1. AP Biology
Nervous Systems
Part 2

2. Membrane Potential and ion concentrations
CYTOSOL
EXTRACELLULAR
FLUID
[Na+]
15 mM
[Na+]
150 mM
[K+]
150 mM
[K+]
5 mM
[Cl–]
120 mM
[Cl–]
10 mM
[A–]
100 mM
Plasma
membrane

3. Na+/K+ pumps Cytoplasmic Na+ bonds to the sodium-potassium pump
EXTRACELLULAR
FLUID
[Na+] high
[K+] low
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
[Na+] low
[K+] high
ATP P
Na+ P
CYTOPLASM
ADP
Cytoplasmic Na+ bonds to
the sodium-potassium pump
Na+ binding stimulates
phosphorylation by ATP.
Phosphorylation causes
the protein to change its
conformation, expelling Na+
to the outside. K+ K+ K+ K+ K+ P P K+
Extracellular K+ binds
to the protein, triggering
release of the phosphate
group.
Loss of the phosphate
restores the protein’s
original conformation.
K+ is released and Na+
sites are receptive again;
the cycle repeats.

4. Stronger depolarizing stimulus
Resting Potential
Stimuli
Stimuli
Stronger depolarizing stimulus
+50
+50
+50
Action
potential
Membrane potential (mV)
Membrane potential (mV)
Membrane potential (mV)
–50
Threshold
–50
Threshold
–50
Threshold
Resting
potential
Resting
potential
Resting
potential
Hyperpolarizations
Depolarizations
–100
–100
–100 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 6
Time (msec)
Time (msec)
Time (msec)
Graded potential hyperpolarizations
Graded potential depolarizations
Action potential

5. Impulse generation Na+ Na+ Na+ Na+ K+
Rising phase of the action potential K+
Falling phase of the action potential
+50
Action
potential
Na+
Na+
Membrane potential
(mV)
–50
Threshold K+
Resting potential
–100
Depolarization
Time
Na+
Na+
Extracellular fluid
Potassium
channel
Activation
gates
Na+ K+
Plasma membrane
Undershoot
Cytosol
Sodium
channel K+
Inactivation
gate
Resting state

6. Propagation Axon Action potential
An action potential is generated as Na+ flows inward across the membrane at one location.
Action
potential K+
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.
Action
potential K+
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.

7. Saltatory Conduction Schwann cell Depolarized region (node of Ranvier)
Cell body
Myelin
sheath
Axon

8. Reflex Arc