Neuroscience: Membrane Potential Quiz

Study Notes

Major intracellular cation is potassium (K+), while sodium (Na+) dominates the extracellular fluid (ECF).
Chloride ions (Cl-) are primarily found with Na+ in the ECF, while phosphate ions and negatively charged proteins are key anions in the intracellular fluid (ICF).
Cells exhibit a net negative charge intracellularly and a net positive charge extracellularly, leading to electrical disequilibrium.
Membrane potential (Vm) arises from concentration gradients and selective membrane permeability.
Active transport of cations generates an electrical gradient, resulting in a resting membrane potential.

Potassium concentration is 150 mM inside and 5 mM outside the cell, with an equilibrium potential (EK) of -90 mV.
Sodium concentration is 150 mM outside and 15 mM inside, yielding an equilibrium potential (ENa) of +60 mV.
The Nernst equation computes equilibrium potentials for ions at 37°C, using respective ion concentrations and charges.

Membrane potential can be calculated using the Goldman equation, incorporating multiple ions' permeability.
The equation considers permeability values: Pk for potassium and PNa for sodium, resulting in a membrane potential of approximately -70 mV.

Action potentials are triggered by sufficient depolarization, generally requiring a threshold of -50 mV.
Sodium (Na+) channels open during depolarization, causing rapid influx and further depolarization toward ENa.
During an action potential, Na+ permeability dominates, while K+ and Cl- become minor contributors.

Initially, Na+ influx causes rapid depolarization.
After reaching peak potential, Na+ channels inactivate, and delayed K+ channels open, leading to repolarization and hyperpolarization.
Hyperpolarization occurs due to prolonged K+ efflux, with potential dipping below resting levels.

The absolute refractory period prevents a second action potential for about 2 msec, due to inactivation of Na+ channels.
Following this is the relative refractory period, during which some Na+ channels reset, allowing for potential action with a stronger stimulus.

Action potentials propagate unidirectionally from the trigger zone to the axon terminal, avoiding overlap due to the refractory period.
Depolarizing graded potentials (excitatory) increase likelihood of firing action potentials; hyperpolarizing graded potentials (inhibitory) decrease this likelihood.
Larger axon diameter and myelination enhance conduction speed through mechanisms like saltatory conduction.

Normal potassium levels (normokalemia) prevent subthreshold graded potentials from eliciting action potentials, while suprathreshold stimuli reliably do.
Hyperkalemia brings the membrane closer to threshold, enabling weak stimuli to trigger action potentials.
Hypokalemia moves the membrane further from threshold, requiring stronger stimuli to elicit action potentials.