Skeletal muscle fiber, with T-tubule labelled in zoomed in image.

T-tubules (transverse tubules) are extensions of the cell membrane that penetrate into the centre of skeletal and cardiac muscle cells.

- T-tubule

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Sarcoplasmic reticulum

Membrane-bound structure found within muscle cells that is similar to the smooth endoplasmic reticulum in other cells.

A cartoon section of skeletal muscle, showing T-tubules running deep into the centre of the cell between two terminal cisternae/junctional SR. The thinner projections, running horizontally between two terminal cisternae are the longitudinal sections of the SR.

Cardiac and skeletal muscle cells contain structures called transverse tubules (T-tubules), which are extensions of the cell membrane that travel into the centre of the cell.


Cell membrane of a muscle cell.

Skeletal muscle fiber, with sarcolemma labeled at upper left.

A special feature of the sarcolemma is that it invaginates into the sarcoplasm of the muscle cell, forming membranous tubules radially and longitudinally within the fiber called T-tubules or transverse tubules.

Cardiac muscle

One of three types of vertebrate muscle tissue, with the other two being skeletal muscle and smooth muscle.

3D rendering showing thick myocardium within the heart wall.
The swirling musculature of the heart ensures effective pumping of blood.
Cardiac muscle
Illustration of a cardiac muscle cell.
Intercalated discs are part of the cardiac muscle cell sarcolemma and they contain gap junctions and desmosomes.
Dog cardiac muscle (400X)

Cardiomyocytes contain T-tubules, pouches of cell membrane that run from the cell surface to the cell's interior which help to improve the efficiency of contraction.

L-type calcium channel

Part of the high-voltage activated family of voltage-dependent calcium channel.

Immunohistochemical analysis of L-type calcium channel Cav1.3 (CACNA1D) in human adrenal cortex. Marked immunoreactivity was detected in the zona glomerulosa. In the figure: ZG = zona glomerulosa, ZF = zona fasciculata, AC = adrenal capsule. Immunohistochemistry was performed according to published methods.
An L-type calcium channel with its subunits labeled along with some drugs known to inhibit the channel.
Alpha subunit of a generic voltage-gated ion channel

In skeletal muscle, there is a very high concentration of L-type calcium channels, situated in the T-tubules.


Smallest functional unit of striated muscle tissue.

Image of sarcomere
Muscle contraction based on sliding filament theory

The action potential then travels along T-tubules (transverse tubules) until it reaches the sarcoplasmic reticulum.

Muscle contraction

Activation of tension-generating sites within muscle cells.

Types of muscle contractions
In vertebrate animals, there are three types of muscle tissues: 1) skeletal, 2) smooth, and 3) cardiac
Organization of skeletal muscle
Structure of neuromuscular junction.
Sliding filament theory: A sarcomere in relaxed (above) and contracted (below) positions
Cross-bridge cycle
Muscle length versus isometric force
Force–velocity relationship: right of the vertical axis concentric contractions (the muscle is shortening), left of the axis eccentric contractions (the muscle is lengthened under load); power developed by the muscle in red. Since power is equal to force times velocity, the muscle generates no power at either isometric force (due to zero velocity) or maximal velocity (due to zero force). The optimal shortening velocity for power generation is approximately one-third of maximum shortening velocity.
Swellings called varicosities belonging to an autonomic neuron innervate the smooth muscle cells.
Cardiac muscle
Key proteins involved in cardiac calcium cycling and excitation-contraction coupling
A simplified image showing earthworm movement via peristalsis
Asynchronous muscles power flight in most insect species. a: Wings b: Wing joint c: Dorsoventral muscles power the upstroke d: Dorsolongitudinal muscles (DLM) power the downstroke. The DLMs are oriented out of the page.
Electrodes touch a frog, and the legs twitch into the upward position

DHPRs are located on the sarcolemma (which includes the surface sarcolemma and the transverse tubules), while the RyRs reside across the SR membrane.

Skeletal muscle

Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system that are mostly attached by tendons to bones of the skeleton.

A top-down view of skeletal muscle
3D rendering of a skeletal muscle fiber
Muscle types by fiber arrangement
Types of pennate muscle. A – unipennate; B – bipennate; 
C – multipennate
ATPase staining of a muscle cross section. Type II fibers are dark, due to the alkaline pH of the preparation. In this example, the size of the type II fibers is considerably less than the type I fibers due to denervation atrophy.
Structure of muscle fibre showing a sarcomere under electron microscope with schematic explanation.
Diagram of sarcoplasmic reticulum with terminal cisternae and T-tubules.
Human embryo showing somites labelled as primitive segments.
When a sarcomere contracts, the Z lines move closer together, and the I band becomes smaller. The A band stays the same width. At full contraction, the thin and thick filaments overlap.
Contraction in more detail
(a) Some ATP is stored in a resting muscle. As contraction starts, it is used up in seconds. More ATP is generated from creatine phosphate for about 15 seconds. (b) Each glucose molecule produces two ATP and two molecules of pyruvic acid, which can be used in aerobic respiration or converted to lactic acid. If oxygen is not available, pyruvic acid is converted to lactic acid, which may contribute to muscle fatigue. This occurs during strenuous exercise when high amounts of energy are needed but oxygen cannot be sufficiently delivered to muscle. (c) Aerobic respiration is the breakdown of glucose in the presence of oxygen (O2) to produce carbon dioxide, water, and ATP. Approximately 95 percent of the ATP required for resting or moderately active muscles is provided by aerobic respiration, which takes place in mitochondria.
Exercise-induced signaling pathways in skeletal muscle that determine specialized characteristics of slow- and fast-twitch muscle fibers
Jogging is one form of aerobic exercise.
In muscular dystrophy, the affected tissues become disorganized and the concentration of dystrophin (green) is greatly reduced.
Prisoner of war exhibiting muscle loss as a result of malnutrition.

T tubules are the pathways for action potentials to signal the sarcoplasmic reticulum to release calcium, causing a muscle contraction.


Myofilaments are the three protein filaments of myofibrils in muscle cells.

Muscle fiber showing thick and thin myofilaments of a myofibril.

This depolarizes the muscle fiber membrane, and the impulse travels to the muscle's sarcoplasmic reticulum via the transverse tubules.

Triad (anatomy)

Skeletal muscle, showing Triad as well as T-tubule.

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.


Protein that in humans is encoded by the TCAP gene.

A representation of the 3D structure of the protein myoglobin showing turquoise α-helices. This protein was the first to have its structure solved by X-ray crystallography. Toward the right-center among the coils, a prosthetic group called a heme group (shown in gray) with a bound oxygen molecule (red).

Telethonin is expressed in cardiac and skeletal muscle at Z-discs and functions to regulate sarcomere assembly, T-tubule function and apoptosis.