• Exam 1 Thursday, 2/10
  • 40 questions
  • No in-person/in-class meeting
  • On Canvas, live at 3:05 PM; open until 10:00 PM

• Electrical communication in neurons
• The action potential

• Resting potential
  • Voltage across neuronal membrane when cell is ‘at-rest’ (not firing)
• Action potential
  • Voltage across neuronal membrane when cell is active or firing

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

• Potassium, \(K^+\)
• Sodium, \(Na^+\)
• Chloride, \(Cl^-\)
• 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

• A balance of forces
  • Force of diffusion
  • Electrostatic force
• Forces cause ion flows across membrane
  • Force of diffusion consistent (over time)
  • Electrostatic force changes
• 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

• Permeable: Permits flow across/through
• Passive \(K^+\) channels open
• [\(K^+\)] concentration inside >> outside
• \(K^+\) flows out
  • Neuron constantly brings \(K^+\) in

https://www.youtube.com/watch?v=I_N82ZvLT-Q

• Organic anions (\(A^-\)) too large to move outside of cell
• \(A^-\) and \(K^+\) largely in balance == no net internal charge
• \(K^+\) outflow creates charge separation: \(K^+\) (outside) <-> \(A^-\) (inside)
• 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 that stops flow == equilibrium potential for \(K^+\)+
  • \(K^+\) positive, so equilibrium potential negative (w/ respect to outside)
  • Equilibrium potential close to neuron’s 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

• 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 adensosine triphosphate (ATP) form of 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

• Regenerate action potential
• \(Na^+\) and \(K^+\) channels exposed to extracellular environment
• Between Nodes of Ranvier, ions can’t move out, so move along
• Nodes of Ranvier -> Action potentials faster & reliable for a given diameter

• Resting potential maintained by balance of forces (diffusion, electrostatic)
• Action potential generated when balance is altered
  • \([Na^+]\) in: rising phase to + peak
  • \([K^+]\) out: falling phase

• More on the action potential
• Review for Exam 1