The olfactory nerves transmit smell information through the cribriform plate to the olfactory bulbs and tracts, which in turn transmit this information to the olfactory cortex in the medial temporal lobes. Smell information is the only sensory modality not transmitted to the thalamus prior to the cortex. In the case of smell, the information first arrives in the olfactory cortex, and is then transmitted to the medial dorsal nucleus of the thalamus.

Loss of smell is referred to as anosmia. Since the olfactory nerve fibers travel through the cribriform plate, and the olfactory tracts lie between the inferior surface of the frontal lobes and the skull base, these structures are susceptible to damage in head trauma and compression by skull base tumors (e.g., olfactory groove meningiomas). More common causes of impaired smell are sinus disease and upper respiratory infections, and rarer causes include Kallman’s syndrome (anosmia and absence of gonadotropin-releasing hormone (GnRH) secreting neurons).

Anosmia may be part of the prodromal phase of neurodegenerative diseases (e.g., Parkinson’s disease and other synucleinopathies (see Ch. 23) and Alzheimer’s disease). In this context, anosmia is noted more commonly in retrospect since it may be attributed to aging when initially present in isolation.

Another scenario in which smell plays a role in neurologic disease is in the olfactory auras associated with temporal lobe seizures: Patients may report a foul odor (e.g., “burning tires”) that precedes temporal lobe seizure onset, presumably due to epileptic activity passing through medial temporal olfactory regions.

CNs 9 and 10 work together to supply the musculature of the pharynx (mostly supplied by CN 10) and transmit visceral afferent information from vascular baroreceptors, and each nerve also has additional individual functions listed below. CN 9 and CN 10 are discussed together since they are difficult to isolate clinically, and are commonly affected together since they both communicate with nuclei in the dorsolateral medulla, both pass through the jugular foramen, and they are adjacent throughout parts of the neck.

CN 9 supplies:

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  One pharyngeal muscle: stylopharyngeus

  
  
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  One gland: parotid

  
  
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  One region of taste: posterior one third of the tongue

  
  
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  One region of visceral sensation: carotid body

  
  
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  Three small regions of somatic sensation: posterior one third of the tongue, pharynx (shared with CN 10), middle ear, and external auditory meatus (shared with CN 7 and CN 10)

The functions of CN 10 include:

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  Motor supply to all muscles of the larynx and pharynx except tensor veli palitini (CN 5), mylohyoid (CN 5), stylohyoid (CN 7), stylopharyngeus (CN 9)

  
  
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  Motor supply to one muscle of the tongue: palatoglossus (all others are innervated by CN 12)

  
  
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  Somatic sensation from:

  
  
  
  
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    The dura mater of the posterior fossa aside from the tentorium (the sensory innervation to the rest of the dura including the tentorium is supplied by CN 5)

    
    
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    The pharynx (shared with CN 9)

    
    
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    The external auditory meatus (shared with CN 7 and CN 9)

  
  
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  Visceral sensation from the aortic arch

  
  
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  Visceral parasympathetic efferent supply to and afferent input from all of the viscera of the thorax and abdomen with the exception of the distal third of the GI tract and genitourinary organs (which receive their parasympathetic supply from sacral spinal cord levels S2–S4).

  
  
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  Taste in the pharynx

There are several functions that are shared across cranial nerves CNs 5, 7, 9, and 10, which are summarized for comparison in Table 14–1. (Mnemonic for some of the miscellaneous muscles: the trigeminal nerve innervates tensor tympani and tensor veli palitini; the seventh nerve innervates stylohoid and stapedius.)