Muscle structure Flashcards
Muscle fibres
- skeletal muscle is made up of this
- large bundles of long cells
Sacrolemma
The cell membrane of muscle fibre cells
Transverse Tubules
The folds created by parts of the sacrolemma inwards across the muscle fibre + stick into the sacroplasm (muscle cell cytoplasm)
What do transverse tubules do
they help to spread the electrical impulses throughout the sacroplasm so they reach all parts of the muscle fibre
Sacroplasmic reticulum
a network of internal membranes which stores and releases calcium ions that are needed for muscle contraction
Multinucleate
contain many nuclei
Myofibrils
Long cylindrical organelles made up of proteins which are highly specialised for contraction
Myofibrils contain..
bundles of thick (made of protein myosin) and thin myofilaments (made of protein actin) which move past each other to make muscles contract
Under electron microscope - see dark and light bands. Which myofilaments are which?
Dark bands = thick myosin filaments and some overlapping thin actin = A bands
Light bands = thin actin filaments only = I bands
Myofibril is made up of short units called…
sacromeres
Z-line
ends of sacromere
M-line
is in middle of each sacromere
H-zone
- around the m-line
- only contains myosin filaments
The sliding filament theory
- this is where the myosin and actin filaments slide over one another to make the sacromeres contract - the myofilaments dont contract
- the simultaneous contraction of lots of sacromeres means the myofibrils and muscle fibres contract
- sacromeres return to original shape as the muscle relaxes.
- A-bands stay same length, I-bands get shorter, H-zones get shorter
Myosin filaments
- have globular heads that are hinged so they can move back and forth
- each myosin has binding sites for actin and ATP
Actin filaments
- has binding sites for the myosin heads - aka the actin-myosin binding sites
- a protein: tropomyosin found between the actin filaments which help the myofilaments move past each other.
Binding sites in resting muscles
- the actin-myosin binding site is blocked by tropomyosin - means myofilaments cant slide past each other as myosin heads cant bind to the actin filaments
The process of muscle contraction (step 1)
- Arrival of an action potential
- action potential from a motor neurone stimulates a muscle cell = depolarises the sarcolemma + depolarisation spreads down the T-tubules to the sarcoplasmic reticulum.
- causes this to release stored Calcium ions into sacroplasm - the influx of calcium into sarcoplasm = muscle contraction.
- Ca2+ ions bind to a protein attached to tropomyosin - causing protein to change shape - pulling the tropomyosin out of actin-myosin binding site on the actin filament - exposing binding site = myosin head
binds - bond is called the an actin-myosin cross bridge
step 2 - movement of the actin filament
- Ca2+ ions activate enzyme ATP hydrolase which hydrolises ATP into ADP + Pi - to provide energy needed for muscle contraction
- energy released from ATP causes myosin head to bend which pulls the actin filament along (in a kinda rowing action) - which shortens the sacromere = muscle contraction.
step 3 - breaking of the bridge
- another ATP breaks the actin-myosin bridge - myosin head detaches from actin filament after its moved.
- Myosin head returns to starting position where it reattaches to a different binding site further along the actin filament, new actin-myosin bridge formed and the cycle is repeated (as long as calcium is present)
step 4 - returning to resting state
- stimulation stops - Ca2+ ions leave binding sites and are moved back via active transport back into the sacroplasmic reticulum (ATP needed)
- tropomyosin move back, blocking the actin-myosin binding sites.
- actin filaments slide back to relaxed position = lengthening sacromere.
