Neuronal Membrane and Resting Potential

Step 1: Individual experiences (e.g., stepping on a thumbtack) trigger nerve signals.
Step 2: Signals travel via sensory nerves to the spinal cord.
Step 3: Information is relayed to interneurons:
- Some interneurons send signals to motor neurons to initiate muscle contractions.
- Others transmit signals to the brain registering painful sensations.

The primary function of the nervous system is to convey information rapidly and effectively.

Effective nervous system operations require electrical signaling due to the long distances and short response times involved.
Example: Touching a hot spoon triggers immediate pain responses to prevent injury.
Neuronal Insulation: Neurons are poorly insulated and conduct electricity poorly (shorted out by conductive fluids).
Nature of Electrical Signaling:
- Neurons use trains of impulses, called action potentials, to transmit signals.
- These impulses encode information based on the timing of occurrences rather than magnitude.
- Resting Potential: A neuron maintains a resting state where the inside is negatively charged compared to the outside (about -65 mV).

Electrical Operations in Aqueous Medium:
- Water (H₂O) acts as a polar solvent and dissolves ionic compounds (e.g., NaCl).
- Substances of interest: Sodium (Na+), Potassium (K+), Calcium (Ca++), Chloride (Cl-).
Ion Characteristics:
- Cations (positively charged): Na+, K+, Ca++
- Anions (negatively charged): Cl-, A-
Hydration Sphere: Ions in solution are surrounded by a shell of water molecules, which affects their behavior.

Cell Membrane:
- Composed of a phospholipid bilayer with hydrophilic phosphate zones facing outward and hydrophobic lipid zones inward.
- Membrane proteins play critical roles in function, aiding in the transportation of ions and other molecules.

Movement Across Membranes:
- Ions require channels for transport due to hydration spheres.
- Transport can occur actively (via pumps) or passively (via channels).

Ion Diffusion:
- Solvent allows for diffusion of ions, leading to electrical potential differences.
- Movement depends on both concentration gradients and selective permeability of the channels.

Electricity Defined:
- Electrical current (I): Movement of charge (positively defined).
- Electrical potential (voltage): Force on a charged particle (difference between anode and cathode).
- Conductance (g): Ability of charge to migrate, while resistance (R) is defined as R = 1/g.
Ohm's Law:
- I = gV, explains the requirements for driving ions across membranes.

Voltmeter Usage:
- Measures electrical potential difference between intracellular and extracellular environments.
- Typical neuronal resting potential is around -65 mV, largely due to ion concentration differences.

At rest, the neuron is primarily influenced by K+ movement and Na+ influences less due to its permeability properties.
Ionic Gradients:
- Sodium-potassium pumps maintain gradients by expelling 3 Na+ for every 2 K+ entering, a crucial ATP-driven function.
Differentiation of Ion Influences:
- The resting membrane potential reflects the weighted average of the equilibrium potentials of permeant ions.

Calculating the equilibrium potential (Eion) for any ion gives insight into ion distribution and electrical gradients across membranes.
Formula: E{ion} = rac{RT}{zF} imes ext{log} rac{[ ext{ion}]o}{[ ext{ion}]_i} where:
- R = gas constant, T = temperature in Kelvin, z = charge of the ion, F = Faraday's constant.

Potassium channels exhibit high permeability influencing resting potentials significantly due to their leakage currents.
Blood-brain barrier limits potassium movement and potassium spatial buffering by astrocytes keeps levels consistent.

The resting potential arises primarily due to selective ion permeability and the action of the sodium-potassium pump ensuring electrochemical gradients.
The typical resting potential is about -70 mV, highlighting the significance of K+ ion concentration internally relative to Na+, which is predominantly found outside the neuron.