- Cells contain a high concentration of potassium ions inside and have membranes that are permeable to potassium, generating a negative resting membrane potential of -70 to -90 mV. - An action potential is initiated when the membrane becomes permeable to sodium ions, causing rapid depolarization that reverses the potential. The membrane then repolarizes as potassium channels open. - Action potentials propagate along axons as the local depolarization opens adjacent sodium channels, causing a regenerative wave. In myelinated axons, action potentials only occur at the nodes of Ranvier, increasing conduction velocity.

1. Membrane Potential (Vm):
- charge difference across the membrane -
K+
Na+
K+
Na+
inside outside
…how can passive
diffusion of potassium
and sodium lead to
development of
negative membrane
potential?

2. Simplest Case
If a membrane were permeable
to only K+ then…
inside outside
K+ K+
K+ would diffuse down its concentration
gradient until the electrical potential
across the membrane countered diffusion.

3. Simplest Case
K+ K+
If a membrane were permeable
to only K+ then…
The electrical potential that counters net
diffusion of K+ is called the K+ equilibrium
potential (EK).
inside outside

4. The Potassium Nernst Potential
Example: If Ko = 5 mM and Ki = 140 mM
EK = -61 log(140/4)
EK = -61 log(35)
EK = -94 mV
EK = 61 log
Ki
Ko
So, if the membrane were permeable only to K+, Vm would be -94
mV
…also called the equilibrium potential

5. Simplest Case
Na+
Na+
If a membrane were permeable to only
Na+ then…
The electrical potential that counters
net diffusion of Na+ is called the Na+
equilibrium potential (ENa).
inside outside
Na+ would diffuse down its
concentration gradient until potential
across the membrane countered
diffusion.

6. The Sodium Nernst Potential
Example: If Nao = 142 mM and Nai = 14 mM
EK = -61 log(14/142)
EK = -61 log(0.1)
EK = +61 mV
EK = 61 log
Nai
Nao
So, if the membrane were permeable only to Na+, Vm would be +61
mV

7. Resting Membrane Potential
0 mV
EK -94 ENa +61
Vm -90 to -70
Why is Vm so close to EK?
Ans. The membrane is far more
permeable to K than Na..

8. The Goldman-Hodgkin-Katz Equation
(also called the Goldman Field Equation)
Calculates Vm when more than one ion is involved.
o
Cl
i
Na
i
K
i
Cl
o
Na
o
K
m
Cl
p
Na
p
K
p
Cl
p
Na
p
K
p
V
]
[
'
]
[
'
]
[
'
]
[
'
]
[
'
]
[
'
log
. -
+
+
-
+
+
+
+
+
+
= 61
NOTE:
P’ = permeability
i
Cl
o
Na
o
K
o
Cl
i
Na
i
K
m
Cl
p
Na
p
K
p
Cl
p
Na
p
K
p
V
]
[
'
]
[
'
]
[
'
]
[
'
]
[
'
]
[
'
log
. -
+
+
-
+
+
+
+
+
+
= -61
or

9. The Goldman-Hodgkin-Katz Equation
The resting membrane potential is closest to the equilibrium
potential for the ion with the highest permeability!

10. ATP
3 Na+
2 K+
ADP
ActiveTransport
K+ Na+
Na+
K+
inside outside
Remember: sodium is pumped out
of the cell, potassium is pumped
in...

11. Resting Membrane Potential Summary

12.  Cells:
 contain high a K+ concentration
 have membranes that are essentially
permeable to K+ at rest
 Membrane electrical potential difference
(membrane potential) is generated by
diffusion of K+ ions and charge separation
 measured in mV (=1/1000th of 1V)
 typically resting membrane potentials in
neurons are -70 to 90 mV
Resting and action potentials
+
+
+
+
+ +
+
+
+
+
+
–
–
–
– –
–
–
–
–
–
–
Voltmeter
– +
0 mV
-80 mV +

13. Nerve Action Potential
 Nerve signals are transmitted by action
potentials, which are rapid changes in the
membrane potential that spread rapidly
along the nerve fiber membrane
 Action potential begins with a sudden change
from the normal resting negative membrane
potential to a positive potential and then
ends with an almost equally rapid change
back to the negative potential

14.  Changes that occur at
the membrane during
the action potential
 Transfer positive
charges to the interior of
the fiber at the onset
and return positive
charges to the exterior
at its end
Nerve Action Potential

15. Stages of action potential
 Resting Stage.This is the resting membrane
potential before the action potential begins.
The membrane is said to be "polarized"
during this stage because of the -90 millivolts
negative membrane potential that is present

16. Depolarization Stage
 The membrane suddenly becomes very
permeable to sodium ions, allowing
tremendous numbers of positively charged
sodium ions to diffuse to the interior of the
axon.
 The normal "polarized" state is neutralized by
the inflowing positively charged sodium ions,
with the potential rising rapidly in the positive
direction
 This is called depolarization

17. Repolarization Stage
 After the membrane becomes highly
permeable to sodium ions, the sodium
channels begin to close and the potassium
channels open more than normal
 Rapid diffusion of potassium ions to the
exterior re-establishes the normal negative
resting membrane potential
 This is called repolarization of the membrane

18. The AP - membrane permeability
• During the upstroke of an action potential:
 Na permeability increases
 due to opening of Na+ channels
 memb. potential approaches ENa
Na+ channels
 K permeability increases
 due to opening of K+ channels
 mem. potential approaches EK
• After hyperpolarization of membrane following an
action potential:
Membrane
hyperpolarized
resting potential
K+ channels
 There is increased K+ conductance
 due to delayed closure of K+ channels
• During the downstroke of an action potential:
 Na permeability decreases
 due to inactivation of Na+ channels
1 ms
+61
0
(mV)
-90
ENa
EK

19. Properties of action potentials
 Action potentials:
 are all-or-none events
 threshold voltage (sudden increase in the
membrane potential) threshold
-70
+60
mV
0
non- myelinated
0 800
400
 have constant conduction velocity
 Fibers with large diameter conduct faster than
small fibers
Fiber diameter (mm)
0 3 6 9
Myelinated
12
75
15
50
25
0
 are initiated by depolarization
 action potentials can be induced in nerve and
muscle by extrinsic stimulation

20. Propagation:
Rest
Opening of Na+ channels generates local current circuit that depolarizes adjacent
membrane, opening more Na+ channels…
Stimulated
(local depolarization)
Propagation
(current spread)

21. Signal Transmission:
Myelination
• Schwann cells surround the nerve
axon forming a myelin sheath
• Sphingomyelin decreases
membrane capacitance and ion flow
5,000-fold
• Sheath is interrupted every 1-3 mm
: node of Ranvier

22. Saltatory Conduction
• AP’s only occur at the nodes (Na
channels concentrated here!)
• increased velocity
• energy conservation

23. Conduction velocity
non-myelinated
myelinated
- non-myelinated vs myelinated -