• 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

• 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
• Claudia (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 (~150 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 not very permeable to Na+
• Concentrated outside neuron (~140 mM) vs. inside (~10 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?