Cardiac excitation and conduction

Cardiac muscle can initiate its own electrical activity without stimulation by a nerve. It is therefore described as myogenic. This electrical activity, or excitation, spreads through the heart in a coordinated sequence so that the atria contract before the ventricles.

The sequence begins at the sinoatrial node (SAN), which acts as the heart’s pacemaker. The SAN generates a wave of excitation that spreads across the atria, causing the atrial muscle to contract.

Conduction from the atria to the ventricles

A layer of non-conducting tissue between the atria and ventricles prevents the excitation from passing directly to the ventricular muscle. Instead, the excitation reaches the atrioventricular node (AVN). The AVN delays its transmission, allowing atrial contraction to finish before ventricular contraction begins.

From the AVN, the excitation travels down the bundle of His, a pathway of Purkyne tissue, towards the apex at the bottom of the heart. It then spreads through Purkyne tissue in the ventricular walls. This causes the ventricles to contract from the apex upwards, helping to force blood out of the heart.

Autonomic control of heart rate

Heart rate changes in response to information about the blood. Receptors detect changes and send signals to the cardiovascular centre in the medulla oblongata, a region of the brain. The cardiovascular centre then sends impulses along autonomic nerves to the heart, adjusting how quickly it beats. The heart is the effector because it carries out the response.

Responses to blood chemistry

Chemoreceptors in the carotid arteries and aorta detect changes in blood chemistry. For example, a rise in carbon dioxide is associated with a fall in blood pH. The receptors signal this change to the cardiovascular centre, which can increase heart rate. Faster circulation helps transport carbon dioxide to the lungs.

Responses to blood pressure

Pressure receptors in the walls of the carotid arteries and aorta detect changes in blood pressure. Their signals to the cardiovascular centre change with the pressure:

• When blood pressure rises, the receptors send more signals. This can lead to a decrease in heart rate, helping to oppose the rise in pressure.
• When blood pressure falls, the receptors send fewer signals. This can lead to an increase in heart rate, helping to oppose the fall in pressure.

These are feedback responses: the change in heart rate helps counter the original change in blood pressure.

Autonomic nerves and the sinoatrial node

The cardiovascular centre adjusts heart rate through the autonomic nervous system. Its nerves affect the sinoatrial node, the part of the heart that generates the excitation initiating each heartbeat.

Sympathetic stimulation increases the rate at which the sinoatrial node generates excitation, so heart rate increases. Parasympathetic stimulation decreases the rate of excitation, so heart rate falls.