Which cranial nerve is involved in taste sensation for the posterior third of the tongue and contributes to the gag reflex?

• A. Facial
• B. Glossopharyngeal
• C. Vagus
• D. Hypoglossal

Answer: (B)

Taste for the posterior third of the tongue is carried by the glossopharyngeal nerve. It provides sensory input from that region and also contributes to the gag reflex as the afferent limb: when the posterior tongue or oropharynx is stimulated, CN IX sends signals to the brainstem to trigger the reflex. The gag response then uses the vagus nerve as the motor (efferent) component to produce the pharyngeal constriction. The facial nerve handles taste from the anterior two-thirds of the tongue, not the posterior third. The vagus nerve can contribute to the gag reflex and has some taste roles in areas like the epiglottis, but it does not convey taste from the posterior tongue itself. The hypoglossal nerve is purely motor to tongue muscles and does not carry taste. So, the nerve involved in taste for the posterior third and in contributing to the gag reflex is the glossopharyngeal nerve.