Control of the heart rate

The heart is able to contract without any external stimulation, a condition refered to as myogenic
The heart rate is normally determined by pacemaker cells in the SAN. The pacemaker initiates a heart beat by passing waves of electrical stimulation first of the atria and then, via the AVN, over the ventricles. A heart with no nervous or hormonal stimulation contracts at a rate of around 100 beats per minute. However, nervous stimulation coordinated by the CV centre in the medulla oblongata overrides this inherent rhythm
Nervous control of the heart beat is mainly by reflex actions which in volve reflex arcs orginiating in sensory receptors
Baroreceptors; are loacted in the carotid sinus and the walls of the heart
Chemoreceptors; sensitive to changes in pH and the concentration of blood carbon dioxide and blood oxygen are located in the walls of the heart, the carotid and aortic arteries, and the medulla oblongata
These recoetors pass their sensory information via sensory neurones to the CV centre from which two nerves, the vagus nerve and the sympathetic nerve, lead to the pacemaker in the SAN. These nerves are part of the autonomic nervous system, the part of the pheripheral nervous system that regualtes body activities that are usually involuntary and performed unconciously. A persons actual heart rate depends in part of the relative activity of these two nerves which act antagonistically;
1. The vagus nerve; runs directly from the CV centre to the pacemaker region in the SAN. It has an inhibitory effect - stimulation causes the heart to slow down
2. The sympathetic nerve goes to the SAN via the spinal cord. It is on excitatory nerve - stimulation accelerates the hearts rate

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Control of the heart rate

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