Resting Membrane Potentials

Overview of Neurons and Electrical Signals

• Introduction to the discussion of how neurons fire electrical signals.

• Focus on concepts related to membrane potentials.

• Specific focus on resting membrane potential.

• Learning outcome: Describe the ionic basis of the resting membrane potential.

Types of Membrane Potentials

• Action Potentials

  • Responsible for sending signals over long distances in axons.

• Graded Potentials

  • Used for short-distance communication.

  • Determine whether a neuron will fire an action potential.

• Resting Membrane Potential

  • Keeps neurons ready to respond like a charged battery.

  • Essential for neuronal function.

Measuring Resting Membrane Potential

• The resting membrane potential can be measured experimentally:

  • Use a microelectrode inserted inside a neuron.

  • An electrode is placed in the extracellular fluid to measure voltage difference.

  • Important to measure potential difference rather than absolute voltages.

  • Typical resting membrane potential for neurons is about -70 millivolts.

Mechanisms of Establishing Membrane Potentials

Basic Cell Model

• Mammalian cells possess a phospholipid bilayer membrane impermeable to water and ions.

• Both sodium and potassium ions initially present in equal concentrations inside and outside of the cell:

  • Results in no membrane potential initially.

Sodium-Potassium Pump (ATPase)

• The sodium-potassium pump operates by:

  • Actively transporting sodium ions out of the cell and potassium ions into the cell using ATP.

  • For this model, it is assumed that one sodium ion is pumped out for every one potassium ion pumped in.

• If this transport functioned perfectly:

  • All potassium ions would be inside the cell and all sodium ions outside, but this does not occur.

  • Charges remain balanced initially, thus retaining no membrane potential.

Leaky Potassium Channels

• Introduction of leaky potassium channels:

  • Always open, allowing potassium ions to leak out along their concentration gradient.

  • Higher concentration of potassium inside the cell leading to a negatively charged interior as positive charges leave.

• Electrochemical Equilibrium:

  • Equilibrium is reached when the electrical gradient balances out the concentration gradient for potassium ions.

  • Represents the resting membrane potential.

  • Analogized to a tug-of-war where both sides exert equal force, maintaining position.

Concentration Gradients and Membrane Potential

Relation Between Potassium Concentration and Membrane Potential

• The size of the initial concentration gradient of potassium ions shapes the resting membrane potential:

  • Small Concentration Gradient:

  • Smaller resting membrane potential is generated.

  • Large Concentration Gradient:

  • Larger resting membrane potential is generated due to the higher number of potassium ions that leak out until equilibrium is met.

Nernst Equation

• The equilibrium potential is defined mathematically by the Nernst equation, modified for ion concentration:

  • Nernst Equation components:

  • R: Gas constant

  • F: Faraday constant

  • T: Temperature in Kelvin

  • Z: Valence of the ion (charge)

  • [ ext{Ion}]: Concentration of the ion inside and outside the membrane.

  • For potassium at physiological temperature (37°C), the equilibrium potential calculates to approximately -90 millivolts.

Dominance of Potassium's Equilibrium Potential

• The resting membrane potential is close to -70 millivolts, reflecting the influence of potassium ions, which establish much of the membrane potential's value.

• Other ions such as sodium and chloride also contribute via leaky channels but are minor in comparison to potassium's effect.

Ion Concentrations Across Neuronal Membranes

• Potassium ions: 150 millimolar (inside) vs. 5 millimolar (outside).

• Sodium ions: 15 millimolar (inside) vs. 150 millimolar (outside).

• Chloride ions: 10 millimolar (inside) vs. 110 millimolar (outside).

• Important to remember these relative concentrations, focusing on potassium's predominance inside the cell.

Goldman Hodgkin Katz Equation

• The Goldman equation predicts membrane potential based on multiple ions:

  • Factors in permeabilities of ions and allows predictions considering multiple species rather than just potassium.

Role of the Sodium-Potassium Pump in Membrane Potential

• The sodium-potassium pump is electrogenic, creating a net negative charge within the cell.

  • However, this contributes minimally (about 5 millivolts) to the resting membrane potential.

• Main role: Establishes concentration gradients that influence membrane potential rather than directly producing the resting potential.

Summary of Resting Membrane Potential

• Dominated by potassium ion permeability and established by the sodium-potassium pump.

• The resting potential is shaped closely by the equilibrium potential for potassium and can deviate due to affects from sodium and chloride channels.

• Final understanding of the ionic basis of the resting membrane potential underscores the importance of potassium permeability.

Practical Implications of Potassium Levels

• Example scenario presented regarding dietary potassium intake (specific focus on bananas):

  • Hypothetical ingestion of more than six bananas discussed.

• Increased extracellular potassium concentration would disrupt normal gradients:

  • Decrease potassium concentration gradient would lead to reduced resting membrane potential (e.g. reaching around -60 millivolts).

• Potential physiological consequences include elevated neuron excitability, possibly leading to seizure risks and action potential firing.

Blood-Brain Barrier Protection

• Importance of the blood-brain barrier in maintaining neuronal homeostasis, selectively regulating ion entry, particularly potassium.

• Contrasted with cardiac cells lacking such barriers, vulnerable to potassium fluctuations leading to arrhythmias.

Conclusion and Outlook

• Final remarks on the interconnectedness of neuronal dynamics and diet.

• Preview of next lecture's focus on graded potentials.