Skeletal muscle fiber, with sarcolemma labeled at upper left.

Cell membrane of a muscle cell.

- Sarcolemma

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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.

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.


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.


Cytoplasm of a muscle cell.

Sarcoplasm shown with a muscle fiber

It has a Golgi apparatus near the nucleus, mitochondria just inside the cell membrane (sarcolemma), and a smooth endoplasmic reticulum (specialized for muscle function and called the sarcoplasmic reticulum).

Action potential

Action potential occurs when the membrane potential of a specific cell location rapidly rises and falls.

As an action potential (nerve impulse) travels down an axon there is a change in electric polarity across the membrane of the axon. In response to a signal from another neuron, sodium- (Na+) and potassium- (K+) gated ion channels open and close as the membrane reaches its threshold potential. Na+ channels open at the beginning of the action potential, and Na+ moves into the axon, causing depolarization. Repolarization occurs when the K+ channels open and K+ moves out of the axon, creating a change in electric polarity between the outside of the cell and the inside. The impulse travels down the axon in one direction only, to the axon terminal where it signals other neurons.
Shape of a typical action potential. The membrane potential remains near a baseline level until at some point in time, it abruptly spikes upward and then rapidly falls.
Approximate plot of a typical action potential shows its various phases as the action potential passes a point on a cell membrane. The membrane potential starts out at approximately −70 mV at time zero. A stimulus is applied at time = 1 ms, which raises the membrane potential above −55 mV (the threshold potential). After the stimulus is applied, the membrane potential rapidly rises to a peak potential of +40 mV at time = 2 ms. Just as quickly, the potential then drops and overshoots to −90 mV at time = 3 ms, and finally the resting potential of −70 mV is reestablished at time = 5 ms.
Action potential propagation along an axon
Ion movement during an action potential.
Key: a) Sodium (Na+) ion. b) Potassium (K+) ion. c) Sodium channel. d) Potassium channel. e) Sodium-potassium pump. In the stages of an action potential, the permeability of the membrane of the neuron changes. At the resting state (1), sodium and potassium ions have limited ability to pass through the membrane, and the neuron has a net negative charge inside. Once the action potential is triggered, the depolarization (2) of the neuron activates sodium channels, allowing sodium ions to pass through the cell membrane into the cell, resulting in a net positive charge in the neuron relative to the extracellular fluid. After the action potential peak is reached, the neuron begins repolarization (3), where the sodium channels close and potassium channels open, allowing potassium ions to cross the membrane into the extracellular fluid, returning the membrane potential to a negative value. Finally, there is a refractory period (4), during which the voltage-dependent ion channels are inactivated while the Na+ and K+ ions return to their resting state distributions across the membrane (1), and the neuron is ready to repeat the process for the next action potential.
When an action potential arrives at the end of the pre-synaptic axon (top), it causes the release of neurotransmitter molecules that open ion channels in the post-synaptic neuron (bottom). The combined excitatory and inhibitory postsynaptic potentials of such inputs can begin a new action potential in the post-synaptic neuron.
In pacemaker potentials, the cell spontaneously depolarizes (straight line with upward slope) until it fires an action potential.
In saltatory conduction, an action potential at one node of Ranvier causes inwards currents that depolarize the membrane at the next node, provoking a new action potential there; the action potential appears to "hop" from node to node.
Comparison of the conduction velocities of myelinated and unmyelinated axons in the cat. The conduction velocity v of myelinated neurons varies roughly linearly with axon diameter d (that is, v ∝ d), whereas the speed of unmyelinated neurons varies roughly as the square root (v ∝√d). The red and blue curves are fits of experimental data, whereas the dotted lines are their theoretical extrapolations.
Cable theory's simplified view of a neuronal fiber. The connected RC circuits correspond to adjacent segments of a passive neurite. The extracellular resistances re (the counterparts of the intracellular resistances ri) are not shown, since they are usually negligibly small; the extracellular medium may be assumed to have the same voltage everywhere.
Electrical synapses between excitable cells allow ions to pass directly from one cell to another, and are much faster than chemical synapses.
Phases of a cardiac action potential. The sharp rise in voltage ("0") corresponds to the influx of sodium ions, whereas the two decays ("1" and "3", respectively) correspond to the sodium-channel inactivation and the repolarizing eflux of potassium ions. The characteristic plateau ("2") results from the opening of voltage-sensitive calcium channels.
Giant axons of the longfin inshore squid (Doryteuthis pealeii) were crucial for scientists to understand the action potential.
As revealed by a patch clamp electrode, an ion channel has two states: open (high conductance) and closed (low conductance).
Tetrodotoxin is a lethal toxin found in pufferfish that inhibits the voltage-sensitive sodium channel, halting action potentials.
Image of two Purkinje cells (labeled as A) drawn by Santiago Ramón y Cajal in 1899. Large trees of dendrites feed into the soma, from which a single axon emerges and moves generally downwards with a few branch points. The smaller cells labeled B are granule cells.
Ribbon diagram of the sodium–potassium pump in its E2-Pi state. The estimated boundaries of the lipid bilayer are shown as blue (intracellular) and red (extracellular) planes.
Equivalent electrical circuit for the Hodgkin–Huxley model of the action potential. Im and Vm represent the current through, and the voltage across, a small patch of membrane, respectively. The Cm represents the capacitance of the membrane patch, whereas the four g's represent the conductances of four types of ions. The two conductances on the left, for potassium (K) and sodium (Na), are shown with arrows to indicate that they can vary with the applied voltage, corresponding to the voltage-sensitive ion channels. The two conductances on the right help determine the resting membrane potential.

