Action Potentials and Membrane Potential

Action potentials are crucial in the nervous and muscular systems.
Cell membrane consists of phospholipids, preventing free ion movement (polar molecules like ions need help to cross).

Sodium-Potassium (Na+/K+) pump transports sodium ions out and potassium ions into the cell, requiring ATP.
Establishes a charge gradient: outside more positive, inside more negative, with concentrations of ions differing between inside and outside.

Resting membrane potential is approximately -60 to -70 mV.
During depolarization, sodium channels open, allowing Na+ influx and raising potential to +30 to +40 mV.

The movement of depolarization along the membrane results from voltage-gated ion channels activating upon reaching threshold (-55 mV).
Different types of channels include:
- Ligand-gated channels (e.g., acetylcholine): open due to chemical signals.
- Mechanically-gated channels: open upon physical deformation (sensory cells).
- Voltage-gated channels: open in response to changes in voltage (propagate action potentials).
- Leakage channels: allow ions to pass passively and help maintain resting potential.

Depolarization: influx of Na+ raises membrane voltage.
Repolarization: K+ outflux restores resting potential, often overshooting to -80 mV (hyperpolarization).
Refractory Period: during hyperpolarization, neuron cannot initiate another action potential.

Continuous conduction in unmyelinated axons involves opening ion channels along the axon, slowing signal transmission.
Saltatory conduction in myelinated axons occurs at nodes of Ranvier, speeding up signal transmission by jumping between nodes.

Graded potentials can vary in magnitude and can lead to an action potential only when threshold is reached.
- Generator potentials: lead to action potentials in unipolar sensory neurons.
- Receptor potentials: lead to neurotransmitter release in other sensory cells.
- Postsynaptic potentials: graded potentials in response to neurotransmitter release at synapses.

Graded potentials can summate in the axon hillock, leading to action potentials if threshold is surpassed.
- Temporal summation: adding potentials over time.
- Spatial summation: adding potentials from multiple locations simultaneously.

Impaired potassium levels can affect action potential generation, impacting muscle and nerve function, potentially leading to fatigue or nervous dysfunction.
Seizures can result from disruption of action potentials, causing overstimulation or failure to properly signal in neurons.