Neurons and Neuron Excitability

Question 0

Diffusible ions distribute themselves so that at equilibrium, concentrations are equal

Problem: Non-diffusible - Protein! But it must equalize.

Answer 0

Gibbs-Donnan Equilibrium

Cl and K ( assymetric –> electric potential)
Cl goes in
K goes out

Question 1

Inside of the membrane is negatively charged

Answer 1

-70 mV

Negativity is only in the vicinity of the membrane
Remember that action potential only occurs near the membrane

Question 2

Transmembrane potassium ion gradient thru non-gated potassium ion leak channels

‘Whatever goes in, goes out’

SINGLE DIFFUSIBLE ION

Answer 2

Nernst Equation: calculates the potential of the membrane of one ion that is diffusible
If 2 ions, not possible
If non-diffusible, not possible

Diffusion pressure = electrical pressure

Reason why K has high conductance
Could move in and out

At equilibrium, no net movement of ions

Question 3

Not readiliy diffusible because at resting state, channels are closed

Readily diffusible

Non-diffusible because of large size

Answer 3

Na

K

Cl

Question 4

Equilibrium potential

Answer 4

K -90 mV (always goes out, leaving the inside of the membrane more negative)
Na +60 mV (goes in)
Cl -70 mV (no movement)

Question 5

Why is it not -90 (equilibrium potential) but -70 mV?

Answer 5

Because K is not the only diffusible (also Na)
Na is virtually closed, not completely closed, certain ions leak in
Cl has no movement

Question 6

Nernst equation can calculate the resting membrane potential

True or False

Answer 6

False
Only measures single diffusible ion

Use Goldman Constant to measure K, Na and Cl -> similar to Nernst but with contribution of Na

Question 7

Losing resting membrane potential leads to

Answer 7

Loss of brain function

Question 8

Na-ATPase pump
-function: prevents membrane potential to become zero by bringing out the Na that went in, and bringing in K that went out

‘Electrogenic pump’ : contributes to the negativity of the membrane, adds 20% to the resting membrane potential

Answer 8

3 Na in

2 K out

Question 9

Is the resting membrane potential in equilibrium?

Answer 9

No
Not an equilibrium potential but a ‘steady state’

If the organism dies, K continuously goes out (cytotoxic edema)

Question 10

Transient shift of the membrane potential in localized area of cell
Occurs where the stimulation occurs
Can only occur if these is resting membrane potential

2 properties:
Graded response: amplitude is proportional to size of stimulus

Answer 10

Local potential

Can be done in 2 ways:
Change the conformation of the protein channel by ligand-gating (permeability of the membrane)
Change the current

Question 11

Synaptic potential

Answer 11

Increase Cl conductance-hyperpolarization
Increase Na conductance-depolarization
Increase K conductance-hyperpolarization
Increase Na and K conductance-depolarization (lesser degree)

Question 12

Space constant

The larger the space constant, the farther along the membrane a voltage change

Answer 12

Larger = Farther

Directly proportional to the transmembrane potential
If the ions won’t go out, space constant is bigger

Connection: Myelin! High resistance, high neurons, farther the distance

Small diameter axon = increase internal axoplasmic resistance = smaller space constant
Connection: Smaller axon = smaller myelin!

Question 13

Local potentials can be summated

Answer 13

True

Spatial summation
Temporal summation

Question 14

Transmembrane resistance

Answer 14

Ability of the membrane to resist the ions to go out

Question 15

Cytoplasmic resistance

Answer 15

Resistance in the plasma
Ions can’t move fast –> tendency to be lost

Inverse with space constant

Question 16

Fleeting
Self-renewing wave of membrane depolarization
Always in positive depolarization
Same amplitude, same configuration
Propagates at a high speed

Answer 16

Action Potential
Concentrated in the initial segment of the axon
Can’t be used with hyperpolarization

What makes it possible: BOTH Na and K voltage gated ion channels!

Question 17

Spike tension

Answer 17

Upward: Influx of Na but not reaching its membrane potential because channel closes (inactivation gate)
Downward: Outflux of K

Question 18

Absolute refractory period

Answer 18

Threshold to one-third of repolarization
Because of the closed and inactivated state of Na channel

Gradually, Na channels become activatable resulting to action potential but needing a higher stimulus (RELATIVE REFRACTORY)

Question 19

Slower fall at end of spike potential

Answer 19

After-depolarization

Question 20

Overshooting of membrane repolarization after reaching a resting level

Answer 20

After-hyperpolarization

Question 21

The wider the nodal length, the wider the diameter

Answer 21

The thicker the myelin, the farther the space constant

Question 22

Salutatory conduction

Answer 22

Current flow

‘Jumping’