NERVOUS SYSTEM: ION CHANNEL

Question 1

Explain how elements of the Hodgkin Huxley (HH) model can be applied to biological membranes

Answer 1

1. Capacitor: plates correspond to inner and outer faces of the membrane
2. Variable resistance (the inverse of conductance, g): corresponds to gated ion channels shown with a switch
3. Electromotive forces: separation of charged ions across the cell membrane, set up by activity of the sodium-potassium ATPase

Question 2

What is a capacitor and explain how the membrane acts as one

Answer 2

stores electrical energy in an electric field by accumulating electric charges on two closely spaced surfaces that are insulated from each other

Question 3

How is the flux of K+ across a cell membrane determined by both the K+ concentration gradient and the membrane potential?

Answer 3

K+ concentration gradient drives K+ out of cell (there is a greater amt of K+ in cell than out of cell)

K+ electrical potential difference drives K+ into cell (The continued efflux of K+ builds up an excess of positive charge on the outside of the cell and leaves behind an excess of negative charge inside the cell)

This buildup of charge leads to a potential difference across the membrane that impedes the further efflux of K+, so eventually an equilibrium is reached: the electrical and chemical driving forces are equal and opposite and as many K+ ions move in as move out

Question 4

Biological membranes are permeable only to molecules with which certain properties (3)

Answer 4

Small
Lipophillic: tend to combine or dissolve in lipids or fats
Uncharged

Question 5

Define: voltage-gated channel

Answer 5

open in response to change in membrane potential

Question 6

Define: ligand-gated

Answer 6

binding of a chemical or hormone opens channel

Question 7

define: facilitated diffusion

Answer 7

the transport of substances across a biological membrane from an area of higher concentration to an area of lower concentration with the help of a transport molecule

Question 8

define: mechanically gated or stretch gated channels

Answer 8

Mechanical stimuli (mechanically-gated or stretch-gated channels) opens channels and ions then diffuse down their gradient

Question 9

NaV and CaV pores are formed by monomers with ___ repeating __ transmembrane regions.

Answer 9

4
6

Question 10

Voltage-gated channels can be inactivated by two different mechanisms; Describe them

Answer 10

1. Many voltage-gated channels enter a refractory (inactivated) state after briefly opening when the membrane is depolarized. They recover from the refractory state and return to the resting state only after the membrane potential is restored to its resting value.
2. Some voltage-dependent Ca2+ channels inactive when the internal Ca2+ level increases following channel opening. The internal Ca2+ binds to calmodulin (CaM), a specific regulatory protein associated with the channel

Question 11

State the different states of gated channels

Answer 11

• closed (resting state)
• open (active state)
• inactive (inactive state until channel has been “reset”)

Question 12

How are the Nav channels gated

Answer 12

Nav channels are essential for action potential generation
Two gates: activation and inactivation
Depolarization (becoming more positive) to threshold opens the activation gate

Inactivation gate then closes, halting ion flow
Inactivation gate cannot be removed until the membrane repolarizes (becomes more negative again)

Question 13

What is the resting membrane potential and why is it that number?

Answer 13

Ions are distributed unequally across the cell membrane: e.g. Na+ is higher outside the cell and K+ is higher inside the cell.

This differential distribution results in a resting membrane potential. All cells have an RMP.

The primary mechanisms that establish the membrane potential are the activity of the Na+/K+ ATPase, K+ leak channels and (to a lesser extent) resting permeability to Na+ and Cl- .

This sets the RMP to about -70 mV in a typical neuron

Question 14

What is the ionic composition of a typical cell?

Answer 14

ECF
- Na+: 135-147 mEq/L
- K+: 3.5-5.0 mEq/L
- Cl-: 95-105 mEq/L
- HCO3-: 22-28 mEq/L
- Ca2+: 2.1-2.8 (total); 1.1-1.4 (ionized)
- Pi: 1.0-1.4 (total); 0.5-0.7 (ionized)

ICF
- Na+: 10-15 mEq/L
- K+: 120-150 mEq/L
- Cl-: 20-30 mEq/L
- HCO3-: 12-16 mEq/L
- Ca2+: 10^-7 (ionized)
- Pi: 0.5-0.7 (ionized)

Question 15

define driving force

Answer 15

The ‘driving force’ is the difference between the membrane charge and the equilibrium potential for a given ion (Vm- Eion) → consider Na+ and K+

Vm is the membrane potential and Eion is dependent on the concentration gradient for that particular ion.
Thus, Vm- Eion represents the net electrochemical driving force acting on the ion.

Question 16

For a positive ion when do the ions flow outward, inward, and ion flow stops?

Answer 16

When Vm - Ex is greater than 0, the ion flows outward

When Vm - Ex is less than 0, the ion flows inward

When Vm - Ex is equal to 0, ion flow stops

Question 17

State the Nernst equation and the definition of it

Answer 17

Eion - predicts the equilibrium potential for a single ion

Eion 61/z *(log ([ion]out/[ion]in))

z = ionic charge

Question 18

What is Eion influenced by?

Answer 18

Concentration gradient
Membrane permeability (channels open or nah)

Question 19

State the
Goldman-Hodgkin-Katz Equation

Answer 19

Vm - membrane potential

Vm = 61log (Px[X]out +Py[Y]out +…. / Px[X]in + Py[y]in +…)