3. The Nerve Impulse or Action
Potential
• Is the electrical current
moving from the
dendrites to cell body to
axon.
• It results from the
movement of ions
(charged particles) into
and out a neuron through
the plasma membrane.
4. Ion channels
• Special proteins in the plasma
membrane allowing specific ions
to pass through.
• Some remain open all the time.
Others will open and close
depending on the conditions of
the cell and its environment.
5. Resting Membrane Potential
• Is the difference in electrical charge on the outside and
inside of the plasma membrane in a resting neuron (not
conducting a nerve impulse).
• The outside has a positive charge and the inside has a
negative charge.
• We refer to this as a polarized membrane.
• A resting neuron is at about -70mV
6.
7. Why is there a difference?
1. There is 30 times more K+ inside the cell than outside
and about 15 times more Na+ outside than inside.
2. There are also large negatively charged proteins trapped
inside the cell. (This is why it is negative inside.)
8.
9. Would you expect to see so
much K+ inside and so much
Na+ outside?
10. Why so much K+ inside?
• Special protein channels called sodium-potassium pumps
moving 3 Na+ out and bringing 2 K+ back in, when the
cell is at rest.
• In a resting cell there are no open channels for Na+ to
easily move back into the cell. However, there are some
K+ channels open at all time.
• Na+ causes the outside to be positive forcing more K+
into the cell. (Lots of potassium ions inside the resting
cell.)
11.
12. Review
• In a resting neuron the inside of the cell has a
__________ charge.
• In a resting neuron potassium ions are in high
concentration __________ of the cell.
• The resting membrane potential of a neuron is about
_____mV.
15. Steps in an action potential
1. The neuron gets stimulated (ex. receives a signal from
another neuron). This stimulation causes a change in the
resting membrane potential.
2. If a neuron is stimulated enough the inside of the cell will
reach a critical level called threshold (about -
55mV).
3. At this point sodium ion channels will open.
18. Depolarization
4. Sodium ions rush into the neuron because of diffusion
forces (high to low) and charge attraction (+ and -).
5. The charge inside the cell eventually reaches about
+30mV. (Relative to the outside of the cell the inside is
now positive and the outside is negative.) At this point
the sodium ion channels close.
• This change in polarization (- inside to +) is called
depolarization (step 4 and 5)
19.
20.
21. Repolarization
6. Next, potassium ion channels open up. This causes K+ to
rush out of the cell.
7. As the K+ leaves it causes the inside of the cell to
become negative again (-70mV). This is referred to as
repolarization (step 6 and 7).
22.
23.
24. Review
• What happens during the depolarization phase of an
action potential?
• What happens during the repolarization phase of an
action potential?
26. All-or-None Principle
• If a stimulus is strong
enough to generate an
action potential (reaches
threshold), the impulse is
conducted along the entire
length of the neuron at the
same strength.
27. Continuous conduction
• Occurs in unmyelinated neurons.
• It is a step-by-step depolarization
of each adjacent area of the axon
(or dendrite) membrane.
• It results form one area
depolarizing causing the next area
to reach threshold and depolarize.
28. Saltatory conduction
• Occurs in myelinated neurons.
• Depolarization only occurs at the nodes of Ranvier.
• The action potential jumps from one node to the next.
• Saltatory conduction will conduct the signal much faster
than continuous conduction.
29. Summary
• On a ½ sheet of paper describe how continuous
conduction is different than saltatory conduction.
• Before writing your answer share your thought with
your neighbor.
