6.5 Flashcards
Several myosin molecules lie together in a thick bundle with their bases attached at a point called the
M-line
2 kinds of myofilaments:
Thick = myosin (dark bands) Thin = actin (light bands)
The myosin molecules have hook-shaped ‘heads’ projecting from the
Bundle
The actin molecules lie together in a long chain, and 2 chains f actin twist together to form a long
Thin filament
The actin filaments attach to each other at a point called the
Z-line
The actin and myosin filaments lie alongside each other and overlap, producing the striped appearance of the
Myofibril
The darker areas are where the actin and myosin filaments
Overlap
The distance between 2 Z-lines is called a
Sarcomere
(Sliding filaments) when muscles contract, the sarcomeres in each Myofibril get
Shorter
(Sliding filaments) the sarcomere gets shorter because the
Actin and myosin filaments slide over each other, pulling the Z-lines closer together
(Sliding filaments) energy from the ATP is needed for
The filaments to slide over each other
(Sliding filaments) this sequence of sarcomeres shortening happens several times, making the actin filament
Slide past the myosin ‘heads’
(Sliding filaments) the 2 kinds of filaments overlap further, pulling the
Z-lines closer together
Stage 1) tropomyosin molecule prevents myosin head from attaching to the
Binding site on the actin molecule
Stage 2) calcium ions released from the endoplasmic reticulum cause the tropomyosin molecule to pull away from the
Binding sites on the actin molecule by entering the sarcoplasm and binding to troponin
Stage 3) myosin head now attaches to the binding site on the
Actin filament
Stage 4) head of myosin changes angle, moving the actin filament along as it does so (power stroke). The ADP molecule is
Released
Stage 5) ATP molecule fixes to myosin head, causing it to
Detach from the actin filament
Stage 6) hydrolysis of ATP to ADP by ATPase provides the energy for the
Myosin head to resume its normal position
Stage 7) head of myosin reattached to a binding site further along the actin filament and the cycle is
Repeated
(ROLE OF CALCIUM IONS) when ATP is present, muscle tissue
Contracts
(ROLE OF CALCIUM IONS) the inhibitor to stop the tissue contracting all the time is called
Tropomyosin
(ROLE OF CALCIUM IONS) tropomyosin fits into binding sites on the actin filaments -
Stopping myosin from binding
(ROLE OF CALCIUM IONS) however, when a muscle contracts, calcium ions enter the sarcoplasm. These bind to the protein troponin which acts to pull the
Tropomyosin out of the way, allowing the myosin ‘heads’ to bind to the actin binding sites
(ROLE OF CALCIUM IONS) in relaxed muscles, calcium ions are actively transported out of the sarcoplasm. They are collected together in the membranes of T-tubules in the
Muscle filaments
(ROLE OF CALCIUM IONS) t-tubules are
Infoldings of membrane that run through the muscle fibres at intervals, close to the endoplasmic reticulum
(STIMULATING A MUSCLE TO CONTRACT) muscles contract when they are stimulated by an
Action potential from a motor neurone
(STIMULATING A MUSCLE TO CONTRACT) a motor neurone synapses with a muscle at a neuromuscular junction. When the transmitter substance fits into specific receptors in the postsynaptic membrane (sarcolemma), sodium ions enter the sarcoplasm -
Depolarising the membrane
(STIMULATING A MUSCLE TO CONTRACT) depolarised membrane causes calcium ions from the T-tubules to enter the
Sarcoplasm and muscle contraction occurs
