Chapter 13 - Neuronal Communication

Question 1

What is cell signalling?

Answer 1

Cell releases chemical which has an effect on the target cell.

Question 2

Structure of a neurone

Answer 2

Cell body, dendrons and dendrites, axon.

Question 3

Types of neurone

Answer 3

Sensory, relay and motor neurones.

Question 4

What is the myelin sheath?

Answer 4

Layers of plasma membrane produced by Schwann cells growing around axon, acts as insulation.

Question 5

Transducer

Answer 5

Converts one form of energy into another e.g. mechanical to electrical.

Question 6

Pacinian corpuscle

Answer 6

Mechanoreceptor.

1. Pacinian corpuscle in resting state.
2. Pressure applied, causes it to change shape and sodium ion channels widen, sodium ions diffuse in.
3. Membrane becomes depolarised, which generates an action potential which is then transmitted along CNS.

Question 7

How is resting potential maintained?

Answer 7

1. Two Na+ out, 3 K+ in via Na-K pump (actively).
2. Na+ diffuses back in down electrochemical gradient. K+ diffuses out.
3. However, most Na+ voltage-gated channels are closed whereas they are open for K+, so it is more positive outside of the axon than inside. Resting potential of -70mV created in axon.

Question 8

How is an action potential generated?

Answer 8

1. Resting potential.
2. Stimulus triggers some Na+ V-G channels to open, Na+ ions diffuse in. Membrane becomes depolarised.
3. At +40mV, Na+ V-G channels close whereas K+ V-G channels open.
4. K+ ions diffuse out of membrane down electrochemical gradient.
5. At first lots of K+ ions leave (hyperpolarisation), then K+ V-G channels close and Na-K pump moves Na+ out and K+ in so back to resting p.d.

Question 9

Refractory period

Answer 9

Short time when axon cannot be excited again after action potential.

Question 10

What is the purpose of the refractory period?

Answer 10

Ensures action potential is unidirectional, does not overlap and occurs as discrete impulses.

Question 11

Saltatory conduction

Answer 11

The action potential is able to jump between Nodes of Ranvier due to the presence of localised circuits created by the Nodes.

Increases speed of transmission.

Question 12

What factors affect the speed of impulse transmission?

Answer 12

Axon diameter: bigger axon provides less resistance to flow of ions.

Temperature: higher temperature results in faster transmission, usually up to 40 degrees C (proteins denature beyond this).

Question 13

All-or-Nothing principle

Answer 13

If the threshold value is not reached, an action potential will not be generated.

Question 14

Structure of a synapse

Answer 14

Synaptic cleft.
Pre-synaptic and post-synaptic neurones.
Synaptic knob.
Synaptic vesicles.
Neurotransmitter receptors.

Question 15

Types of neurotransmitter

Answer 15

Excitatory - cause depolarisation of post-synaptic neurone, trigger action potential.

Inhibitory - cause hyperpolarisation of post-synaptic neurone, prevents action potential being triggered.

Question 16

Transmission across synapse.

Answer 16

1. Action potential reaches synaptic knob, depolarises so Ca2+ channels open and diffuse into knob.
2. Causes synaptic vesicles to release acetylcholine into synaptic cleft via exocytosis.
3. Acetylcholine binds to specific receptors on post-synaptic membrane, action potential is triggered.
4. Acetylcholine broken down by acetylcholinesterase into choline and ethanoic acid, and is recycled.

Question 17

Spatial summation

Answer 17

Several pre-synaptic neurones connect to single post-synaptic neurone.

Each release neurotransmitter, threshold value is reached and action potential is triggered.

Question 18

Temporal summation

Answer 18

Single pre-synaptic neurone releases neurotransmitter several times over short period.

Threshold value reached, action potential triggered.

Question 19

Effect of stimulatory drugs on the nervous system.

Answer 19

Create more action potential by:

• Mimicking shape of neurotransmitter
• Stimulating release of neurotransmitter
• Inhibiting enzyme that breaks down neurotransmitter

Question 20

Effect of inhibitory drugs on nervous system.

Answer 20

Blocks receptors so neurotransmitter cannot bind.