Action potential propagation and synaptic transmission

Question 1

What types of gated channels are located on a neuron and where?

Answer 1

Axon hillock: VG Na+ and K+ channels
Axon: VG Na+ and K+ channels
Axon terminals: VG Ca2+ channels

Question 2

What is action potential propagation?

Answer 2

• During rapid depolarisation a ‘flood’ of Na+ enters the Axon Hillock
• Na+ diffuses from the points of entry
• Bringing the axon’s initial segment membrane to threshold
• Which stimulates an action potential at the initial segment

Question 3

Describe action potential propagataion in unmyelinated axons

Answer 3

1. An action potential develops at the initial segment, the membrane potential at this site depolarises to +30mV (now in the absolute refractory period)
2. As the sodium ions entering spread away from the open VG channels, a graded depolarisation quickly brings the membrane in segment 2 to threshold
3. An action potential develops in segment 2. The first segment starts to repolarise
4. As the sodium ions entering at segment 2 spread laterally, a graded depolarisation quickly brings the membrane in segment 3 to threshold.
   The action potential can only move forwards, not backward.

Question 4

Why do we have myelinated axons?

Answer 4

Because action potentials propagate along unmyelinated axons relatively slowly. Some axons can be very long so a slow action potential is not adequate for all of our needs. Myelination dramatically increases conduction velocity.

Question 5

Describe action potential propagation in myelinated axons

Answer 5

1. An action potential develops at the initial segment
2. A local current produces a graded depolarisation that brings the axolemma at node 1 to threshold.
3. Anaction potential develops at node 1. The initial segment begins to depolarise.
4. A local current produces a graded depolarisation that brings the axolemma at node 2 to threshold.

Question 6

What is the absolute refractory period?

Answer 6

Includes rapid depolarisation and repolarisation.
- A second action potential cannot be generated in this period
- Occurs when VG Na+ channels are open or become inactive

Question 7

What is the relative refractory period?

Answer 7

Includes hyper polarisation.
- A second action potential can be generated only if the stimulus is much larger than normal
- Occurs when some VG Na+ channels begin to shift from inactive to closed state
Note: VG Na+ channels cannot open when inactive. They only open from a closed state.

Question 8

List the key features of a chemical synapse

Answer 8

Presynaptic axon terminal:
- VG Ca2+ channels
- Synaptic vesicles filled with neurotransmitter
Synaptic cleft:
- A space neurotransmitter diffuses across
- Enzymes that deactivate neurotransmitter are present in the cleft
Postsynaptic cell:
- Chemically gated ion channels

Question 9

Describe synaptic transmission: 1. Axon terminal depolarised

Answer 9

• The action potential deopolarises the axon terminal membrane to threshold (-60mV)
• … causing VG Ca2+ channels to open
• Ca2+ moving down its electrochemical gradient into the axon terminal

Question 10

Describe synaptic transmission: 2. Release of neurotransmitter

Answer 10

• Ca2+ interacts with vesicles
• … causing them to release neurotransmitter (eg. ACh) into the synaptic cleft
• Neurotransmiter diffuses across the synaptic cleft

Question 11

Describe synaptic transmission: 3. Formation of local potentials

Answer 11

Neurotransmitter binds to chemically gated ion channels on the post-synaptic cell.
- Excitatory neurotransmitter (eg. ACh or NE) opens Na+ channels to cause EPSPs
- Inhibitory neurotransmitter (eg. GABA) opens K+ channels to cause IPSPs

Question 12

Describe termination, end of synaptic transmission

Answer 12

Synaptic transmission ends when:
- Neurotransmitter unbinds from chemically-gated channels
- Enzymes in the synaptic cleft degrade neurotransmitter
- Portions of the degraded neurotransmitter are recycled back into the axon terminal

Question 13

What are the key features of a neruomuscular junction (NMJ)?

Answer 13

Key features of NMJ:
- A specialised type of chemical synapse
- Between axon terminal of a motor neuron and skeletal muscle fibre
- Is a cholinergenic synapse (neurotransmitter is ACh) so is always excitatory
- Summation is usually not needed - one synaptic transmission typically results in bringing muscle membrane to threshold

Question 14

Neuron-neuron synapse vs Neuromuscular junction

Answer 14

Neuron-Neuron synapse:
- Synapses are tiny, each synapse may be one of thousands on the post-synaptic cell
- Requires summation: Single presynaptic AP will rarely bring postsynaptic cell to threshold
- Inputs may be excitatory or inhibatory (EPSPs & IPSPs)
- Variety of neurotransmitters
Neuromuscular junction:
- Synapses are huge, each muscle fibre receives input from only one neuron at one site.
- No summation required: AP from motor neuron very likely to bring muscle to threshold
- Inputs are only excitatory
- Only acetylcholine (ACh) used.

Question 15

Describe electrical synapses

Answer 15

• Electrical synapses allow depolarisation to pass faster across gap junctions
• Relatively rare - allow no opportunity for signal modulation
  Example: found in heart to facilitate coordinated waves of depolarisation and contraction in cardiac muscle