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?

Explanation:

Basically  Na+ is an extracellular ions, while K+ is intracellular. Therefore the influx of Na+ produce depolarization and excitability of a neuron, while  K- ions outflow  from the axoplasm ensures re-polarization to return neuron to its resting state.

Therefore an Athletes , loosing a lot of Na+, and drinking water  experiences  hyponatremia. Therefore the neuron  in this case experiences low Na+ concentration , with reduction in solute potential of the axoplasm, , with consequent  high water potential.

Therefore the neuron swells up with water, and difficult to depolarize to propagate action potential.This leads to loss of sensory  control in the athletes, leading to confusion.

i think it's c