Triad (anatomy)
Encyclopedia
In the histology
of skeletal muscle
, a triad is the structure formed by a T tubule with a sarcoplasmic reticulum (SR) known as the terminal cisterna on either side. Each skeletal muscle fiber has many thousands of triads, visible in muscle fibers that have been sectioned longitudinally. (This property holds because T tubules run perpendicular to the longitudinal axis of the muscle fiber.) In mammals, triads are typically located at the A-I junction; that is, the junction between the A
and I bands
of the sarcomere
, which is the smallest unit of a muscle fiber.
Triads form the anatomical basis of excitation-contraction coupling
, whereby a stimulus excites the muscle and causes it to contract. A stimulus, in the form of positively charged current, is transmitted from the neuromuscular junction
down the length of the T tubules, activating dihydropyridine receptors (DHPRs). Their activation causes 1) a negligible influx of calcium
and 2) a mechanical interaction with calcium-conducting ryanodine receptor
s (RyRs) on the adjacent SR membrane. Activation of RyRs causes the release of calcium from the SR, which subsequently initiates a cascade of events leading to muscle contraction
. These muscle contractions are caused by calcium's bonding to troponin and unmasking the binding sites covered by the troponin-tropomyosin complex on the actin myofilament and allowing the myosin cross-bridges to connect with the actin.
Histology
Histology is the study of the microscopic anatomy of cells and tissues of plants and animals. It is performed by examining cells and tissues commonly by sectioning and staining; followed by examination under a light microscope or electron microscope...
of skeletal muscle
Skeletal muscle
Skeletal muscle is a form of striated muscle tissue existing under control of the somatic nervous system- i.e. it is voluntarily controlled. It is one of three major muscle types, the others being cardiac and smooth muscle...
, a triad is the structure formed by a T tubule with a sarcoplasmic reticulum (SR) known as the terminal cisterna on either side. Each skeletal muscle fiber has many thousands of triads, visible in muscle fibers that have been sectioned longitudinally. (This property holds because T tubules run perpendicular to the longitudinal axis of the muscle fiber.) In mammals, triads are typically located at the A-I junction; that is, the junction between the A
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...
and I bands
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...
of 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...
, which is the smallest unit of a muscle fiber.
Triads form the anatomical basis of 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...
, whereby a stimulus excites the muscle and causes it to contract. A stimulus, in the form of positively charged current, is transmitted from the neuromuscular junction
Neuromuscular junction
A neuromuscular junction is the synapse or junction of the axon terminal of a motor neuron with the motor end plate, the highly-excitable region of muscle fiber plasma membrane responsible for initiation of action potentials across the muscle's surface, ultimately causing the muscle to contract...
down the length of the T tubules, activating dihydropyridine receptors (DHPRs). Their activation causes 1) a negligible influx of calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
and 2) a mechanical interaction with calcium-conducting ryanodine receptor
Ryanodine receptor
Ryanodine receptors form a class of intracellular calcium channels in various forms of excitable animal tissue like muscles and neurons...
s (RyRs) on the adjacent SR membrane. Activation of RyRs causes the release of calcium from the SR, which subsequently initiates a cascade of events leading to 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...
. These muscle contractions are caused by calcium's bonding to troponin and unmasking the binding sites covered by the troponin-tropomyosin complex on the actin myofilament and allowing the myosin cross-bridges to connect with the actin.