Lecture 9: Nerve 2

Question 1

What is summation?

Answer 1

All local voltage changes add up at the axon hillock

Question 2

What is spatial summation?

Answer 2

Summation of local potentials from different inputs that occur close enough together in space

Question 3

What is temporal summation?

Answer 3

Summation of local potentials from different inputs that occur closer enough in time

Question 4

What must occur in order for an action potential to fire?

Answer 4

The net threshold at the axon hillock must reach -60 mV (threshold) to generate an action potential

Question 5

Describe what steps occur in an action potential:

Answer 5

1. Cell at RMP -70mV
   - Na+/K+ -pump (ATPase)
2. Input to the cell from outside
   - Reaches threshold (-60mV)
3. DEPOLARISATION
   - Voltage-gated Na+ channels open
   - Na+ moves into cell
   - Cell becomes more positive
   - Reaches +30mV
4. REPOLARISATION
   - Voltage-gated Na+ channels close
   - Voltage-gated K+ channels open
   - K+ leaves the cell
   - Cell becomes more negative
5. HYPERPOLARISATION
   - Voltage-gated K+ channels begin to close slowly at -40mV
   - RMP reestablished
   - Na/K+ (ATPase)

Question 6

What is the refractory period?

Answer 6

• Not able to generate another action potential during this period
• The voltage-gated Na+ channels are either already open (causing depolarisation) or inactive (during hyperpolarisation).

Question 7

Describe the propagation of an action potential:

Answer 7

• The action potential regenerates down the length of the axon fast
• Because of the refractory period the action potential can only move forward

Question 8

What is myelin?

Answer 8

Myelin is made from a specialised cells, called 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

Question 9

How does myelin affect the speed of an action potential?

Answer 9

The myelin sheath increases the speed of action potentials down the axon.
* Unmyelinated axons are fast: ~1 m/s, 3.6 km/h
* Myelinated axons are really fast: ~100-150 m/s, 360-540 km/h

Question 10

What protection does myelin provide for the axon?

Answer 10

It protects the axon from the spread of electrical current to unintended target cells (insulation)

Question 11

What is multiple sclerosis?

Answer 11

• Multiple sclerosis is a demyelinating disease affecting the CNS.
• It might result from (immune) destruction of the myelin‐producing cells (oligodendrocytes)
• Lack of myelin impairs nerve impulse conduction leading to a variety of sensory, motor, visual, and autonomic deficits, such as weakness, loss of coordination, visual impairment, speech disturbances

Question 12

What are the differences between an electrical synapse and a chemical synapse?

Answer 12

Electrical synapse:

• Very fast
• Ion flow through gap junction from cell to cell
• Can be bi-directional
• Can be opened by voltage, pH, Ca2+ and receptors

Chemical synapse:

• Complex series of events - slower
• Relies upon neurotransmitter crossing the synaptic cleft
• Synapse strength can increase/decrease
• One way conduction

Question 13

What occurs at the neuronal output?

Answer 13

Chemical synaptic transmission:

1. Depolarisation of axon terminal → voltage-gated Ca2+ channels open → Ca2+ enters axon terminal
2. Ca2+ triggers neurotransmitter to be released from vesicles into the 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