Muscle contraction Flashcards
What is the cell membrane of muscle fibre cells called?
The sarcolemma
What is the cytoplasm of a muscle cell called?
The sarcoplasm
What does the sarcoplasmic reticulum do?
Stores and releases calcium ions needed for muscle contraction
Structure of the sarcolemma
Parts of the sarcolemma fold inwards across the muscle fibre and stick into the sarcoplasm
These folds are called transverse (T) tubules which help spread impulses throughout the sarcoplasm so they reach all parts of the muscle fibre
What is a myofibril?
Loong, cylindrical organelles that are made up of lots of proteins which are highly specialised for muscle contraction
What do myofibrils contain?
Bundles of thick and thin myofilaments that move past each other to make muscles contract
What proteins are the thick and thin myofilaments made up of?
Thick myofilaments - myosin
Thin myofilaments - actin
If you look at a myofibril through a microscope what do you see?
Dark and light bands
The dark bands contain the thick myosin myofilaments and some overlapping thin actin filaments. These are called A bands
Light bands contain thin actin filaments only. These are called I bands
The H zone only contains myosin filaments
What is the sliding filament theory?
Myosin and actin filaments slide over one another to make the sarcomeres contract
The myofilaments themselves don’t contract
The simultaneous contraction of lots of sarcomeres means the myofibrils and muscle fibres contract
The sarcomeres return to their original length as the muscle relaxes
What is a sarcomere?
A myofibril is made up of many short units called sarcomeres
In the middle of the sarcomere is an M-line which is the middle of the myosin filaments
Around the M-line is the H-zone which only contains myosin filaments
What is tropomyosin?
A protein that blocks the actin-myosin binding site when muscle is in a resting state
It means the myofilaments can’t slide past each other
Why do myosin filaments have globular heads and binding sites?
The globular heads are hinged so they can move back and forth
Each myosin head has a binding site for actin and a binding site for ATP
Actin filaments have binding sites for myosin heads called actin-myosin binding sites
Tropomyosin is found between actin filaments and it helps the myofilaments move past each other
How do an influx of calcium ions trigger muscle contraction?
When an action potential from a motor neurone stimulates a muscle cell the sarcolemma becomes depolarised. The depolarisation spreads down the T tubules into the sarcoplasmic reticulum which releases calcium ions into the sarcoplasm
Calcium ions bind to a protien attached to tropomyosin causing the protein to change shape
This pulls the tropomyosin out of the actin-myosin binding site on the actin filament
This exposes the binding site allowing the myosin head to bind, forming an actin-myosin cross bridge
Calcium ions activate ATP hydrolase to break down ATP to ADP + Pi to provide energy needed for muscle contraction
This energy causes the myosin head to bend which pulls the actin filament along
Another ATP molecule provides energy to break the actin-myosin cross bridge
The myosin head then reattaches to a different binding site further along the actin filament forming a new actin-myosin cross bridge
What happens when muscle cells stop being stimulated?
When muscle cells are no longer stimulated calcium ions leave the binding site via active transport back into the sarcoplasmic reticulum
This allows the tropomyosin to block the actin-myosin binding site again
So no actin-myosin cross bridges can form
Sarcomere lengthens as actin filaments relax
How is ATP regenerated for muscle contraction?
Oxidative phosphorylation during aerobic respiration
Glycolysis during anaerobic respiration
ATP-Phosphocreatine (PCr) system
