6.2 NERVOUS COORDINATION

Question 1

What is the structure of a motor neurone?

Answer 1

• Body
• Dendrites
• Schwann Cells
• Nodes of Ranvier
• Axon
• Myelin sheath

Question 2

What is the function of the axon body?

Answer 2

Contains organelles, makes protein and neurotransmitters.

Question 3

What is the function of the dendrites?

Answer 3

They carry nerve impulses to the axon.

Question 4

What is the function of the axon?

Answer 4

Long fibres that carry impulses away from the axon body.

Question 5

What is the function of the Schwann cells?

Answer 5

• Protect the axon
• Insulate the axon with the myelin sheath
• Carries out phagocytosis to remove debris and regenerate nerves.

Question 6

What is the resting potential?

Answer 6

The electrical potential across the membrane when not conducting an impulse.

Question 7

How is the resting potential maintained across an axon membrane?

Answer 7

• The sodium and potassium voltage-gated ion channels are closed.
• Sodium potassium pump actively transports 3 sodium ions out and 2 potassium ions into the axon.
• The membrane is more permeable to potassium ions leaking out than sodium ions.
• Sodium and potassium ions also move by simple diffusion.
• All these together allow the outside of the membrane to be more positive than the inside, creating and maintaining the action potential at -70mV.

Question 8

How does the membrane reach the action potential?

Answer 8

1. Stimulus causes depolarisation, where voltage-gated sodium ion channels open.
2. Influx of sodium ions into the axon causes the potential difference to get more positive.
3. Once the difference reaches -55mV, more channels open and there is a greater influx of sodium ions.
4. It will reach the point where the potential difference is +30mV, at which an action potential is sent.

Question 9

How do you go from the action potential back to the resting potential?

Answer 9

1. After +30 mV, sodium ion channels close.
2. Potassium ion channels open, causing potassium ions to rush out of the axon.
3. This makes the potential difference more negative until it reaches -70mV: resting potential.

Question 10

Why does the resting potential not return immediately?

Answer 10

The membrane is more permeable to the leakage of potassium, so too many potassium ions leave making the potential difference < 70mV: hyperpolarisation.

Question 11

How do you go from hyperpolarisation to the resting potential?

Answer 11

Step 1, of how to achieve the resting potential with the sodium/potassium pump.

Question 12

What is the period of hyperpolarisation called?

Answer 12

The refractory period

Question 13

What is the importance of the refractory period?

Answer 13

• Limits the frequency of impulses.

- Allows for the discrete sending of impulses.

Question 14

What is the all-or-nothing principle?

Answer 14

The threshold for an action potential is +30mV, any voltage >30mV will still send the same action potential.

Question 15

What is the impulse passage of an unmyelinated axon?

Answer 15

A wave of depolarisation and action potentials travel down the whole axon in sequence until it reaches the end.

Question 16

What is the impulse passage of a myelinated axon?

Answer 16

• Myelin sheath is an insulator so action potentials cannot be sent in its’ presence.

Depolarisation jumps between nodes of Ranvier, so missed long parts of the axon length, making the speed of conductance quicker than in an unmyelinated axon.

This is called ‘Saltatory Conduction’.

Question 17

What increases the speed of conduction?

Answer 17

• The presence of the myelin sheath.
• The bigger the diameter of the axon, so less leakage to ions.
• Higher temperature so more diffusion of ions and faster depolarisation.

Question 18

How is an impulse sent across a cholinergic synapse? STEP 1

Answer 18

Action potential reaches the pre-synaptic membrane, so Ca²⁺ channels open and calcium ions move into the pre-synaptic knob.

Question 19

How is an impulse sent across a cholinergic synapse? STEP 2

Answer 19

The calcium ion influx causes vesicles to bind to the pre-synaptic membrane and release neurotransmitters into the synaptic gap.

Question 20

How is an impulse sent across a cholinergic synapse? STEP 3

Answer 20

Acetylcholine diffuses across the gap and binds to receptors on sodium ion channels on the postsynaptic membrane.

Question 21

How is an impulse sent across a cholinergic synapse? STEP 4

Answer 21

This causes sodium ion channels to open, so they move into the postsynaptic axon and depolarise it. This creates an action potential in the postsynaptic neurone.

Question 22

How is an impulse sent across a cholinergic synapse? STEP 5

Answer 22

Then the acetylcholine breaks down into acetyl & choline which diffuses back to the presynaptic membrane to be used again.