What I expect you to do:
Be able to explain how the characteristics of cardiac muscle make a coordinated heartbeat possible.
Be able to describe the important structures of the heart that are involved in regulating the heartbeat, and to locate them on a diagram.
Be able to describe the sequence and timing of events that occur in order as the heart beats, to identify why each event is important, and to explain how each event leads to the next.
Be able to explain how the body adjusts the heart rate to meet varying needs.
Be able to answer questions about the process confidently.
The regular rhythm of the heartbeat is maintained by a number of specialized tissues within the heart. Together, these tissues are known as the pacemaker.
Cardiac muscle is "self-excitable", meaning that the cells can contract without any external signal, and "autorhythmic", meaning that they will do so spontaneously at intervals. Cardiac muscle cells are also arranged in such a way that the excitation of one cell will tend to stimulate its neighbours to contract. The intercalated disks between adjacent muscle cells provide the close contact necessary for this propagation of a signal. This means that a small sample of cardiac muscle taken from a live vertebrate will continue to contract and relax independent of any outside signal, provided the correct environmental conditions are maintained. The contraction of isolated heart muscle tends to be slower than the normal heartbeat, however, and much more irregular. The regular, highly coordinated actions of the atria and ventricles during systole require much stricter control.
The regular resting beat of the heart is controlled by the sinoatrial (SA) node, which is found in the wall of the heart where the anterior vena cava meets the right atrium. Cells of the SA node (like all cardiac muscle) have electrical charges across their membranes. They are more negative inside than outside. The charge is created by active transport of sodium and potassium (as it is in nerve cells). If this charge is reduced below a threshold level, channels open that allow the charged particles to rush across the membrane, eliminating the charge. This is what causes the muscle fibrils to contract. Active transport is then used to restore the charge.
To see an animation of the wave of electrical activity as it travels through the heart muscle, click here
In cells of the SA node, the membranes are leaky, so that the charge will spontaneously drop below the threshold approximately 0.8 seconds after it is created. Thus, cells of the SA node discharge rhythmically about 70 times per minute.
When the cells of the SA node contract, they stimulate surrounding cardiac muscles cells to follow suit. A wave of contraction spreads over the surface of the entire atria, causing them to push blood into the ventricles.
The muscle cells of the ventricle are separated from those of the atria by fibrous ring that does not conduct the signal. Only a small knot of tissue in the atrial wall, the atrioventricular (AV) node has connections extending through this fibrous ring. The AV node gives rise to the Bundle of His, which is made up of highly specialised cells called Purkinje fibres. These extend all the way to the base of the ventricles.
When the AV node receives a signal from the contracting atrial muscles, it passes the signal to the Bundle of His, but not before slowing its transmission by about 50 milliseconds. This allows time for the atria to empty before the ventricles begin to contract. The Purkinje fibres then rapidly carry the signal to muscle fibres throughout the ventricles, starting from the bottom up. Ventricular muscles contract in a highly coordinated beat, creating high blood pressure and forcing blood out into the pulmonary artery and the aorta.
Both the AV node cells and the Purkinje fibres are also autorhythmic, and will contract spontaneously. Their rhythm, however, tends to be about half the rate of the SA node. Thus, under normal circmstances, the SA node sets the rate for the rest of the heart. In an emergency, such as damage to the SA node or failed transmission of the signal, the AV node and Purkinje fibres will act as an accessory pacemaker. This can sometimes maintain life in an emergency, such as a heart attack or disruption of the SA node by an electrical shock.
The SA node is also supplied with a rich nerve supply. Nerves from the sympathetic (stress response) system stimulate the SA node with epinephrine (adrenaline) and cause its frequency to increase. These are called cardioaccelerator nerves. Nerves from the vagus nerve of the parasympathetic system (relaxation response) cause the rhythm of the SA node to slow down. These antagonistic nerves allow your body to adjust your heart rate to meet the current demands for oxygen.