The Dorsal Vagal Shift, or the Frozen Response
Of the three part autonomic nervous system, the 'oldest' is the unmyelinated dorsal vagal part of the parasympathetic nervous system. It regulates mostly visceral organs below the diaphragm, such as the bladder and bowel, but it connects to the heart and lungs as well. It consists mostly of unmyelinated fibers, so it's response is the slowest. Its overall tendency is toward shut-down or freezing. When it comes to regulating human behavior, it can be thought of as a crude instrument. However, constant sympathetic arousal is often unsustainable or ineffective. Therefore a dorsal vagal shift is very common in modern life, but often unrecognized, because to others it can appear quite erroneously as being well-behaved or peaceful.
The Dorsal, or 'Emergency' Brake
The dorsal brake is in contrast to the (ventral) vagal brake. In very different species, such as reptiles, the dorsal vagal system is responsible for the 'dive reflex' and immobilization as a defense (playing dead). In humans, such defenses are less useful, but they exist still. When sympathetic tone drives the body in an unsustainable way, physiology demands some respite and it often comes in the form of shut-down. An example is sudden death in athletes, described below.
Sudden Death During Exercise
Strenuous exercise will both increase the sympathetic tone and minimize the fine tuned ventral vagal brake. However it will also produce endorphins which will disguise the nature of autonomic situation. Sudden death while exercising seems to be related to 1) exercise is taken over a fairly long period maximizing sympathetic tone and decreasing vagal brake, 2) the exerciser ignores sensations of distress either with the aid of endorphins or will based goals or pressure from a coach or from competition 2) the dorsal emergency brake slams on causing cardiac arrest.
Signs of Dorsal Shift
|Freezing||This term is used when someone doesn't move. But the term implies also a drop in temperature. This is probably more than the peripheral vasoconstriction of sympathetic shift, it possibly represents a decrease in the production of body heat|
|Speechlessness||The organs of speech, run by the ventral vagal system seem unable to co-ordinate when the dorsal vagal system is strongly dominant.|
|Dissociation||This is possibly just the psychological correlate of freezing. With no movement thought and perception loses its anchor in the body and the here and now.|
|Involuntary Defecation||This is in extreme distress.|
|Involuntary Urination||This is in extreme distress.|
|Fainting||Plain fainting is also known as vaso-vagal syndrome. While there is a large danger of injury from an unprotected fall, people who faint often feel fine afterwards. Fainting may work as an autonomic reset.|
|Shock||Shock is circulatory collapse. It is life threatening. We also speak of emotional shock from hearing or witnessing something, it may be akin to fainting. Shock is usually notable for speechlessness.|
|Sense of Effort||The muscles and limbs feel heavy|