T-tubule
Encyclopedia
A T-tubule is a deep invagination
of the sarcolemma
, which is the plasma membrane, only found in skeletal
and cardiac muscle
cells. These invaginations allow depolarization
of the membrane to quickly penetrate to the interior of the cell.
(the muscle fiber's plasma membrane) which invaginates perpendicular to the length of the fiber to form what is called a T-tubule. At these invaginations the sarcolemma is studded with a large number of L-type calcium channels.
In skeletal muscle cells specifically, T-tubule invaginations are typically located at the junction overlap between the A and I bands of the sarcomere
, and together with a pair of terminal cisternae
(bulbous enlarged areas of the sarcoplasmic reticulum) it forms an arrangement called a triad
.
Cardiac muscle has a similar structure; instead of a triad there is a diad
, which is composed of a T-tubule and a single terminal cisterna, and it occurs at the Z line.
It is physiologically important for excitation-contraction coupling (see section below) that the T-tubules are positioned close to the terminal cisternae
of the sarcoplasmic reticulum as the triad
or diad
arrangement allows physical and functional contact by voltage dependent L-type calcium channels. So, an action potential
along the sarcolemma causes calcium channels to open in the terminal cisternae
/sarcoplasmic reticulum which enables calcium to move from the sarcoplasmic reticulum into cytoplasm and the intracellular calcium concentration to increase.
T-tubules are the major sites for the coupling of excitation and contraction
, which is the process whereby the spreading depolarization is converted into force production by muscle fibers. The L-type calcium channels in T-tubules activate in response to electrical stimulation; their opening allows calcium
to flow down its electrochemical gradient
and into the cell. Activation of the L-type channel also causes a mechanical interaction between it and calcium-release channels
located on the adjacent sarcoplasmic reticulum membrane.
In skeletal muscle
, the influx of calcium through the L-type calcium channel on the T-tubule contributes little to excitation-contraction coupling, whereas it is crucial to the proper function of cardiac muscle
(see Cardiac action potential
). Conversely, the mechanical interaction between the T-tubule's L-type calcium channel and the calcium-release channel is critical to proper skeletal muscle contraction
, whereas it contributes little to the contraction of cardiac muscle.
(for skeletal muscle
) or formamide
(mainly for cardiac muscle
). Addition of these chemicals to the solution surrounding muscle cells causes the cells to shrink; when the chemical is withdrawn the cells rapidly expand before returning to their normal size. The rapid expansion is thought to cause the t-tubules to detach from the surface membrane, which reseals, and to reseal within the cell. This technique can be used to investigate the function of the t-tubules.
There is some evidence that heart failure precipitates the loss of the T-tubule network, again indicating their importance.
Invagination
Invagination means to fold inward or to sheath. In biology, this can refer to a number of processes.* Invagination is the morphogenetic processes by which an embryo takes form, and is the initial step of gastrulation, the massive reorganization of the embryo from a simple spherical ball of cells,...
of the sarcolemma
Sarcolemma
The sarcolemma is the cell membrane of a muscle cell . It consists of a true cell membrane, called the plasma membrane, and an outer coat made up of a thin layer of polysaccharide material that contains numerous thin collagen fibrils...
, which is the plasma membrane, only found in skeletal
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...
and 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...
cells. These invaginations allow 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...
of the membrane to quickly penetrate to the interior of the cell.
Structure
Each muscle fiber is surrounded by a sarcolemmaSarcolemma
The sarcolemma is the cell membrane of a muscle cell . It consists of a true cell membrane, called the plasma membrane, and an outer coat made up of a thin layer of polysaccharide material that contains numerous thin collagen fibrils...
(the muscle fiber's plasma membrane) which invaginates perpendicular to the length of the fiber to form what is called a T-tubule. At these invaginations the sarcolemma is studded with a large number of L-type calcium channels.
In skeletal muscle cells specifically, T-tubule invaginations are typically located at the junction overlap between the A and I 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...
, and together with a pair of terminal cisternae
Terminal cisternae
Terminal cisternae are enlarged areas of the sarcoplasmic reticulum surrounding the transverse tubules. These discrete regions within the muscle cell store calcium and release it when an action potential courses down the transverse tubules, eliciting muscle contraction...
(bulbous enlarged areas of the sarcoplasmic reticulum) it forms an arrangement called a triad
Triad (anatomy)
In the histology of skeletal muscle, a triad is the structure formed by a T tubule with a sarcoplasmic reticulum 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...
