Neurophysiology II

2025-09-18

Rick Gilmore

Department of Psychology

Prelude

Today’s topics

  • Warm-up
  • Neurophysiology II

Warm-up

Which of the following glial cells contribute to the blood/brain barrier?

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

Which of the following glial cells contribute to the blood/brain barrier?

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

Neurons are unusual among cells because they…

  • A. Have unusually long lives.
  • B. Have a membrane.
  • C. Have a nucleus and chromosomes.
  • D. Are born throughout the lifespan.

Neurons are unusual among cells because they…

  • A. Have unusually long lives.
  • B. Have a membrane.
  • C. Have a nucleus and chromosomes.
  • D. Are born throughout the lifespan.

Electrical potential (voltage) is analogous to…

  • A. Size
  • B. Mass
  • C. Charge
  • D. Pressure

Electrical potential (voltage) is analogous to…

  • A. Size
  • B. Mass
  • C. Charge
  • D. Pressure

Neurophysiology II

Why should we care?

  • Neurons send electrical signals (action potentials) over long distances
  • Enables big, multicellular organisms to behave
  • How do neurons create action potentials?
  • How does the neuron “reset” after it fires an action potential?

Resting membrane potential

  • Voltage across neuron’s membrane
  • -65-70 mV (~1/20th of a 1.5 V battery)
  • Voltage \(\rightarrow\) current can flow

Why is there a membrane potential?

  • Na+/K+ pump creates ion concentration differences
    • \([K^+]_{inside} >> [K^+]_{outside}\)
    • \([Na^+]_{outside} >> [Na^+]_{inside}\)

Na+/K+ “pump”

  • Also known as Na+/K+-ATPase
  • Moves ions across membrane
  • With metabolic “help” (energy expenditure)

Why is there a membrane potential?

  • Concentration differences create two forces of diffusion
  • Diffusion from high concentration to low
    • \(K^+\) pushed out
    • \(Na^+\) pushed in

https://en.wikipedia.org/wiki/Membrane_potential

Force of diffusion

Wikipedia

A practical illustration of the force of diffusion.

Wikipedia

Simulating diffusion

Why is there a membrane potential?

  • Large organic anions \(A^-\) can’t diffuse out.
  • Outflow of \(K^+\) creates +/- charge separation along membrane
  • Charge separation creates a voltage (potential)

https://en.wikipedia.org/wiki/Membrane_potential

Why is there a membrane potential?

  • Electrostatic force balances force of diffusion
    • Voltage build-up along membrane slows \(K^+\) outflow
    • Specific voltage that stops flow == equilibrium potential for \(K^+\)
    • \(K^+\) positive, so equilibrium potential negative (w/ respect to outside)

(Aside) on equilibrium potential

  • Can be calculated based on ion concentrations
  • Nernst equation: \(\frac{RT}{nF}\ln\frac{[K^+]_o}{[K^+]_i}\)
  • Related to electronic circuit model of membrane

Wikipedia

Neuron resting potential ≠ \(K^+\) equilibrium potential

  • Resting potential not just due to \(K^+\)
  • \(Na^+\) inflow contributes
  • Resting potential == net effects of all ion flows across membrane
    • Calculate with Goldman-Hodgkin-Katz equation

\(Na^+\) role

  • \(Na^+\) concentrated outside neuron
  • Membrane at rest not very permeable to \(Na^+\)
  • Some, but not much \(Na^+\) flows in

\(Na^+\) role

  • \(Na^+\) Equilibrium potential is positive
  • \(Na^+\) has equilibrium potential ~ + 60 mV
  • Would need positive membrane potential to keep \(Na^+\) from flowing in

\(Na^+\) role

  • \(Na^+\) inflow makes membrane potential more positive (less negative) than it would be with \(K^+\) outflow alone
  • Resting potential (-55-70mV) more positive than \(K^+\) equilibrium potential

Summary of forces

Ion Concentration gradient Force of diffusion Sign of electrostatic force
\(K^+\) \([K^+]_{i} >> [K^+]_{o}\) outward -
\(Na^+\) \([Na^+]_{i} << [Na^+]_{o}\) inward -

Summary: Why is there a membrane potential?

  • Na+/K+ pump creates concentration differences
  • K+ flow out (slowly), creating charge separation
  • Charge separation \(\rightarrow\) voltage (membrane potential)
  • Voltage has negative sign (positive \(K^+\) exit; negative \(A^-\) remain)

Summary: Why is there a membrane potential?

  • Na+ ions flow in (very slowly, < \(K^+\) flows out)
  • Reducing charge separation (a bit), makes membrane potential less negative
  • Net result
    • Balance of diffusion and electrical forces acting on \(K^+\) and \(Na^+\)

(Aside) Khan Academy video

– khanacademymedicine (2012)

What happens if something changes?

  • “Something” == ion channels open (or close)
  • Easier for \(Na^+\) to enter? Or \(K^+\) to exit?
  • Some action!

