Organization of Chemosensory Systems Within the Brains of Bony Fishes

Abstract

The chemical senses in all vertebrates can be separated according to end organ, mode of innervation, and primary terminus within the CNS. According to these criteria, three exteroceptive chemical senses can be distinguished: olfaction, gustation, and the common chemical sense (Parker 1922; Atema 1980; Finger 1983b). The olfactory sense is mediated via olfactory receptors, themselves neurons with axons extending into the CNS and terminating in the olfactory bulb, the rostralmost extension of the forebrain (Fig. 13.1). In contrast, gustation is mediated through specialized axonless neuroepithelial receptors called taste buds. The chemosensory elements of the taste buds synapse on the distal processes of ganglionic cells of either the facial, glossopharyngeal, or vagal nerves, which terminate centrally in the special visceral sensory column of the medulla (Fig. 13.1). The common chemical sense is the least characterized, but appears to be mediated by either isolated chemosensory cells (Lane and Whitear 1982) innervated by the somatic sensory (trigeminal and spinal) nerves or by free nerve endings of these same nerves. In most, and especially aquatic, vertebrates, these chemical senses mediate different sorts of behavior (Atema 1980). The olfactory sense is associated with functions such as intraspecific social interactions or being alerted to the presence of food, whereas gustation is used in food localization and determination of palatability. In general, stimulation of the common chemical sense results in avoidance, but in at least some specialized species, such as sea robins, this sense is used for food localization (Parker 1922; Silver and Finger 1984).