Sarcoplasmic reticulum

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.

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

- Sarcoplasmic reticulum

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Muscle cell

Also known as a myocyte when referring to either a cardiac muscle cell , or a smooth muscle cell as these are both small cells.

General structure of a skeletal muscle cell and neuromuscular junction: 1. Axon

2. Neuromuscular junction

3. Skeletal muscle fiber

4. Myofibril
Diagram of skeletal muscle fiber structure

The cytoplasm in a muscle cell is termed the sarcoplasm; the smooth endoplasmic reticulum of a muscle cell is termed the sarcoplasmic reticulum; and the cell membrane in a muscle cell is termed the sarcolemma.

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)

Electrical stimulation in the form of a cardiac action potential triggers the release of calcium from the cell's internal calcium store, the sarcoplasmic reticulum.


Cell membrane of a muscle cell.

Skeletal muscle fiber, with sarcolemma labeled at upper left.

On either side of the transverse tubules are terminal cisternal enlargements of the sarcoplasmic reticulum (termed endoplasmic reticulum in nonmuscle cells).


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

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

Through these mechanisms, T-tubules allow heart muscle cells to contract more forcefully by synchronising calcium release from the sarcoplasmic reticulum throughout the cell.

Rigor mortis

Third stage of death.

Corpses of victims of the 1991 Bangladesh cyclone in Sandwip displaying signs of rigor mortis
Timeline of postmortem changes (stages of death).

Calcium is released into the cytosol due to the deterioration of the sarcoplasmic reticulum.

Calcium ATPase

Plasma membrane Ca2+ ATPase (PMCA)

Rendered image of the Ca2+ pump

Sarcoplasmic reticulum Ca2+ ATPase (SERCA)

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.

While the muscle fiber does not have smooth endoplasmic cisternae, it contains sarcoplasmic reticulum.

Ryanodine receptor

Ryanodine receptors (RyR for short) form a class of intracellular calcium channels in various forms of excitable animal tissue like muscles and neurons.


Ryanodine receptors mediate the release of calcium ions from the sarcoplasmic reticulum and endoplasmic reticulum, an essential step in muscle contraction.

Beta-1 adrenergic receptor

Beta-adrenergic receptor, and also denotes the human gene encoding it.

β2 adrenoceptor shown binding carazolol (yellow) on its extracellular site. β2 stimulates cells to increase energy production and utilization. The membrane the receptor is bound to in cells is shown with a gray stripe.

Phospholamban's phosphorylation deactivates its function which is normally inhibition of SERCA on the sarcoplasmic reticulum (SR) in cardiac myocytes.


Micropeptide protein that in humans is encoded by the PLN gene.

Micropeptides can be transcribed from 5'UTRs, small genes, polycistronic mRNAs, or mis-annotated lncRNA.

In the unphosphorylated state, phospholamban is an inhibitor of cardiac muscle sarcoplasmic reticulum Ca2+-ATPase (SERCA2) which transports calcium from cytosol into the sarcoplasmic reticulum.