Action potential

Wikipedia

Phases of the action potential

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

Wikipedia

Phases of the action potential

Phase Neuron State
Rise to threshold + input makes membrane potential more +
Rising phase Voltage-gated \(Na^+\) channels open, \(Na^+\) flows in

Phases of the action potential

Phase Neuron State
Peak Voltage-gated \(Na^+\) channels close and deactivate; voltage-gated \(K^+\) channels open
Falling phase \(K^+\) flows out

Phases of the action potential

Phase Neuron State
Refractory period \(Na^+\)/\(K^+\) pump restores [\(Na^+\)], [\(K^+\)]; voltage-gated \(K^+\) channels close

Refractory period (phase)

  • Absolute
  • Relative

Absolute refractory phase

  • Voltage-gated \(K^+\) channels close
    • Driving force on \(K^+\) tiny or absent
  • \(Na^+\)/\(K^+\) pump restores concentration balance
  • No AP can be generated

Relative refractory phase

  • Can generate AP with larg(er) stimulus
  • Some voltage-gated \(Na^+\) ‘de-inactivate’
    • can open with large input
    • membrane potential is more negative than resting potential

Generating action potentials

  • Axon hillock
    • Portion of soma adjacent to axon
    • Sums inputs to soma from many dendrites

Generating action potentials

  • 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

Simulating the action potential

How action potentials propagate

  • Propagation
    • move down axon, away from soma, toward axon terminals.
  • Axon is like an electrical cable

Nodes of Ranvier

  • Regenerate action potential
  • \(Na^+\) and \(K^+\) channels exposed to extracellular environment
  • Between Nodes of Ranvier, ions can’t move out, so move along

Wikipedia contributors (2025a)

Nodes of Ranvier

  • Action potentials faster & more reliable for a given axon diameter

Wikipedia contributors (2025a)

How action potentials propagate

  • Propagation \(\rightarrow\) move
    • move down axon, away from soma, toward axon terminals
  • Axon like an electrical cable

Propagation

  • Unmyelinated axons
    • Each segment “excites” the next

Wikipedia contributors (2025b)

Propagation

  • Myelinated axon
    • AP “jumps” between Nodes of Ranvier
    • voltage-gated \(Na^+\), \(K^+\) channels exposed
    • current flows through/down myelinated segments

Wikipedia contributors (2025b)

Why does AP flow in one direction, away from soma?

  • Soma does not have (many) voltage-gated \(Na^+\) channels.
  • Soma is not myelinated.
  • Refractory periods mean polarization works only in one direction.

Why does AP flow in one direction, away from soma?

  • Soma does not have (many) voltage-gated \(Na^+\) channels.
  • Soma is not myelinated.
  • Refractory periods mean polarization works only in one direction.

How fast are APs?

  • Axons carry information at different rates
    • More myelin -> faster
    • Larger diameter axon -> faster
  • Somatosensory information faster than pain (nociception)

Wikipedia contributors (2025b)

Information processing

  • Action potential amplitudes don’t vary (much)
    • All or none
    • \(Na^+\)/\(K^+\) pumps working all the time
    • \([K^+]\) & \([Na^+]\) don’t vary much, so
    • \(V_{K^+}\) & \(V_{Na^+}\) don’t vary much

Information processing

  • Action potential frequency and timing vary
    • Rate vs. timing codes
    • Neurons use both

Eyherabide et al. (2009)

Main points

  • Resting potential due ion concentration differences, balance of forces
  • Change in membrane permeability to \(Na^+\)–voltage-gated \(Na^+\) channels, triggers action potential.
    • Rapid rise to peak, fall, refractory
  • Action potentials propagate down axon

Next time

  • Neurophysiology III
  • Exam 1 review

Resources

About

This talk was produced using Quarto, using the RStudio Integrated Development Environment (IDE), version 2025.5.1.513.

The source files are in R and R Markdown, then rendered to HTML using the revealJS framework. The HTML slides are hosted in a GitHub repo and served by GitHub pages: https://psu-psychology.github.io/psych-260-2025-fall/

References

Eyherabide, H. G., Rokem, A., Herz, A. V. M., Samengo, I., Eyherabide, H. G., Rokem, A., … Samengo, I. (2009). Bursts generate a non-reducible spike-pattern code. Frontiers in Neuroscience, 3, 1. https://doi.org/10.3389/neuro.01.002.2009
khanacademymedicine. (2012). Membrane potentials - part 1 | circulatory system physiology | NCLEX-RN | khan academy. Youtube. Retrieved from https://www.youtube.com/watch?v=PtKAeihnbv0&t=1s
Wikipedia contributors. (2025a, June 13). Node of ranvier. Retrieved from https://en.wikipedia.org/wiki/Node_of_Ranvier
Wikipedia contributors. (2025b, August 30). Nerve conduction velocity. Retrieved from https://en.wikipedia.org/wiki/Nerve_conduction_velocity