Bipolar neurons of olfactory nerves synapse on secondary olfactory neurons in olfactory bulbs.
Details
Figure 29.3 Topography of olfactory nerves, bulbs, and tracts. CLINICALLY ORIENTED ANATOMY, FIGURE 9.6.
The ceilings of the nasal cavities are lined by a special olfactory epithelium that includes the peripheral processes of primary olfactory neurons (olfactory cells). These are bipolar neurons whose cell bodies are in the olfactory epithelium just inferior to the cribriform plate of the ethmoid bone. The peripheral processes of the primary olfactory neurons are expanded into rounded knobs that contain cilia and receptor sites where aromatic molecules can bind. The central processes of the primary olfactory neurons are collected into about 20 fiber bundles that pass through the cribriform foramina of the ethmoid bone to enter the cranial cavity. These bundles of nerve fibers are collectively known as the olfactory nerves.
Figure 29.4 CLINICALLY ORIENTED ANATOMY, FIGURE 9.6.
Olfactory nerves are tiny structures that do not attach directly to the forebrain. They synapse on secondary olfactory neurons (mitral cells) in the olfactory bulbs, bilateral structures located on either side of the crista galli in the anterior cranial fossa, just above the nasal cavities. The bulbs are the distal enlargements of the olfactory tracts. The bulbs and tracts are extensions of the forebrain, so they technically are parts of the central nervous system. The olfactory tracts contain the axons of secondary olfactory neurons that relay smell information to the primary olfactory area of the cerebral cortex. Smell signals are sent directly to the cortex without first synapsing in the thalamus. This is an exception = all other sensory pathways synapse in the thalamus before reaching the cortex.
Cranial nerve II: Optic nerve
Functions
Vision (special sensory).
Skull opening
Optic canal.
Nucleus of origin/destination in CNS
Cell bodies of ganglion cells (afferent neurons) are located in the retinas = these are nuclei of the brain that have migrated into the eyes to be close to the surface of the head. Axons of ganglion cells project to the lateral geniculate nuclei of the thalamus.
Details
Figure 29.5 Topography of optic nerves, chiasm, and tracts. GRANT’S ATLAS OF ANATOMY, 14TH ED., FIGURE 9.6.
The optic nerves are located in the orbits. They extend from the eyeballs through the optic canals to the cross- shaped optic chiasm (= from the Greek letter chi χ). Proximal to the chiasm, within the cranial cavity, the optic tracts pass dorsally to the thalamus.
The optic nerves are not true nerves at all—they are tracts of afferent fibers that connect the retinas (equivalents of brain nuclei that migrated toward the surface of the head) with the diencephalon. Thus, the optic nerves are really parts of the central nervous system = like the olfactory tracts, they are paired anterior extensions of the brain. The nerve fibers within the optic nerves, chiasm, and optic tracts are the axons of ganglion cells whose cell bodies are in the retina. Ganglion cell axons leave the eyeball at the optic disc. Since the optic nerves are outgrowths of the brain, they are surrounded by all three layers of meninges = dura, arachnoid, and pia mater. Within the optic chiasm, nerve fibers from the medial (nasal) halves of the retinas cross-over (decussate)—the fibers from the lateral (temporal) halves of the retinas remain un-crossed in the chiasma. From the chiasm, two optic tracts are formed and these transmit the ganglion cell axons to the lateral geniculate nuclei of the thalamus.
Cranial nerve VIII: Vestibulocochlear nerve
Functions
Hearing and balance (special sensory).
Skull opening
Internal acoustic meatus.
Attachment to brainstem
Pons/ medulla junction.
Nucleus of origin/destination in CNS
Vestibular nerve fibers project to vestibular nuclei in the pons and medulla.
Cochlear nerve fibers project to cochlear nuclei in the brainstem at the pons/medulla junction.
Details
Figure 29.6 Vestibulocochlear nerve. GRANT’S ATLAS OF ANATOM Y, 14TH ED., FIGURE 9.16.
CN VIII leaves the brainstem at the junction of the pons and medulla—posterior to the facial nerve. It contains afferent fibers from organs within the internal ear: the cochlea (hearing) and the utricle, saccule, and semicircular ducts (balance and equilibrium). CN VIII occupies the internal acoustic meatus along with the facial nerve. Two sets of fibers make up CN VIII and these fibers are kept separate throughout.
The vestibular portion of CN VIII (also called the vestibular nerve) contains bipolar neurons. The cell bodies of these neurons are in the vestibular ganglion, situated in the internal acoustic meatus. The peripheral processes of the bipolar neurons penetrate the temporal bone to be distributed to receptors in the utricle, saccule, and semicircular ducts = structures involved with balance, located in the internal ear. The central processes of the neurons enter the brainstem and synapse on vestibular nuclei.
The cochlear portion of CN VIII (also called the cochlear nerve) also contains bipolar neurons whose cell bodies are in the spiral ganglia, lodged in the bony central core of the cochlea in the internal ear. The peripheral processes of these neurons are distributed to hair cells in the spiral organ (of Corti) in the cochlea. Central processes of neurons enter the brainstem and synapse on cochlear nuclei.
