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Lecture 3

EXSC 224 Lecture Notes - Lecture 3: Resting Potential, Axon Hillock, Axon Terminal

5 pages21 viewsFall 2016

Department
Exercise Science
Course Code
EXSC 224
Professor
Thompson
Lecture
3

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Quiz:
1. Name the cell types that create myelin and what part of the neuron are they associated?
a. Glial cells - oligodendrocytes and schwann cells on the axons. Most important
thing that they do is SPEED
2. When a neuron establishes a resting membrane potential, where is the Na and the K
a. Na - extracellular fluid; K - cytosol (intracellular fluid)
3. Where on a neuron will you find ligand gated and mechanical gated channels?
a. Dendrites and Cell Body (soma) - associated with neurotransmitters
4. A positive change in membrane potential due to a stimulus is called:
a. Depolarization
5. Where on a neuron will you find Graded Potentials?
a. Dendrites and Cell Body (soma)
6. Where on a neuron will you find voltage gated channels?
a. Axons (axon hillock; axon terminal)
Action potential (AP)
Graded potentials are essential for causing action potentials
Graded potentials are INCOMING
Action potentials are OUTGOING
Action potential is caused by permeability changes in the plasma membrane
Notice the unit of time - milliseconds!
Takes about 4 milliseconds for an action potential to complete
You can have at least 250 action potentials in one second
-55mV is the threshold you must reach to trigger an action potential
If you don’t reach threshold, nothing happens
Resting state >> Depolarization >> Repolarization >> Hyperpolarization >> resting states
When you hyperpolarize the potassium channel closes
Phase 1: Resting State
Only leakage channels for na and K are open
All gated na and K channels are closed
Working to establish equilibrium & maintain membrane potential at-70mV
Threshold - opens up sodium gated channels
Na rushes into cytosol, making the cell very positive (+30mV at peak)
At peak, the sodium voltage gated channels close
Then, potassium voltage gated channels open at +30mV peak, K+ rushes out
Properties of gated channels -
Each Na channel has two voltage sensitive gates
Activation gates - closed at rest; open with depolarization
Inactivation gates
Open at rest; block channel once it is open
Each K channel has one voltage-sensitive gate
Closed at rest & opens slowly with depolarization
Phase 2: Depolarization
Depolarization ALWAYS means a positive deflection
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We use this term for action potentials AND graded potentials
Local currents open voltage-gated Na channels
You met the threshold
Na influx causes more depolarization
At threshold (-55 to -50 MV) positive feedback leads to opening of all Na
channels, and a reversal of membrane polarity to +30mV (spike of action
potential)
At peak, the sodium voltage gated channels close
Then, potassium voltage gated channels open at +30mV peak, K+ rushes out
Phase 3: Repolarizing phase
Reestablish a negative membrane potential
Na channel slow inactivation gates close
Membrane permeability to Na declines to resting levels
Slow voltage-sensitive K+ gates open
K+ exits the cell and internal negativity is restored
Phase 4: Hyperpolarization
This term is used in both graded and action potentials
Temporary
Some K+ channels remain open, allowing excessive K+ efflux
This causes after-hyperpolarization of the membrane (undershoot)
Now sodium (Na+) is inside and potassium is outside (K+)
Role of the Sodium-Potassium Pump
● Repolarization
Restores the resting electrical conditions of the neuron
Does not restore the resting ionic conditions
Ionic redistribution back to resting conditions is restored by the thousands
of sodium-potassium pumps
Propagation of an Action Potential
Na influx causes a patch of the axonal membrane to depolarize
Local currents occur
Na+ channels toward the point of origin are inactivated and not affected by the
local currents
Local currents affect adjacent areas in the forward direction
Depolarization opens voltage-gated channels and triggers an AP
Repolarization wave follows the depolarization wave
At threshold:
Membrane is depolarized by 15 to 20 mV
Na permeability increases
Na influx exceeds K+ efflux
The positive feedback cycle begins
Subthreshold stimulus - non adequate
Threshold stimulus
Action potential - all-or-none phenomenon
Coding for stimulus intensity
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