Which nerve mediates the afferent limb of the gag reflex and provides sensation to the posterior third of the tongue?

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The glossopharyngeal nerve is the one that carries sensory information from the back of the tongue and also provides the afferent input for the gag reflex. Its sensory fibers supply the posterior third of the tongue, giving the taste and general sensation there, and they detect touch when the oropharynx is stimulated. This sensory signal travels to the brainstem to trigger the gag reflex, with the motor (efferent) response then carried out by the vagus nerve to constrict the pharyngeal muscles.

In contrast, the vagus nerve handles the motor portion of the gag reflex and sensory input from other parts of the pharynx and larynx, not the posterior tongue. The facial nerve manages taste for the anterior two-thirds of the tongue, while the hypoglossal nerve controls tongue movement. So the posterior tongue sensation and the afferent gag signal fit best with the glossopharyngeal nerve.