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Lecture 3

EXSC 224 Lecture Notes - Autonomic Nervous System, Exocytosis, Saltatory Conduction

5 Pages
16 Views
Fall 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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Description
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 ■ 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 ○ All action potentials are alike and are independent of stimulus intensity ● How does the CNS tell the difference between a weak stimulus and strong one? ○ Action potentials dont get bigger, but we have more of them in the same amount of time ■ Frequency of action potentials/ rate coding ● Reflects the intensity of the stimulus ○ Strong stimuli can generate action potentials more often than weaker stimuli ○ The CNS determines stimulus intensity by the frequency of impulses ● Absolute refractory period ○ Time from the opening of the Na channels until the resetting of the channels ■ Depolarized ○ Ensures that each AP
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