2017-02-12 11:54:30

Measuring potentials in actual neurons

Today's Topics

  • The neuron at rest
  • The neuron in action

Resting potential

Resting potential arises from

  • A balance of forces
    • Force of diffusion
    • Electrostatic force
  • Forces cause ion flows across membrane
  • Ion channels allow ion flow

Ion channels

  • Openings in neural membrane
  • Selective
  • Vary in permeability
  • Types
    • Passive/leak
    • Voltage-gated
    • Ligand-gated (chemically-gated)
    • Transporters/pumps

Ion channels

Neuron at rest permeable to K+

  • Passive K+ channels open
  • [K+] concentration inside >> outside
  • K+ flows out

Force of diffusion

Force of diffusion

Neuron at rest permeable to K+

  • Organic anions (A-) to large to move outside of cell
  • A- and K+ largely in balance == no net internal charge
  • K+ outflow creates charge separation: K+ <-> A-
  • Charge separation creates a voltage
  • Outside +/inside -
  • Voltage build-up stops outflow of K+

The resting potential

Balance of forces in the neuron at rest

  • Force of diffusion
    • K+ moves from high concentration (inside) to low (outside)

Balance of forces in the neuron at rest

  • Electrostatic force
    • Voltage build-up stops K+ outflow
    • Specific voltage called equilibrium potential for K+
    • K+ positive, so equilibrium potential negative (w/ respect to outside)
    • Equilibrium potential close to neuron resting potential

Equilibrium potential and Nernst equation

Building on intuition

Equilibrium potentials calculated under typical conditions

Ion [inside] [outside] Voltage
K+ ~150 mM ~4 mM ~ -90 mV
Na+ ~10 mM ~140 mM ~ +55-60 mV
Cl- ~10 mM ~110 mM - 65-80 mV

Resting potential ≠ K+ equilibrium potential

  • Resting potential not just due to K+
  • Other ions flow
  • Resting potential == net effects of all ion flows across membrane

Goldman-Hodgkin-Katz equation

Na+ role

  • Na+ concentrated outside neuron
  • Membrane at rest not very permeable to Na+
  • Some, but not much Na+ flows in
  • Na+ has equilibrium potential ~ + 60 mV
  • Equilibrium potential is positive (with respect to outside)
  • Would need positive interior to keep Na+ from flowing in

Electrical circuit model

Summary of forces

Ion Concentration gradient Electrostatic force Permeability
K+ Inside >> Outside - (pulls K+ in) Higher
Na+ Outside >> Inside - (pulls Na+ in) Lower

Party On

  • Annie (A-) was having a party.
    • Used to date Nate (Na+), but now sees Karl (K+)
  • Hired bouncers called
    • "The Channels"
    • Let Karl and friends in or out, keep Nate out
  • Annie's friends (A-) and Karl's (K+) mostly inside
  • Nate and friends (Na+) mostly outside
  • Claudia (Cl-) tagging along

What happens if something changes?

  • Easier for Karl [K+] to exit?
  • Easier for Nate [Na+] to enter?
  • Some action!

Action potential

Action potential

  • Threshold of excitation
  • Increase (rising phase/depolarization)
  • Peak
    • at positive voltage
  • Decline (falling phase/repolarization)
  • Return to resting potential (refractory period)

Action potential break-down

Phase Neuron State
Rise to threshold + input makes membrane potential more +
Rising phase Voltage-gated Na+ channels open, Na+ enters
Peak Voltage-gated Na+ channels close and deactivate; voltage-gated K+ channels open
Falling phase K+ exits
Refractory period Na+/K+ pump restores [Na+], [K+]; voltage-gated K+ channels close

What's a Na+/K+ pump?

  • Enzyme (Na+/K+ ATP-ase) embedded in neuron membrane
  • Pumps Na+ and K+ against concentration gradients
  • Na+ out; K+ in
  • Uses ATP or chemical energy

Example in another domain

Refractory periods

  • Absolute
    • Cannot generate action potential (AP) no matter the size of the stimulus
    • Voltage-gated Na+ channels inactivated, reactivate in time.
  • Relative
    • Can generate AP with larg(er) stimulus
    • Some voltage-gated K+ channels still open
  • Refractory periods put 'spaces' between APs

Generating APs

  • Axon hillock
    • Portion of soma adjacent to axon
    • Integrates/sums input to soma
  • Axon initial segment
    • Umyelinated portion of axon adjacent to soma
    • Voltage-gated Na+ and K+ channels exposed
    • If sum of input to soma > threshold, voltage-gated Na+ channels open

Axon hillock, axon initial segment

Next time

  • How action potentials propagate
  • Review for Exam 1