Generation & Propagation of action potential

1. DILSHANA FATHIMA
M.Sc. BIOCHEMISTRY

2.  Dendrite conducts signal from a sensory cell or
neighbouring neuron towards the cell body.
 Axon conducts signal away from cell body to another
neuron or effector cell.
 Axon ending relays signal to next neuron or effector
cell.

3.  Difference in voltage b/w the inside & outside of the
cell as measured across the cell membrane.
 When a neuron is not being stimulated, it maintains a
resting potential
Ranges from -40mV to -90mV
Average about -70mV

4.  Is the entire series of charges which contribute
towards the changes in membrane potential.
 Occurs in response to a threshold stimulus
 Either ocurs completely/it does not occur at
all(all/none principle)
 Has 2 main phases:
Depolarising phase: -ve memberane potential
becomes less –ve reaches zero & then becomes +ve
Repolarising phase: the memberane potential is
restored to the resting state of -70mV

5.  Following the repolarising phase, there may be an
after hyperpolarising phase, during which the
memberane potential temporarily becomes more –ve
than the resting level
 Caused by volage gated ion channels
 Voltage gated Na+ channels
 Activation & inactivation phase
 At rest, activation gate closed, inactivation
gate open
 Transient influx of Na+ causes the
membrane to depolarize

6.  Voltage gated K+ channels
 Single activation gate i.e, closed in the
resting state
 K+ channels opens slowly
 Efflux of K+ repolarizes the memberane
• The after hyperpolarizing phase occurs when the
voltage gated K+ channels remain open after the
repolarizing phase ends
• Action potential occurs in the memberane of the
axon when depolarisation reaches a certain level –
Threshold(abt -55mV in the neuron)

7.  Threshold in a particular neuron is usually constant
 An action potential will occur in response to a
threshold stimulus & not to a subthreshold stimulus
 Several action potentials will form in response to a
supra threshold stimulus

11.  When a stimulus causes the memberane of the axon to
depolarise to threshold,
 voltage gated Na+ channels open rapidly
 rapid influx of Na+ ions into the cell
 inside of the cell membrane become more +ve than
outside
This change is called depolarisation
 Membrane potential changes from -55mV to +30mV

12.  Each voltage gated Na+ channels have 2 separate
gates, an activation gate(AG) & an inactivation
gate(IAG)
 In resting state,
IAG is open
AG is closed
Na+ cannot move into the cell
 In activated state,
Both AG & IAG are open
Inflow of Na+

13.  As more channels open
Na+ inflow increases
Membrane depolarises further
More Na+ channels open
 This is an example for +ve feedback mechanism

14.  After the AG of the voltage gated channels open, the
IAG closes & it is in an inactivated state
 In addition to opening voltage gated Na+ channels, a
threshold level depolarisation also opens voltage gated
K+ channels
 Opening of voltage gated K+ channels occurs at about
the same time the voltage gated Na+ channels closes
 The slower opening of voltage gated K+ channels &
closing of previously open Na+ channels produce the
repolarising phase

15.  Slowing of Na+ inflow & acceleration of K+ outflow
causes the memberane potential to change from
+30mV to -70mV
 Repolarisation allows inactivated Na+ channels to
revert to resting state

16. • While the voltage gated K+ channels are open,
outflow of K+ may be large enough to cause an after
hyperpolarising phase of an action potential
• During this phase, the voltage gated K+ channels
remain open & the memberane potential becomes
even more –ve (-90mV)
• As the voltage gated K+ channels close, the
memberane potential returns to the resting level of -
70mV

17.  The period of time after an action potential begins
during which an excitable cell cannot generate another
action potential in response to a normal threshold
level
 During an absolute refractory period, even a strong
stimulus cannot initiate a 2nd action potential
 This period coincides with the period of Na+ channel
activation & inactivation
 Large diameter axons have a larger surface area &
have a brief absolute refractory period of about
0.4mSec

18.  Small diameter axons have absolute refractory periods
as long as 4mSec
 The relative refractory period is the period of time
during which a 2nd action potential can be initiated,
but only by a larger than normal stimulus
 It coincides with the period when the voltage gated
K+ channels are still open after inactivated Na+
channels have returned to their resting state

19.  To communicate information, action potentials must
travel from where they arise at the trigger zone of the
axon to the axon terminals
 It is not decremental
 Keeps its strength as it spreads along the membrane
 This mode of conduction is called propagation

20.  Each action potential in its rising phase, reflects a
reversal in membrane polarity
 +ve charges due to influx of Na+ can depolarise the
adjacent region to threshold
 So the next region produces its own action potential
 The previous region repolarises back to the resting
membrane potential
 Signal does not go back towards the cell body

22.  2 ways to increase velocity of conduction
Axon has a larger diameter
Axon is myelinated
 There are 2 types of conduction
1. Continuous conduction
2. Saltatory conduction

23.  Involves step by step depolarisation & repolarisation
 Ions flow through their voltage gated channels
 Occurs in unmyelinated axons & in muscle fibres
 Action potential propagates only a relatively short
distance in a few milliseconds

24.  Occurs along myelinated axons
 Occurs because of uneven distribution of voltage
gated channels
 Few voltage gated channels are present in regions
where a myelin sheath covers the axolemma
 At the Nodes of Ranvier, the axolemma has many
voltage gated channels

25.  Current carried by Na+ & K+ flows across the
membrane mainly at the nodes
 When an action potential propagates along a
myelinated axon, an electric current flows through the
extracellular fluid surrounding the myelin sheath &
through the cytosol from 1 node to the next
 Action potential at the 1st node generates ionic
currents in the cytosol & extracellular fluid that
depolarize the membrane to threshold opening Na+
channels at the 2nd node

26.  The resulting ion flow through the opened channels
constitutes an action potential at the 2nd node
 Then the action potential at the 2nd node generates an
ionic current that opens voltage gated Na+ channels at
the 3rd node & so on
 Each node repolarizes after it depolarises

28.  Amount of myelination
 Action potentials propagate more rapidly along myelinated
axons than along unmyelinated axons.
 Axon diameter
 Large diameter axons propagate action potentials faster than
smaller ones due to their large surface area.
 Temperature
 Axons propagate action potentials at lower speed when
cooled.