• Announcement: Exam 1 next Friday
• Why brains?
• The neuron at rest

• A. Astrocytes
• B. Oligodendrocytes
• C. Schwann cells
• D. Microglia

• A. Astrocytes
• B. Oligodendrocytes
• C. Schwann cells
• D. Microglia

• A. 100 million
• B. 100 billion
• C. ~86 billion
• D. ~86 million

• A. 100 million
• B. 100 billion
• C. ~86 billion
• D. ~86 million

• A. The axon
• B. The terminal button
• C. The soma
• D. The dendrites

• A. The axon
• B. The terminal button
• C. The soma
• D. The dendrites

• 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

• For neurons
• Bigger bodies
• Live longer
• Do more, do it faster

• Electrical
  • Fast(er)
  • Within neurons
• Chemical
  • Slow(er)
  • 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
  • Inside - Outside = potential

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

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

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

• 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+

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

• 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

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

• 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

• 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

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

• The action potential
• How action potentials propagate