Lecture 3

Question 1

movement of electric charge movement of electric charge (ions)

Answer 1

electrical current

Question 2

force exerted on a charged particle

Answer 2

electrical potential

Question 3

relative ability of an electric charge to migrate from one point to the next

Answer 3

electrical conductance

Question 4

relative inability of a charge to migrate

Answer 4

electrical resistance

Question 5

Ohm’s Law

Answer 5

V (voltage) = I (current) * R (resistance)

Question 6

Equation for conductance

Answer 6

g (conductance) = 1/R

Question 7

difference between membrane potential (Vm) and the equilibrium potential of a particular ion (Eion )

Answer 7

driving force

Question 8

ionic current

Answer 8

conductance X driving force (Gion[Vm-Eion])

Question 9

What are the currents of Na, K and Cl at rest?

Answer 9

• Ina is inward (negative)
• Ik is outward (positive)
• Icl is approximately zero

Question 10

What changes during an action potential?

Answer 10

• membrane permeability to Na and K
• conductance
• ionic currents

Question 11

___ current is depolarizing. ___ ions move in and ___ ions move out.

Answer 11

inward
positive
negative

Question 12

___ current is hyperpolarizing. ___ ions move out and ___ ions move in.

Answer 12

outward
positive
negative

Question 13

Depolarizing current shows a ____ deflection on a current vs. time chart.

Answer 13

downward

Question 14

What does tetrodotoxin do?

Answer 14

Blocks sodium current

Question 15

What does tetraethylammonium do?

Answer 15

Blocks potassium current

Question 16

activated by the influx of Na +, counteracts the effect of that cation by allowing the discharge of K + (resets the membrane potential)

Answer 16

K+ delayed rectifier channels

Question 17

During resting potential, potassium channels are ____, and permeability to potassium is ____. Resting membrane potential is near ___.

Answer 17

always open
highest
Ek

Question 18

During depolarization, permeability to Na+ _____. Membrane potential _____.

Answer 18

increased

rises

Question 19

Voltage-gated Na+ channels open in response to ________.

Answer 19

depolarization above threshold (~-50mV)

Question 20

The voltage-gated Na+ channel is a single long polypeptide with __ domains each with __ transmembrane alpha-helices. It is ___ more permeable to Na+ than K+ due to ____.

Answer 20

4
6
12x
hydration (potassium is too big when hydrated)

Question 21

The __ alpha-helice in sodium channels is the voltage sensor. It twists away from the inside when the cell is depolarized to ___. The twists causes the channel to open and ____. Vm ___ as a result.

Answer 21

S4
-40mV
Na+ to rush in
rises

Question 22

Following depolarization, Na+ channels become ____. K+ channels, on the other hand, only ___.

Answer 22

inactivated

close

Question 23

the period immediately following the firing of a nerve fiber when it cannot be stimulated no matter how strong the stimulus; due to the increase in K+ permeability

Answer 23

absolute refractory period

Question 24

What de-inactivates the sodium channels?

Answer 24

repolarization

Question 25

What happens during the rising phase of an action potential?

Answer 25

- voltage-gated sodium channels open - potassium channels close - sodium influx

Question 26

What happens during the falling phase of an action potential?

Answer 26

- sodium channels inactivate - potassium channels open - cell becomes more negative

Question 27

4 polypeptide subunits joined to form a pore

Answer 27

voltage-gated potassium channels

Question 28

What is an afterhyperpolarization?

Answer 28

voltage-gated K+ channels take a few msec to return to the closed state; efflux of K+ from the cell is higher than at rest; membrane potential becomes more negative than at rest

Question 29

a period during which you can trigger an AP by applying a larger than normal stimulus; some (but not all) Na+ channels have been reset

Answer 29

relative refractory period

Question 30

What is spatial buffering of potassium and why is is important?

Answer 30

repolarization of neurons tends to raise potassium concentration in the extracellular fluid; potassium ions are taken up at one region of the astrocyte and then distributed throughout the cytoplasm of the cell, and further to its neighbors via gap junctions.

Question 31

How does lidocaine work?

Answer 31

blocks sodium channel pore (from inside), blocks action potentials in sensory axons so that pain signals do not reach the brain

Question 32

How does scorpion toxin work?

Answer 32

It blocks sodium channel inactivation --> muscle stiffness

Question 33

How are action potentials propagated?

Answer 33

Na+ channels locally open in response to stimulus generating an action potential; resulting current flows passively along the length of the axon

Question 34

Why doesn't the action potential travel backwards?

Answer 34

upstream Na+ channels inactivate, K+ channels open --> temporary hyperpolarization; also delay in sodium channel reopening

Question 35

What is membrane resistance (Rm)?

Answer 35

of channels open for ions to cross the membrane

Question 36

How is membrane resistance related to the surface area of membrane and conduction speed?

Answer 36

- increased SA --> increased leaky channels --> decreased resistance - increased resistance --> increased conduction speed

Question 37

How is internal (axial) resistance (Ri) related to axon volume?

Answer 37

increased diameter --> increased volume --> decreased resistance

Question 38

How is internal (axial) resistance (Ri) related to conduction speed?

Answer 38

decreased resistance --> increased speed

Question 39

What does myelin do?

Answer 39

acts as insulation to decrease current loss through leak channels

Question 40

How do the Nodes of Ranvier work?

Answer 40

voltage-gated sodium channels present in nodes only; action potential “recharges” as it propagates

Question 41

action potential “jumps” from node to node

Answer 41

saltatory conduction

Question 42

damage or loss of myelin over time

Answer 42

demyelination

Question 43

myelin never forms correctly

Answer 43

dysmyelination

Question 44

slowly progressive CNS disease; characterized by patches of demyelination in the CNS

Answer 44

multiple sclerosis

Question 45

What are the symptoms of MS?

Answer 45

- weakness and clumsiness - stiffness and gait disturbances - visual disturbances - mental disturbances, including lack of judgment, emotional liability, sudden weeping or laughter

Question 46

What is the epidemiology of MS?

Answer 46

- affects mainly Caucasians - average age of onset is 28 - 30 years - female:male ratio = 2:1 - chronic illness leading to cumulative disability - complex disorder (combination of genes and environment) - mediated by autoimmune processes

Question 47

What causes MS?

Answer 47

- myelin proteins recognized as foreign by T cells - lymphocytes and macrophage cross BBB and destroy oligodendrocytes - molecular mimicry (?): antigen initiates an immune response, antibodies cross-react with a self protein that “mimics” the antigen

Question 48

Is is possible to recover from MS?

Answer 48

- occasional re-myelination - some demyelinated axons express higher than normal densities of sodium channels in demyelinated regions (restore conduction --> remission) - degeneration of axons is irreversible; permanent loss of function

Question 49

How is MS treated?

Answer 49

- no cure - enhance endogenous remyelination - limit damage to oligodendrocytes - transplant exogenous myelin-forming cells (stem cells)