The Function of the Diving Reflex
The function of the diving reflex is to extend survival under water by conserving oxygen. It involves apnoea, bradycardia and selective vasoconstriction. (Gooden, 1994; Tipton, 1989)
Apnoea: temporary cessation of breathing
Bradycardia: an abnormally slow heart rate
Vasoconstriction: oxygen and warmth preservation via the narrowing of blood vessels
Essentially, when you jump into cold water and trigger this reflex, you experience apnoea, which immediately reduces oxygen loss through the lungs. At the same time, the blood vessels in your limbs narrow to conserve oxygen for vital organs like your brain and heart (vasoconstriction). Baroreceptors, located in the neck (carotid sinus) and near the heart (aortic arch), detect this rise in blood pressure in your core, caused by the vasoconstriction. These receptors then signal to your brain that the blood pressure is too high. The brain reacts by lowering sympathetic nervous system activity, which normally increases the heart rate and blood vessel tension. This leads to bradycardia, reducing the body's oxygen consumption. Additionally, this response reduces resistance in the blood vessels of your lungs, enhancing oxygen exchange efficiency. In this way, oxygen is spared and you can stay underwater longer without oxygen.
A little explanation of the image below. The cutaneous receptors in the skin sense the cold water and send a signal via the nerves in the face (Trigeminal nerve) to the trigeminal ganglion. The signal goes from here to the pons and via interneurons to the medulla. This is located caudal to the pons. The Vagus nerve sends a signal to the heart and lungs. Bradycardia occurs in the heart and bronchoconstriction in the lungs.
Bronchoconstriction: constriction of the bronchi the lungs
N. Vagus: this is the 10th cranial nerve that goes to the organs
Trigemental ganglion: the ganglion of the 5th cranial nerve: N. trigeminus.
Triggers and Variability of the Diving Reflex
The diving reflex can be triggered by immersing the nose area alone, stimulating the ophthalmic division of the trigeminal nerve (5th cranial nerve), and interestingly, the response is less prominent with increasing age. This may relate to the apparent reduction in parasympathetic activity within older people (Gooden, 1994; Panneton, 2013; Tipton, 1989).
Research by Siebke and colleagues (1975) indicates that the diving reflex is significantly related to the survival rates of both adults and children following prolonged immersion in water. Notably, children tend to survive longer than adults when deprived of oxygen under water, likely due to a more pronounced diving response. (Gooden, 1994)
Additionally, the response effectiveness is modified by variables including water temperature and the specific location of contact with water. For instance, immersion may occur intranasally or through stimulation of the forehead and cheek regions. (Choate et al., 2014)