Smooth muscle can only be stimulated to contract by the CNS.

In smooth muscle, actin filaments and associated myosin attach to regular Z lines producing a striation pattern.

Calcium-dependent phosphorylation of myosin light chains is responsible for the contraction of smooth muscle.

Smooth muscle cells are multi-nucleated like cardiac and skeletal muscle cells.

Myocytes in single-unit smooth muscle are not electrically coupled to each other.

The set length of the muscle is one factor that determines the level of tension developed.

The muscle contracts at its maximum velocity of shortening.

The tension produced is the same as for a single

twitch but maintained for a longer time.

The muscle shortens under a fixed load.

The muscle shortens but does not generate tension.

The main elastic (spring) component within the sarcomere is the endomysium.

In adult humans, a large muscle fibre (one cell) contains around 20 myofibrils.

Skeletal muscle fibres contain hundreds of nuclei and these are located just below the sarcolemma in healthy cells.

Absence of 

the protein

titin makes skeletal muscle fibres more elastic. 

Within a skeletal muscle fibre, each fibril is enveloped

by a sarcolemma membrane.

During the cross-bridge cycle, ATP is broken down to provide energy for the movement of the myosin heads.

During sliding the A band shortens.

Z-lines are found at each end of the sarcomere.

Sliding is caused by the myosin heads interacting with specific sites on the actin molecule.

Muscle works by the relative sliding of the thin filament (actin) over the thick filament (myosin).

The sarcoplasmic reticulum is responsible for Ca²⁺ uptake.

Excitation-contraction coupling.

An action potential conducted down the t-tubules.

An action potential in the surface membrane.

Increase in the sarcoplasmic concentration of Ca²⁺.

Tropomyosin pulls the myosin filament towards the Z-line.

The actin and myosin filaments are both moving.  

The actin filament moves along the fixed myosin filament.

The titin molecule pulls the actin filaments towards the Z-line.

The thick myosin filament moves over the thin actin filament.

A-Band

Sarcomere      

Z-line to Z-line

I Band

M-line to M-line

A bigger diameter muscle

fibre can generate more tension because the thick filaments get thicker.

A muscle can generate more

tension when multiple motor units are recruited.

A sarcomere will not

develop as much tension when fully stretched, as there is less overlap between

the thick and thin filaments.

When

a whole muscle is stretched to its maximum, there is passive tension due to the

elastic elements of the muscle.

A

rapid series of muscle twitches can generate more tension than a single twitch.

Skeletal muscle fibres are never coupled via gap

junctions.

Cardiac and smooth muscle are under voluntary control.

They do not require an influx of Ca²⁺ from the extracellular fluid to initiate contraction.

Each individual cardiac/smooth muscle cell has its own nerve supply.

Cardiac and smooth muscle contract more quickly.

The type of actin and titin expressed.

The properties of the muscle protein titin e.g. fast or slow titin.

The amount of ATP available in the fibres.    

The rate at which myosin can hydrolyse ATP.

The density of calcium channels present in the sarcolemma.