The body has an extrinsic cardiac control center in the , a portion of the brain that controls internal organs, which can alter the beat of the heart by way of the autonomic nervous system.

*the autonomic system is divided into the parasympathetic division, which promotes functions of a resting state, and the sympathetic division, which brings about responses to increased activity or .

*the parasympathetic division decreases sa and av activity when we are inactive, and the sympathetic system increases these nodes' activity when we are active or excited.

*the hormones epinephrine and norepinephrine, which are released by the , also stimulate the heart.

when we are frightened, the heart pumps faster and stronger due *to stimulation, and because of the release of epinephrine and norepinephrine.

1)Medulla oblongata

This is the part of the central nervous system located in the lower part of the brain (in the brainstem). Medulla oblongata is responsible for the regulation of main autonomic (involuntary) functions such as respiratory, cardiac (heart rate), vomiting, vasomotor (blood pressure). It connects higher areas of the brain with the spinal cord (motor and sensory neurons travel through the medulla).

2)Stress

Parasympathetic and sympathetic systems are both part of the autonomic nervous system with opposite functions, but they work together to maintain organism’s homeostasis.

Sympathetic system is also known as a part of fight and flight reaction because of its neuronal and hormonal stress response. This system is responsible for priming the body for action, particularly in situations threatening survival.

On the other hand, parasympathetic system is also involved in body's unconscious actions but for stimulation of "rest-and-digest” response. It regulates activities that occur when the body is at rest, especially after eating, salivation, urination, digestion and defecation.

3)Nodal

Both of them (parasympathetic and sympathetic systems) regulate the heart rate, more precisely heart's internal pacemaker activity. The nerves of the autonomic nervous system send signals to the SA node to either accelerate (sympathetic) or slow down (parasympathetic) heart rate depending on need. So, as a result sympathetic system increases heart rate and the contractility of cardiac cells (myocytes) while parasympathetic does the opposite. SA sends signal to AV node.

4)Adrenal medulla

Epinephrine (adrenalin) and norepinephrine (noradrenaline) are class of catecholamine with the role to control many body functions including heart rate. Both of them are part of the fight and flight response, meaning that they increase the heart rate. The difference is that epinephrine binds to both,  alpha and beta receptors, while norepinephrine acts mostly on alpha receptors.

5)Sympathetic

As a result of sympathetic nervous system, released hormones increase arousal and alertness, promote vigilance, and enhance focuses attention. Also, heart rate and blood pressure are increased, glucose is released from energy stores so it can be used, blood flow to skeletal muscle is increased while, blood flow to the gastrointestinal system is decreased. The brain and body are mobilized for action.  

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