Neurotransmitters & Control of Heart Function

Cardiac output is the rate at which the heart pumps blood. Cardiac output is modified to meet the changing physiological needs of the body.

Cardiac output = stroke volume x heart rate

The most important control on heart rate is by the autonomic nervous system and negative feedback control mechanisms. The heart is supplied by both sympathetic and parasympathetic nerves.

There are two control centres for regulating the heart and both are found in the medulla oblongata in the brain stem.

The cardioacceleratory centre (CAC) is responsible for increasing cardiac output whilst the cardioinhibitory centre (CIC) reduces cardiac output.

From the cardioacceleratory centre sympathetic fibres pass down the spinal cord and leave via the sympathetic accelerator nerves to the sino-atrial node, (also known as the sinus node) atrio-ventricular node and cardiac muscle.

When this centre is stimulated impulses are transmitted through the sympathetic nerves to the heart which causes an increase in heart rate and force of contractions.

Parasympathetic nerve fibres originate in the cardioinhibitory centre and reach the sino-atrial node and atrio-ventricular node via the vagus nerve. Parasympathetic stimulation causes a decrease in heart rate.

At rest the usual heart rate is about 60-80 beats per minute. This is less than the intrinsic rate of discharge of the cardiac pacemaker, the sino-atrial node. Therefore in the resting state the parasympathetic influence is dominant and this is known as the vagal brake.

 

Brain showing cardioacceleratory centre (CAC) and cardioinhibitory centre (CIC) in the medulla and parasympathetic and sympathetic fibres reaching the heart:

Neurotransmitters involved at the neuromuscular junctions:

Nerve fibres
Neurotransmitter
Sympathetic nerve fibres
Nor-adrenaline
Parasympathetic nerve fibres
Acetylcholine