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.

Diseases that affect heart muscle, known as cardiomyopathies, can affect how well the heart works. Keeping heart muscle healthy is essential for overall well-being.

Structure

Further information: Heart § Structure

Cardiac muscle, also called myocardium, is the main part 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. The epicardium helps protect the heart.

Inside the cardiac muscle, special cells called cardiomyocytes work together. They contract in a coordinated way, letting the heart pump blood well. The heart muscle needs a lot of energy. It gets this energy from 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. These help the cells work together. The cells have a striped look under a microscope and contain many energy-making structures called mitochondria. This helps the heart stay strong and keep beating.

Development

Humans are born with a certain number of heart muscle cells, called cardiomyocytes. These cells get bigger as the heart grows during childhood. Some new cells form over time, but most of the cells we are born with stay with us our whole lives.

These heart cells can also grow in response to things like exercise, heart disease, or injury. They change shape in different ways to help the heart stay strong.

Physiology

Main article: Cardiac excitation-contraction coupling

Further information: Autorhythmicity and Myocardial contractility

Cardiac muscle works like skeletal muscle, which you may already know about. Both types of muscle tighten to do their jobs. In both cases, this tightening 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 tighten.

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 repeats, keeping blood flowing throughout the body.

Clinical significance

Further information: Cardiomyopathy

Diseases that affect the heart muscle, called cardiomyopathies, can make many people very sick. One common problem is coronary artery disease, where the blood supply to the heart gets smaller because the coronary arteries become 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, some 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 make it harder for the heart to work properly.