Increased HCN channel driven inward rectification in benign cramp fasciculation syndrome

Muscle cramps are a common complaint associated with sudden painful involuntary contractions of a muscle. The mechanisms responsible for muscle cramps are still not clear. Axonal excitability and multi-unit electromyography studies were performed in 20 patients suffering from benign cramp fasciculation syndrome, not currently on medication. The measures of axonal excitability suggested greater inward rectification, indicative of an increase in Ih. Mathematical modelling suggested that the data were best explained by depolarization of the voltage dependence of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. Parameters associated with polarization of resting membrane potential were not changed. These findings suggest that a role for HCN channels may become apparent during the rhythmic discharge associated with a voluntary contraction. Consistent with this view, patients had higher motor unit discharge rates than healthy controls during maximal voluntary effort. Muscle cramps are a common complaint, but the underlying mechanisms remain unclear. Czesnik et al. examine the factors affecting excitability of peripheral axons in 20 patients with benign cramp fasciculation syndrome. Results suggest that increased HCN channel-driven inward rectification leads to both increased axonal excitability and increased motor unit discharge.