oxygen deprivation.
The felt experience is a deep relaxation response.
You can play around with this using the Nordic dunk. We've personally found that a dunk directly to the top of the head creates a more intense sense of contrast and cold shock, while tipping your head forward and letting it hit the back of the neck tends to trigger - after the initial cold shock - more of a parasympathetic response. In a way, you can feel it moving the stress from your head and neck down through your feet. Try to visualize that as you try MDR with the Nordic dunk and see if you could use as a tool to move whatever stresses may be stuck in your body down your spine, legs and out through your feet.
Here's a few more details on what the Mammalian Dive Reflex triggers:
Trigeminal Nerve Activation:
Submersion in cold water, particularly the face, activates the trigeminal nerve (fifth cranial nerve).
Vagus Nerve Stimulation:
The trigeminal nerve signals the brain, which then activates the vagus nerve (tenth cranial nerve), part of the parasympathetic nervous system.
Bradycardia and Apnea:
The vagus nerve slows the heart rate (bradycardia) and causes the individual to hold their breath (apnea).
Peripheral Vasoconstriction:
Blood vessels in the extremities constrict, diverting blood flow to the brain and heart.
Spleen Contraction:
In some mammals, the spleen may contract, releasing stored red blood cells to enhance oxygen-carrying capacity.
Increased Oxygen-Carrying Capacity:
Aquatic mammals have adaptations like increased blood volume, hemoglobin, and myoglobin to enhance their oxygen storage and utilization.