Excitability & Neuronal APs

Question 1

Describe the transmission of a subthreshold depolarization down an axon.

Answer 1

A subthreshold depolarization will decrease with increased distance from the current electrode (decreases down the length of an axon)

Question 2

How do membrane and axial resistances affect the length constant?

Answer 2

The length constant depends on the ratio of membrane resistance to axial resistance (rm/ra). Therefore, a higher membrane resistance and a lower axial resistance will result in faster transmission.

Question 3

Describes the distance over which a subthreshold depolarization (local response) will spread and influence the next segment of membrane.

Answer 3

length (space) constant

Question 4

What is the effect of tetrodotoxin (TTX)?

Answer 4

it blocks the Na+ channel pore, thus blocking the passage of Na+ and inhibiting the firing of APs

Question 5

For Na+ channels, are gating kinetics dependent on time, voltage, or both?

Answer 5

both!

Question 6

Na+ channels exhibit regenerative depolarization… explain.

Answer 6

Depolarization increases Na+ permeability, which causes further depolarization (positive feedback effect).

Question 7

What do class I antiarryhthmic drugs do?

Answer 7

They bind to the inactivated state of the Na+ channel to prevent reactivation and lengthen the refractory period

Question 8

What are examples of conditions caused by mutations in Na+ channels?

Answer 8

• Hyperkalemic Periodic Paralysis
• Paramyotonia congenita
• Long Q-T Syndrome (LQT3)
• All are caused by failure of Na+ channels to inactivate

Question 9

What are refractory periods in nerves and the heart based on?

Answer 9

the voltage-dependent characteristics of Na+ channels

Question 10

What is the overall effect of hyperkalemia in terms of the neuronal action potential?

Answer 10

it causes RMP to become more positive, thus making less Na+ channels available, thus slowing conduction

Question 11

Hyperkalemia alters the ________, while hypercalcemia alters the _________.

Answer 11

activation state of Na+ channels; threshold

Question 12

What is the overall effect of hypercalcemia in terms of the neuronal action potential?

Answer 12

it increases the threshold for Na+ channel activation, thus decreasing membrane excitability

Question 13

How does respiratory acidosis affect plasma [Ca2+]?

Answer 13

It increases free plasma [Ca2+] by decreasing Ca2+ bound to proteins in serum. (change in pH alters binding capacity of albumin for Ca)

Question 14

AP conduction is directly related to which factors?

Answer 14

axonal diameter, length constant, myelination, and membrane resistance

Question 15

AP conduction is inversely related to which factors?

Answer 15

axial (internal) resistance

Question 16

Is a local depolarization the same thing as an action potential?

Answer 16

no!

Question 17

How does the myelin sheath increase membrane resistance?

Answer 17

it physically blocks Na+ channels, thus increasing membrane resistance and allowing the action potential to reach the end of the axon

Question 18

What are post-synaptic potentials

Answer 18

• local, graded responses propagated passively

- NOT action potentials

Question 19

What are the excitatory NTs that contribute to EPSPs?

Answer 19

ACh and glutamate

Question 20

What are the inhibitory NTs that contribute to IPSPs?

Answer 20

GABA and glycine

Question 21

What is the difference between an ionotropic and metabotropic channel?

Answer 21

• ionotropic: ligand-gated, more of a fast/direct response

- metabotropic: GPCRs, more of a slow/modulatory response

Question 22

A sequence of APs that occur in the same place very close in time:

Answer 22

temporal summation

Question 23

Two or more APs happening at the same time in different locations:

Answer 23

spatial summation