Question

What would happen to the current if you remove Ca^2+ from the extracellular space? What would happen if you gradually increase the [Ca^2+] back to the physiological level?

Answer 1

Effects of calcium ions on action potential :

The membrane of the excitable tissue is capable of maintaining two different states namely, either the resting stage or action stage.These two states are defined in terms of membrane permeability to ions.The permeability of ions is low in resting state where as high in action state.Several experimental results suggest that the membrane potential may be dependent on the state of membrane calcium.If the calcium ions are associated with the membrane, resting state takes place while action state or action potential takes place if the ions are dissociated from the membrane. When the membrane calcium is removed the sodium and potassium increases thus causing action potential.

It is expected that the dissociation of the membrane calcium will be hindered or prevented when the concentration of calcium in the external solution or extracellular fluid is high, and accelerated when the concentration of calcium is low in ECF. Thus the cell tends to stay in resting or action stage depending upon the external concentration of the calcium.

If the calcium concentration is slowly reduced from the extracellular fluid , the membrane is depolarized and initiates action potential.

If the Ca2+ ions in the extracellular fluid [Ca2+]_ECF is too low (hypocalcemia) , voltage gated ions start opening spontaneously causing nerve and muscle cells to become hyperactive.This syndrome of involuntary muscle spams due to low Ca2+ in extracellular fluid is called Hypocalcemia tetany.

When [Ca2+]_ECF is too high (hypercalcemia) in the extracellular fluid, voltage gated channels don't open easily and there is depressed nervous system function.Another problem of hypercalcemia is that calcium can combine with phosphate ions forming deposits of Calcium phosphate causing stones in blood vessels and kidneys. Hypercalcemia is a result of overactive parathyroid glands.

Calcium in extracellular fluid is influenced by dietary intake, Ca2+ absorption in small intestine and by excretion in urine.Importantly the bone contains 99% of calcium in the body, so they provide as a reservoir of Ca2+ that can be used to maintain calcium levels in extracellular fluids.

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