3.6.3 Flashcards
What is the neuromuscular junction?
The synapse between a motor neurone and a muscle fibre cell.
What is the key to understanding how a myofibril contracts?
Understanding how a sarcomere contracts.
What remains the same length during contraction and relaxation of a muscle fibre?
Actin and myosin filaments.
What changes during the contraction and relaxation of a muscle fibre?
The pattern of light and dark bands of a sarcomere.
What theory suggests that actin and myosin filaments slide past one another?
The sliding filament theory.
What forms when muscle contracts?
Crossbridges between actin filaments and myosin heads.
What prevents crossbridges from forming when muscle is relaxed?
Tropomyosin covers myosin binding sites on actin molecules.
What role does troponin play in muscle contraction?
Troponin binds to tropomyosin and calcium ions, altering tropomyosin’s position.
What triggers the release of calcium ions in muscle fibres?
A nerve impulse arriving at the synapse.
What happens when calcium ions bind to troponin?
Tropomyosin moves away from the myosin binding site on actin.
What enzyme is located in each myosin head?
ATPase.
What is released when ATP is hydrolyzed during muscle contraction?
ADP and inorganic phosphate (P_i).
What happens to the myosin head when an actinomyosin crossbridge forms?
The myosin head tilts, pulling the actin molecule over the myosin.
What is the result of repeated cycles of forming and releasing actinomyosin crossbridges?
Continued contraction of the muscle.
What is required to actively transport calcium ions into the endoplasmic reticulum?
Energy released from the hydrolysis of ATP.
Fill in the blank: The sliding filament theory describes how _______ and _______ slide past each other.
actin; myosin
True or False: The length of actin and myosin filaments changes during muscle contraction.
False.
describe how actin and myosin move past each other
1- When action potential from motor neurone stimulates muscle cell, it depolarises sarcolemma
Depolarisation spreads down T-tubules
= sarcoplasmic reticulum releases Ca2+ into sarcoplasm
-Influx of Ca2+ … which bind to troponin , altering its tertiary structure, the change in shape causes causes tropomyosin molecules to move away from the actin-myosin binding site, exposing binding sites
-Myosin heads, have ADP attached, attach to binding site on actin, creating actin-myosin cross bridge
- Ca2+ activate ATPase to hydrolyse ATP to ADP
-Energy released causes myosin heads to bend, pulling actin filament and forming a power stroke
-Another ATP molecule is hydrolysed releasing energy for the myosin head to detach from actin
- the myosin head rechecks to original position.
With ATP myosin head can reattach to actin
Myosin head then reattaches to different binding sites further along actin
New actin-myosin cross bridge is formed & cycle is repeated
