Muscles

Question 1

What are muscles

Answer 1

Specialised effectors that bring about movement through contraction.

Question 2

Explain the structure of muscle

Answer 2

1. Many muscle fibres bunched together- each fibre surrounded by a sarcolemma
2. Each fibre- multinucleate
3. Fibre- rich in mitochondria
4. Sarcolemma folds inwards- transverse tubules (T- tubules)
5. The bulk of the muscle fibre is filled with highly specialised contractile units called myofibrils.

Question 3

What are the two units that make up myofibrils

Answer 3

Myosin and Actin

Question 4

What is the m-line

Answer 4

Thin disc that links myosin

Question 5

What is the z-line

Answer 5

Where actin filaments slot between outer edges

Question 6

What is a sarcomere

Answer 6

A section of myofibril between two z-lines

Question 7

What is the A-band

Answer 7

Also known as antisotropic- areas where thinner actin penetrates between the myosin filaments

Question 8

What is the I- band

Answer 8

Areas that contain only actin

Question 9

What is the H-zone

Answer 9

Area that contains only myosin

Question 10

What is the mechanism called that allows myosin filaments to slide past each other, reducing the overall length of the sarcomere

Answer 10

Sliding filament mechanism

Question 11

Explain the process of muscle contraction

Answer 11

1. An action potential stimulates the muscle fibre as it travels through its extensive system of T-tubules
2. An action potential causes the calcium ion channels in the sarcoplasmic reticulum to open.
3. This causes the calcium ions that have been stored in the sarcoplamic reticulum to diffuse into the sarcoplasm, down a concentration gradient.
4. The calcium ions cause the anciliary protein tropomyosin, that normally covers the binding sites on the actin filaments to be moved, enabling the myosin bulbous heads to link with the actin filaments, causing actomyosin bridges.
5. Once attached, the myosin heads change angle, rock back 45 degrees, and pull the actin filaments over the adjacent myosin filaments.
6. An ATP molecule attaches to each myosin head, and the energy released from its hydrolysis enables the myosin head to detach from the stationary actin binding site, and return to its original position.
7. The detached myosin head repeats the process of attachment, rotation and release around 5 times every second.
8. The cycle continues as long as the muscle fibre receives nervous stimulation and calcium ions are present.

Question 12

What changes occur in the muscle fibre when it is contracted

Answer 12

-Sarcomere (distance between two z-lines) become shorter
-H-zone becomes shorter
-I- band becomes shorter
-A band remains the same length