- 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.
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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
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