Topic 5: Nervous Transmission

Question 1

What are action potentials?

Answer 1

nerve impulses

Question 2

What do nerve action potentials require?

Answer 2

• A membrane potential:

An electrical charge difference across cell membrane – like a battery

• And Ion Channels: allow ions to move by diffusion

Question 2

If there is no action potential, the resting cell has a resting membrane potential of?

Answer 2

-70mv

Question 3

What does actual resting membrane potential depend on?

Answer 3

the relative leakage channel numbers

Question 3

What do leakage channels consist of and what is their function

Answer 3

• Cytosol high in K+ & interstitial fluid high in Na+ (sodium –potassium pumps)
• Leakage lets K+ through easily and Na+ poorly
• inside is negative relative to outside

Question 3

Explain what action potential (AP) is

Answer 3

• Series of active events
• Channels actively open & close
• Some initial event is required to reach a voltage threshold (~ - 55 mv)
• Stimulus (Any event bringing membrane to threshold)

Question 4

In action potential: What are the 2 phases that can occur after the arrival of a stimulus

Answer 4

Depolarizing phase
Repolarizing phase

Question 5

What happens in the Depolarizing phase

Answer 5

• membrane potential rises and becomes positive

Question 6

What happens in the Repolarizing phase

Answer 6

• potential restored to resting value
• may overshoot = hyperpolarizing phase
• then recovery to rest.

Question 7

In Action potential events, What happens if Stimulus is strong enough to reach threshold?

Answer 7

1. Na+ channels open
2. K+ channels then open
3. Sodium/potassium pump

Question 8

In action potential events, What happens if the Na+ channels open

Answer 8

• Na+ ions enter the cell from extracellular fluid
• depolarization (positive membrane potential)

Question 9

In action potential events, What happens when K+ channels open

Answer 9

• K+ leave the cell
  
  • repolarization (negative membrane potential)

Question 10

In action potential events what does the sodium/potassium pump restore?

Answer 10

restores original ion concentrations

Question 11

Explain the all or none law

Answer 11

• This sequence is always the same
• If threshold is reached then the action potentials are always the same size.
• Stimulus must reach threshold to start
• After one AP there is a short period before next can be triggered = refractory period
• A strong stimulus changes the frequency of action potentials not the size of the peak

Question 12

What effect does caffeine have on nervous transmission

Answer 12

Caffeine=a stimulant found in coffee, tea, cocoa, cola etc.

Acts by lowering the threshold level of the nerve

increases the possibility for a stimulus to result in a nerve potential

Question 13

What effect does alcohol have on nervous transmission

Answer 13

Alcohol=acts as a depressant

increases the nerve threshold level

decreases the possibility of a stimulus to result in a nerve action potential

Question 14

What effect does Local Anaesthetics have on nervous transmission

Answer 14

Local Anaesthetics = Generation of nerve action potentials can be prevented by local anaesthetics such as novocaine and xylocaine.

These drugs prevent the opening of sodium gated channels in the nerve cell membrane

pain messages are not carried from the site of injury back to the CNS.

Question 15

Conduction of nerve impulses:

each section triggers the next by using what?

Answer 15

Local currents

Question 16

Conduction of nerve impulses:

What is the function of the refractory period in conduction of nerve impulses

Answer 16

refractory period keeps it going the right direction

Question 17

Conduction of nerve impulses:

Function of unmyelinated fibre in conduction of nerve impulses

Answer 17

Unmyelinated fibre = continuous conduction

Question 18

Conduction of nerve impulses:

Function of fibre with myelin in conduction of nerve impulses

Answer 18

With myelin = saltatory conduction

• can only be triggered at Nodes of Ranvier
• Myelinated fibers faster & larger neurons faster

Question 19

What is synaptic transmission and how is it triggered?

Answer 19

• Sequence of events at synapse
• Triggered by voltage change of the action potential

Question 20

A sending neuron is described as:

Answer 20

Sending neuron = presynaptic

Question 21

A receiving neuron is described as:

Answer 21

Receiving neuron = postsynaptic

Question 22

What are the 3 presynaptic events?

Answer 22

1. Action potential arrives at presynaptic end bulb
2. Opens voltage gated Ca2+ channels

• Ca2+ flows into cell from synaptic cleft
• increased Ca2+ concentration in presynaptic cell
• exocytosis of synaptic vesicles

3. Neurotransmitter released into cleft, diffuse across to postsynaptic cell membrane

Question 22

Provide a definition for a neurotransmitter

Answer 22

Neurotransmitter = chemical that carries signal across synaptic cleft

Question 23

Describe the 4 post synaptic events

Answer 23

4. NT binding at postsynaptic receptors
5. Chemical trigger of ion channels
6. May depolarize or hyperpolarize postsynaptic cell membrane
7. If threshold reached at axon hillock

• then postsynaptic cell action potential results

Question 24

Explain Termination of Synaptic Transmission

Answer 24

Neurotransmitter must be removed from the cleft:

• Diffusion
• Destroyed by enzymes in cleft
• Transported back into presynaptic cell
• Destroyed by neuroglia

Question 25

State 5 neurotransmitters

Answer 25

1. Acetylcholine (Ach):
   - Common in PNS
   - May be stimulatory or inhibitory
2. Amino Acids:
   - glutamate, aspartate, gamma aminobutyric acid (GABA), glycine
3. Modified amino acids: norepinephrine (NE), dopamine (DA), serotonin (5-HT)
4. Neuropeptides: endorphins
5. Nitric oxide (NO)

Question 26

What 3 chemicals effect synaptic transmission and why?

Answer 26

1. Botulinum toxin
   - inhibits release of acetylcholine
   - inhibits muscle contraction (paralysis)
2. Curare
   - competes for acetylcholine receptor sites on muscle cells
   - muscle relaxant ( used in surgical procedures)
3. Nerve agents
   - inactivates acetylcholinesterase
   - leads to tetanus (paralysis)