The cardiac muscle of the heart is referred to as myogenic because its contraction is initiated from within the muscle itself, rather than by external nervous impulses.

The initial stimulus for heart contraction originates from a distinct group of cells known as the sinoatrial node (SAN), located within the right atrium of the heart.

1. The sinoatrial node (SAN) initiates a wave of electrical excitation that spreads across both atria
2. Atria contract
3. The signal is passed to the atrioventricular node (AVN), followed by a short delay
4. The AVN conducts the excitation through the bundle of His
5. The bundle of His conducts the excitation through the atrioventricular septum to the base of the ventricles
6. The signal spreads up the Purkyne fibres and the ventricles contract

The heart rate needs to increase during exercise to provide the extra oxygen required for increased respiration and to meet the higher demands for oxygen in active muscles.

The changes in heart rate are controlled by a region of the brain known as the medulla oblongata.

The sympathetic nervous system is responsible for increasing the heart rate.

The parasympathetic nervous system is responsible for decreasing the heart rate.

Chemoreceptors are sensitive to changes in blood pH resulting from changes in carbon dioxide concentration, and respond to these changes in blood pH by adjusting the heart rate accordingly.

These chemoreceptors are located in the walls of the carotid arteries and the aorta.

1. During exercise, blood has a higher concentration of carbon dioxide
2. Chemoreceptors in the carotid arteries and aorta detect this and increase the frequency of nervous impulses sent to the medulla oblongata
3. The medulla oblongata increases the rate of impulses to the sinoatrial node (SAN) via the sympathetic nervous system
4. Heart rate increases
5. The increased blood flow helps remove excess carbon dioxide via the lungs
6. Carbon dioxide concentration reduces

Pressure receptors, or baroreceptors, detect changes in blood pressure and transmit impulses to the medulla oblongata, which can then adjust heart rate to maintain blood pressure within normal ranges.

These pressure receptors are located in the walls of the carotid arteries and the aorta. 

Increasing blood pressure when it is too low:

1. Pressure receptors detect that blood pressure is too low
2. Pressure receptors transmit more signals to the centre in the medulla oblongata that increases heart rate
3. The medulla oblongata increases heart rate via the sympathetic nervous system

Decreasing blood pressure when it is too high:

1 Pressure receptors detect that blood pressure is too high

2 Pressure receptors transmit more signals to the centre in the medulla oblongata that decreases heart rate

3 The medulla oblongata decreases heart rate via the parasympathetic nervous system

Hormones, such as adrenaline and noradrenaline, can influence heart rate.

In times of stress, these hormones are released and cause the SAN in the heart to increase the heart rate as part of the 'fight or flight' response.