Synapses and Skeletal Muscles

Question 1

How do synapses ensure that nerve impulses only travel in one direction?

Answer 1

Presynaptic membrane is where acetylcholine is released and the post-synaptic membrane has receptors

Question 2

If calcium ion channels are closed, how will it affect muscle contraction?

Answer 2

1. Prevents influx of calcium ions (pre-synaptic membrane)
2. Synaptic vesicles don’t fuse with membrane
3. Neurotransmitter does not diffuse across synapse
   receptors on post-synaptic membrane
4. No depolarisation, Na+ channels don’t open

Question 3

What is the role of calcium ions and ATP in the contraction of a myofibril?

Answer 3

• Calcium ions diffuse into myofibrils from sarcoplasmic reticulum
• Cause movement of tropomyosin
• Causes exposure of the binding sites on the actin
• Myosin heads attach to binding sites on actin
• Hydrolysis of ATP on myosin heads causes myosin heads to bend;
• Pulling actin molecules;
• Attachment of a new ATP molecule to each myosin head causes myosin heads to detach (from actin sites).

Question 4

Why do both slow and fast muscle fibres contain ATPase?

Answer 4

1. Hydrolysis of ATP;
2. Contraction requires energy
3. Use of ATP by myosin.

Question 5

What is the difference in ATPase for fast and slow muscle fibres?

Answer 5

• Fast muscles involve the rapid hydrolysis of ATPase

- Slow muscles involve the rapid synthesis of ATPase

Question 6

What does actin attach to?

Answer 6

The Z line at the end of the sarcomere

Question 7

What does ATPase do?

Answer 7

Hydrolyses ATP

Question 8

What does tropomyosin do?

Answer 8

Covers the binding site on actin in a relaxed myofibril

Question 9

Why is muscle contraction prevented in myosin molecules cannot bind to other myosin molecules?

Answer 9

1. Can’t form myosin
2. Can’t move actin
3. Myosin moves
4. Can’t pull Z lines together.

Question 10

If there are many mitochondria, slow or fast fibres?

Answer 10

Slow because it uses aerobic respiration which occurs in the mitochondria

Question 11

Why are the leg muscles of athletes bigger than those of non-athletes?

Answer 11

• They don’t have more fibres
• The diameter of each fibre is larger due to excercise
• Slow fibres with wider diameter than fast fibres

Question 12

What is the role of phosphocreatine in a muscle contraction?

Answer 12

Provides phosphates that can be used to make ATP for energy

Question 13

What is spatial summation

Answer 13

This is where many PREsynaptic neurones being stimulated by low-frequency action potentials combine their small amounts of neurotransmitters to one POSTsynaptic neurone to exceed its threshold.

Question 14

What is temporal summation?

Answer 14

when one presynaptic neurone releases neurotransmitter many times over a period of time. The total amount of neurotransmitter released may exceed the threshold value of the postsynaptic neurone. Higher frequency of action potential = threshold exceeded quickly.