Neuronal Activity

Page 2: Learning Outcomes

• Understand Ohm's law.

• Define the relationship between Coulomb forces and diffusion (Nernst equation) for ion movement.

• Identify components determining neuronal electrical properties.

• Differentiate between:

  • Ligand-gated ion channels

  • Voltage-gated ion channels

  • Ion pumps

• Explain ion pumps and membrane permeabilities in resting potential production.

• Describe the role of voltage-gated ion channels during action potentials (depolarization, repolarization, refractory period).

Page 3: Preface

• Introduction to neuronal activity concepts.

Page 4: Analogies in Neuronal Activity

• Neuronal activity explained similarly to electrical circuits:

  • Voltage, current, resistance, capacitance

  • Components include: VNa, VK, RNa, VM

  • Comparisons: voltage source (battery), conductor, insulator (brain/computer).

Page 5: Relevance of Concepts

• Physical concepts underpin neuronal activity.

• Understanding neuron communication using physical rules.

• Core concepts also applicable to heart & muscle cells.

• Implications:

  • EEG measurements

  • Deep brain stimulation

  • Brain-computer interfaces

  • Toxicology, etc.

Page 6: Electricity Basics

• Overview of fundamental electrical concepts related to neurons.

Page 7: Definition of Charge

• Charged particles can be:

  • Positive (e.g., Na+, K+, Ca2+)

  • Negative (e.g., Cl-, e-)

• Charge measurement: units of Coulomb [C].

• Charges include electrons (in metal) & ions (in tissues).

Page 8: Definition of Voltage

• Voltage defined as the electrical potential difference due to charge accumulation.

• Measured in Volts [V].

• 1 Volt: charge of 1 Coulomb moved by 1 Joule of energy.

• Biological voltages are typically in millivolts [mV].

Page 9: Definition of Capacitance

• Capacitance defined as the ability of an insulator to maintain voltage when charge accumulates.

• Relationship: Voltage = Charge / Capacitance (V = Q / C).

• Measured in Farads [F].

Page 10: Definitions of Current & Resistance

• Current: rate of charge flow, measured in Amperes [A]: 1 A = 1 C/s.

• Good conductors allow easy charge movement.

• Resistance: opposition to charge flow, measured in Ohms [Ω].

Page 11: Ohm’s Law

• Relation: Voltage = Current * Resistance (V = I.R).

• Demonstrates how resistance impacts current flow:

  • High resistance = low current

  • Low resistance = high current.

• Analogy: fluid flow through a system (Poiseuille’s Law).

Page 12: Electrical Properties of Neurons

• Identifying key components affecting neuronal electrical properties.

Page 13: Lipid Bilayer

• Acts as an insulator; ions cannot pass easily through.

• Accumulation of ions leads to a voltage known as the membrane potential (VM).

Page 14: Membrane Potential (VM)

• Membrane potential defined as the voltage difference between neuron inside & outside.

Page 15: Ion Pumps

• Use ATP to transport ions against their concentration gradient.

• Establish different ion concentrations.

• Example: Na+/K+ antiporter.

Page 16: Ion Channels

• Tiny pores allowing ions to pass without active transport.

• Numerous ion channels influence membrane permeability for specific ions:

  • Ligand-gated channels respond to bound ligands.

  • Voltage-gated channels respond to membrane voltage.

Page 17: Voltage-Gated Ion Channels

• Selectivity filter permits specific ion passage (e.g., Na+).

• Control opening/closing via voltage sensing domains.

• Delayed inactivation for emergency shutdown.

Page 18: Voltage-Dependent Behaviour

• Ion channels transition between states determined by membrane potential:

  • Open state allows conduction.

  • States are influenced by voltage: open, closed, inactivated.

Page 19: Membrane Resistance for Ions

• State of ion channels dictates permeability for ions.

Page 20: Ionic Membrane Currents

• Overview of ionic currents related to neuronal membrane activity.

Page 21: Voltage

• Drift occurs due to voltage differences causing ion movement.

• Driven by Coulomb forces affecting equally charged ions.

• affects equally charged ions in the same way

Page 22: Concentration Difference

• Diffusion arises from concentration differences, moving from high to low concentration.

• Specific to ion types (Na+, K+, Ca2+, Cl-).

Page 23: Nernst Equation

• Describes voltage differences and concentration differences simultaneously.

• Voltage (Vion) drives current through resistance corresponding to ion concentration differences.

• Vion: Nernst potential or reversal potential.

• nerst equation: Vion[mV]=25/z ln (in/out)

Page 24: Nernst Potentials for Na+ and K+

• Concentration differences function like batteries for each ion type.

• Membrane currents are driven by these battery-like behaviors.

Page 25: Modified Ohm’s Law Including Nernst Potential

• Ion flow through resistance (Rion) determined by membrane potential (Coulomb force) and Nernst potential (diffusion force).

• Concentration difference acts similarly to a battery in series.

• Iion=Vm-Vion/Rion

Page 26: Physiological Membrane

• Membrane potential (VM) lies between sodium (VNa) and potassium (VK) reversal potentials:

  • (VM-VNa)<0: Na+ influx only.

  • (VM-VK)>0: K+ efflux only.

Page 27: Relationship Between Ionic Currents and Membrane Voltage

• Currents of Na+ inflow & K+ outflow charge the membrane capacitance, affecting voltage changes:

  • More Na+ inflow = more positive potential.

  • More K+ outflow = more negative potential.

Page 28: Resting Potential

• Defined as the membrane potential in an unperturbed state around –70 mV for neurons.

Page 29: Relative Membrane Permeabilities

• Membrane potential (VM) influenced by relative permeabilities:

  • Higher Na+ permeability shifts VM closer to VNa.

  • Higher K+ permeability shifts VM closer to VK.

Page 30: Resting Potential Characteristics

• Resting potential indicates relative permeabilities for Na+ and K+; questioning which is larger.

• a nureon is -70mV

Page 31: Opening State of Ion Channels

• Analyzing the voltage-dependency of channels indicates ones likely to be open during resting state.

Page 32: Action Potential (AP)

• AP characterized by change in membrane potential (VM).

• Triggered by signals raising potential above threshold.

Page 33: Timing During AP

• AP is composed of three phases:

  • Depolarization

  • Repolarization

  • Hyperpolarization

• Refractory period affected by these phases.

Page 34: Driving Factors of AP

• AP processes depend on previous voltage-gated Na+ and K+ channels and ionic currents.

• Noting Na+ channels react faster to voltage changes than K+ channels.

• Ap duration is 0.5-1ms, Refractory period 2-5ms (absolute) 5-20ms(relative)

Page 35: Resting Potential During AP

• Few K+ channels open; most Na+ channels closed.

• Na+ influx equals K+ efflux leading to negative resting VM.

• na+ chnnels react faster to boltage changes than k+ channels

Page 36: Depolarization Phase

• Rapid Na+ channel opening leads to Na+ influx, overcoming modest K+ efflux, causing positive feedback and increased VM.

Page 37: Repolarization Phase

• Na+ channels inactivate while K+ channels open, leading to significant K+ efflux, dropping VM.

Page 38: Hyperpolarization Phase

• K+ channels close, Na+ channels recover; minimal net influx of ions, slowly raising VM back towards resting state.

Page 39: Return to Resting Potential

• Few K+ channels are open; most Na+ channels closed, and equilibrium reached between Na+ influx and K+ efflux.