Paper 2 - Muscles

Question 1

The process of muscle contraction

Answer 1

1. An action potential in a motor neurone stimulates the release of ACh, that then binds to receptors on the sarcolemma.
2. This depolarises the sacrolemma and spreads through the transverse tubules to the sarcoplasmic reticulum = Ca2+ released into sarcoplasm.
3. Ca2+ binds to tropomyosin, changing its shape. This exposes the binding sites on the actin molecules.
4. Myosin heads with ADP attached attach to the binding sites of actin, forming an actin myosin cross bridge.
5. Myosin heads change their angle, pulling the actin filaments as they do = power stroke. This uses energy released from hydrolysed ATP.
6. An ATP molecules attaches to each myosin head causing it to become reattached from the actin filament.
7. ATPase hydrolysed ATP into ADP providing the energy for the myosin head to go back to its normal position.
8. Myosin heads reattach further along the actin filaments and the cycle continues if the concentration of Ca2+ remains high.

Question 2

Antagonistic pairs

Answer 2

Contracting muscle = agonist

Relaxing muscle = antagonist

Question 3

Structure of Skeletal Muscle

Answer 3

Many highly specialised cells that form skeletal muscle tissues

Plasma membrane is called the sarcolemma, cytoplasm is called sarcoplasm

Transverse tubules = folded parts of sarcolemma

Sacroplasmic reticulum store calcium ions and releases these when depolarisation occur.

Contains myofibrils = specialised protein filaments

Actin = thinner protein fibre that contains binding sites for myosin

Myosin = thicker protein

Question 4

Structure of sarcomere

Question 5

Compare neuromuscular junction to cholinergic synapse - similarities

Answer 5

• Both use ACh
• Both have receptors
• Both use AChE to hydrolyse the neurotransmitter

Question 6

Compare the neuromuscular junction to a cholinergic synapse - differences

Answer 6

Neuromuscular junction:

• Only excitatory
• Only links neurones to muscles
• Only motor neurons are involved
• The NT binds to receptors on receptors on the membrane of a muscle fibre

Cholinergic synapse:

• Can be excitatory or inhibitory
• Links neurones to neurones, or neurones to other effectors
• Motor,sensory and relay neurones may be involved
• NT binds to receptors on membrane of post-synaptic neurone

Question 7

What is the sliding filament theory?

Answer 7

In muscle contraction the thin actin filaments slide inwards over the thicker myosin filaments
= shortening of each sarcomere
= each muscle fibre becomes shorter and so muscle contracts

Question 8

Role of calcium ions

Answer 8

• Opening of their channels allow them to diffuse into the myofibrils sarcoplasm

= tropomyosin molecules move out of binding sites on actin

• Activate ATPase in the myosin head

Question 9

Role of ATP

Answer 9

Hydrolysis of ATP provides energy for:

• Myosin head to change angle and pull actin along
• Myosin head to detach from actin and return to its original position
• Reabsorption of calcium ions into the sarcoplasmic reticulum by active transport

Question 10

Slow twitch muscle fibres

Answer 10

• Contract more slowly
• Can work for longer without getting tired e.g, marathons
• Energy is released slowly though aerobic respiration
• Has a large store of myoglobin - red colour
• Rich supply of blood vessels to deliver oxygen and glucose for aerobic respiration
• Many mitochondria to produce ATP found at edge of fibres = short diffusion pathway for oxygen

Question 11

Fast twitch muscle fibres

Answer 11

• Contract very quickly = get tired quickly
• Good for short bursts of speed and power e.g, sprinting
• High proportions in muscles which need to do short bursts of intense activity
• Energy is released quickly through anaerobic respiration using glycogen
• Thicker and more numerous myosin filaments
• High concentration of glycogen
• A store of PCr so ATP can be generated quickly when needed
• Few myoglobin - whitish colour