Final answer:
The olfactory nerve (CN I) is the only cranial nerve that directly transmits sensory information about smells from the nose to the brainstem without a thalamic relay to the cortex.
Step-by-step explanation:
The only cranial nerve that lacks a thalamic relay to the cortex is the olfactory nerve (CN I). Unlike the other cranial nerves, the olfactory nerve transmits sensory information about smells directly from the nose to the brainstem without the intermediate relay through the thalamus. This unique feature distinguishes it from other sensory cranial nerves, such as the optic (CN II) and the vestibulocochlear (CN VIII), which do rely on thalamic relays to reach the cortex.
The olfactory nerve's direct transmission to the brain is essential for the processing of olfactory, or smell, information. The other cranial nerves have various functions, including motor and sensory roles. For example, the oculomotor nerve (CN III) is involved in controlling the eyelid and some eye movements, and both the facial (CN VII) and glossopharyngeal (CN IX) nerves have roles in taste sensation as well as motor functions.
Preganglionic parasympathetic fibers are found in a few cranial nerves, like the oculomotor, facial, glossopharyngeal, and vagus nerves. These fibers are crucial for the autonomic control of various body functions, such as pupillary constriction initiated by oculomotor fibers and salivation initiated by facial and glossopharyngeal fibers.