Exam #1: Action Potential Flashcards
What is the depolarization phase of an action potential? What ion is responsible for depolarization?
Portion of the Na+ channels are activated, resulting in the rising phase of the action potential (increase in membrane potential)
What is the overshoot of an action potential? What ion is responsible for over shoot?
Activation of Na+ channels leads to depolarization above 0mV
What is the repolarization phase of an action potential? What ion is responsible for repolarization?
Potassium channel activation leads to the efflux of K+ & decreased in membrane potential
What is the undershoot of an action potential? What ion is responsible for undershoot?
Prolonged opening of K+ channels leads to a dip in membrane potential below resting membrane potential
What is the hyperpolarization phase of an action potential? What ion is responsible for hyperpolarization?
Prolonged opening of K+ channels leads to a dip in membrane potential below resting membrane potential
Draw an action potential. Label ion movement for each phase of the action potential.
N/A
What happens to the conductance of Na+ & K+ in the action potential?
- Pk > PNa at rest
- PNa > Pk during depolarization
- PNa > Pk at over shoot
- PNa waning during repolarization
- Pk > PNa during hyperpolarization/ undershoot
- Pk > PNa at rest
What are the three possible states for the rapidly inactivating Na+ channel?
Sodium channels are voltage-gated and have 2x gates, an activation gate & an inactivation gate. The two gates give rise to three states:
1) Resting= activation gate closed & inactivation gate open
2) Activated= both open
3) Inactivated= activation gate open & inactivation gate closed
What is the absolute refractory period?
A second response is not possible regardless of strength or duration of the stimulus
- Na+ channel inactivation
What is the relative refractory period?
A second response can be elicited but at a greater cost i.e. strength or duration
- K+ channel prolonged opening
What factors control the speed of action potential conduction?
Size
How does axon diameter relate to speed of action potential conduction?
Larger= faster
Directly proportional to length constant
Inversely proportional to time constant
Where are voltage-gated Na+ channels located in myelinated & unmyelinated nerves?
Nodes of Ranvier
Which conducts more rapidly, myelinated or unmyelinated nerves? Why?
Myelinated
- Myelination greatly decreases axon membrane capacitance and increases membrane resistance
Which conducts more efficiently, myelinated or unmyelinated nerves?
Myelinated
What is saltatory conduction?
Jumping of current from node to node
Which causes spontaneous muscle twitches, hypo or hypercalcemia?
Hypocalcemia
What causes Trousseau’s sign?
Much more sensitive and specific sign of hypocalcemia than Chvostek’s sign
- BP cuff inflation leading to muscle spasm of arm & hand
What causes Chvostek’s sign?
A possible sign of latent tetnay associated with hypocalcemia
- Contraction of muscles of the eye, mouth, nose elicited by tapping over facial nerve in front of ear
What change can cause muscle weakness, fatigue, mental confusion, and coma?
Hypercalcemia
What cell type is responsible for myelination in the CNS?
Oligodendrocytes
What cell type is responsible for myelination in the PNS?
Schwann Cells
What is Guillian-Barre Syndrome? What part of the nervous system is affected?
Demyelination of the PNS (Schwann cells)
- Autoimmune disease
- Can undergo remyelination
What is Multiple Sclerosis? What part of the nervous system is affected by MS?
Demyelination of the CNS (oligodendrocytes & axons)
- Autoimmune disease
- Cannot undergo remyelination
What is the time constant?
Amount of time it takes for the voltage to change by a certain percentage
What is the time constant a function of?
Product of the axon’s electrical resistance (R) & capacitance (C)
What is the length constant?
Distance between current injection site & point where that voltage has decayed 63%
What two tissues types are considered excitable?
Nerve & muscle
What is TTX?
Tetrodotoxin from the puffer fish, a Na+ channel blocker
What is the mechanism of action of Lidocaine? What is Lidocaine used for?
- Na+ channel antagonist
- Local anesthesia
What is the definition of AP threshold?
Lowest voltage or minimal depolarization needed to drive Na+ channels into the fast, positive, feedback loop
What factors influence AP threshold?
Na+ Channel
K+ Channel
Ca++ ECF
What are the CATS of hypocalcemia?
Convulsions
Arrhythmia
Tetany
Stridor
How does Ca++ change threshold?
External Ca++ influences AP threshold by changing Na+ open probability (% of channels that will open)
Experimentally, if an axon is stimulated somewhere in the middle, which direction will current flow? Does this happen in life?
- Both
- No, the axon hillock has the lowest AP threshold
- K+ channels in subsequent Nodes of Ranvier prevent backward direction of AP propagation