In such cases, the released neurotransmitter is acetylcholine, which binds to the acetylcholine receptor, an integral membrane protein in the membrane (the sarcolemma) of the muscle fiber.

Anrep effect

Autoregulation method in which myocardial contractility increases with afterload.

Sustained myocardial stretch activates tension-dependent Na+/H+ exchangers, bringing Na+ ions into the sarcolemma.


Protein that in humans is encoded by the SPTAN1 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).

Alpha II-spectrin is expressed in a variety of tissues, and is highly expressed in cardiac muscle at Z-disc structures, costameres and at the sarcolemma membrane.

Mitochondrial disease

Group of disorders caused by mitochondrial dysfunction.

Micrograph showing ragged red fibers, a finding seen in various types of mitochondrial diseases. Muscle biopsy. Gomori trichrome stain.
Example of a pedigree for a genetic trait inherited by mitochondrial DNA in animals and humans. Offspring of the males with the trait don't inherit the trait. Offspring of the females with the trait always inherit the trait (independently from their own gender).

"Ragged Red Fibers" are clumps of diseased mitochondria that accumulate in the subsarcolemmal region of the muscle fiber and appear when muscle is stained with modified Gömöri trichrome stain

Striated muscle tissue

Cardiac muscle (heart muscle)

Micrograph of HPS stained skeletal striated muscle (fibularis longus).

Each muscle fiber contains sarcolemma, sarcoplasm, and sarcoplasmic reticulum.

Bar-headed goose

Goose that breeds in Central Asia in colonies of thousands near mountain lakes and winters in South Asia, as far south as peninsular India.

At Chilika Lake
Bar-headed goose wing flapping in Hadinaru lake Mysore
Swimming at Sylvan Heights Waterfowl Park
With glossy ibis Plegadis falcinellus at Keoladeo National Park, Bharatpur, Rajasthan, India.
Head detail
Preening and resting at Keoladeo National Park
At Keoladeo National Park
At Keoladeo National Park
Bharatpur, India.
Flying off at Bharatpur, India.
Flying off at Bharatpur, Rajasthan, India.
At Keoladeo National Park, Bharatpur, Rajasthan, India.
Bar-headed geese near Hetampur, West Bengal, India
Bar-headed goose with Canada geese near Cincinnati, Ohio, U.S., July 2015
In Karnataka, India
Anser indicus eggs - MHNT

Compared to lowland birds, mitochondria (the main site of oxygen consumption) in the flight muscle of bar-headed geese are significantly closer to the sarcolemma, decreasing the intracellular diffusion distance of oxygen from the capillaries to the mitochondria.