Identify the cranial nerves carrying SVA fibers.
Identify the cranial nerves carrying SSA fibers.
Identify the ganglia associated with SVA and SSA cranial nerves.
Identify the specialized receptors for the SVA and SSA afferents.
Discuss the olfactory pathway and associated structures.
Discuss the visual pathway and associated structures.
Discuss the gustatory pathway and associated structures.
Discuss the vestibular pathway and associated structures.
Special Visceral Afferents (SVA)
CN I
CN I contains only SVA fibers that function in the sense of smell or olfaction. The olfactory system develops from the embryonic nasal placode and consists of the olfactory epithelium, olfactory bulb, and tract, as well as the olfactory associated cortical areas, sometimes called the rhinencephalon ( ).
Specialized receptors of the olfactory system are stimulated by chemicals that generate odors (odorants). Odorants must be drawn into the nasal cavity in order to activate the sensory neurons.
Olfactory receptors (ORs) are present on the cilia of sensory neurons within the olfactory epithelium.
ORs are G-protein coupled receptors that bind odorants. Binding of the odorant to the receptor activates the G-protein. Activation of the G-protein receptor ultimately generates an action potential (AP) via second messenger systems. The olfactory signal is then transduced to the brain via the olfactory nerve.
The bipolar olfactory neurons, along with their processes, are located in specialized epithelium (olfactory epithelium) in the roof of the superior concha of the nasal cavity, beneath the cribriform plate ().
The first-order bipolar neurons have a single dendrite that communicates with the olfactory mucosa as olfactory knobs. The olfactory knob or terminal end of the dendrite has cilia that are embedded in the apical mucosa.
A single, unmyelinated axon arises from the opposite end of the olfactory neurons. The axons are SVA fibers that transmit olfactory sensation. They form bundles that collectively create CN I.
Unlike most sensory systems, the first-order neurons of CN I do not reside in a sensory ganglion.
Sensory neurons are replaced when aged or damaged via differentiation of basal cells that reside near the lamina propria.
The olfactory threads (SVA fibers) pass through the small openings in the cribriform plate of the ethmoid bone and terminate in the ipsilateral olfactory bulb where the second-order neurons and interneurons are located ().
The olfactory bulb consists of mitral cells (primary projection neurons), granule cells (interneurons), and tufted cells (projection neurons).
The olfactory bulb constitutes an enlargement of the rostral aspect of the olfactory tract. The olfactory tract extending from the olfactory bulb represents the axons of the second-order neurons (mitral and tufted cells) ().
The olfactory tract projects to the olfactory associated areas of the cortex (rhinencephalon).
In man, most of the second-order neurons project to the lateral olfactory area, which consists of the uncus and the hippocampal gyrus. Other projections include the medial aspect of the frontal lobe where connections to the limbic system are responsible for emotional responses to odors (). See Clinical Correlation Box 15.1.
Fig. 15.2 Fiber bundles in the olfactory mucosa pass from the nasal cavity through the cribriform plate of the ethmoid bone into the anterior cranial fossa, where they synapse in the olfactory bulb. (Reproduced with permission from Gilroy AM, MacPherson BR. Atlas of Anatomy. Third Edition. © Thieme 2016. Illustrations by Markus Voll and Karl Wesker.)
Fig. 15.4 Specialized neurons in the olfactory bulb, called mitral cells, form apical dendrites that receive synaptic contact from the axons of thousands of primary sensory cells. The dendrite and the synapses make up the olfactory glomeruli. Axons from sensory cells with the same receptor protein form glomeruli with only one or a small number of mitral cells. The basal axons of the mitral cells form the olfactory tract. The axons that run in the olfactory tract not only project primarily to the olfactory cortex but are also distributed to other nuclei in the central nervous system. (Reproduced with permission from Baker EW. Anatomy for Dental Medicine. Second Edition. © Thieme 2015. Illustrations by Markus Voll and Karl Wesker.)
Fig. 15.5 The olfactory tract projects to the olfactory associated areas of the cortex (rhinencephalon). In man, most of the second-order neurons project to the lateral olfactory area which includes the uncus and the hippocampal gyrus. (Reproduced with permission from Baker EW. Anatomy for Dental Medicine. Second Edition. © Thieme 2015. Illustrations by Markus Voll and Karl Wesker.)
Anosmia or loss of smell can occur from trauma, inflammation, rhinitis, and aging. One of the chief complaints in individuals who suffer from anosmia is the complete or altered loss of taste. In severe head injuries the olfactory bulbs can be torn away from the olfactory nerves. Depending on the etiology, loss of smell can be transient or permanent. Treatment includes resolution of underlying condition with medication or surgery if there is an anatomic abnormality. In some instances, anosmia resolves on its own.
CN VII
The SVA fibers in CN VII carry taste sensation from the anterior two-thirds of the tongue as well as the hard and soft palates (). The five taste sensations are: sweet, bitter, salty, sour, and umami (Japanese word for pleasant).
Fig. 15.6 SVA fibers carried in CN VII carry taste sensation from the anterior two-thirds of the tongue and the palate. SVA, special visceral afferent. (Reproduced with permission from Gilroy AM, MacPherson BR, Ross LM. Atlas of Anatomy. Second Edition. © Thieme 2012. Illustrations by Markus Voll and Karl Wesker.)
Afferent fibers transmit sensory information from chemoreceptors in the tongue.
The chemoreceptors (taste receptor cells) lie within the taste buds (fungiform) of the tongue (see Chapters 9 and 19).
Salivary fluid containing dissolved substances enters the taste bud through a taste pore and bathes the microvilli associated with the taste receptor cells.
Taste receptor cells have a limited lifespan and are replaced every 1 to 2 weeks by the differentiation of basal cells that migrate from the surrounding epithelium.
The generation of APs from taste signals involves several methods depending on the type of taste sensation.
Taste buds communicate with peripheral processes from the first-order pseudounipolar neurons (). These afferent fibers run with the lingual nerve and then chorda tympani to the sensory cell body located in the geniculate ganglion.
The central process of the first-order neuron emerges from the geniculate ganglion and enters the facial canal. They eventually enter the brainstem as part of the nervous intermedius. The fibers then join the rostral part of the solitary tract to synapse on second-order neurons in the solitary nucleus ( a, b).
Axons from the cell bodies in the solitary nucleus will ascend both ipsilaterally and contralaterally to the ventral posteromedial (VPM) of the thalamus where they synapse on third-order neurons.
The axons of third-order neurons project via the posterior limb of the internal capsule to the area of the cortex responsible for taste.
Fig. 15.7 Taste buds communicate with peripheral processes of the first-order pseudounipolar neurons. These afferents run in the lingual nerve and chorda tympani. (Reproduced with permission from Baker EW. Anatomy for Dental Medicine. Second Edition. © Thieme 2015. Illustrations by Markus Voll and Karl Wesker.)
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