Cardiac Muscle Diagram

The cardiac muscle or the myocardium forms the musculature of the heart. These are striated and involuntary muscles that are supplied by autonomic nerve fibres. They form the middle layer of the heart wall and are composed of cardiac muscle fibres. The other two layers are the pericardium (outer layer) and the endocardium (inner layer). Let’s learn more about the cardiac muscle with the help of a diagram.

Diagram of Cardiac Muscle

Cardiac muscle diagram

Cardiac Muscle – Description

  • The myocardium or cardiac muscle forms the bulk portion of the heart and it is mainly responsible for pumping action. This muscle has several layers of cardiomyocytes or cardiac muscle cells.
  • The myocardium has 3 types of muscle fibres — muscle fibres that form the contractile unit, muscle fibres that form the pacemaker and muscle fibres that form the conductive system.
  • The contractile unit is made up of striated cardiac muscles that resemble skeletal muscles. These muscle fibres are bound by the sarcolemma. Also, it has a nucleus that is centrally placed. The myofibrils are placed in the sarcoplasm.
  • The sarcomere of the myocardium contains contractile proteins named – myosin, actin, tropomyosin and troponin.
  • The sarcotubular system in the myocardium or cardiac muscle is identical to that of the skeletal muscle. The main difference between cardiac and skeletal muscles is that the former is branched and the latter is not branched.
  • A tough double membranous structure called the intercalated disk is present between the branches of adjacent cardiac muscle fibres. It is formed by the fusion of the membrane of those branching cardiac muscles.
  • The intercalated disk forms an adherens junction that plays a vital role in muscle contraction.
  • There are also a few microscopic structures called the T-tubules that run from the cell surface to the deep interior part. They are seen continuous with the cell membrane and are found close to the sarcoplasmic reticulum.
  • The sarcoplasmic reticulum is a membrane-bound component found within the cardiac muscle cells. They act as a storehouse for calcium. Usually, a single T-tubule pairs with a part of the sarcoplasmic reticulum to form a diad.
  • The cytoplasmic continuity present between the neighbouring cells is called the syncytium. However, the cytoplasmic continuity is not seen in the cardiac muscle and the muscle fibres are separated by the cell membrane. Thus, it is more of a physiological syncytium.
  • At the sides, the membranes of neighbouring cardiac muscle fibres fuse to the gap junction. This gap junction facilitates a rapid action potential from one cardiac muscle fibre to another.
  • The cardiac muscle fibres that form the pacemaker have lesser striations. It is made of P cells or pacemaker cells. They help in generating the impulse for a heartbeat.
  • The conductive portion is made up of slightly modified cardiac muscle fibres that help in conducting impulses through various components. The AV node, SA node, bundle branches, bundle of His and Purkinje fibres are the main components of the cardiac conduction system.

Explore: Muscular Tissue

Summary of the Cardiac Muscle Description









Number of Nucleus


Myofibrils, Sarcomere, Cross-striations



Broad and short microstructures

Source of calcium

Sarcoplasmic reticulum

Sarcotubular system


Nerve supply

Autonomous nerves

Properties of the Cardiac Muscle

The four main properties of the cardiac muscle are –

  1. Excitability – It is the basic ability of a tissue (living) to respond to a particular stimulus.
  2. Rhythmicity – It is the ability of tissues to produce their own impulses periodically.
  3. Conductivity – It is the ability of tissues to conduct impulses to the neighbouring cell.
  4. Contractility – It is the ability of tissues to shorten in length when receiving a stimulus.

These properties of the cardiac muscle cells aid in the proper functioning of the heart.

Frequently Asked Questions


What is myocardium?

Myocardium or cardiac muscle is a type of vertebrate muscle tissue apart from smooth and skeletal muscle tissues. Myocardium forms the thick middle layer of the heart while the inner layer is called endocardium and the outer layer is called pericardium.

What are intercalated discs?

Intercalated discs are microscopic structures unique to the cardiac muscle. The cardiomyocytes are interconnected by the intercalated discs to form a single functional cardiac syncytium and aid in the rapid transmission of electrical impulses.

What are T-tubules?

Transverse tubules or T-tubules are microscopic extensions of the sarcolemma. They penetrate the centre of cardiac muscle cells and lie close to the sarcoplasmic reticulum. They rapidly transmit electric impulses called action potentials from the surface of the cell to its core.

Also Read: Difference between Cardiac Muscle and Skeletal Muscle

Keep exploring BYJU’S Biology for more such exciting diagram topics