Question

The Guyton and Hall Textbook of Medical Physiology, p. 76, says:

For example, a high extracellular fluid calcium ion concentration decreases membrane permeability to sodium ions and simultaneously reduces excitability.

Why does this hold?

Naively, a high extracellular concentration of calcium ions should facilitate their flow through the calcium channels. Since the flow contributes to the action potential, the neurons should actually become hyperexcitable in the such a situation.

Answer 1

A high extracellular calcium concentration decreases neuron excitability because it reduces membrane permeability to sodium ions, inhibiting their influx during an action potential. Calcium ions also alter the voltage sensitivity of sodium channels, making them less likely to open and thus reduce excitability.

Step-by-step explanation:

The question pertains to why a high extracellular fluid calcium ion concentration decreases the excitability of neurons, instead of making them hyperexcitable, given that it could theoretically facilitate the flow of calcium through calcium channels.

The reason behind this is that a high concentration of extracellular calcium ions affects the membrane permeability to sodium ions.

The sodium (Na+) ion channels are affected because calcium competes with sodium for entry into these channels, and when the extracellular calcium concentration is high, it can inhibit sodium entry.

This leads to a decrease in the influx of sodium ions during the action potential, hence reducing excitability.

Additionally, calcium ions affect the voltage sensitivity of sodium channels, making them less likely to open, thereby reducing the likelihood of an action potential being generated.

Moreover, it is important to note that while calcium ions do contribute to action potentials, their primary role in neurotransmission occurs at the synaptic cleft, where they assist in neurotransmitter release from vesicles into the synaptic gap, rather than directly causing the action potential in the neuron.