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
8.
9.
10.
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
21.
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
27.
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.