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Cell Function: Fundamental for signaling within cells.
Membrane Electrical Properties: Developed through ion gradients across membranes leading to voltage differences (membrane potentials).

Definition: Voltage difference across cell membranes due to ion gradients.
Presence: All cells posses membrane potentials which determine the movement of ions and molecules across membranes.
Significance: Vital for the function in excitable cells (neurons, muscles) and non-excitable cells (regulation of cell volume, signaling pathways).

Neuromuscular Junctions
- Electrical Signal: Travels along the motor neuron.
- Neurotransmitter Release: Acetylcholine (ACh) released, stimulating muscle membrane potential change leading to contraction.
Venus Flytrap Mechanism
- Mechanosensitive Ion Channels: Open upon insect touch, allowing positively charged ions to enter and changing membrane potential, causing trap closure.

Sodium-Potassium Pump
- Function: Moves 3 Na+ out and 2 K+ into the cell using ATP.
- Outcome: Establishes concentration gradients (high Na+ outside, high K+ inside).
Selective Permeability of Membrane
- Resting State: More permeable to K+ due to potassium leak channels.
- Effect: K+ moves out, causing change in membrane potential (becomes more negative).
Trapped Negatively Charged Anions
- Nature: Large anions (e.g., proteins, amino acids) unable to cross the membrane.
- Result: Contributes to negatively charged interior of the cell (Gibbs Donnan effect).

Definition: Charge separation across the membrane, with interior being more negatively charged compared to the external environment.
Ion Movement: Small ion movements generate significant changes in membrane potential.

Steady State Value: Typically around -70 mV in neurons.
Establishment Factors:
- Sodium-Potassium Pump creates concentration gradients.
- Potassium leak channels are the primary contributor by allowing K+ to move out, thus generating charge separation.
Electrochemical Equilibrium Concept: Current flux of ions leads to a balanced state but not true electrochemical equilibrium as ions still tend to move across.

Purpose: Defines equilibrium potential for a single ion, balancing concentration gradient and electrical gradient.
Form: $Ex = rac{RT}{zF} imes ext{log} rac{[X]{inside}}{[X]_{outside}}$
- Where R = gas constant, T = temperature (K), z = ion charge, F = Faraday's constant.
Simplified version: At ~30ºC, $E{K} = -60 imes ext{log}{10} rac{[K]{inside}}{[K]{outside}}$
Example for Potassium:
- Concentration: 140 mM inside, 5 mM outside results in Nernst potential of approximately -86.8 mV.

Purpose: Describes membrane potential considering multiple ions and their permeabilities.
Form: $Vm = rac{PK[K]{inside} + P{Na}[Na]{inside} + P{Cl}[Cl]{outside}}{PK + P{Na} + P{Cl}}$
- Where P represents relative permeabilities for each ion.
Key Contributors:
- Potassium influences resting potential the most, followed by sodium and chloride.
- Resulting typical resting potential is around -70 mV (not at equilibrium due to active transport).

Definition: Rapid, transient changes in membrane potential.
Process: Beginnings at resting potential, depolarizes to approximately +30-40 mV before repolarizing and after hyperpolarization occurs.
All-or-None Principle: Action potentials occur if a threshold (-50 mV) is reached; otherwise, nothing occurs.

Depolarization Phase: Voltage-gated sodium channels open, allowing Na+ influx, leading to rapid depolarization.
Repolarization Phase: Voltage-gated sodium channels inactivate, while voltage-gated potassium channels open, allowing K+ to exit, returning membrane potential to negative values.
Hyperpolarization Phase: Due to slow closing of potassium channels, potential dips below resting membrane potential.

Voltage-Gated Sodium Channels:
- States: Closed, open, inactivated.
- Function in depolarization and establishment of a refractory period preventing backward movement of action potentials.
Voltage-Gated Potassium Channels: Add to repolarization and after hyperpolarization phases.

Signal Propagation: Action potentials propagate effectively without amplitude loss along axons due to local depolarization triggering adjacent segments.
Distinct Features:
- Unidirectional propagation due to inactivation of sodium channels behind the advancing action potential wave.
- Myelination Role: Myelin sheath enables saltatory conduction, increasing propagation speed and minimizing energy expenditure for ion gradient restoration.
Conduction Velocities: Range from 1 m/s (slow, unmyelinated) to 120 m/s (fast, myelinated). Typical in mammals: 50-60 m/s.

Presynaptic Terminals: Arrival of action potential opens voltage-gated calcium channels, leading to calcium influx.
Neurotransmitter Release: Calcium influx triggers fusion of synaptic vesicles with presynaptic membrane to release neurotransmitters (e.g., acetylcholine).
Role of Calcium: Amount entering determines neurotransmitter release extent; prolonged stimulation can deplete vesicle availability.

Excitatory Postsynaptic Potential (EPSP): Result of Na+ influx through ligand-gated channels, depolarizing the postsynaptic cell.
Inhibitory Postsynaptic Potential (IPSP): Result of K+ efflux or Cl- influx, leading to hyperpolarization of the postsynaptic cell.
Postsynaptic Potentials: Graded responses based on neurotransmitter release size and proximity from synapse; if strong enough, can engage action potential at trigger zone.

Membrane Properties: Electrical signals are dynamic; resting potentials are crucial for action potentials.
Action Potentials: Underpin extensive physiological functions, including rapid communication across the nervous system.
Neurotransmission: Converts electrical signals to chemical signals and back into electrical signals across synapses leading to various bodily responses, including muscle contractions and hormone release.
Central Theme: Understanding membrane potentials is key to comprehending cellular and systemic signal processing in biology.