Muscle structure

Question 1

Muscle fibres

Answer 1

• skeletal muscle is made up of this

- large bundles of long cells

Question 2

Sacrolemma

Answer 2

The cell membrane of muscle fibre cells

Question 3

Transverse Tubules

Answer 3

The folds created by parts of the sacrolemma inwards across the muscle fibre + stick into the sacroplasm (muscle cell cytoplasm)

Question 4

What do transverse tubules do

Answer 4

they help to spread the electrical impulses throughout the sacroplasm so they reach all parts of the muscle fibre

Question 5

Sacroplasmic reticulum

Answer 5

a network of internal membranes which stores and releases calcium ions that are needed for muscle contraction

Question 6

Multinucleate

Answer 6

contain many nuclei

Question 7

Myofibrils

Answer 7

Long cylindrical organelles made up of proteins which are highly specialised for contraction

Question 8

Myofibrils contain..

Answer 8

bundles of thick (made of protein myosin) and thin myofilaments (made of protein actin) which move past each other to make muscles contract

Question 9

Under electron microscope - see dark and light bands. Which myofilaments are which?

Answer 9

Dark bands = thick myosin filaments and some overlapping thin actin = A bands
Light bands = thin actin filaments only = I bands

Question 10

Myofibril is made up of short units called…

Answer 10

sacromeres

Question 11

Z-line

Answer 11

ends of sacromere

Question 12

M-line

Answer 12

is in middle of each sacromere

Question 13

H-zone

Answer 13

• around the m-line

- only contains myosin filaments

Question 14

The sliding filament theory

Answer 14

• 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

Question 15

Myosin filaments

Answer 15

• have globular heads that are hinged so they can move back and forth
• each myosin has binding sites for actin and ATP

Question 16

Actin filaments

Answer 16

• 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.

Question 17

Binding sites in resting muscles

Answer 17

• 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

Question 18

The process of muscle contraction (step 1)

- Arrival of an action potential

Answer 18

• 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

Question 19

step 2 - movement of the actin filament

Answer 19

• 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.

Question 20

step 3 - breaking of the bridge

Answer 20

• 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)

Question 21

step 4 - returning to resting state

Answer 21

• 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.