The muscle fibreLet’s go back to the individual muscle fibre

The fibres are covered by a membrane called sarcolemmaBetween the muscle fibre and the sarcolemma membrane is a gel-like fluid called sarcoplasmAnd then, as already learned, on the outside of each fibre’s sarcolemma is the connective endomysium tissue, that helps the fibres to bind

SarcoplasmSarcoplasm is liquid gold, it contains:– Mitochondria (where aerobic energy is

made)– Myoglobin (carries oxygen to the

mitochondria)– Energy nutrients (fat/protein/carbs)– Adenosine triphosphate (immediate

energy)– Enzymes (chemicals to speed up energy

production)– Actin and Myosin myofilaments (proteins

that help with contraction)

The muscle fibreEach muscle fibre is actually made up of long strands called myofibrilsEach myofibril contains many smaller units called SarcomeresSarcomeres are responsible for muscular contraction, they contain two filaments called actin and myosinYes, we’re finally down to the smallest unit of the muscle!

SarcomeresThis is a contractile unitIt’s important to grasp that each myofibril contains many sarcomere, joined end to end

SarcomeresThe sarcomeres are lined up end to end along the myofibril.Where the abut each other is called the Z-LineActin is a contractile protein (filament) that attaches to each of the z-linesMyosin is another contractile protein that sits between the actin filaments

SarcomeresI-Band = Actin onlyA-Band = both, length of myosinH-zone = myosin only, the gap between acton filaments (space for contraction)

SarcomeresMyosin filaments have cross bridges – just picture little hockey stick type things.

SarcomeresWhen the sarcomere contracts, the curved end of the hockey stick grabs onto the actin and pulls it towards the centre of the sarcomere.This happens in all sarcomeres at the same time, making the muscle contract

Sarcomeres

The muscle will relax when the myosin and actin lose contact with each other and the sarcomere returns to normal size.Myosin doesn’t change in length during a contraction, actin does

Muscular toneThere’s never a time when ALL myosin cross bridges have detached from the actin, some stay in contactThis gives us muscular tone – the muscle is never completely relaxed.Because they’re already partially connected, they can be activated quickly.Tone also helps with posture.