Neuro 3b – The Action Potential
Anil Chopra
1. Explain in general terms what function of the action is.
Transmit information reliably and quickly over long distances by the following
mechanisms.



Changes in the permeability due to ion channels being open or closed.
If the permeability changes, ions cross the membrane down their
electrochemical gradient
The membrane potential therefore moves towards the equilibrium potential for
that ion.
The changes in membrane potential are not due to ion pumps.
2. Examples of other types of excitable cells (in addition to neurons) in which
action potentials occur.
Muscle fibres and come sensory receptors can also be excitable using action
potentials.
3. Define the following terms as they apply to other action potentials.
Threshold: this is the level at which the membrane has to be depolarised to initiate a
full action potential. One will not start unless the membrane is depolarised to the
threshold level (-55mV). This is the all or nothing rule.
Refractory period: this is the time in which the neuron cannot generate another
action potential as the resting potential is still being restored. The Na channels are
therefore inactive.
“All or Nothing” Behaviour: this is a rule of the neuron in that action potentials will
always be of the same size and are only generated if the threshold level is reached. If
the neuron is stimulated but the threshold level of depolarisation has not been reached,
then no action potential is generated.
Depolarisation: this is a reduction of potential difference across the membrane,
usually the inside of the membrane becomes positive.
Hyperpolarisation: this is where the potential difference across the membrane is
increased, usually involving the inside of the membrane being more negative.
Saltatory Conduction: a process that only occurs in myelinated neurons.
Depolarisation occurs at the unmyelinated nodes of Ranvier so action potential
seemingly jumps from node to node. This gradually speeds up the conduction
velocity.
4. Define the Following terms as they apply to membrane channels involved in
producing an action potential.
Voltage- Gated Channel: membrane-spanning channels that open/close in response to
changes in membrane potentials.
Channel Inactivation: this is when a gate blocks the Na channels in repolarisation and
the absolute refractory period. It stops any more Na from entering the cell.
Positive feedback: this is what occurs when the threshold level is reached. As the
membrane becomes depolarised, Na channels open, Na ions flood in which cause
more depolarisation.
5. Outline the sequence of events during a typical action potential in a neuron.
Equilibrium potential for Na
4) Resting potential
5) Hyperpolarisation
3) Upstroke Phase
2) Stimulus/Foot
1) Resting potential
Equilibrium potential for K
(1) Resting Potential – Membrane permeable to K+ but impermeable to Na+ so
membrane potential is close to that of K+ (-90mv). It is -70mV.
(2) Stimulus/Foot – stimulus slightly depolarises membrane to threshold level.
(3) Upstroke Phase – voltage gated Na channels open and membrane becomes
permeable to Na+ ions. Na+ ions flux in down their concentration gradient.
(4) Repolarisation phase - voltage gated Na channels inactivate so Na entry stops.
K+ channels open so K+ ions flow out of the causing membrane potential to go down
to that of K+.
(5) Hyperpolarisation – K+ channels still open so that K+ ions still leaving cell until
gates close and resting potential in restored.
6. State the size and duration of a typical action potential.
Size: from -70mV up to +30mV Duration: around 4 milliseconds
7. Define the term regenerative as applied to action potentials and its significance
for the spread of the potential along the axon.
Regenerative: more and more Na channels keep opening and Na+ ion entering the
axon until the depolarisation reaches a point where the voltage-gated Na+ channels
inactivate and become voltage insensitive. This allows the K+ gates to open and so the
membrane regenerates its polarised state. As the action potential travels in one
direction only as the regions behind them are in absolute refractory period.
8. Explain how conduction of the action potential occurs (conduction here means
spread along the axon/transmission propagation)
Local current flow
depolarizes adjacent
region toward
threshold
Active area at
peak of action
potential
Direction of propagation of action potential
Remainder of axon at resting
potential
Adjacent area
at resting
potential




The depolarisation spreads along the axon by Na+ ions enter the cell and K+ ions
leaving.
Only a small number of ions cross causing a 0.1% concentration change.
The current that is initiated induces depolarisation in the part of the axon just after
(adjacent) to it.
Just behind the action potential, the region returns to resting potential with the
help of the Na+/K+ ion pump.
9. List two structural features that affect the conduction velocity along normal
axons. Briefly explain why they affect velocity as they do.
Velocity affected by…
Myelination: increases the speed by saltatory conduction. The depolarisation only
occurs at the nodes of Ranvier and so the action potential can jump from node to
node.
Axon diameter: the larger the diameter, the less resistance there is to the flow of the
action potential.
Large diameter myelinated axon  120m/s
Small diameter unmyelinated axon  1m/s
Local current flow
depolarizes new
adjacent region toward
threshold
New active area
at peak of action
potential
Old active region
returning to resting
potential
New adjacent
area at resting
potential
10. List at least one pathological condition that affects conduction velocity.
- Cold
- Multiple
- Sclerosis
- Anoxia
- Some drugs