Sarcomeres slide


Do contracting smooth muscles follow the sliding filament model?

Smooth muscles experience cross-bridge cycling when they contract but it has some differences from the sliding filament model. Smooth muscles are not made of sarcomeres but have actin and myosin filaments which are less organised than those in skeletal and cardiac muscles. Ca2+ binds to calmodulin (instead of troponin C) which activates myosin light chain kinase (MLCK). This complex catalyses a change in conformation of myosin heads with an increase in its ATPase activity that allow them to bind with actins. Calponin and caldesmon block the ATPase activity of myosin heads but are released with Ca2+ binding. Cross-bridge cycling also occurs less frequently in smooth muscle.

Why are muscle cells multinucleated?

The many nuclei can coordinate simultaneous contraction of actin and myosin filaments along the whole length of the muscle fibre. If muscle cells are separate and have only one nuclei, there is a need to communicate with each other via cell signalling. This would slow down contraction unnecessarily.

Why is it that when a person dies, their muscle becomes stiff to movement?

Muscles become more resistant to stretch after death because of the accumulation of Ca2+ that is no longer taken up into the sarcoplasmic reticulum (no active transport mechanism). The cross-bridges are permanently formed. This causes muscle to become stiff and it is difficulty to induce forced movement of muscles externally.

Why is it important to warm-up before exercising?

This is to induce treppe so that muscle contraction will be more efficient and stronger when the exercise regime begins.



A neurotransmitter of the neuromuscular junction that causes depolarisation at the post-synaptic terminal, i.e. the target muscle. It eventually results in muscular contraction.


The enzyme that breaks down acetylcholine at the synaptic cleft that causes muscle relaxation.

Actin filament

The thin filament of myofilaments that consist of intertwined G actin molecules called troponin and tropomyosin.

Action potential

An electric signal consisting of depolarization and subsequent repolarization of a nerve or muscle cell membrane; travels along the membrane and functions as a signal to initiate an activity e.g. a muscle cell contraction.

Afterload (cardiac muscle)

Made of arterial pressure which opposes blood outflow from the heart. The heart has to overcome both the preload and the afterload when contracting to expel blood from the heart to around the body.

All or nothing law

A property of action potentials. They will only propagate once the membrane reaches a certain electrical potential. Below this threshold level, depolarisation will not occur at all.


Adenosine triphosphate (ATP) is a molecule that provides cell energy for metabolism with the release of phosphate ion. This converts ATP to adenosine diphosphate (ADP).

Calcium ions

Calcium ions (Ca2+ ions) are required for muscular contraction.


Cross bridge is the binding of myosin heads with exposed actin binding sites, and the power stroke occur by myosin heads to drag the actin filaments along with it. This happens when calcium ions move troponin-tropomysin complex from the actin binding sites. To release Myosin head fron...


A change in cell membrane potential, usually tending to more positive potential. Large enough depolarisations set off action potentials.


An electrically-insulating layer that separates muscle cells from each other.


The external connective tissue wrapped around a muscle.


A bundle of individual cells that make up a skeletal muscle.

Gap junction

An intercellular connection between cells that facilitate the movement of ions, allowing cells to communicate with each other.

Intercalated disc

A double membrane that links cardiac muscles together.

Isometric contraction

When the muscle contracts without changing its length. Tension is increased in the muscle but the force is absorbed by the sarcomere components. The tension will not be able to move a load.

Isotonic contraction

When muscle contracts and either shortens (concentric contraction) or lengthens (eccentric contraction). Its length changes but the tension produced is maintained. The internal tension has enough force to overcome and move a load.

Motor unit

An entity consisting of a single motor neuron (spefically, α motor neuron) and the motor fibres it innervates. It groups with other motor units and coordinate contractions of the muscle.


A cylindrical bundle of contractile myofilaments.


The contractile proteins in myofibrils which are made of actin and myosin filaments.

Myosin filament

The thick filament of myofilament that consists of 200 to 500 myosin molecules bundled together with the heads projecting outward in a spiral array.


The connective tissue layer that surrounds the fascicles.

Power stroke

The sliding movement of actin filaments over myosin filaments

Preload (cardiac muscles)

The initial sarcomere length and end-diastolic volume that acts together as a force the cardiac muscles have to resist before contraction.


The activation of more motor units within a muscle by more frequent depolarisation. Also known as multiple motor unit (MMU) summation.


The cell membrane of muscle cells


The repeating contractile unit of a myofibril.

Sarcoplasmic reticulum

The endoplasmic reticulum of muscle cells.

Schwann cell

A glial cell that supports neurons and other cells in the peripheral nervous system.


The additive effect of stronger contractions of the muscle as the frequency of action potential arrivals increases.

Synaptic cleft

The tiny gap that separates neurons from each other.

Synaptic vesicle

A small, enclosed space surrounded by a bilayer of membrane. In a synaptic vesicle, it transports neurotransmitters from one neuron to another.


The invagination of sarcolemma that projects deep down into muscle cells.

Temporal summation

A similar phenomenon to treppe. However, action potential arrives at a faster rate and muscles have little relaxation phases. As a result, muscle tension builds up. Also known as wave summation.


The magnitude of pulling force the muscle exerts in order to move a load.

Tetanus (complete)

A complete tetanus occurs when twitches fuse into a continuous contraction due to the very high frequency of action potential arrivals.

Tetanus (incomplete)

The building up of tension in muscles during a temporal succession. Muscle does not experience a continuous contraction but instead the contractions oscillate.


A staircase phenomenon composed of many successive twitches. This occurs due to incomplete muscle relaxation before the next action potential arrives.


A filament that coils around the actin filament. It covers the binding sites of actin filaments and prevents myosin heads from binding to it when the muscle is unstimulated.


A molecule that attaches to tropomyosin strands and facilitate tropomyosin movement so that myosin heads can bind to the exposed actin binding sites.


A short period of time when a muscle contracts and relaxes fully.

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