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Title: Physiology Lesson 2: Action Potentials
Description: University Major Level Physiology Notes describing action potentials, a function of neurons Villanova University
Description: University Major Level Physiology Notes describing action potentials, a function of neurons Villanova University
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Lecture 3: Action Potentials
No t e b o o k:
Cre at e d :
Physiology
8/16/2012 12:03 AM
U p d at e d :
8/16/2012 9:37 AM
Polarization: changing the membrane potential of the cell
Depolarization: reducing of the membrane potential (removing polarization, making
potential LESS NEGATIVE aka MORE POSITIVE)
Hyperpolarization: increase of membrane potential (adding polarization, Making
MORE NEGATIVE)
Repolarization: return back to resting membrane potential (rmp)
Depolarizing above the threshold (15-20 mV above rmp) leads to an Action Potential (AP)
adding any more voltage will not increase action potential
AP is an all or nothing response
below threshold yields no AP
Scenario: Someone has low K+ levels in blood, what would this do?
Answer: Hyperpolarize cells, because a low [K+] outside would pull K+ out of the cell
making it more negative inside
...
Na+ Channels use
positive feedback to let in more and more Na+
...
K+ Channels also open when the cell is depolarized, but much more slowly than the Na+
ones, but there is still depolarization because there is much more Na+ in than K+ out
After Na+ channels close, K+ channels remain open to repolarize the cell
K+ channels close once inside becomes slightly more negative than rmp
...
The Na+/K+ then restores the rmp
Action Potential (AP) Features
All or None
...
Same size and same duration like everytime
Refractory Periods: while a segment of the axon is producing an AP, membrane is
absolutely or relatively resistant to further stimulation
Absolute Refractory Period: due to the inactivated Na+ Channels, a second AP
is not allowed
...
" This a a very short time sequence,
It is in the sequence of refractory periods
Relative Refractory Period: due to the continued outward diffusion of K+, AP is
possible, but depolarization must overcome the continued outward diffusion of K+
...
Occurs Second in the
sequence of refractory periods (it comes after the absolute)
Action Potential -Conduction
Action potentials (APs) normally originate in the axon hillock
Action Potentials are conducted in axons of 2 types:
Naked Axon: current spreads
...
The Region that just produced an AP is now refractory
...
Myelin is interrupted by Nodes of Ranvier where axon membrane is
exposed to interstitial fluid and thus can exchange ions necessary for AP
conduction
...
Insulated areas (with myelin sheath) passively pass on a (+) Charge to the
next node
...
Can also cut axons
leads to loss of APs
Cause isn't quite known
APs are conducted, but can't pass to next region
Question: What would Happen if Na+ Channels stop opening?
Local Anesthetics (Novocaine, Lidocaine and Xylocaine) stop conduction when put on a
sensory nerve (hits the Na+ Channels to stop AP at that point, thats why you don't feel
pain)
Title: Physiology Lesson 2: Action Potentials
Description: University Major Level Physiology Notes describing action potentials, a function of neurons Villanova University
Description: University Major Level Physiology Notes describing action potentials, a function of neurons Villanova University