Ac

The Resting Membrane Potential (RMP)

The cell membrane functions like a charged battery, with an interior resting potential of -70mV. The outside of the cell contains high levels of sodium (Na+) while the inside is rich in potassium (K+). The sodium-potassium pump plays a crucial role, actively transporting 3 Na+ out and 2 K+ in, thus maintaining the resting potential. Potassium leaks out more easily than sodium, leading to a negative interior as anions remain trapped inside.

Hypokalemia

Hypokalemia occurs when extracellular potassium levels drop below 3.5 mmol/L, resulting in hyperpolarization of the cell as K+ efflux increases. Clinically, this leads to increased excitation thresholds (negative bathmotropy), causing lethargy and muscle weakness. In cardiac tissue, it heightens the risk of lethal arrhythmias due to slowed repolarization, observable via U-waves and flattened T-waves on ECG.

Voltage-Gated Channels & Action Potential

Voltage-gated channels open at specific threshold voltages, initiating the action potential sequence: depolarization occurs as Na+ rushes in, followed by repolarization as K+ exits, returning the cell to its resting state. This unidirectional flow is crucial for signal propagation.

Calcium Channels: L, T, N-Type

L-type channels are essential for cardiac muscle contraction, while T-type channels support the pacemaker cells in the heart. N-type channels are located at synapses, facilitating neurotransmitter release.

Tissue Specialization: Nerve vs. Cardiac

Nerve cells generate fast action potentials (1-2 ms), targeting speed. Conversely, cardiac muscle uses L-type channels to create extended signals (100x longer), allowing sufficient time for heart contractions before reset.

CCB-Induced Edema

Selective arteriolar vasodilation by calcium channel blockers (CCBs) increases capillary hydrostatic pressure, resulting in fluid leakage into interstitial spaces, leading to symmetric swelling typically in the ankles and feet. Pitting edema may occur, and management includes leg elevation and compression stockings. Adding an ACE inhibitor can mitigate edema by enhancing venous dilation.