.
Cardiac muscle has a similar structure; instead of a triad there is a diad
Diad
The diad is a structure in the cardiac myocyte located at the sarcomere Z-line. It is composed of a single t-tubule paired with a terminal cisterna of the sarcoplasmic reticulum. The diad plays an important role in excitation-contraction coupling by juxtaposing an inlet for the action potential...
, which is composed of a T-tubule and a single terminal cisterna, and it occurs at the Z line.
It is physiologically important for excitation-contraction coupling (see section below) that the T-tubules are positioned close to the terminal cisternae
Terminal cisternae
Terminal cisternae are enlarged areas of the sarcoplasmic reticulum surrounding the transverse tubules. These discrete regions within the muscle cell store calcium and release it when an action potential courses down the transverse tubules, eliciting muscle contraction...
of the sarcoplasmic reticulum as the triad
Triad (anatomy)
In the histology of skeletal muscle, a triad is the structure formed by a T tubule with a sarcoplasmic reticulum 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...
or diad
Diad
The diad is a structure in the cardiac myocyte located at the sarcomere Z-line. It is composed of a single t-tubule paired with a terminal cisterna of the sarcoplasmic reticulum. The diad plays an important role in excitation-contraction coupling by juxtaposing an inlet for the action potential...
arrangement allows physical and functional contact by voltage dependent L-type calcium channels. So, an action potential
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...
along the sarcolemma causes calcium channels to open in the terminal cisternae
Terminal cisternae
Terminal cisternae are enlarged areas of the sarcoplasmic reticulum surrounding the transverse tubules. These discrete regions within the muscle cell store calcium and release it when an action potential courses down the transverse tubules, eliciting muscle contraction...
/sarcoplasmic reticulum which enables calcium to move from the sarcoplasmic reticulum into cytoplasm and the intracellular calcium concentration to increase.
Excitation-contraction coupling
See Excitation-contraction couplingExcitation-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...
T-tubules are the major sites for the coupling of excitation and 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...
, which is the process whereby the spreading depolarization is converted into force production by muscle fibers. The L-type calcium channels in T-tubules activate in response to electrical stimulation; their opening allows 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...
to flow down its electrochemical gradient
Electrochemical gradient
An electrochemical gradient is a spatial variation of both electrical potential and chemical concentration across a membrane; that is, a combination of the membrane potential and the pH gradient...
and into the cell. Activation of the L-type channel also causes a mechanical interaction between it and calcium-release channels
Ryanodine receptor
Ryanodine receptors form a class of intracellular calcium channels in various forms of excitable animal tissue like muscles and neurons...
located on the adjacent sarcoplasmic reticulum membrane.
In 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...
, the influx of calcium through the L-type calcium channel on the T-tubule contributes little to excitation-contraction coupling, whereas it is crucial to the proper function of 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...
(see Cardiac action potential
Cardiac action potential
In electrocardiography, the cardiac action potential is a specialized action potential in the heart, necessary for the electrical conduction system of the heart....
). Conversely, the mechanical interaction between the T-tubule's L-type calcium channel and the calcium-release channel is critical to proper skeletal 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...
, whereas it contributes little to the contraction of cardiac muscle.
Detubulation
It is possible to physically and functionally uncouple T-tubules from the surface membrane using a technique known as detubulation. This relies on osmotically active chemicals, such as glycerolGlycerol
Glycerol is a simple polyol compound. It is a colorless, odorless, viscous liquid that is widely used in pharmaceutical formulations. Glycerol has three hydroxyl groups that are responsible for its solubility in water and its hygroscopic nature. The glycerol backbone is central to all lipids...
(for 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...
) or formamide
Formamide
Formamide, also known as methanamide, is an amide derived from formic acid. It is a clear liquid which is miscible with water and has an ammonia-like odor. It is used primarily for manufacturing sulfa drugs and synthesizing vitamins and as a softener for paper and fiber...
(mainly for 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...
). Addition of these chemicals to the solution surrounding muscle cells causes the cells to shrink; when the chemical is withdrawn the cells rapidly expand before returning to their normal size. The rapid expansion is thought to cause the t-tubules to detach from the surface membrane, which reseals, and to reseal within the cell. This technique can be used to investigate the function of the t-tubules.
There is some evidence that heart failure precipitates the loss of the T-tubule network, again indicating their importance.