Magnesium and Smooth Muscle

The following video gives an excellent introduction to smooth muscle. There’s no need to understand all the details, just get a feel for the general process and in particular the section on ‘myogenic activity’ – smooth muscle is able to contract without input from the nervous system.

Smooth Muscle Contraction

Smooth muscle contracts when calcium ions (Ca2+) enter the smooth muscle cell in response to a nerve impulse or a hormonal signal.  Magnesium and calcium are both divalent cations with very similar properties; they are also mutual antagonists.  Specifically, the threshold at which calcium enters the smooth muscle cell is determined by the level of magnesium ions (Mg2+) in the blood (free extracellular Mg).  As free magnesium levels fall the threshold for smooth muscle contraction falls.

Similarly, the level of Mg2+ within the cell (intracellular Mg) also determines the threshold for smooth muscle contraction as the two divalent cations compete for binding sites.

If free extracellular or intracellular magnesium levels are low there will be abnormal smooth muscle contraction.  Magnesium modulates smooth muscle contraction.

NMDA receptors convey signals to smooth muscle in addition to their function in visceral pain and the enteric nervous system. We next look at Magnesium and NMDA Receptors.