Smooth muscle myosin is inactivated when myosin-light-chain phosphatase

removes the phosphate residue attached by myosin-light-chain kinase

Intracellular calcium binds to calmodulin (CaM) and the Ca-CaM complex activates a kinase.

An increase in cytosolic calcium induces calmodulin binding to myosin-light chain kinase.

The regulation of the actin-myosin interaction occurs by calcium binding to troponin.

Phosphorylation of the myosin light chain converts the myosin into its active form which can then bind to F-actin.

Isotonic contraction.

Isometric contraction.

Isometric concentric contraction.

Eccentric contraction.

Isotonic concentric contraction.

Sliding is caused by the cross-bridges formed between the myosin head and specific sites on the actin molecule

ATP hydrolysis provides the energy for the myosin head to unbind from the actin binding site.

Every cycle, four molecules of ATP are used by each myosin head.

The cross-bridges are activated once during a contraction.

The troponin complex is an ATP sensor.

The amplitude of the action potentials coming from the motor neuron is larger during a tetanus.

In a twitch (which occurs when a muscle is activated by a single action potential) there is not enough time to take up all the elastic tension before the muscle starts to relax again.

The myosin heads move more slowly during a twitch.

The CaATPase pumps on the sarcoplasmic reticulum work faster during a single twitch, lowering the calcium concentration more quickly than in the case of the larger tetanus.

The rate of diffusion of Ca²⁺ molecules from the sarcoplasmic reticulum to the contractile proteins is slower during a single twitch.

Myosin and Titin

Collagen and Troponin

Actin and Myosin

Tropomyosin and Alpha-actinin

Actin and Titin

Whole muscle is surrounded by the TENDOPIMYSIUM, fascicles by the PERIMYSIUM and individual fibres by the ENDOMYSIUM.

Whole muscle is surrounded by the EPIMYSIUM, fascicles by the ENDOMYSIUM and individual fibres by the PERIMYSIUM.

Whole muscle is surrounded by the EPIMYSIUM, fascicles by the PERIMYSIUM and individual fibres by the ENDOMYSIUM.

Whole muscle is surrounded by the PERIMYSIUM, fascicles by the EPIMYSIUM and individual fibres by the ENDOMYSIUM.

Whole muscle is surrounded by the ENDOMYSIUM, fascicles by the PERIMYSIUM and individual fibres by the EPIMYSIUM.

The concentration difference between intracellular Ca²⁺ stored in the sarcoplasmic reticulum  and sarcoplasmic [Ca^2+]  is so small (approx 1 to 1.5 times) that ATP powered Ca²⁺ pumps are required to move Ca²⁺ 

from the SR

into the cytoplasm.

This is possible because Ca²⁺ is released from mitochondrial stores close to the contractile proteins.

The rate of onset of muscle contraction is dependent on the diffusion of intracellular ATP and not the diffusion of Ca²⁺.

The concentration difference between intracellular and extracellular Ca²⁺ is so large (about 1 to 20,000) that the Ca²⁺ diffuses much more rapidly than expected.

Membrane invaginations (t-tubular system) carry the action potential into the middle of the muscle fibre triggering an almost simultaneous release of Ca²⁺ from the SR throughout the fibre.

The question cannot be answered because we cannot measure the fibre type ratios in living athletes.

It is the high density of mitochondria present in both power and endurance athletes which determines their elite performance characteristics.

There is little individual variation in fibre type proportions.

Elite power athletes are

born with muscles which are all fast fibres, whereas elite endurance athletes have all slow fibres.

Elite power athletes like high jumpers are born with a higher

than 50% fast-fibre profile, and marathon runners have a higher than 50% slow fibre

profile in key muscles.

The sarcoplasmic reticulum of cardiac muscle is more extensive than that of skeletal muscle.

Skeletal and

cardiac myocytes have extensive networks of t-tubules lined with voltage

sensors.

Cardiac myocytes

are electrically coupled, whereas skeletal muscle fibres do not communicate via

electrical signals between each other.

Dyads are typical in cardiomyocytes, while triads are typical in skeletal muscle.

Mitochondria are more abundant in cardiac muscle than in skeletal muscle.

The voltage sensor depolarizes the SR membrane which

opens the calcium release channels.

Calcium entering through the voltage

sensor activates the ryanodine receptor via calcium-induced calcium release.

The 

ryanodine receptor is opened

by the voltage sensor.

The 

voltage sensor is located in

the membrane of the sarcoplasmic reticulum.

The voltage sensor is a

voltage-gated sodium channel in the sarcolemma.