5.3 - Neuronal communication

Question 1

What is distinctive about the function of myelinated neurones (in comparison to non-myelinated neurones)?

Answer 1

Action potential conduction is faster in myelinated neurones

Question 2

What do myelinated neurones have that non-myelinated neurones not have that allow myelinated neurones to have faster conduction?

Answer 2

Schwann Cells that produce myelin - provides electrical stimulation

Question 3

What do myelinated neurones prevent?

Answer 3

Depolarisation / movement of ions

Question 4

Where can depolarisation/action potentials only occur?

Answer 4

Where Na+ channels are present

Question 5

In comparison to non-myelinated neurones, what do myelinated neurones have in terms of axon length and Na+ channels?

Answer 5

Longer axons

NO Na+ channels

Question 6

Where can depolarisation/ion movement only take place in myelinated neurones?

Answer 6

Nodes of Ranvier

Question 7

How is the action potential transmitted across myelinated neurones?

Answer 7

Saltatory conduction: action potential jumps from one Node of Ranvier to another (enhances local current effect)

Question 8

Describe the resting potential stage of the action potential (9)

Answer 8

• Sodium-Potassium pump
• 3Na+ ions pumped out of neuron
• 2K+ ions pumped into neuron
• by Active Transport
• K+ channels are open
• Some K+ diffuses back out
• Fewer Na+ channels open, so less Na+ diffuses out
• Voltage-Gated Na+ channels closed
• Membrane potential approx -70mV

Question 9

Describe the Threshold and Depolarisation stage of the action potential (9)

Answer 9

• Stimulus causes some sodium ion channels to open
• Generator potential: membrane potential changes (makes inside of axon LESS negative)
• Some voltage-gated Na+ channels open
• Na+ diffuses into axon
• Causes more voltage-gated Na+ channels to open (positive feedback)
• Threshold is reached
• All voltage-gated Na+ channels open
• Na+ diffuses into the axon
• Depolarisation ends at approx +30mV

Question 10

Describe the Re-polarisation stage of the action potential (2)

Answer 10

• Voltage-gated K+ ion channels open

- K+ diffuses out of axon

Question 11

Describe the Hyperpolarisation stage of the action potential (3)

Answer 11

• Voltage-gated K+ ion channels are slow to close
• Membrane potential more negative than resting potential
• Resting potential established by the sodium-potassium pump

Question 12

Give 9 roles of synapses

Answer 12

1. Allows neurones to communicate/cell signalling
2. Ensures transmission between neurones is unidirectional / in one direction only
3. Allows convergence / impulses from more than one neurone to be passed to a single neurone
4. Allows divergence / impulses from a single neurone to be passed to more than one neurone
5. Filters out low level stimuli / ensures that only stimulation is strong enough will be passed on
6. Prevents fatigue / prevents over-stimulation
7. Allows many low level stimuli to be amplified
8. Presence of inhibitory and stimulatory responses allows impulse to follow a specific path
9. Allows memory/learning/decision making

Question 13

When an action potential is travelling across a neurone where does it travel to in order to deliver the impulse to a neighbouring neurone?

Answer 13

The presynaptic membrane

Question 14

When the action potential arrives at the presynaptic membrane of the synapse, what happens?

Answer 14

Calcium (Ca2+) Channels open

Question 15

What happens when the Calcium channels in the presynaptic membrane open?

Answer 15

Ca2+ diffuse into the presynaptic knob

Question 16

When Ca2+ ions diffuse into the presynaptic knob, what does this initiate?

Answer 16

The neurotransmitter Acetylcholine, is stimulated and travels in vesicles to the presynaptic membrane and fuse with it

Question 17

How does acetylcholine reach the post-synaptic neurone?

Answer 17

Acetylcholine is released by exocytosis into the synaptic cleft and diffuses across it

Question 18

Once Acetylcholine diffuses across the synaptic cleft, what happens?

Answer 18

Acetylcholine binds to receptors on the post-synaptic membrane which causes Na+ channels to open

Question 19

When Na+ channels open in the post-synaptic membrane, what happens?

Answer 19

Na+ diffuses into the post-synaptic membrane and if a threshold is reached an action potential will be generated

Question 20

How is acetylcholine recycled and re-synthesised after transmission across a synapse occurs?

Answer 20

• Acetylcholine is hydrolysed by acetylcholinesterase
• Products diffuse back across the synaptic cleft to the presynaptic knob
• Products re-synthesised into acetylcholine

Question 21

How can a synapse be inhibited (6)?

Answer 21

• Inhibitor binds to/occupies/competes with acetylcholine
• This prevents acetylcholine binding / blocks receptor site
• Ion channels /sodium channels don’t open
• Na+ cannot enter/K+ cannot leave the neurone
• Insufficient depolarisation / excitatory postsynaptic potential/generator potential
• Post synaptic membrane does not reach threshold

Question 22

Why are synapses unidirectional (3) ?

Answer 22

• Acetylcholine released from the presynaptic side
• Diffusion from higher concentration to lower concentration
• Receptors only on the postsynaptic membrane

Question 23

What condition must a stimulus meet to generate an action potential?

Answer 23

Stimuli must reach THRESHOLD

Question 24

What is the all-or-nothing law?

Answer 24

Either an action potential occurs or it doesn’t

Question 25

What is true about the magnitude of action potentials?

Answer 25

They’re all the same magnitude no matter how strong the stimulus

Question 26

What can a strong stimulus initiate?

Answer 26

Produce MANY action potentials in rapid succession