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Chapter 4

BIOB33H3 Chapter Notes - Chapter 4: Resting Potential, Central Nervous System, Axon Hillock


Department
Biological Sciences
Course Code
BIOB33H3
Professor
Connie Soros
Chapter
4

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Chapter 4 Neuronal Physiology
Terms
- excitable tissues: capable of producing electrical signals (transient, rapid changes in membrane
potential), nerve & muscle
- resting membrane potential: the membrane potential that exists when no net changes in potential
are occurring
- graded potentials: local changes in membrane potential that vary in magnitude (flow of ions)
- action potentials: brief, rapid reversals in membrane potential, which can spread throughout the
membrane (flow of ions)
- voltage-gated channels: membrane channels that open or close in response to changes in
potential
- polarization: a membrane that has potential is polarized
- depolarization: a decrease in membrane potential (inside becomes more positive)
- triggering event: event that initiates a depolarization (stimulus like light or touch, chemical
messenger)
- hyperpolarization: an increase in membrane potential (inside becomes more negative)
- repolarization: return to resting potential after a depolarization
Graded Potentials
- magnitude of graded potential related to magnitude of triggering event
1. stronger trigger greater magnitude of change in potential
- current flow (movement of charges)
1. when a graded potential occurs, a piece of the membrane (called the active area) has a
different potential than the rest of the membrane (which is at resting potential, called the inactive
area)
a. current flows between active area and adjacent inactive areas (opposite charges
attract)
b. previously inactive areas become active and more current flow occurs
2. spread of graded potential is decremental
a. decreases as it moves along the membrane
b. current leaks into ECF
c. function as signals over short distances
3. usually not an actual reversal of charges - just a reduction in potential (inside becomes less
negative than before, small depolarization)
4. important for nerve and muscle cells (e.g., postsynaptic potentials)

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Action Potentials (AP)
- can be transmitted over long distances without losing strength
- Depolarization
1. triggering event causes depolarization to occur relatively slowly until threshold potential is
reached (about -50 to -55 mV)
a. once threshold is reached, membrane quickly depolarizes to +30 mV
(1) when triggering event begins depolarization, some of the voltage-gated Na+
channels open, Na+ flows into cell (proteins that make up the channel have
charged portions, shape change occurs as those charges interact with charges
surrounding the membrane)
(2) this further depolarizes the membrane, causing even more Na+ channels to
open
(3) at threshold all the Na+ channels are open and there is an explosive increase
in Na+ permeability (P Na+)
(4) at peak depolarization, the Na+ channels close (the channel is constructed so
that the same depolarization that opens them also closes them)
- Repolarization begins
1. as Na+ channels close, K+ channels open (P K+ increases) due to delayed voltage-gated
response to the depolarization, K+ flows out of cell
2. this restores internal negativity
3. as repolarization progresses...
a. Na+ channels resume original conformation (closed but capable of opening)
b. newly opened K+ channels close
(1) hyperpolarization occurs before channels close (membrane even more
negative than at resting potential)
(2) resting potential restored
- AP lasts about 1 millisecond
- ion gradient restored
1. Na+-K+ pump restores ion gradients
a. important for long term maintenance of gradient
b. not necessary between APs
(1) ion shifts during AP are not so great that they wipe out concentration
gradients, so many APs can occur in succession
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