L8. Summation, Action Potentials, Neuronal Output

Question 1

Axon hillock summation?

Answer 1

All local (graded) charges add up (summate) at the axon hillock
- Spatial summation = summation of local potentials from different inputs that occur close enough together in space
- Temporal summation = summation of local potentials from different inputs that occur close enough in time
If the net threshold at the axon hillock reaches -60mV then an action potential will fire

Question 2

Depolarisation?

Answer 2

1. • Cell at RMP -70mV
     - Na+/K+ pump (ATPase)
2. • Input to the cell from outside
     - Reaches threshold (-60mV)
     - Voltage-gated Na+ channels open
     - Large amounts of Na+ move into cell
     - Membrane potential reaches +30mV

Question 3

Repolarisation?

Answer 3

3. • Voltage-gated Na+ channels close
     - Voltage-gated K+ channels open
     - K+ leaves the cell
     - The cell becomes more negative

Question 4

Hyperpolarisation?

Answer 4

4. • Voltage-gated K+ channels begin to close slowly at -40mV
     - RMP reestablished
     - Na+/K+ pump (ATPase)

Question 5

Refractory period?

Answer 5

A period of inactivity after a neuron has fired in which another AP can’t be generated, because voltage-gated Na+ channels are either open (causing depolarisation) or inactive (hyper-polarisation)

Question 6

Action potential - propagation?

Answer 6

The action potential regenerates down the length of the axon fast

Because of the refractory period the action potential can only move forward

Question 7

Myelin?

Answer 7

• Made from oligodendrocytes (CNS) or Schwann cells (peripheral) and is wrapped around the axon
• The action potential regenerates at each gap (Nodes of Ranvier) between the myelin sheath
• Myelin sheath increases the speed of action potentials down the axon

Question 8

Disease - Multiple Sclerosis?

Answer 8

• A demyelinating disease affecting the CNS caused by the destruction of the oligodendrocytes

Question 9

Neuronal output - synaptic transmission?

Answer 9

1. Depolarisation of axon terminal –> voltage-gated Ca2+ channels open –> Ca2+ enters axon terminal
2. Ca2+ triggers neurotransmitters to be released from vesicles into synaptic cleft
3. Neurotransmitter diffuses across the synaptic cleft
4. Neurotransmitter binds to its receptor (chemically-gated ion channel) on the post-synaptic membrane
5. Na+ enters the post-synaptic cell
   –> depolarises post-synaptic cell

Question 10

Electrical vs Chemical Synapse?

Answer 10

Electrical synapse (gap junction) =
- Very fast
- Ions flow through gap junction from cell to cell
- Can be bidirectional
- Can be opened by voltage, pH, Ca2+ and receptors

Chemical synapse (nerve terminal) =
- Slower than electrical synapse
- Complex series of events
- Relies upon neurotransmitter crossing the synaptic cleft
- One way conduction
- Synapse - strength can increase/decrease