CHAPTER 15. THE GUSTATORY AND OLFACTORY SYSTEMS

OBJECTIVES:
  1. BE ABLE TO IDENTIFY THE CENTRAL GUSTATORY PATHS AND NUCLEI.
  2. BE ABLE TO IDENTIFY THE ANATOMICAL FEATURES OF THE OLFACTORY PATH IN GROSS SPECIMENS.

Gustatory

The taste buds of the anterior two-thirds of the tongue are innervated by the facial nerve, of the posterior third by the glossopharyngeal, and of the epiglottic and palatal regions by the vagus. The primary neurons are unipolar cells located in the geniculate, inferior (petrosal), and inferior (nodose) ganglia, respectively.

Taste impulses from these ganglia enter the brainstem, pass to the solitary tract, and synapse in the rostral part of the solitary nucleus (Sls. 17, 18). This part of the solitary nucleus is called the gustatory nucleus.

Secondary connections of the gustatory nucleus travel near the contralateral medical lemniscus and central tegmental tract to the ventral posteromedial nucleus (Sls. 35 , 36). The most medial part of the VPM, the parvicellular part, receives the projections from the gustatory nucleus. From here fibers travel via the posterior limb of the internal capsule to the cortical gustatory area.

Observe the primary gustatory area in the cerebral cortex. It is located in the parietal operculum (Pl. 4) and the adjacent part of the insula (Pl. 14).

Olfactory

The central branches of the bipolar olfactory neurons form the axons of the olfactory nerves. These non-myelinated fibers become collected into about 20 bundles which traverse the foramina in the cribriform plate of the ethmoid bone. Collectively these bundles form the olfactory nerve and they terminate in the olfactory bulb (Pls. 6, 7, 8).

Identify the olfactory tract (Pls. 6, 7, 8, 11), the narrow band that continues posteriorly from the olfactory bulb along the olfactory sulcus. It consists chiefly of the efferent fibers of the bulb although it does contain clumps of neurons that form the anterior olfactory nucleus as well as centrifugal fibers from the contralateral anterior olfactory nucleus and from neurons in the basal forebrain.

Identify the olfactory trigone (Pl. 11) at the posterior end of the olfactory tract where the fibers of the tract diverge to form two bundles, the lateral and medial olfactory striae, which border the anterior perforated substance. The fibers of the medial olfactory stria arise chiefly in the anterior olfactory nucleus and project via the anterior or olfactory part of the anterior commissure to the contralateral olfactory bulb. The medial olfactory stria becomes buried in the anterior perforated substance shortly after emerging from the olfactory trigone.

The lateral olfactory stria (Pl. 11) carries the efferent projections of the olfactory bulb toward the limen of the insula where they bend medially to enter the temporal lobe in the vicinity of the uncus, which is the primary olfactory area. The olfactory association area is in the orbitofrontal part of the prefrontal cortex. It is located in the posterolateral part of the orbital gyri (Pl. 6). Impulses reach it either from the medial dorsal nucleus of the thalamus, which receives direct connections from the uncal region and projects to the prefrontal cortex, or via the uncinate fasciculus which directly connects the temporal lobe and orbitofrontal cortex.

Lesions in the olfactory area of the orbitofrontal cortex result in the loss of the ability to discriminate different odors.