The heart pumps blood toward the body by performing a series of alternate contraction and relaxation movements. In order for these contraction and relaxation movements to occur, electrical impulses must be transmitted through the heart muscle.
Within the heart is present an electrical system that controls how and when the heart muscle contracts and relaxes. This electrical system is often assessed by a simple test called an electrocardiogram are also called ECG.
In this post, we take a brief look at the electrical system of the heart and discuss how electricity in the heart is transmitted.
Parts of the electrical system
The electrical system of the heart has got three main parts to it.
1. The Sino-atrial node
2. The atrioventricular node
3. The His-Purkinje system
The Sino-Atrial Node (SAN)
The sino-atrial node is located at the top of the right atrium and is responsible for generating electricity in the heart. In other words, the sino-atrial node behaves like a generator and is considered the natural pacemaker of the heart. Activity in the sino-atrial node is triggered by blood entering the right atrium from the superior and inferior vena cava.
If the sino atrial node is generating electrical impulses at the rate of 75 impulses per minute, then the heart rate will be 75 beats per minute (bpm). This will be experienced as the heartbeat or pulse. Normally, the sino-atrial node generates electrical impulses at the rate of 60 – 100 bpm
The Atrio-Ventricular Node (AVN)
Once electrical activity is generated in the sino atrial node, it is transmitted through a series of small ‘cables’ (called internodal tracts) to the centre of the heart all the way to the atrioventricular node. The atrioventricular node is located in the muscular inter-atrial septum (the part of the heart muscle that separates a right and the left atrium) at close proximity to the tricuspid valve. The atrioventricular node acts like a electrical transmitter.
Once the electrical impulses reached the atrioventricular node, they remain there for a fraction of a second before they are transmitted further down the heart. During this period, the heart muscle fills up with blood in preparation for pumping action.
The His-Purkinje System
From the atrioventricular node, the electrical impulses are transmitted to a small bundle of nerve fibers known as the His bundle. The His bundle then divides into a series of fibers that travel through the muscle and supply the left and the right ventricle. These fibers are known as the Purkinje fibers. This entire system is called the His-Purkinje system.
The impulses reached the left ventricle through a bundle known as the left bundle while they reach the right ventricle through the right bundle. Contraction of the left ventricle occurs a few milliseconds before the contraction of the right ventricle.
Once the electrical signal has passed through the heart muscle, the muscle relaxes. The sino-atrial node then generates the next electrical impulse. In this way, electrical activity of the heart can result in contraction and relaxation of the heart muscle.
The electrical activity of the heart can be studied to a test called as the electrocardiogram. If you wish to read more about this, click here.