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BIOLOGY 55-204-HUMAN PHYS MIDTERM 2. EXAM
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BIOLOGY 55-204-HUMAN PHYS MIDTERM 2. EXAM/BIOLOGY 55-204-HUMAN PHYS MIDTERM 2. EXAM
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Page 1HUMAN PHYS MIDTERM 2
Voltage gated channels: a class of transmembrane proteins that form ion channels that are
activated by changes in the electrical membrane potential near the channel; respond to a
change in the mb potential
- Open for action & graded potential (action-open everywhere because positive
feedback that trigures all of them to open; graded-open only near stimuli)
K+ voltage gated channels are either open or close (not complicated)
Na+ voltage gated channels
- have 2 gates (activation and inactivation gate)
- 3 different positions they can be in depending on which gates are opened or closed
At rest: activation gate is closed, inactivation gate is open
At threshold: activation gate is open, inactivation gate is open
Inactivated: activation gate is open, inactivation gate is closed
- All triggered by the same voltage change
- Activation gate is the fastest, then inactivation gate, then potassium channel
Graded Potential
- Depolarizing or hyperpolarizing
- Decremental as you move away from the source
Action Potential
- Always depolarizing
- Must hit threshold (-50mV) so that it can fire
- Always will be the same strength when fired
Steps
1) Starts at resting membrane potential
2) Grade potential usually triggers a change in voltage that allows the increase of
sodium permeability
3) Once threshold is hit, permeability of Na+ drastically increases (positive
feedback mechanism)
4) Rising phase: all depolarizing due to sodium flooding into cell; sodium gates
are activated
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5) Absolute refractory period: permeability of Na+ drops off and permeability of
K+ increases as the sodium gates are inactivated
- ABSOLUTELY cannot fire another action potential
6) Falling phase: membrane is repolarized through a K+ efflux; sodium gates are
inactivated Near the end of the falling phase, the sodium gates begin to open again
7) Relative refractory period: might be able to fire an action potential but might need
a greater stimulus to fire the action potential and it will be smaller
- Including hyperpolarization and depolarization
- Sodium gates are closing
- Potassium gates are still working
- Hyperpolarization: potassium floods in and overshoots before coming back to
resting membrane potential (Cl- ions also contribute to this)
Hyperpolarization is any time the potential goes below resting
- Depolarization: K+ channels close
- HAPPENS IN THE MIDDLE OF THE FALLING PHASE AND WILL CONTINUE UNTIL
RESTING IS ACHIEVED
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- Channels are opened as we move down the axon in one direction
- Are only activated to the right of the open gates as gates to the left are in a
refractory period and are closed and inactivated (unable to open)
Myelin
- Not all are myelated
- Insulates the axon that prevents ion leakage
- Helps move action potential along
Neuron
- A nerve cell
Flow of information:
1) Input zone: where incoming signals from other neurons are received (found in
dendrites and cell body)
- Just graded potentials present in dendrites and cell body
2) Trigger zone: where action potentials are initiated (found in axon hillock)
- Only action potentials present in the rest of axon
3) Conducting zone: part that conducts action potentials in undiminishing fashion,
often over long distances (found in axon)
4) Output zone: part that released neurotransmitters that influences other cells (found
in axon terminals)
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