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Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs.Cranial nerves relay information between the brain and parts of the body, primarily to and from regions of the head and neck, including the special senses of vision, taste, smell, and hearing.
Two of the cranial nerves show chiasmas: (1) the chiasma of the optic tract (i.e., cranial nerve II), which originates from the eyes and inserts on the optic tectum of the midbrain; and (2) the trochlear nerve (i.e., cranial nerve IV), which originates in the ventral midbrain and innervates one of the six muscles that rotate the eye (i.e., the ...
In mammals and birds and other vertebrates with frontal eyes, the optic nerves do blend in the optic chiasm, and only part of the nerve fibres cross the midline. [5] The drawings of Cajal suggest that the axons of the optic nerve may branch in the optic chiasm, and thus give off a branch both in the ipsi- and contralateral optic tract. [5]
Sometimes: cranial accessory, spinal accessory. Mainly motor Cranial and Spinal Roots Located in the jugular foramen. Controls the sternocleidomastoid and trapezius muscles, and overlaps with functions of the vagus nerve (CN X). Symptoms of damage: inability to shrug, weak head movement. XII Hypoglossal: Mainly motor Medulla
In neuroanatomy, the optic chiasm, or optic chiasma (/ ɒ p t ɪ k k aɪ æ z əm /; from Greek χίασμα 'crossing', from Ancient Greek χιάζω 'to mark with an X'), is the part of the brain where the optic nerves cross. It is located at the bottom of the brain immediately inferior to the hypothalamus. [1]
Fibers from the optic nerves cross over in the optic chiasm with some fibers passing to the contralateral optic nerve tract. This is the basis of the " swinging-flashlight test ". Loss of accommodation and continued pupillary dilation can indicate the presence of a lesion on the oculomotor nerve.
However, most typically, the optic nerve is grouped with the other eleven cranial nerves and is considered to be part of the peripheral nervous system. The optic nerve is ensheathed in all three meningeal layers (dura, arachnoid, and pia mater) rather than the epineurium, perineurium, and endoneurium found in peripheral nerves.
The corticobulbar (or corticonuclear) tract is a two-neuron white matter motor pathway connecting the motor cortex in the cerebral cortex to the medullary pyramids, which are part of the brainstem's medulla oblongata (also called "bulbar") region, and are primarily involved in carrying the motor function of the non-oculomotor cranial nerves, like muscles of the face, head and neck.