Muscle Structure & Contraction AIM To understand the structure and histology of muscle and the sliding filament theory of muscle contraction

What are the 3 types of muscle called? Skeletal Smooth Cardiac

Skeletal muscle Skeletal muscles attach to bones and act in antagonistic pairs  one muscle to move the bone in one direction and another to move it back the other way These muscles usually contract voluntarily (act together with the nervous system) Able to carry out a single contraction (twitch) or a long, sustained contraction (tetanus)

Cardiac muscle found only in the heart,  Adapted for endurance and reliability  It can stretch in a limited way, like smooth muscle, and contract with the force of a skeletal muscle  It is a twitch muscle only and contracts involuntarily.

Smooth muscle found in the digestive system, blood vessels, bladder, airways and, in a female, the uterus Smooth muscle has the ability to stretch and maintain tension for long periods of time It contracts involuntarily: you do not have to think about contracting it because the nervous system controls it automatically

The Gross structure of muscles A single skeletal muscle, e.g.triceps muscle, is attached at its: 1.Origin - a fixed non movable part of the skeleton e.g. the shoulder and humerous 2.Insertion – a movable part of the skeleton e.g. the ulna Attachment is by inelastic collagen tendons As the triceps contracts, the insertion is pulled toward the origin and the arm is straightened or extended at the elbow The triceps is an extensor Because skeletal muscle exerts force only when it contracts, a second muscle (a flexor) is needed to bend the joint The biceps muscle is the flexor of the lower arm Together, the biceps and triceps make up an antagonistic pair of muscles

The antagonistic action of the biceps and triceps muscles

What are muscles made of? Skeletal muscle is made up of thousands of cylindrical muscle fibres running from origin to insertion Each fibre is surrounded by a membrane (sarcolemma) Fibres are bound together by connective tissue with B.V. and nerves running through Each muscle fibre contains: 1) myofibrils that are stacked lengthwise and run the entire length of the fibre 2) mitochondria 3) an extensive endoplasmic (sarcoplasmic) reticulum 4) many nuclei near the surface

The Structure of a Muscle Fibre

The structure of Myofibrils The cross striations seen are Z lines They occur every 2.5um in a relaxed muscle The interval between is a sarcomere Thin filaments of actin extend the length of each sarcomere In the centre of the sarcomere the actin is interspersed with thick filaments of myosin The actin and myosin filaments are crosslinked

Myofibrils appear darker where the filaments overlap – A bands 6 actins filaments surround each myosin filament The light bands in between where there is only actin are the I bands There is a small band where it is only myosin filaments and this makes the H zone The Z lines run through the middle of each I band

The Arrangement of filaments in a myofibril

Huxley and Hanson (1954) Observations during muscle contraction (twitch) –I band shortens –A band does not change length –Z lines come closer together (sarcomere shortens) –H zone shortens Muscle contracts by the actin and myosin filaments sliding past each other shortening the sarcomeres Sliding Filament Theory of Muscle contraction

Sliding Filament Theory Actin Filament – 2 helical strands G-actin twisted around to make a fibre Myosin filament - a long myosin rod with 2 heads appearing at intervals along the rod Where the actin and myosin filaments overlap the myosin heads can attach to the actin filaments Troponin molecules as part of tropomyosin are attached to the actin filament

Ca release from the sarcoplasmic reticulum into the sarcoplasm cause a rise in Ca concentration Ca binds to the troponin, causing a change in shape of the tropomyosin Tropomyosin moves to expose the myosin binding sites on the actin Myosin heads form actomyosin cross-bridges Sliding Filament Theory

The myosin heads become attached to the actin at a certain angle and a cross bridge forms The myosin head swivels to a different angle moving the actin filaments towards the centre of the sarcomere The myosin binds an ATP which causes it to detach from the actin The ATP is hydrolysed to ADP and the energy is used to return the myosin head back to its original angle The process is repeated It is a ratchet or rowing mechanism Sliding Filament Theory

The ratchet mechanism of muscle contraction

Ca ions are actively taken back into the sarcoplasmic reticulum Only small amounts of ATP are found in resting muscles Instead there are larger amounts of creatine phosphate (CP) ATP is recycled by using the P from the CP to phoshorylate ADP CP is restored by oxidation of fatty acids or glycogen to yield ATP which can then rephosphorylate the CP