If we want to understand why fainting occurs, we must understand how the central nervous system regulates the heartbeat.

The Autonomic Nervous System
The Autonomic Nervous System is the division of the nervous system that regulates our visceral functions, such as the heartbeat, and gland activity. It is also known as the Involuntary Nervous System since it is not under our control; as we cannot decide how fast or how slow our heart should beat or how much acid our stomach should secrete.

The Autonomic Nervous system is organized into two parts; the SYMPATHETIC and PARASYMPATHETIC, which bring about opposite effects on the same organ, e.g. the sympathetic quickens the heartbeat while the parasympathetic slows it. The sympathetic increases the strength of the heart contraction, while parasympathetic decreases the heart contraction. The sympathetic constricts the blood vessels, while the parasympathetic dilates it.

How does the nervous system control heart beat?
The normal average heart beat in a resting teen is 70-80 beats per minute, During sleep it goes down to 50-60 beats per minute, and it accelerates to above 100 with emotional excitement.

Increased heart rate is produced by increased sympathetic activity and decreased parasympathetic activity.

During sleep the parasympathetic is in control and the sympathetic is suppressed so the heart rate becomes low. During fear the sympathetic takes command and the parasympathetic is suppressed so the heart rate will be faster.

The Parasympathetic Pathways
The parasympathetic nerves originate from the Medulla in the brain, and specifically from the Nucleus Ambiguous. The nerve, which carries those fibers, is called the Vagus.

The Vagus Nerve innervates the Sinus node and the AV node. The Vagus therefore can slow the heartbeat by inhibiting the activity of the Sinus Node, which is the main switch of the heartbeat. If the Vagus activity is very high it may even cease the activity of the sinus node and cause complete cessation of the heart beat for seconds (Sinus Arrest).

The parasympathetic activity dominates the sympathetic activity at the Sinus node, i.e. when the sinus node is subjected to combined parasympathetic and sympathetic stimulation; the heartbeat slows under the influence of the Parasympathetic.

The AV Node is located at the junction between the atria (upper cardiac chambers) and the ventricles (lower cardiac chambers), hence the name Atrio-Ventricular Node. It enables the transmission of the electric impulse from the atria to the ventricles. The Vagus nerve results in delay of this transmission and may result in transient heart block.

The Sympathetic Pathways
The Sympathetic Pathways originate from the spinal cord and travel over the great heart vessels to reach the heart. Upon reaching the heart they are distributed over the heart chambers like an extensive net. Accompaning the coronary arteries, they penetrate the heart muscle (myocardium),

The effects of the sympathetic diminish gradually after cessation of stimulation, while the effects of the parasympathetic diminish abruptly after cessation of stimulation. For example, in a frightening situation it takes your heart beat a longer time to go back to normal even after the danger is over, while your heart beat briskly increases as you wake up, here a parasympathetic which is dominant during sleep abruptly gives way.