Sensory Receptors and Primary Neurons

Cranial nerve I is the collection of approximately 6 million bipolar receptor cells whose cell bodies, dendrites, and initial axon segments are located within the olfactory neuroepithelium and whose axons project through the cribriform plate of the ethmoid bone to the anterior cranial fossa (Fig. 7-1 (Figure Not Available) ). '4] The olfactory neuroepithelium is a pseudostratified columnar epithelium supported by a highly vascularized lamina propria and is situated on the cribriform plate as well as on segments of the superior septum and both the superior and middle turbinates. Each bipolar receptor cell sends 3 to 50 cilia (which lack dynein arms and commonly have a 9 + 2 microtubule arrangement) from its dendritic knob into the overlying mucus. The cilia, which can radiate across the epithelial surface for over 30 pm, contain the receptor sites where odorants bind and

TABLE 7-1 -- CLI

NICO-ANATOMICAL CORRELATION OF DISORDERS OF THE OLFACTORY NERVE

Anatomic Site of

Typical CN I Finding

Other Neurological and Medical Findings

Common Etiologies

Damage

Sensory receptors and

Hyposmia or anosmia

With trauma, rare nasal leaks of CSF

Head trauma

primary neuron

Dysosmia

Upper respiratory infections

Can be unilateral

Nasal or sinus disease

Toxic exposure

Secondary neurons

Hyposmia or anosmia

Foster Kennedy syndrome

Meningioma

Olfactory bulb cells

Dysosmia

Disinhibition, change in personality

Neurodegenerative diseases (Alzheimer's disease,

Anterior olfactory

Can be unilateral

Gait dyspraxia, disinhibition, change in personality

Parkinsonsdisease, Huntingtons disease)

nucleus

Medial and lateral

Frontal lobe tumors

striae

Pituitary tumors

Aneurysms

Medial dorsal nucleus

Decreased odor

Signs of Wernicke-Korsakoff syndrome: ataxia, extraocular

Wernicke-Korsakoff syndrome

of thalamus

identification

paresis, nystagmus,memory problems including confabulations

Normal or increased

Infarctions

odor thresholds*

Primary and

Decreased odor

Lip smacking, automatisms during seizures

Epilepsy

secondary olfactory

identification

cortex

Normal or increased

Neurodegenerative disorders

odor thresholds*

Dementia, memory loss

Alzheimer's disease

Tremor, bradykinesia

Parkinson's disease

Chorea, dementia

Huntington's disease

Contralateral weakness, aphasia, homonymous quadrant visual

Tumors or infarcts

field defects

* Because of bilateral cortical and subcortical representation of olfactory function, unilateral lesions at this level generally do not cause clinically meaningful olfactory dysfunction.

provide a relatively large area for such binding. Assuming 6 million bipolar receptor cells, each containing 20 cilia 30 pm long and with an average diameter of 0.5 pm, the surface area of the cilia proves to be nearly 9 square inches.

In addition to bipolar receptor cells, the olfactory epithelium contains several other major cell types (see Fig. 7-1 (Figure Not Available) ). The sustentacular or supporting cells, whose apical ends have microvilli that extend into the olfactory mucus, span the distance from the epithelial surface to the basal laminae and function to (1) mechanically isolate the bipolar receptor cells from one another, (2) secrete mucopolysaccarides, (3) transport molecules across the epithelium, and (4) detoxify and degrade odorants.^ Some of the basal cells, located near the basement membrane, serve as precursors for the generation of other cell types within the neuroepithelium. The duct cells of Bowman's glands line passages through which most of the olfactory mucus is secreted, whereas the microvillar cells, located at the surface of the epithelium, send tufts of microvilli into the nasal mucus. The function of these flask-shaped cells is unknown. However, they number about 600,000 in humans'4 and have an axon-like process that extends to the olfactory bulb. '6

The constituent bipolar neurons of CN I are unique in three respects. First, their apical processes are more or less directly exposed to the external environment. Second, unlike nearly all other neuronal cells, they have the propensity to regenerate from basal cells after being damaged; indeed, some of these cells die and replenish themselves at regular intervals, although others are more long-lived. M Third, each of these cells serves as both a receptor cell and as a first-order neuron, projecting an axon directly from the nasal cavity into the brain without an intervening synapse. Unlike the other major cell types of the epithelium, these cells have relatively little xenobiotic-metabolizing capacity '8 and are a primary route of invasion into the central nervous system of toxic agents and viruses. Indeed, it is believed that the majority of neurovirulent viruses first enter the CNS through this pathway.

Within the lamina propria, the unmyelinated and unbranched axons of the olfactory receptor cells coalesce into bundles of approximately 200 axons, each surrounded by ensheathing or Schwann cell mesoaxons. Axons within these bundles are in direct contact with one another and may interact metabolically and electrically. '9! These bundles in turn combine with other bundles to form the olfactory fila, which traverse the cribriform plate of the ethmoid bone through 50 or so foramina. The fila, on exiting from the cribriform plate, form a dense layer of axons on the surface of the olfactory bulb. From here, the receptor cell axons branch and synapse with second-order neurons within the glomeruli of the bulb.

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