• Quiz 1
• Why brains?
• The resting potential

• Escherichia Coli (E. Coli)
• Paramecium
• Caenorhabditis Elegans (C. Elegans)

Sterling & Laughlin, 2015

• Tiny, single-celled bacterium
• Feeds on glucose
• Chemo ("taste") receptors on surface membrane
• Flagellum for movement
• Food concentration regulates duration of "move" phase
• ~4 ms for chemical signal to diffuse from anterior/posterior

• 300K larger than E. Coli
• Propulsion through coordinated beating of cilia
• Diffusion from head to tail ~40 s!
• Use electrical signaling instead
  • \(Na^+\) channel opens (e.g., when stretched)
  • Voltage-gated \(Ca^{++}\) channels open, \(Ca^{++}\) enters, triggers cilia
  • Signal across cell within ms

• ~10x larger than paramecium
• 302 neurons + 56 glial cells (out of 959)
• Swim, forage, mate

• Electrical
  • Fast(er)
  • Within neurons
• Chemical
  • Diffusion slow(er)
  • Within & between neurons

• Electrical potential (== voltage)
  • Think of potential energy
  • Voltage ~ pressure
  • Energy that will be released if something changes

\[E = IR\]

• Current flow (\(I\)) across membrane
• Membrane varies in permeability (\(R\)) to ion flow

• Membrane stores (& releases) charge like capacitor

• Resting potential
• Action potential

• Measurement
  • Electrode on inside
  • Electrode on outside (reference)
  • Inside - Outside = potential

• Neuron (and other cells) have potential energy
  • Inside is -60-70 mV, with respect to outside
  • About 1/20th typical AAA battery
• Like charges repel, opposites attract, so
  • Positively charged particles pulled in
  • Negatively charged particles pushed out

• Ions
• Ion channels
• Separation between charges
• A balance of forces

• Potassium, \(K^+\)
• Sodium, \(Na^+\)
• Chloride, \(Cl^-\)
• Calcium, \(Ca^{++}\)
• Organic anions, \(A^-\)

• 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
• Claude (\(Cl^-\)) tagging along

• Macromolecules that form openings in membrane
• Different types of subunits

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

• Passive \(K^+\) channels open
• \(K^+\) flows out
• \(K^+\) outflow creates charge separation from A-
• Charge separation creates voltage
• Voltage prevents \(K^+\) concentration from equalizing b/w inside and out

• Force of diffusion
  • \(K^+\) moves from high concentration (~140 mM inside) to low (~4 mM outside)
  • Movement of charged particles == current

• Electrostatic pressure
  • Voltage build-up stops \(K^+\) outflow
  • Voltage called "reversal potential"
  • \(K^+\) positive, so reversal potential negative (w/ respect to outside)
  • Reversal potential close to resting potential

• \(Na^+\) also has reversal potential
• Membrane at rest has low \(Na^+\) permeability
• Concentrated outside neuron (~145 mM) vs. inside (~12 mM)
• Some \(Na^+\) flows in
• Equilibrium potential is positive (with respect to outside)

• "Driving Force" on a given ion depends on its equilibrium potential.
• Driving force larger if membrane potential far from equilibrium potential for ion.
• Equilibrium potential
  • Voltage that keeps current (inside/outside) concentrations the same
  • Voltage membrane potential will approach if only that ion flows

• See also https://en.wikipedia.org/wiki/Membrane_potential
• Or, what could make Annie's party go haywire?