Transcript chapter 8 neuronal physiology B

Graded potential vs action potential
• graded potentials – current flow, amplitude
- post-synaptic potentials
• Action potentials
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Trigger zone
threshold voltage
All-or-none
Channel activity
Refractory period
Coding for stimulus intensity
Graded potential – amplitude decreases away from point of channel opening
Graded potential
• Amplitude is directly proportional to the
strength of the stimulating event
• Depolarization wave known as local
current flow
• Strength depends on how much charge
enters the cell
• Decreases in strength as it travels through
the cytoplasm
• Can be depolarizing or hyperpolarizing
Graded potential – amplitude decreases away from point of channel opening
Action potential
-Initiated if a threshold depolarization
is reached at the trigger zone
-Also called a spike
-Always the same amplitude
-Amplitude does not diminish
-“All or none”, essential for long
distance propogation
overshoot
After hyperpolarization
(undershoot)
Movement of ions through channels during the action potential
(ion conductance)
The voltage gated Na+ channels
• Has two gates: an activation gate and an
inactivation gate (gate can be opened, gate can
be locked)
Refractory period
• Another action potential
cannot fire when the
inactivation gate is latched.
• Action potentials cannot
overlap
• The absolute refractory
period
• Limits the rate of spike
frequency
• Important for frequency
coding
Resting membrane potential is not restored,
Na+ channel gates are unlatched (inactivation
Is removed)
Coding for stimulus intensity
Coding for stimulus intensity
NO lecture on Friday
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Today
How are action potentials propagated?
Factors that affect conduction speed
The consequences of abnormal levels of
extracellular K+
Coding for stimulus intensity
Coding for stimulus intensity
How does an action potential
spread?
What causes adjacent voltage gated sodium channels to open?
Low resistance to current flow in large diameter axons. (wide water pipe)
Mammalian axons are very thin, and action potential are conducted quickly.
Mammalian axons are insulated by myelin sheaths (myelinated axons).
What will abnormally high levels of
extracellular K+ cause?
Abnormally high levels of K+ will
cause
• Resting membrane potential will be shifted
toward action potential threshold
• Get too much excitation (too many action
potentials)
Abnormally low K+
• Membrane resting potential hyperpolarizes
• Cell is far away from threshold for action
potential