ACTION POTENTIALS
What are four main characteristics of APs?
- APs are electrical signals that propagate without decrement along axons - are "all or none" because of the "reaching threshold" component (they'll either reach the threshold and fire or they won't) - Have refractory periods (i.e. cell need to come back to rest before another action potential can be fired) - Are unidirectional.
What is simple definition of an action potential?
- An AP is a wave of depolarisation followed immediately by a wave of repolarisation.
What causes depolarisation and depolarisation during an AP?
- During an AP, depolarisation is due to the movement of Na+ INTO the nerve cell. - Repolarisation is due to the movement of K+ OUT of the cell.
What are the two main characteristics of an axon that allow the fast transmission of APs?
- Myelination (whether or not axon is myelinated) - Diametre of the axon (current will travel down a larger/wider axon more quickly because there is less resistance)
Why can actions potentials only move one way along the axon?
- because of *absolute refractory period* of the voltage gated Na+ channel. - This can be described as the Unidirectional propagation of the AP.
Does the resting membrane potential of a neuron change if the extracellular K+ is increased from 4 mM to 6 mM? A) YES. The resting membrane potential moves towards threshold B) YES. The resting membrane potential moves further away from threshold. C) No. There is no effect on the resting membrane potential.
ANSWER: A) Yes. The resting membrane potential moves towards threshold (depolarize).
In which of the following would the rate of impulse conduction be greatest? A. a myelinated fiber 20 microns in diameter. D. a nonmyelinated fiber 2 microns in diameter E. a non-myelinated fiber 25 cm long B. a non-myelinated fiber 20 microns in diameter C. a myelinated fiber 2 microns in diameter
ANSWER: A) a myelinated fiber 20 microns in diameter.
Would an increase in the number of open K+ channels near the initial segment of a neuron affect the size of the stimulus required to initiate an action potential? A) yes, the stimulus required would be greater B) yes, the stimulus required would be smaller C) no
ANSWER: A) yes, the stimulus required would be greater
When chemically gated Cl- channels open in a neuron with a resting membrane potential of - 55 mV: A) Chloride ions exit the cell moving the membrane potential closer to threshold B) Chloride ions enter the cell moving the membrane potential further from threshold C) Chloride ions enter and exit the cell equally with no change in the resting membrane potential.
ANSWER: B) Chloride ions enter the cell moving the membrane potential further from threshold
Rob was diagnosed with a disease in which the peripheral neurons lose their myelin sheath. In this disease, voluntary movement of the limbs is slowed because: A) the action potential amplitude is decreased. B) the axon conductance is slowed. C) the rate of opening of ligand gated Na+ channels is decreased. D) the Na+ channel density is decreased along the fiber.
ANSWER: B) the axon conductance is slowed.
What is responsible for the repolarization phase of an action potential in a nerve? A) the influx of calcium ions via the voltage gated calcium channel B) the efflux of potassium ions via the voltage gated potassium channel C) the exit of sodium ions via the voltage gated sodium channel D) the entry of potassium ions via the ATP gated potassium channel
ANSWER: B) the efflux of potassium ions via the voltage gated potassium channel
Which of the following channels must be localized to the node of Ranvier for normal propagation of the action potential along the axon? A) voltage gated K+ channel B) voltage gated Na+ channel C) voltage gated Cl- channel D) A and B E) B and C
ANSWER: D) A and B
When the membrane of the axon terminal depolarizes: A) neurotransmitter is secreted. B) K+ enters the nerve terminal C) Ca+ enters the nerve terminal D) A and B E) A and C
ANSWER: E) A and C Neurotransmitter is secreted and Ca+ enters the nerve terminal.
Which of the following statements correctly describes action potentials? A) Amplitude varies with size of the initiating event B) Has a threshold C) Has no refractory period D) Is only an excitatory event E) B and D
ANSWER: E) B and D
If the voltage-gated Na+ channel is modified so it inactivates more rapidly, how will the action potential change? A) the depolarization peak will reach a higher voltage B) the depolarization peak will remain the same C) the depolarization peak will reach a lower voltage
ASNWER: C) the depolarization peak will reach a lower voltage
The voltage gated channels that allow an AP to occur are located in an ________.
AXON
What happens when the membrane potential goes above threshold at the Axon Initial Segment?
An AP will be sent down the axon.
True or false: APs will travel slower down a large axon that is myelinated.
FALSE: APs will travel FASTER down a large axon that is myelinated.
What is the role of myelination in APs?
Gaps in the myelin (called nodes of ranvier) allow ions to enter the cell, increasing the speed of the AP.
True or false: Dendrites and cell bodies tend to have ligand-gated ion channels.
TRUE: These are the parts of the cell that are going to be responsive to neurotransmitter.
What is an action potential (AP)?
The change in electrical potential associated with the passage of an impulse along the membrane of a muscle cell or nerve cell.
Where do action potentials take place?
Unidirectionally along an axon.
