Cranial Nerves I & II

Cranial nerves can be thought of as modified spinal nerves, since the “general” functional fiber types found in spinal nerves also are found in cranial nerves but are supplemented by “special” afferent or efferent fibers. fibers conveying olfaction (in cranial nerve I) and taste (in cranial nerves VII, IX, and X) are classified as special visceral afferent, while the designation of special somatic afferent is applied to fibers conveying vision (cranial nerve II) and equilibrium and hearing (cranial nerve VIII). Skeletal muscles that arise from the branchial (pharyngeal) arches are innervated by fibers of cranial nerves V, VII, IX, and X; these are classified as special visceral efferent fibers.

The 12 pairs of cranial nerves are commonly identified either by name or by Roman or Arabic numeral.

Olfactory nerve (CN I or 1)

Bipolar cells in the nasal mucosa give rise to axons that enter the cranial cavity through foramina in the cribriform plate of the ethmoid bone. These cells and their axons, totaling about 20–24 in number, make up the olfactory nerve. Once in the cranial cavity, the fibers terminate in a small oval structure resting on the cribriform plate called the olfactory bulb. As stated above, the functional component of olfactory fibers is special visceral afferent. Injury or disease of the olfactory nerve may result in anosmia, an inability to detect odours; this may also dull the sense of taste.

Optic nerve (CN II or 2)

Rods and cones in the retina of the eye receive information from the visual fields and, through intermediary cells, convey this input to retinal ganglion cells. Ganglion cell axons converge at the optic disc, pass through the sclera, and form the optic nerve. The optic nerve from each eye enters the skull via the optic foramen and joins its opposite to form the optic chiasm. At the chiasm, fibers from the nasal halves of each retina cross, while those from the temporal halves remain uncrossed. In this way the optic tracts, which extend from the chiasm to the thalamus, contain fibers conveying information from both eyes. Injury to one optic nerve therefore results in total blindness in that eye, while damage to the optic tract on one side results in partial blindness in both eyes.

Optic fibers also participate in accommodation of the lens and in the pupillary light reflex. Since the subarachnoid space around the brain is continuous with that around the optic nerve, increases in intracranial pressure can result in papilledema, or damage to the optic nerve, as it exits the bulb of the eye.

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