Lateral Zone
H. Taste
The taste cells, just as the receptive neurons in the olfactory system, are in a very hostile environment and they are constantly being replaced by new cells, and yet their functions continue unabated. Taste plays a critical role in determining what an infant eats. In most animals, gustatory sensations from each foodstuff greatly influence the animal’s intake, but in humans, this is not necessarily the case. We are affected by environmental pressure that produces bizarre eating habits that could lead to extremes such as obesity or anorexia.
1. Lingual Taste Buds
The tongue is a very mobile muscular organ that is located in the oral a cavity. It is divided into a root, a body, an apex, a dorsal surface, and an inferior surface. The body and apex of the tongue are very mobile. The tongue has at least three main functions: (1) forming words, (2) moving food from the oral cavity to pharynx by passing the bolus along the palate, and (3) taste.
1. Lingual Papillae
a. Circumvallate papillae are large and flat, lie in the terminal sulcus, and consist of a wall with a deep central depression. The walls contain- taste buds.
b. Foliate papillae are small lateral folds of the lingual mucosa, which contains the taste buds.
c. Filiform papillae are long and numerous and contain sensory nerve endings from cranial nerve V3 (anterior two-thirds of the dorsum and apex of tongue) and CN IX (posterior third of tongue); taste buds are not present in these processes.
d. Fungiform papillae (mushroom-shaped) are pink in color and are located among the filiform papillae contain taste buds; they are most numerous at the apex and sides of the body of the tongue. There are also a few taste buds in the oral surface of the soft palate, the posterior wall of oral pharynx, and the epiglottis.
2. Subjective Taste
Classically, there are four groups of subjective taste qualities, each localized to a zone of the tongue: sweet and salt at the tip of tongue, sSour at the sides, bitter at the pharyngeal end. In reality, all parts of the tongue respond to the four kinds of taste. Bitter compounds are especially protective because they make the indi-vidual aware of toxic substances, and they use both ligand-gated channels and G-protein-coupled receptors. That permits them to be activated by many compounds.
Chorda tympani of cranial nerve VII is more sensitive to sweet or salty, whereas glossopharyngeal fibers are more sensitive to sour or bitter. The gustatory nerve endings usually end in many taste buds, with there being one predominate response in each nerve that matches the function of that region (e.g., sweet from the apex and bitter from the pharyngeal border)
3. Second-Order Neurons
The second-order neurons originate in cranial nerves VII, IX, and X and participate in reflexes, including swallowing, or coughing. The axons from the second-order ascend uncrossed in the central tegmental tract to the medial part of the ventral posterior medial nucleus (VPM) of the thalamus that then projects onto the gusta-tory cortex in the insula and parietal operculum.
Gustatory information projects from these areas onto the orbital cortex, where there is an overlap of olfactory, gustatory, and hypothalamic sensations that produce a strong response to many gustatory stimuli. Gustatory information also reaches the -parabrachial nucleus in the pontine reticular formation and is then distributed with nociceptive and visceral information onto the hypothalamus, amygdale, and septum.
4. Cortical Neurons
As one ascends in the gustatory system, the neurons are more sensitive to particular chemical sensations. Cells in the brain stem and thalamus respond to more than one taste, whereas in the cortical regions, cells show a selectivity that is not seen in the subcortical regions.
V. Guidelines for Localizing Disease to and within the Brain Stem
1. Mental status is not directly involved (except that lesions involving the reticular formation in the upper brain stem might alter the level of consciousness).
2. No muscle atrophy is presented, except that relevant to local involvement of cra-nial nerves. Limb weakness, if present, involves central control and spasticity.
3. Ipsilateral lower motor neuron findings relevant to cranial nerve motor function are often associated with contralateral upper motor neuron findings.
4. Deep tendon reflexes are increased in a bilateral or unilateral manner. A unilat-eral or bilatunilat-eral sign of Babinski might be present.
5. Extraocular movements are affected. Involvement of horizontal eye movements is indicative of lower pontine involvement. Involvement of vertical eye move-ments is indicative of an upper midbrain level of involvement.
6. Vertigo nausea and vomiting in association with other neurological symptoms is almost always indicative of brain stem pathology at the lower pontine or upper medullary level. Vertigo alone or in association with tinnitus and ipsilateral hear-ing deficit is indicative of disease outside of the brain stem at the level of the cochlea or cranial nerve VIII.
7. The combination of selective ipsilateral deficits in pain and temperature over the face in association with selective deficits in pain and temperature over the contralateral body is always indicative of a lateral tegmental upper medullary or lower pontine level, indicating involvement of the spinal tract and nucleus of V.
8. Horner’s syndrome in association with ipsilateral cranial nerve findings and the selective pain deficits indicated in item 7 similarly localizes the disease to the lateral tegmental areas of the lower pons or upper medulla.
9. The combination of cranial nerves V, VII, and VIII findings with ipsilateral cerebellar findings localizes the disease to the cerebellar–pontine angle and strongly suggests a vestibular Schwannoma or, less likely, a meningioma in this location.
10. Example of consequences of vascular lesions in the brain stem. Other examples are included on the CD that accompanies this text.