Which cranial nerve is responsible for motor functions of the tongue and assists in swallowing and speech?

The hypoglossal nerve is the cranial nerve specifically responsible for controlling the motor functions of the tongue. This includes movements that are essential for swallowing and speech. The tongue's role in these processes is critical, as it aids in manipulating food during chewing, forming a bolus for swallowing, and articulating sounds for speech.

The other cranial nerves mentioned do not perform these specific functions related to the motor control of the tongue. The optic nerve is primarily responsible for vision, while the vagus nerve extends its influence over various autonomic functions, including some aspects of digestion and heart rate, but does not control the tongue's movements. The facial nerve mainly oversees the muscles of facial expression and taste sensations, which do not involve direct motor control of the tongue itself. Thus, the hypoglossal nerve stands out as the one that directly governs the actions necessary for effective swallowing and speech.