Question

9. How does hyperventilation or a calcium deficiency cause muscle tetany? Which is the more likely cause? Explain?

Answer 1

Tetany is the incomplete involuntary state of muscle contraction, which may occur due to the diseased state or other physiological conditions that result in increased frequency of action potentials of muscle or nerve cells.

Calcium deficiency is the more likely cause of tetany becuase the decreased calcium concentration cause increased permeability of sodium ions, which causes increased frequency of generation of action potentials.

Hyperventilation (reduces CO2) increase the pH of blood due to decreased carbon dioxide levels.

H2O + CO2 <-----> H2CO3 <----> HCO3 +H

During hyperventilation the CO2 levels in the circulation decrease, so the dissociation of carbonic acid (H2CO3) into bicarbonate and hydrogen ions reduces. The decreased H+ level raises the blood pH (More H+ ions means less pH and vice versa). During acidic conditions, the fraction of calcium that exist in ioized state is decreased, which again causes increased rate of action potential generation.

Muscle twitch and tetany:

In the muscle contraction curve, the latent period is denoted by the initial raise of the curve, and the contraction phase is represented by the upward deflection. The downward deflection of the curve denotes the relaxation phase. When we apply a single stimulus, the muscle fibre completely follows all these phases (rapid and short), and we get a neat curve representing various phases of muscle contraction, this is called as muscle twitch.

If the impulses are given repeatedly (multiple stimuli) at high frequency, each successive stimuli fall during contraction period of previous twitch. So that, the muscle fibre remains in contracted state and fails to reach the relaxation period, this is called as tetanus. This can be clearly observed in the graph produced by a tetanic muscle, which shows the absence of relaxation wave.