Suzanne I Sollars
Department of Psychology, University of Nebraska Omaha, Omaha, NE, USA

https://doi.org/10.36866/pn.61.28

One of the most fundamental, but least scientifically understood, of the sensory systems is taste. The taste system helps to guide each of us toward food and fluid selection vital to important regulatory functions such as energy maintenance, hormonal status and electrolyte balance. Taste perception is the product of a multifaceted and complex sensory system. When foods and fluids are consumed, they generally enter the front of the mouth, move throughout the oral cavity, and proceed to the back of the mouth prior to swallowing. Contact time with any particular area of the mouth is variable, dependent upon the texture or content of the ingested substance. Throughout the oral cavity are taste receptor cells, clustered into groupings of taste buds and subserved by four taste nerves. Although regional specializations exist across the oral cavity, each of the nerves conducts multiple sensitivities corresponding to the various classes of chemical stimuli. Ultimately, tastes must be coded in the brain as reliably corresponding to each particular food or fluid via the integration of this complex information.

A closer examination of individual regional differences and similarities in the oral cavity further demonstrates the difficult task of understanding how the brain ultimately codes the array of information received from taste receptor cells. For example, in humans taste buds are spread across the tongue, but they are also present on the soft palate, a region in the back part of the oral cavity on the roof of the mouth. While we know quite a lot about taste buds on the tongue, much less information is available about taste receptors on the palate. The presence of these taste buds can easily be demonstrated by application of a small amount of salt water or sugar water to the soft palate. With the head tilted backward, apply one of the solutions to the posterior palate with an eyedropper, taking care to avoid getting solution onto the tongue. The majority of those who participate in this demonstration report tasting both the salt and the sugar solutions when contact occurs on the palate alone. Furthermore, after closing the mouth and tasting the solutions on the tongue, participants invariably report a qualitatively different taste than when the solution was on the palate alone. While these reported differences in taste perception are merely anecdotal evidence, they suggest some sort of differential processing of taste perception on the tongue versus the palate.

To better understand the coding of taste stimuli, the rodent model is widely used in taste experiments to obtain data on the electrophysiological processing of chemical stimuli. In the rat, taste buds are found across the tongue and palate in areas similar to those in humans. However, taste buds on the palate have a wider distribution in the rat than in the human. Rats’ taste buds are distributed in a region directly behind the incisor teeth called the nasoincisor duct, in a region called the geschmacksstreifen (German for ‘taste stripe’), and also on the soft palate which is directly behind the geschmacksstreifen. The greater superficial petrosal nerve (GSP) innervates each of these areas, with cell bodies contained within the geniculate ganglion (Fig. 1). Early experiments on the electrophysiological properties of the GSP indicated a higher sensitivity of this nerve to sucrose than observed in other taste nerves (Nejad, 1986). Subsequent experiments have also shown strong neural responses to other stimuli, such as salt (Sollars & Hill, 1998). In a recent experiment (Sollars & Hill, 2005), we demonstrated that only a subset of individual nerve fibres respond strongly to sucrose, while some do not respond at all to this stimulus (Fig. 2). Other fibres are more responsive to salt, quinine, or acid. Fibres of the other taste nerves are also differentially responsive to various stimuli, yet the pattern and strength of particular chemical responses varies between the nerves (Frank, 1991; Lundy & Contreras, 1999; Sollars & Hill, 2005).

Therein lies a small example of the complicated dynamics of taste sensory coding. Multiple receptor subtypes, continual receptor cell turnover, potential temporal coding based on stimulus contact time or location of receptors in the oral cavity, and variability in neural representation across nerves come together to form a relatively stable interpretation of chemical stimulus representation in the cortex. The ‘taste map’ is yet to be determined, but with each nuance of recent advances in the field, we come ever closer to a better understanding of what makes this system so complex.

References

Frank ME (1991). Taste-responsive neurons of the glossopharyngeal nerve of the rat. J Neurophysiol 65, 1452–1463.

Lundy RF Jr & Contreras RJ (1999). Gustatory neuron types in rat geniculate ganglion. J Neurophysiol 82, 2970–2988.

Nejad MS (1986). The neural activities of the greater superficial petrosal nerve of the rat in response to chemical stimulation of the palate. Chem Senses 11, 283–293.

Sollars SI & Hill DL (1998). Taste responses in the greater superficial petrosal nerve: substantial sodium salt and amiloride sensitivities demonstrated in two rat strains. Behav Neurosci 112, 991–1000.

Sollars SI & Hill DL (2005). In vivo recordings from rat geniculate ganglia: taste response properties of individual greater superficial petrosal and chorda tympani neurones. J Physiol 564.3, 877–893.