Duus' Topical Diagnosis in Neurology © 2005 Thieme All Rights Reserved

4 128 · 4 Brainstem

Olfactory System (CN I)

The olfactory pathway (Figs. 4.7 and 4.8) is composed of the olfactory epithelium of the nose, the fila olfactoria (olfactory nerve = CN I), the olfactory bulb and tract, and a cortical area (the paleocortex) extending from the uncus of the temporal lobe across the anterior perforated substance to the medial surface of the frontal lobe under the genu of the corpus callosum.

The olfactory epithelium occupies an area of about 2 cm2 in the roof of each nasal cavity, overlying portions of the superior nasal concha and of the nasal septum. It contains receptor cells, supportive cells, and glands (Bowman’s glands) that secrete a serous fluid, the so-called olfactory mucus, in which aro- matic substances are probably dissolved. The sensory cells (olfactory cells)are bipolar cells whose peripheral processes terminate in the olfactory hairs of the olfactory epithelium.

Fila olfactoria and olfactory bulb. The central processes (neurites) of the olfactory cells coalesce into bundles containing hundreds of unmyelinated fibers surrounded by a Schwann-cell sheath. These fila olfactoria, about 20 on either side, are, in fact, the olfactory nerves (CN I is thus composed of peripheral nerve fibers, but is not a single peripheral nerve in the usual sense). They pass through small holes in the cribriform (“sievelike”) plate and enter the olfactory bulb, where they form the first synapse of the olfactory pathway. Although it is not physically located in the cerebral cortex, the olfactory bulb is actually a piece of the telencephalon. Within it, complex synapses are made onto the dendrites of mitral cells, tufted cells, and granule cells.

Olfactory pathway. The first neuron of the olfactory pathway is the bipolar olfactory cell; the second neurons are the mitral and tufted cells of the olfactory bulb. The neurites of these cells form the olfactory tract (2nd neuron), which lies adjacent to and just below the frontobasal (orbitofrontal) cortex. The olfactory tract divides into the lateral and medial olfactory striae in front of the anterior perforated substance; another portion of it terminates in the olfactory trigone, which also lies in front of the anterior perforated substance. The fibers of the lateral stria travel by way of the limen insulae to the amygdala, semilunar gyrus, and ambient gyrus (prepyriform area). This is the site of the 3rd neuron, which projects to the anterior portion of the parahippocampal gyrus (Brod- mann area 28, containing the cortical projection fields and association area of the olfactory system). The fibers of the medial stria terminate on nuclei of the septal area below the genu of the corpus callosum (subcallosal area) and in front of the anterior commissure. Fibers emerging from these nuclei project, in turn, to the opposite hemisphere and to the limbic system. The olfactory path-

Longitudinal striae Striae medullares of the thalamus Medial olfactory stria

Subcallosal area Habenulo- interpedun- cular tract

Habenular nucleus

Olfactory bulb Inter- peduncular nucleus Medial forebrain bundle Tegmental nuclei Dorsal Olfactory epithelium longitudinal Lateral Area 28 bipolar olfactory cells fasciculus olfactory (entorhinal stria area) Uncus with Prepiriform Reticular amygdaloid body area formation Fig. 4.7 The olfactory nerve and tract and the olfactory pathway

Olfactory bulb Olfactory tract

Medial olfactory stria Temporal pole Lateral olfactory stria

Limen insulae Prepiriform Anterior area perforated substance Amygdaloid body Ambient gyrus Diagonal Semilunar band of gyrus Broca Uncus

Fig. 4.8 The olfactory nerve and tract as seen from below