Topic 6: skeletal muscle

Question 1

What is the role of the following in muscle contraction: phosphocreatine

Answer 1

In times when aerobic respiration cannot create enough ATP to meet this demand, anaerobic respiration occurs. The chemical
phosphocreatine, which is stored in muscles, assists this by providing phosphate to regenerate ATP from ADP.

Question 2

Describe the sliding filament theory

Answer 2

When an action potential reaches a muscle, it stimulates a response.

Calcium ions (are released from sarcoplasmic reticulum) enter and cause the protein tropomyosin, that block binding sites for the myosin head of the actin, to move and uncover the binding sites.

Whilst ADP is attached to the myosin head, it can bind to the binding site on the actin to form a cross-bridge.
The angle created in this cross-bridge creates tension and as a result, the actin filament is pulled and slides along the myosin.
In doing so the ADP molecule is released.

An ATP molecule can then bind to the myosin head and causes it to change shape slightly and as a result, it detaches from the actin.
Within the sarcoplasm, there is the enzyme ATPase, which is activated by the calcium ions, to hydrolyse the ATP back into ADP, to allow the myosin heads to recock and re-attach to the actin filament at a binding site further along.

Question 3

Properties of slow-twitch fibres

Answer 3

Structure: contains a large store of myoglobin, a rich blood supply and many mitochondria

Location: calf muscles

General properties: contract slower and can respire aerobically for longer periods of time due to the rich blood supply and myoglobin oxygen store. These muscles are adapted for endurance work like, marathons

Question 4

Properties of fast-twitch fibres

Answer 4

Structure: thicker and more myosin filaments, a large store of glycogen, a store of phosphocreatine to help make ATP from ADP and a high concentration of enzymes involved in anaerobic respiration.

Location: Biceps

General properties: contract faster to provide short bursts of powerful contraction. These are adapted for intense exercise such as sprinting

Question 5

State what would happen to the
- H zone
- A band
- Z lines
when sarcomere contracts

Answer 5

• size of H zone decreases
• size of the A band remains the same because myosin is constant and does not move
• distance between Z lines
• I band decreases

Question 6

Compare neuromuscular junction with cholinergic synapse

Answer 6

Neuromuscular junction
- unidirectional due to neurotransmitter receptors only being on postsynaptic membrane
- Only excitatory
- Connects motor neurone to muscles
- This is the end point for the action potential
- acetylcholine binds to receptors on muscle fibres membranes

Cholinergic synapse
- could be excitatory or inhibitory
- unidirectional due to neurotransmitter receptors only being on postsynaptic membrane
- connects 2 neurons, which could be sensory, relay or motor
- a new action potential is generated in the next neuron
- acetylcholine binds to receptors on postsynaptic membrane of a neuron

Question 7

Define how muscles work. Key words

Answer 7

muscles act in antagonistic pairs against an incompressible skeleton to create movement. This can be automatic as part of a reflex or controlled by conscious thought

Question 8

What are myofibrils made of?

Answer 8

Myofibril made up of fused cells that share nuclei and cytoplasm, known as sarcoplasm, and there is a high number of mitochondria

Question 9

What two types of protein that make up myofibril to form a sarcomere?

Answer 9

myosin
actin

Question 10

Which protein is the thicker filament?

Answer 10

myosin

Question 11

How long does sliding filament action go on for?

Answer 11

Entire process repeats continually whilst the calcium ions remain high, and therefore whilst the muscle remains stimulated by the nervous system