1. Action Potential & Propagation
DENT/OBHS 131 Neuroscience
2009

2. Ionic basis of APs action potential:
faithfully transmit information along the membrane (axon) of excitable cells
allow rapid communication between distant parts of a neuron

3. Learning Objectives
Describe the roles of both sodium and potassium ions / voltage-gated channels before, during and after the action potential
Understand how the resistive & capacitive properties of neurons influence electrical signaling
Compare and contrast local circuit and saltatory propagation of action potentials

4. How many distinct ion channels are necessary for the AP?1 2 3 4 5

5. 3 phases of the action potential
Resting
i.e. RMP
Depolarization
reversal of membrane potential
Repolarization
return of membrane potential to RMP

6. General rule relationship between:
ENa
+67
membrane
potential (mV)
relationship between:
membrane potential
ion equilibrium potentials
if the membrane becomes more permeable to one ion over other ions then the membrane potential will move towards the equilibrium potential for that ion (basis of AP)
RMP
ECl
-90 EK
-98

7. Depolarization rapid opening of Na-selective channels
entry of Na “down” its electrochemical gradient
1. membrane more permeable to Na than K
2. membrane potential moves (rapidly) towards ENa
3. because ENa is positive, the AP overshoots zero
4. At the peak of the AP Na is the primary ion determining the membrane potential

8. Repolarization closure (inactivation) of Na-selective channels
slower opening of K-selective channel
1. membrane more permeable to K than Na
2. K moves out of cell
3. membrane potential moves towards EK

9. 2 independent channels
selective agents block the 2 components

10. Voltage-gated ion channels
the opening and closing of AP Na and K channels are controlled by changes in the membrane potential

11. What triggers an AP? all-or-none threshold
AP are not graded potentials
threshold
in order for an AP to occur the membrane must be depolarized beyond a threshold level
inward Na overcomes resting outward K movement
electrical stimulation
synaptic activation

12. APs are regenerative activation of Na channels is cyclical
initial depolarization
opening of Na channels
Na entry
etc..

13. Learning Objective #1
Describe the roles of both sodium and potassium ions / voltage-gated channels before, during and after the action potential

14. AP review

15. Learning Objective #2
Understand how the resistive & capacitive properties of neurons influence electrical signaling

16. How does an AP move? Propagation
Aps are conducted along excitable cell membranes away from their point of origin
e.g. down the axon from cell soma to terminal

17. Resistance ≈ how far it can get
membrane resistance (rm)
axon / dendrite
diameter (d)
axial, or internal, resistance (ri) rm ri
ength constant  =
“leaky pipe”

18. Fat axons are fastest!

19. Capacitance ≈speed “bulk” solutions IN and OUT are neutral
the transmembrane potential difference exists within a narrow band just across the membrane
a capacitor
separates / stores charge
to change membrane potential
must add or remove charge
this takes time

20. Summary Capacitance - speed (time constant)
Resistance - distance (length constant)
How does neuron deal with these properties in order to have efficient AP propagation?

21. Unmyelinated axons local circuit propagation slow
of the membrane during the AP is not restricted to a single spot
the inward current carried by Na ions during the AP depolarizes adjacent portions of the membrane beyond threshold and the regenerative AP travels along the membrane

22. Refractory period
following a single AP a second AP cannot be generated at the same site for some time (absolute versus relative)
Na channels need to recover from inactivation
open K channels oppose inward Na movement

23. Myelination local circuit propagation is slow (< 2 m/s)
In motor neurons propagation is fast 100 m/s
Schwann cell / oligodendrocyte
envelop axons / layer of insulation
increase membrane resistance
less leaky
eliminate capacitance
less discharge
Nodes of Ranvier
discontinuity in myelin sheath (every few 200+ m)

24. Saltatory conduction APs are only generated at Nodes of Ranvier
high density of Na / K channels
current flows rapidly between nodes
little current leakage between nodes
AP “jumps” down fiber as successive nodal membrane capacitances are discharged

25. Learning Objective #3
Compare and contrast local circuit and saltatory propagation of action potentials

26. propagation review
Press button

28. How can AP rise so fast (< 1 ms)?
m= rmcm

29. Membrane time constant
changing the membrane potential takes time
charging a capacitor is not instantaneous
inject current V
record voltage I
m = rmcm ≈ 50 ms
axon/dendrite