Special Senses: Taste And Smell
The so-called special senses comprise the sensations of taste, smell, vision, hearing and balance. The receptors involved in taste and smell are chemoreceptors, those in vision are photoreceptors, and those in hearing and balance are mechanoreceptors.
The sensations of taste and smell are two modalities of sense that are very closely related. What the layperson calls ‘taste’ is really a combination of taste and smell, and probably a number of other modalities. Taken together, a better term would be flavour. The modalities of flavour are taste (gustation), smell (olfaction), touch (texture), temperature (thermoreception) and common chemical sense (chemoreception).
Taste buds (the gustatory end organs) (Fig. 56a) are found in the tongue, soft palate, pharynx, larynx and epiglottis, and are unevenly distributed around these regions. Those in the tongue are associated with three of the four types of papillae (fungiform, foliate and circumvallate) (Fig. 56b). Those associated with the other oral tissues are found on the smooth epithelial surfaces. The different papillae occupy specific areas of the tongue. Their associated taste buds are innervated by either the glossopharyngeal (IX) nerve (posterior one- third of the tongue) or the chorda tympani branch of the facial (VII) nerve (anterior two-thirds of the tongue). In humans, the number of taste buds varies considerably: on average in the range 2000–5000, but can be as low as 500 or as high as 20 000 (Fig. 56c). Each taste bud is made up of 50–150 neuroepithelial cells arranged in a compact, pear-shaped structure (intragemmal cells). There is general agreement that there are four types of intragemmal cells: basal, type I (dark cells), intermediate and type II (light cells). Each taste bud comprises a dynamic system in which there is a rapid turnover of cells within each bud. The lifespan of an individual receptor cell is about 10 days. There is a small opening in the surface, the taste pore, where the cells have access to the gustatory stimuli (Fig. 56a).
Since Aristotle (384–322 bc), people have tried to categorize taste into primary or basic qualities of taste. The four qualities that have stood the test of time are sweet, sour, salt and bitter, with a fifth categorized by the taste of monosodium glutamate (umami). The mechanisms involved in the process of transduction of the signals that eventually produce these basic sensations of taste are complex. In common with many other receptor cells, gustatory receptor cells use specifically localized ion channel and receptor sites for transduction. Unlike many other receptor cells, there is no single membrane transduction event and the different basic tastes utilize different ionic mechanisms (Fig. 56d). Most gustatory stimuli are water soluble and non-volatile and are either already dissolved or are dissolved in saliva during mastication.
The common chemical sense has been defined as the sensation caused by the stimulation of epithelial or mucosal free nerve endings by chemicals. Evidence suggests that these are polymodal nociceptors and that, in the mouth, the major contributor to this sense is the trigeminal (V) nerve. The trigeminal innervates almost all regions of the mouth, including the floor of the mouth, the tongue, the hard and soft palate, and the mucosa of the lips and cheek. These nerve endings are stimulated by a number of different chemicals, such as menthol, peppermint, and capsaicin and piperine (found in chilli peppers and black peppers, respectively).
The human olfactory organ, the olfactory epithelium or mucosa, is a sheet of cells, 100–200-μm thick, situated high in the back of the nasal cavity and on the thin bony partition (the central septum) of the nasal passage. The olfactory system responds to airborne, volatile molecules that gain access to the olfactory epithelium with the in-and-out air flow through and behind the nose. The odour molecules are distributed over the receptor sheet in an irregular pattern by the turbulence of the air flow set up by the turbinate bones (Fig. 56e).
The olfactory epithelium contains specialized, elongated nerve cells (olfactory receptors) (Fig. 56e). These cells have very thin fibres that run upwards in bundles through perforations in the skull (the cribriform plate) above the roof of the nasal cavity. These bundles of nerves constitute the olfactory (I) nerve. They extend only a very short distance, ending in the olfactory bulbs, a pair of swellings underneath the frontal lobes. The other end of each olfactory receptor, pointing down into the nasal cavity, is extended into a long process, ending in a knob carrying several hairs (cilia) between 20 and 200 μm in length. These cilia are bathed in a thin (35-μm thick) layer of mucus, secreted by specialized cells in the olfactory epithelium, in which the molecules of odorous substances dissolve. The molecules diffuse through the surface layer of mucus and stimulate the olfactory receptors. Hydrophilic (water-soluble) molecules dissolve readily in the mucus, but the diffusion of less soluble molecules is assisted by ‘odour-binding proteins’ in the mucus, which are also thought to assist in removing odour molecules from the receptor cells. The mucus layer moves across the surface of the olfactory mucosa at 10–60 mm/min towards the nasopharynx. This flow of mucus also assists in the removal of odours after they have been sensed. In the membrane of the cilia are olfactory receptor proteins, which interact with the smelly molecules, and initiate a cascade reaction inside the cell that leads to a change in the rate of impulses.
Humans are able to distinguish 10 000 or more different odours. Individual olfactory receptor neurones fire off spontaneously at between 3 and 60 impulses per second. When stimulated with particular odours, they increase their firing frequency. Each receptor cell responds, although not equally, to many different types of odour.