Muscles are organs, They are specialised effectors that contract to cause movement

There are three basic types of muscle:-

Found only inheart tissue

Found involuntary muscletissue e.g. biceps

Found ininvoluntary muscle tissuee.g. stomach, artery walls

Skeletal Smooth Cardiac

Appearance Striated, multinucleate fibres

Discrete uninucleate spindle shaped cells. No striations

Striated, branched fibres with intercalated discs between cells (discrete lines)

Distribution Attached to bone found throughout the body

Lining of the gut & blood vessels, the iris & ciliary body in the eye.

Wall of the heart

Nervous control

Voluntary, conscious control

Involuntary, automatic control: reflex actions

Myogenic & involuntary control

•Voluntary muscle that is under conscious control

• It is also called striated muscle because of its striped appearance.

•Muscles are attached to bones by tendons and are made up of hundreds of specialised cells called muscle fibres, which are several cms long

•the surface membrane of each muscle fibre is called the sarcolemma and is deeply folded to form transverse or T-tubules.

•The cytoplasm is called the sarcoplasm and contains many mitochondria.

•Many nuclei are found just below the sarcolemma, making muscle fibres multinucleate.

•The sarcoplasm also contains long specialised units called myofibrils that run the length of the fibre.

Musclefibres

nuclei

sarcolemma

sarcoplasm

sarcolemma

nuclei

myofibril

Ultrastructure of myofibrils

•Each myofibril consists of 2 types of proteins, often called myofilaments

•Thick myofilaments are made of the large protein myosin, whilst

•Thin myofilaments are composed of the smaller protein actin

Muscle fibre > myofibrils myofilaments (actin & myosin)

(light) actin & (dark) myosin myofilaments

• Myosin and actin filaments lie adjacent to one another.

• Myosin filaments are linked by a thin disc, the M line, which runs perpendicular to the filaments

M line

Myosin protein filament

• Actin filaments are linked in a similar way, held together by a disc called the Z line.

Z line

Actin proteinfilament

nucleus

Infoldings of sarcolemma form T tubules

actin and myosin filaments overlap

sarcoplasm

mitochondria

• The actin filaments slot between the myosin filaments.

• The thicker myosin filaments appear darker than the thinner actin and give the skeletal muscle its striated/banded pattern under the microscope.

• The section of myofibril between 2 Z-lines forms the basic contractile unit and is called a sarcomere.

The part of the myofibril•containing only actin is called the I-band (isotropic)•containing myosin, including the overlap with actin, is called the A-band (anisotropic)•Within the A-band containing myosin only is called the H-zone•Detailed inspection shows a regular arrangement in the area of overlapping myosin and actin. Each myosin being surrounded by 6 actin filaments in a hexagonal pattern.

Z lineZ line

one sarcomere

M lineI-band

H-band

A-band

M

When a muscle contracts the actin filaments are pulled over the myosin filaments (towards the M line), reducing the length of each sarcomere and therefore the whole muscle.

This process, known as the

sliding filament mechanism.

How does this occur?

Bulbous heads extend from the myosin filaments, which are able to attach to receptor complementary receptor sites on the actin filaments.

myosin head

myosin filament

receptor site

actin filament

At rest the actin receptor sites are blocked by an accessory protein called tropomyosin, which prevents the myosin heads from binding.

tropomyosinblocking actinreceptor sites

1. An action potential arrives at the neuromuscular junction causing the release of neurotransmitter substance that crosses the synaptic cleft and depolarises the post synaptic membrane. The AP travels along the sarcoplasm through the T tubule system.

2. The AP causes calcium ion channels in the sarcoplasmic reticulum (ER of muscle fibres) to open.

3. Ca2+ stored in the sarcoplasmic reticulum diffuse into the sarcoplasm down their concentration gradient.

4. Ca2+ cause the tropomyosin to move, freeing the actin binding sites for the myosin heads to attach, forming actomyosin bridges.

5. When attached the myosin heads change shape, rotating back to an angle of 45o. This pulls the actin filaments over the adjacent myosin filaments (about 10nm).

The sliding filament mechanism

6. ATP attaches to each myosin head & is hydrolysed, releasing energy which causes the myosin head to detach from the stationary actin binding site and return to its original position.

7. The detached myosin heads repeat the process, attachment, rotation, release, about 5 times per second.

8. The cycle will continue as long as the muscle receives nervous stimulation and Ca2+ is present.

Each individual sarcomere (about 2.5m) moves a very short distance, but the combined movement of all the sarcomeres lined end to end in a

muscle fibre, all contracting at the same time, is considerable.

The movement of the actin filaments across the myosin filaments causes the following changes:

• the sarcomere shortens,

• the Z lines become closer together, so the

I bands become shorter

• The H zone becomes shorter

However the A band (length of myosin)

does not change in length

H zone

Myosin heads

• The synapse between the axon of a motor neurone and a muscle is called the neuromuscular junction.

• The part of the muscle fibre that forms the post-synaptic membrane is called the motor end plate.

• It is infolded to increase the surface area for more neurotransmitter receptors.

How an action potentialcauses a muscle to contract

Muscle fibres are innervated by a motor neurone

Motor end plate

Motor end plate

myelin sheath

neurone axon

mitochondria

vesiclesynaptic cleft

presynaptic membrane

postsynaptic membrane

I bandLight band

sarcoplasm

sarcoplasmic reticulum

sarcolemma

A bandDark band

Myofibril contains repeating dark and light bands, responsible for the striped appearance

• There are many neuromuscular junctions along each muscle fibre. Each receives an AP at the same time, ensuring the whole muscle contracts at the same time, rather than a wave gradually spreading across it.

• The strength of the contraction depends on: – How long the muscle is stimulated– How many muscle fibres are stimulated and

contracting.

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