Membrane Potentials and Action Potentials

Question 1

Lipid Membranes

Answer 1

-doesn’t allow Ions to Pass through Unless via Specialized Transport Mechanisms
o Protein Molecules and Membrane Structure – trans membrane domains
o Hydrophilic pores made by proteins through the membrane
o No transmembrane protein = no ion flux

Question 2

Active vs. Passive Transport

Answer 2

• Active Transport Mechanisms – consume energy to move ions against concentration gradient
• Passive Transport Mechanisms – channel mediated diffusion based on concentration gradients; no energy required

Question 3

Measuring Ionic Flux

Answer 3

– flux and direction determined by 2 driving forces (voltage and concentration gradient)

Question 4

Ionic Channels

Answer 4

– passive, facilitated diffusion mechanism
o Selectivity to K+ or Na+
o Gating – either voltage or ligand
o Resting potential uses K+ and Na+ leak channels
o Action potential uses K+ and Na+ voltage-gated channels

Question 5

Ionic Flux Generating Membrane Potential

Answer 5

o K+ ions pushing out by diffusion (chemical driving force)

o Electrical potential builds up due to K+ moving out

Question 6

At Rest: Electrical-chemical Equilibrium

Answer 6

o All nerve cells are negatively charged
o Balance between chemical (concentration) force and electrical (membrane potential) force
o Resting Potential = -60mV
o At equilibrium there is NO NET FLUX
o Nernst Equation – at equilibrium (no net flux) the potential is determined by
 E = 59 log [K]o/[K]in

Question 7

Resting Membrane Potential

Answer 7

– dynamic balance between K+ leak outward and Na+ leakage inward
o K+ In = 155; K+ Out = 4
o Na+ In = 12; Na+ Out = 145
o Resting Potential VARIES = -60mV when awake; -70mV when sleeping
o High Na+ outside; High K+ inside
o Depolarized = inside becomes more positive
o Hyperpolarized = inside becomes more negative

Question 8

Threshold

Answer 8

stimulus opens up some voltage gated Na+ channels; if the influx of Na+ reaches threshold then more channels are opened triggering an action potential

Question 9

Action Potential - Depolarization

Answer 9

o Action potential triggers Na+ channels to open, K+ channels remain closed
o Initial Positive Feedback
 Inward Na+ Current  Membrane Depolarization  opening of voltage-gated Na+ channels  MORE inward Na+ current

Question 10

Action Potential - Repolarization

Answer 10

o Inactivation (closure) of voltage gated Na+ channels; K+ channels open to repolarize the membrane potential back to resting membrane potential

Question 11

Refractory Periods

Answer 11

o Absolute – all Na+ channels inactive
 Makes sure there is one-way propagation of action potentials
 Second response NOT possible
o Relative – some Na+ channels recovered
 Helps limit the frequency of action potentials
 Second response can be elicited but at a greater cost (strength or duration)

Question 12

Axonal Action Potential Factors

Answer 12

– all or nothing; unidirectional
o	Conduction velocity – speed of propagation
	Units: meters/sec
	Differs for different axons
o	Membrane capacitance and myelination
o	Activation kinetics of the Na+ channel

Question 13

Conduction Velocity Depends on:

Answer 13

o Internal diameter of axon and internal resistance
o Membrane capacitance
o Amount of myelination
o Activation kinetics of Na+

Question 14

Space Constant

Answer 14

• Space Constant (λ) = √(Rm / Ri)
o Ri = 1 / diameter
o Larger diameter = lower Ri (internal resistance) = higher conduction velocity

Question 15

Capacitance

Answer 15

– slows down the generation of action potential
o Property of biological membranes ability to store charge
o Larger the capacitance the larger the time constant

Question 16

Time Constant

Answer 16

• Time constant (τ) = Rm x Cm
o If the onset time (ti) for generating an action potential is longer due to larger membrane capacitance, the velocity is reduced
o Larger time constant = slower the velocity

Question 17

Ideal Space Constant/Capacitance/Time Constant

Answer 17

o Want large space constant (conduction velocity)
o Want small time constant
o Want small capacitance  myelination decreases capacitance

Question 18

Myelination

Answer 18

o ~1mm “jumps”; action potential jumps from node to node  salutatory conduction
o Effect: salutatory conductance and reduced capacitance