Cardiac muscle

Cardiac muscle, also called heart muscle or myocardium, is one of three types of vertebrate muscle tissues, along with skeletal muscle and smooth muscle. It is a special kind of involuntary, striated muscle that makes up the main part of the wall of the heart. This muscle forms a thick middle layer between the outer layer of the heart wall, called the pericardium, and the inner layer, called the endocardium. It gets its blood supply through the coronary circulation.

Cardiac muscle is made up of individual cardiac muscle cells connected by special structures called intercalated discs and supported by collagen fibers and other substances known as the extracellular matrix. Like skeletal muscle, cardiac muscle contracts, but it works in a slightly different way. When an electrical signal, called a cardiac action potential, reaches the muscle cell, it causes calcium to be released from a storage area inside the cell, known as the sarcoplasmic reticulum. This increase in calcium helps the cell's tiny thread-like structures, called myofilaments, to slide past each other in a process known as excitation-contraction coupling.

Diseases that affect heart muscle, known as cardiomyopathies, are very important to understand because they can affect how well the heart works. Some of these diseases, like angina and myocardial infarction, are caused when the blood supply to the heart muscle is not enough, a condition known as ischemic. Keeping heart muscle healthy is essential for overall well-being.

Structure

Further information: Heart § Structure

Cardiac muscle, also known as myocardium, makes up the bulk of the heart. The heart wall has three layers: the inner endocardium, the middle layer of cardiac muscle, and the outer epicardium. The endocardium lines the heart’s chambers and covers the heart valves, while the epicardium forms part of a protective sac around the heart.

Inside the cardiac muscle, special cells called cardiomyocytes work together. They contract in a coordinated way, allowing the heart to pump blood effectively. The heart muscle needs a lot of energy, so it has its own blood supply through coronary arteries that bring oxygen and nutrients, and coronary veins that carry waste away.

Cardiac muscle cells are connected by structures called intercalated discs, which help them work together. These cells have a striped appearance under a microscope and contain many energy-producing structures called mitochondria. This helps the heart muscle stay strong and keep beating without tiring.

Development

Humans are born with a certain number of heart muscle cells, called cardiomyocytes. These cells grow larger as the heart develops during childhood. While some new cells form over time, most of the cells we are born with stay with us throughout life.

These heart cells can also grow in response to factors like exercise, heart disease, or injury. They change shape in different ways depending on what the heart needs — either stretching longer or becoming thicker, which helps the heart adapt and stay strong.

Physiology

Main article: Cardiac excitation-contraction coupling

Further information: Autorhythmicity and Myocardial contractility

Cardiac muscle works in a similar way to skeletal muscle, which you might be more familiar with. Both types of muscle contract, or tighten, to do their jobs. In both cases, this contraction starts with tiny particles called ions moving across the cell's outer layer, or cell membrane. This movement creates an electrical signal that tells the muscle to contract.

The heart has a special pattern of beating called the cardiac cycle. It has two main parts: when the heart relaxes and fills with blood, called diastole, and when it contracts strongly to pump blood out, called systole. After pumping, the heart relaxes again to get ready for the next beat. This cycle happens over and over, keeping blood flowing throughout the body.

Clinical significance

Further information: Cardiomyopathy

Diseases that affect the heart muscle, called cardiomyopathies, are a major cause of illness in many countries. One common problem is coronary artery disease, where the blood supply to the heart becomes reduced because the coronary arteries narrow. This can cause chest pain during activity, known as angina pectoris. If an artery becomes blocked, it can lead to a myocardial infarction, or heart attack, which can damage the heart muscle.

Heart muscle can also be harmed by infections, certain medicines, high blood pressure, or other health issues. When the heart cannot pump enough blood, it is called heart failure. These conditions can weaken the heart and affect its ability to work properly.