Neuroscience Chapter 3: Neuronal Membrane

Neuronal Membrane Overview

• Neuron communication relies on electrical signals known as action potentials, requiring a baseline resting membrane potential.
• Resting membrane potential is the electrified state of neuronal cells, vital for action potentials to occur.

Key Components of Neuronal Membrane

• Cytosol and extracellular fluid primarily consist of water, a polar solvent essential for dissolving ions.
• Ions are charged atoms or molecules; cations have a positive charge, while anions have a negative charge.
• The phospholipid bilayer forms a barrier that isolates neuronal cytosol from extracellular fluid, distinguishing hydrophilic (water-soluble) and hydrophobic (water-insoluble) substances.

Electrical Principles in Neuron Function

• Ohm’s law explains the relationship between current (I), conductance (g), and potential (V): I = gV.
• Voltage measures the potential energy generated by separated charges, with net charge differences leading to varying voltages.
• Current signifies the flow of electric charge between two points, dependent on voltage and resistance.

Membrane Potential Dynamics

• Neurons maintain a resting membrane potential that can change rapidly compared to other cell types.
• The energy required to separate opposite charges across membranes contributes to potential energy.

Ionic Basis of Resting Membrane Potential

• The equilibrium potential (Eion) depicts no net ion movement across membranes without channel proteins.
• The presence of K+ channels leads to ionic flow down concentration gradients, altering charge/voltage across the membrane.
• The Nernst equation calculates equilibrium potentials, accounting for ion charge, temperature, and concentration ratios.

Ion Distribution and Movement

• Ion concentrations vary, with K+ primarily inside and Na+ along with Ca2+ predominantly outside the cell.
• Sodium-potassium pumps (3 Na+ out, 2 K+ in) actively transport ions against their gradients, consuming up to 70% of brain ATP.

Membrane Permeability

• Neuronal membranes are selectively permeable, with K+ permeability 25 times greater than Na+ due to more leakage channels.
• More potassium diffuses out than sodium diffuses in, contributing to the negative internal charge and establishing resting membrane potential.

Role of Ion Channels

• Membrane permeability relies on distinct ion concentrations, channel presence, and ion-specific conductance.
• Gated channels allow ion diffusion along concentration and electrical gradients; potassium channels are central to resting membrane potential regulation.
• Various types of K+ channels exist, such as the Shaker potassium channel, identified in fruit flies.

Potassium Channel Structure and Function

• Potassium channels consist of four subunits, forming a selective filter for K+ ions through the pore loop area.
• MacKinnon's research on K+ channels won the 2003 Nobel Prize, revealing that specific mutations lead to neurological disorders.

Importance of Extracellular K+ Concentration

• The resting membrane potential closely aligns with EK due to high K+ permeability, making it sensitive to extracellular K+ changes.
• Elevated extracellular K+ levels can cause membrane depolarization, significantly affecting neuronal activity.