Muscles & Neurones

Question 1

What is the shape of an action potential curve?

Answer 1

Question 2

What is an action potential?

Answer 2

A self-propagating wave of electrical activity which passes along neurones as a way of transmitting information

Question 3

What are the stages of an action potential?

Answer 3

1. Depolarisation
2. Repolarisation
3. Hyperpolarisation

Question 4

How is resting potential maintained?

Answer 4

1. Na⁺/K⁺ pumps - active transport of ions
   
   • 3 Na⁺ out of the axon, 2 K⁺ into the axon
2. K⁺ “leak” gates - diffusion
   
   • K⁺ diffuses out of the axon, down a concentration gradient

Question 5

What happens during an action potential?

Answer 5

Depolarisation:

• Stimulus opens Na⁺ channels (if threshold is reached)
• Na⁺ ions diffuse into the axon

Repolarisation:

• Na⁺ channels close and K⁺ channels open
• K⁺ ions diffuse out of the axon

Hyperpolarisation:

• K⁺ channels close
• Resting potential is restored by the Na⁺/K⁺ pump and K⁺ “leak” gates

Question 6

What is the myelin sheath?

Answer 6

Schwann cells wrap around axon, cytoplasm rich in a lipid (myelin)

Acts as an electrical insulated to the axon

Question 7

How does an action potential travel down a myelinated axon?

Answer 7

Jumps between nodes of Ranvier (saltatory conduction)

Forms localised circuits between nodes

Question 8

What is the refractory period?

What is its purpose?

Answer 8

It is the period during which a new action potential cannot be formed.

It ensures:

1. Unidirectionality of impulses
2. Impulses are discrete

Question 9

What is the all or nothing principle?

Answer 9

If threshold is reached, an action potential is always formed

These action potentials are always the same size

Increasing the strength of the stimulus increases the frequency of the action potentials not the size

Question 10

What factors affect speed of transmission of an action potential?

Answer 10

Temperature

Axon diameter

Myelinated axon

Question 11

How does an impulse pass across a synapse?

(cholinergic synpse)

Answer 11

1. Action potential arrives at presynaptic neurone, opening Ca²⁺ channels
2. Ca²⁺ ions diffuse into the presynaptic neurone and cause vesicles to bind to the presynaptic membrane
3. Acetylcholine is released into the synapse and diffuses to the postsynaptic neurone
4. Acetylcholine binds to receptors on the postsynaptic neurone and causing Na⁺ ion channels to open
5. Na⁺ ions diffuse into the postsynaptic neurone generating an action potential
6. Acetylcholine is broken down by acetylcholinesterase into ethanoic acid and choline which are absorbed by the presynaptic neurone
7. Na⁺ channels close on the postsynaptic neurone and ATP is used to reform aceytlcholine

Question 12

What ensures unidirectionality of synapses?

Answer 12

1. Only neurotransmitters in the presynaptic neurone
2. The neurotransmitter diffuses down a concentration gradient
3. There are only receptors on the postsynaptic neurone

Question 13

What are the types of summation? How do they work?

Answer 13

Spatial - multiple presynaptic neurones releasing neurotransmitters

Temporal - presynaptic neurone carrying high frequency action potentials

Question 14

What is the structure of a muscle?

Answer 14

Question 15

What is the name and structure of the thick filament?

Answer 15

Myosin

Question 16

What is the name and structure of the thin filament?

Answer 16

Actin

Question 17

What is the name and structure of the individual units of a myofibril?

Answer 17

Sarcomere

Question 18

What are the two types of twitch fibre?

Answer 18

1. Slow
2. Fast

Question 19

What are the characteristics of slow twitch fibres?

Answer 19

• Slow contractions
• Aerobic respiration
• Take a long time to fatigue
• Rich in myoglobin

Question 20

What are the characteristics of fast twitch fibres?

Answer 20

• Fast contractions
• Anaerobic respiration
• Fatigue quickly
• Little myoglobin

Question 21

What are the sources of energy in muscles?

Answer 21

1. Aerobic respiration of glucose - slow
2. Anaerobic respiration of glucose - fast but produces lactic acid
3. Phosphocreatine (PCr) system - fast but runs out quickly

Question 22

What is a neuromuscular junction?

Answer 22

Where a muscle a motor neurone meet, they act the same as a cholinergic synapse

Question 23

Why are there many neuromuscular junctions along a muscle?

Answer 23

So all the muscles cells contract simultaneously

Question 24

How does a muscle contract?

(Assume the action potential has already been tansmitted across the neuromuscular junction and been generated in the muscle)

Answer 24

1. The action potential travels down the T-tubules to the sarcoplasmic reticulum
2. Ca²⁺ channels open and Ca²⁺ ions diffuse into the sarcoplasm
3. Ca²⁺ binds to troponin and causes the tropomyosin to change shape, exposing the binding sites on the actin
   
   • Ca²⁺ ions also activate ATPase (hydrolyses ATP)
4. The myosin head binds to the actin, forming cross-bridges
5. Energy from ATP is used to change the position of the myosin head producing a power-stroke
6. Energy from ATP is also used to break the cross-bridges
7. The head returns to its original position and can now bind further along the actin
8. This repeats as long as Ca²⁺ ions are present

Question 25

What happens when nervous stimulation of a muscle ceases?

Answer 25

1. The Ca²⁺ channels close
2. Ca²⁺ is actively transported into the sarcoplasmic reticulum
3. Tropomyosin returns to its original shape and troponin blocks the binding site on the actin
4. The myosin cannot bind so the muscle relaxes