action potential

Question 1

how is action potential measured in neurons?

Answer 1

a stimulating and a recording microelectrode are inserted into a neuron. The stimulating electrode sends current through the cell. The cell hyperpolarizes then repolarizes until the overcorrection reaches the threshold and creates an action potential

Question 2

what is the initial potential of a neuron?

Answer 2

-65mV

Question 3

what happens to nerve impulses if depolarization gets greater (when already above threshold)?

Answer 3

it produces more spikes at a higher frequency

Question 4

what are the phases of the action potential?

Answer 4

rising, falling, overshoot, and undershoot phase

Question 5

what periods occur during the undershoot phase?

Answer 5

the refractory period (when it is not possible for the axon to have an action potential) and the relative refractory period (when it requires greater stimulation for the axon to produce an action potential)

Question 6

which ion is essential for the generation of AP and how does its concentration extracellularly affect the action potential?

Answer 6

sodium. lowered external sodium results in smaller and slower action potentials

Question 7

what are the three voltage-sensitive mechanisms that generate action potentials?

Answer 7

activation of Na conductance, delayed activation of K conductance and inactivation of Na conductance

Question 8

what causes the undershoot of the membrane potential?

Answer 8

the voltage gated K channels gradually close and the membrane potential returns to the resting potential

Question 9

what is the initial consequence of a stimulus that depolarizes the membrane potential above threshold?

Answer 9

voltage dependent Na+ channels open and ions rush into the cell down their concentration (inward current) therefore depolarizing the cell

Question 10

describe the delayed activation of K+ channels.

Answer 10

after a delay, the voltage dependent K+ channels open and ions rush out of the cell (outward current) therefore hyperpolarizing the cell.

Question 11

why can’t neurons fire during the refractory period? What ends this period?

Answer 11

because the Na+ channels are inactivated as the voltage gated K+ channels begin to close. this period is ended when the inactivation gates open and the channel closes

Question 12

what is measured by a voltage clamp and how does it work?

Answer 12

a voltage clamp measures the current flowing across a neuron membrane by bringing the voltage of the neuron down to a certain voltage. When the gated Na+ channels open, the voltage clamp device passes current in the opposite direction of the Na+ influx to keep the voltage at the command level

Question 13

what was found using the voltage clamp technique?

Answer 13

that there was an early inward current of sodium and a late outward current of potassium. this constructed a mathematical model of Na+ and K+ conductances that predicted properties of the action potential

Question 14

how was it determined that the early inward current was sodium and the late outward current was potassium?

Answer 14

drugs that selectively block Na+ (TTX) and K+ (TEA) channels were added at different times. it was shown that these drugs also blocked late and early currents respectively

Question 15

how is the voltage clamp technique continually used?

Answer 15

during the development of new pharmaceuticals and to study how genetic mutations affecting channels alter membrane currents.

Question 16

what is active flow in regards to action potential?

Answer 16

the opening of voltage gating channels for the influx of Na+

Question 17

what is passive current flow in action potentials?

Answer 17

depolarization wave that precedes the action potential from positive ions entering via current passively moving down the exon

Question 18

why does a signal only transmit in one direction down the axon?

Answer 18

because the section of axon from where the potential is coming from is in its refractory period and therefore the passive current towards that area cannot generate an action potential.

Question 19

how does hyperpolarization of the action potential dissipate?

Answer 19

potassium channels close and pumps restore ion concentrations to more normal values

Question 20

why are many axons myelinated?

Answer 20

to increase the rate of action potential more efficient down the membrane by insulating it from the leak of passive flow, by decreasing the number of voltage gated channels and by decreasing the capacitance.

Question 21

what is the equation for capacitance?

Answer 21

C= sigma A/d (d is total thickness of the dielectric membrane)

Question 22

what are the gaps between schwann cells called? what happens there that does not happen under the myelin sheath?

Answer 22

the nodes of ranvier. this is where action potentials are generated because there are Na and K channels.

Question 23

what happens under the myelin sheath to propagate the action potential and why does a nerve need breaks in the myelin sheath?

Answer 23

there is passive potentials that travel under the myelin sheath that need to be “boosted” at the nodes of ranvier to continue the passive current flow down the axon.

Question 24

what is the jumping of action potentials from node to node called?

Answer 24

saltatory conduction

Question 25

what is the conduction velocity of unmyelinated nerves and myelinated nerves?

Answer 25

unmyelinated: 0.5-10 m/s
myelinated: 150 m/s

Question 26

what is the pathology of multiple sclerosis caused by?

Answer 26

demyelinating of the neurons disrupts saltatory conduction and slows or blocks transmission of nerve impulses in the CNS