Cranial Neuropathies I, V and VII–XII

Nerve fibers mediating the sense of smell have their cells of origin in the mucous membranes of the upper and posterior parts of the nasal cavity. Axons of olfactory cells form nerve bundles, which are unmyelinated fibers that converge to form small fascicles enwrapped by Schwann's cells to constitute the olfactory nerve that passes through openings in the cribriform plate of the ethmoid bone into the olfactory bulb. The olfactory tract divides into medial and lateral olfactory striae. The medial stria contains fibers from the anterior olfactory nucleus, which pass to the contralateral hemisphere via the anterior commissure. During quiet breathing, little of the air entering the nostrils reaches the olfactory mucosa; sniffing carries the air into the olfactory crypt. A unilateral lesion distal to the decussation of the olfactory fibers is usually asymptomatic due to bilateral cortical representation. Anosmia due to head injury is dependent on the severity of the trauma and recovers in about one-quarter of patients.