The diad is a structure in the cardiac myocyte

Myocyte

A myocyte is the type of cell found in muscles. They arise from myoblasts.Each myocyte contains myofibrils, which are long, long chains of sarcomeres, the contractile units of the cell....

 located at the sarcomere

Sarcomere

A sarcomere is the basic unit of a muscle. Muscles are composed of tubular muscle cells . Muscle cells are composed of tubular myofibrils. Myofibrils are composed of repeating sections of sarcomeres, which appear under the microscope as dark and light bands...

 Z-line

Z-line

Z-line can refer to:*the borders that separate and link sarcomeres within a muscle*a ban on a user's IP address for the purposes of blocking their access to Internet Relay Chat. See Z-line....

. It is composed of a single t-tubule

T-tubule

A T-tubule is a deep invagination of the sarcolemma, which is the plasma membrane, only found in skeletal and cardiac muscle cells...

 paired with a terminal cisterna

Cisterna

A cisterna comprises a flattened membrane disk that makes up the Golgi apparatus. A typical Golgi has anywhere from 3 to 7 cisternae stacked upon each other like a stack of dinner plates, but there are usually around 6...

 of the sarcoplasmic reticulum. The diad plays an important role in excitation-contraction coupling

Excitation-contraction coupling

Excitation-contraction coupling is a term coined in 1952 to describe the physiological process of converting an electrical stimulus to a mechanical response . This process is fundamental to muscle physiology, whereby the electrical stimulus is usually an action potential and the mechanical...

 by juxtaposing an inlet for the action potential near a source of Ca²⁺ ions. This way, the wave of depolarization

Depolarization

In biology, depolarization is a change in a cell's membrane potential, making it more positive, or less negative. In neurons and some other cells, a large enough depolarization may result in an action potential...

 can be coupled to calcium-mediated cardiac muscle

Cardiac muscle

Cardiac muscle is a type of involuntary striated muscle found in the walls and histologic foundation of the heart, specifically the myocardium. Cardiac muscle is one of three major types of muscle, the others being skeletal and smooth muscle...

 contraction

Muscle contraction

Muscle fiber generates tension through the action of actin and myosin cross-bridge cycling. While under tension, the muscle may lengthen, shorten, or remain the same...

via the sliding filament mechanism.