The vestibular nuclei give rise to various projections, including projections through the medial longitudinal fasciculus (see
Fig. 4.6C to E ) to the oculomotor nuclei. Therefore, it is not surprising that the vestibular nuclei exert powerful control over eye movements (the vestibule-ocular reflex). Other projections give rise to the lateral and medial vestibulospinal tracts, which, respectively, provide for the activation of trunk and neck muscles and thereby contribute to equilibrium and to head movements (vestibulocolic reflex). There are vestibular pathways to the cerebellum, the reticular formation, and the contra-lateral vestibular complex, as well as to the thalamus. The pathway to the thalamus, via a projection to the cerebral cortex, mediates conscious sensation of vestibular activity. Vestibular reflexes and clinical tests of vestibular function are described in
The Chemical Senses
The senses of gustation (taste) and olfaction (smell) help detect chemical stimuli that are present either in food and drink or in the air. In the evolution of humans, these chemical senses apparently did not have the survival value of some of the other senses, but they contribute considerably to quality of life and food selection, and they are important stimulants of digestion. In other animals, the chemical senses have greater survival value, and their activation evokes a number of social behaviors, including mating, territoriality, and feeding.
The stimuli that we commonly know as tastes are actually mixtures of five elementary taste qualities: salty, sweet, sour, bitter, and umami.
Taste stimuli that are particularly effective in eliciting these sensations are, respectively, sodium chloride, sucrose, hydrochloric acid, quinine, and mono-sodium glutamate. Umami has been described as having a proteinaceous, meaty character.
bThe existence of a sixth, taste, fat (free fatty acids), is currently being debated.
CHAPTER 8 The Special Senses
The sensation of taste depends on the activation of chemoreceptors located in taste buds. A taste bud consists of a group of 50 to 150 receptor cells, as well as supporting cells and basal cells (Fig. 8.29A ). The chemoreceptor cells synapse at their bases with primary afferent nerve fibers, and their apices have microvilli that extend toward a taste pore. Chemoreceptor cells live only about 10 days. They are continuously replaced by new chemoreceptor cells that differentiate from basal cells located near the base of the taste bud.
Chemoreceptor molecules, each specialized for one type of taste stimulus, sit on the microvilli of chemoreceptor cells and detect molecules that diffuse into the taste pore from the overlying mucus of the tongue, part of which originates from glands adjacent to the taste buds. Some stimuli can pass directly into the cell to depolarize it (Na+ for salty and H+ for sour) or open cation channels to generate a receptor potential (also salty and sour), whereas others (sucrose, quinine, and glutamate for sweet, bitter, and umami) activate a second messenger that can either open cation channels or directly activate intracellular Ca++ stores (see
Fig. 8.29B ). In each case, depolarization of the receptor results in the release of an excitatory neurotransmitter and, consequently, action potentials in the primary afferent nerve fiber that are transmitted to the CNS.
Coding of taste, however, is not based entirely on the selectivity of the chemoreceptors for the different primary qualities because each cell responds to a range of stimuli, although most intensely to one. Because most natural tastes have chemicals that effect responses from a number of chemoreceptors, recognition of taste quality appears to depend on the patterned input from a population of chemoreceptors, each responding differentially to the components of the stimulus. The intensity of the stimulus is reflected in the total amount of activity evoked.
Distribution and Innervation of Taste Buds