260-2019-01-31-neurophys-II

• Exam 1 in-class next Thursday
• Mind maps/concept maps
• Warm-up
• Electrical communication in neurons
• The action potential
• Wherefore brains

Neural communication

• 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

• Ions (charged atoms)
• Ion channels
• Separation between charges
• A balance of forces

• Potassium, $K^+$
• Sodium, $Na^+$
• Chloride, $Cl^-$
• Organic anions, $A^-$

• 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 for specific ions
• Vary in permeability (how readily ions flow)
• Types
  • Passive/leak (always open)
  • 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!

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

• 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

• 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

• 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

• Putting it all together
• How action potentials propagate
• Review for Exam 1

• 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