Question

q4.10. marathon runners can lose a great deal of na⁺ (through sweat). some runners compensate by drinking large quantities of water (far more than they lost). in extreme cases, this causes mental confusion and loss of muscle coordination. what is happening to the neurons of such a runner? the person has run out of neurotransmitter by firing too many action potentials. na⁺ loss and dilution results in increased k⁺ entry into neurons through na⁺ channels. na⁺ loss and dilution has lowered the na⁺ concentration outside of neurons so that less na⁺ flows into neurons through open na⁺ channels. neutral oxygen molecules (o₂) are entering through oxygen channels and depolarize the neuron.

Explanation:

When marathon runners lose a lot of $Na^+$ through sweat and then drink excessive water, it dilutes the extracellular fluid. The normal functioning of neurons depends on the proper balance of ions, especially $Na^+$. A lower $Na^+$ concentration outside neurons due to loss and dilution means that when the $Na^+$ channels open during an action - potential, less $Na^+$ will flow into the neurons. This disrupts the normal generation and propagation of action - potentials, leading to mental confusion and loss of muscle coordination. The other options are incorrect as there is no evidence to suggest running out of neurotransmitter due to action - potentials, $K^+$ entering through $Na^+$ channels, or oxygen molecules entering through oxygen channels and depolarizing neurons in this context.

Answer:

$Na^+$ loss and dilution has lowered the $Na^+$ concentration outside of neurons so that less $Na^+$ flows into neurons through open $Na^+$ channels.