Lecture 5.2: Action Potentials

Question 1

Resting Membrane Potential

Answer 1

All cells have an electrical potential (voltage) difference across their plasma membrane (membrane potential)

Membrane potential of cells under resting condition is defined as the resting membrane potential

Changes in the membrane potential provide the basis of signalling in the nervous system and in many other types of cells

Question 2

Factors Influencing Resting Potential

Answer 2

The ionic permeability of the cell membrane
The ionic concentrations on either side of the cell membrane
K+ is the main ion affecting the resting membrane potential
• K+concentration gradient
• Selective permeability of membrane to K+

Question 3

Depolarisation

Answer 3

A decrease in the absolute value of the membrane potential

Cell interior becomes less negative

E.g. -70mv to +40mv

Opening Na+ or Ca2+ channels will cause depolarisation

Question 4

Hyperpolarisation

Answer 4

An increase in the absolute value of the membrane potential.

Cell interior becomes more negative

E.g. -70mv to -90mv

Opening K+ or Cl- channels will cause hyperpolarisation

Question 5

Equilibrium potentials for physiological ions

Answer 5

EK= - 95 mV
ENa = + 70 mV
ECl = - 96 mV
ECa = + 122 mV

Question 6

How do neurones “communicate”?

Answer 6

Neurons (nerve cells) communicate through electrical signals

Incoming signals are received across intercellular gaps called synapses (via neurotransmitters) on extensions of the cell membrane called dendrites

Question 7

Where do synaptic connections occur?

Answer 7

• nerve cell – nerve cell
• nerve cell – muscle cell
• nerve cell – gland cell
• sensory cell – nerve cell

Question 8

Synaptic Transmission

Answer 8

At the synapse, a chemical transmitter released from the presynaptic cell binds to receptors on the postsynaptic membrane

Question 9

Sympathetic Nervous System

Answer 9

Prepares the body for fight and flight response
Excitatory (this is a massive oversimplification)

Question 10

Parasympathetic Nervous System

Answer 10

Restores the body to a calm and composed state and prevents it from overworking
Inhibitory (this is a massive oversimplification)

Question 11

Excitatory Synapses

Answer 11

• Excitatory transmitters open ligand-gated channels permeable to Na+ ,Ca2+, sometimes cations in general (nAChR)
• Cause membrane depolarisation called an Excitatory postsynaptic potential (EPSP)

1. Longer time course than action potential
2. Graded with amount of transmitter
3. Transmitters include: Acetylcholine, Glutamate

Question 12

Inhibitory Synapses

Answer 12

• Inhibitory transmitters open ligand-gated channels permeable to K+ or Cl-
• Cause membrane hyperpolarization
• The resulting change in membrane potential is called an Inhibitory
postsynaptic potential (IPSP)

Transmitters include: Glycine, g-aminobutyric acid (GABA)

Question 13

Summation of inputs: where are they received?

Answer 13

Excitatory and inhibitory inputs are received at dendrites

Question 14

Summation of inputs: where are the summed?

Answer 14

Inputs are summed in the perikaryon at the axon hillock

Question 15

Summation of inputs: when is an action potential generated?

Answer 15

If the summed input above a threshold of ≈ -55mV, an action potential is started in the axon

Question 16

Initiation of an action potential

Answer 16

If the axon hillock membrane potential exceeds the threshold of -55mV

Voltage-gated sodium channels open as the cell depolarises

Positive feedback

“All or nothing”

Question 17

Phases of an action potential 1: Depolarisation + AP peak

Answer 17

Sodium channels are fast to open

Rapid influx of Na+ ions cause the upstroke of the AP

Membrane potential goes towards ENa

Na channels also inactivate rapidly stopping Na+ influx

Question 18

Phases of an action potential 2: Repolarisation

Answer 18

Na channels inactivate and close
Depolarisation causes K+ channels to open
Membrane potential goes to EK

Question 19

Phases of an action potential 3: Hyperpolarisation + Threshold Reestablished

Answer 19

Efflux of K+ leads to an ‘undershoot’ and a short period of hyperpolarisation

At this time the membrane is refractory – unable to conduct another AP

Resting membrane potential is restored

Na+-K+-ATPase restores the resting ion balance

Question 20

Absolute Refractory Period

Answer 20

Most VSGCs (voltage-gated sodium channels) are in the inactivated state

Question 21

Relative Refractory Period

Answer 21

VGSCs are resetting to the closed conformation

Hyperpolarisation of the membrane makes membrane less excitable

Question 22

At the synapse

Answer 22

As an AP reaches the synapse the change in membrane potential triggers Ca2+ entry

Increased Ca2+ causes release of neurotransmitters stored in vesicles into the
synapse (via exocytosis)

Transmits a chemical message to down-stream cells

Neurotransmitters are the broken down in the synapse (e.g. acetylcholine broken down by acetylcholinesterase, taken up by presynaptic neurone again via endocytosis)