AP Biology
Nervous Systems
Part 2
Animation
• http://www.youtube.com/watch?v=90cj4NX8
7Yk
Membrane Potential and ion
concentrations
CYTOSOL
EXTRACELLULAR
FLUID
[Na+]
15 mM
[Na+]
150 mM
[K+]
150 mM
[K+]
5 mM
[Cl–]
[Cl–]
120 mM
10 mM
[A–]
100 mM
Plasma
membrane
Na+/K+ pumps
EXTRACELLULAR [Na+] high
FLUID
[K+] low
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
CYTOPLASM
[Na+] low
[K+] high
Na+
Cytoplasmic Na+ bonds to
the sodium-potassium pump
P
ATP
P
ADP
Na+ binding stimulates
phosphorylation by ATP.
Phosphorylation causes
the protein to change its
conformation, expelling Na+
to the outside.
Loss of the phosphate
restores the protein’s
original conformation.
K+ is released and Na+
sites are receptive again;
the cycle repeats.
P
P
Extracellular K+ binds
to the protein, triggering
release of the phosphate
group.
Resting Potential
Stimuli
Stimuli
+50
–50 Threshold
Resting
potential
Hyperpolarizations
0 1 2 3 4 5
Time (msec)
Graded potential hyperpolarizations
+50
Membrane potential (mV)
0
Membrane potential (mV)
Membrane potential (mV)
+50
–100
Stronger depolarizing stimulus
0
–50 Threshold
Resting
potential
–100
0
–50 Threshold
Resting
potential
Depolarizations
0 1 2 3 4 5
Time (msec)
Graded potential depolarizations
Action
potential
–100
0 1 2 3 4 5 6
Time (msec)
Action potential
Impulse generation
Na+
Na+
Na+
Na+
K+
K+
Rising phase of the action potential
Falling phase of the action potential
Na+
Na+
Membrane potential
(mV)
+50
0
–50
K+
Action
potential
–100
Threshold
Resting potential
Time
Depolarization
Na+
Na+
Extracellular fluid
Na+
Potassium
channel
Activation
gates
K+
Plasma membrane
Cytosol
Resting state
Undershoot
Sodium
channel
K+
Inactivation
gate
Propagation
Axon
Action
potential
Na+
An action potential is generated as Na+ flows inward
across the membrane at one location.
K+
Action
potential
Na+
K+
The depolarization of the action potential spreads to the
neighboring region of the membrane, re-initiating the
action potential there. To the left of this region, the
membrane is repolarizing as K+ flows outward.
K+
Action
potential
Na+
K+
The depolarization-repolarization process is repeated in the
next region of the membrane. In this way, local currents of
ions across the plasma membrane cause the action
potential to be propagated along the length of the axon.
Saltatory
Conduction
Schwann cell
Depolarized region
(node of Ranvier)
Cell body
Myelin
sheath
Axon
Reflex Arc