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

There are three basic types of muscle:- Found only in heart tissue Found in voluntary muscle tissue e.g. biceps Found in involuntary muscle tissue e.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

skeletal muscle Voluntary muscle that is under conscious control It is also called striated muscle because of its striped appearance.

structure of skeletal muscle 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.

Muscle fibres

nuclei sarcolemma

sarcolemma sarcoplasm myofibril nuclei

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

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

Infoldings of sarcolemma actin and myosin filaments overlap mitochondria nucleus sarcoplasm Infoldings of sarcolemma form T tubules

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 line Z line one sarcomere H-band M line I-band A-band

M

Muscle contraction sliding filament mechanism. 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. receptor site actin filament myosin head myosin filament

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

The sliding filament mechanism 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).

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 2.5mm) 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.

does not change in length 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

How an action potential causes a muscle to contract 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.

striated muscle fibres are stimulated by motor neurones innervated by a motor neurone Motor end plate

Motor end plate

motor end plate in section myelin sheath synaptic cleft vesicle neurone axon presynaptic membrane mitochondria postsynaptic membrane sarcolemma sarcoplasm sarcoplasmic reticulum Myofibril contains repeating dark and light bands, responsible for the striped appearance A band Dark band I band Light band

The strength of the contraction depends on: 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.

REVIEW

the structure and function Give an account of the structure and function of skeletal muscle