OLFACTION AND TASTE

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The sense of smell is due to stimulation of olfactory chemoreceptors, which allow discrimination between a vast number of sometimes subtly different odors. The olfactory receptors (OC) are nerve cells located in a patch of epithelium, the olfactory mucosa (see below), lining the upper part of the nasal septum in each nostril. Bowman's glands (Bg) in the sub-mucosa, secrete a mucous layer covering the epithelial surface. Small basal cells with large nuclei, serve as stem cells and divide to continuously produce olfactory receptor neurones. The dendrites of the olfactory receptors have cilia-like protuberances the olfactory knobs (Ok) which stick into the mucus overlying the epithelium. Their axons run together in small bundles out of the epithelium, and through the overlying ethmoid bone. Supporting cells lie amongst the olfactory receptors. Odorant molecules, usually small, volatile, molecules, dissolve in the mucus and combine with specific Olfactory Binding Proteins secreted into the mucus. These help to solubilize the lipophilic odorant molecules and transfer them to the olfactory receptor molecules on the cell membranes of the receptor cell dendrites.

Each receptor cell expresses one variant of a enormous family of genes which, like the antibodies of the immune system, can stereospecifically bind with particular odorant molecules. This binding [activates a G_o protein, which via generation of cAMP,] causes the opening of Na⁺ channels in the membrane, depolarization of the olfactory cells (receptor potential), and generation of action potentials, which run along the axons of the olfactory cells, into the brain.

The axons of the olfactory cells penetrate up through the ethmoid bone, into the olfactory bulb which lies below the frontal lobe of the cerebral hemispheres. Here they synapse on dendrites of the mitral (e) and tufted cells (d), to form complex synaptic structures, glomeruli, in the bulb. Receptor cells expressing the same sub-type of receptor molecule, tend to converge on to a particular glomerulus. There are about as many glomeruli as receptor molecule sub-types. Glomeruli of olfactory terminals (a, b) and of mitral and tufted cell terminals (c) are shown separately here, but of course, interweave with each other.

The response to a given odorant sets up in the olfactory bulbs, a specific pattern of activity which may serve to code for different odors. Small peri-glomerular cells and granule cells mediate lateral inhibition between glomeruli and mitral cells, thereby helping to enhance the difference between response patterns to similar odors.

Axons of the mitral and tufted cells in the olfactory bulb, run along the olfactory tract into the hypothalamus (feeding and sexual reflexes etc.), pyriform cortex, limbic system (memory, emotional responses etc.), and related areas.

The sense of smell is important in the recognition of food, mates, family etc., and in the regulation of appetite and other quite subtle behavioural responses.

Sexually related responses to smell are mediated by a separate epithelial area enclosed in a blindly ending pit, the vomeronasal organ, opening into the nasal cavity. The receptor cells here have microvilli instead of olfactory knobs, and project to the accessory olfactory bulb, thence to the olfactory cortex and amygdala.

Loss or reduction of the sense of smell (anosmia or hyposmia) may be due to damage to the olfactory mucosa (e.g. in smoking) or to the olfactory bulbs or tracts. Central nervous system disorders (e.g. some types of epileptic seizures) can cause parosmia (disturbed sense of smell). Olfactory receptor cells are short-lived and constantly die and are replaced throughout life. Some investigators believe that alterations in the olfactory system can give an early warning of impending Alzheimer's disease.

The sense of taste is mediated by small, barrel-shaped taste buds, located mainly on papillae on the tongue and pharynx.

There are only four fundamental taste categories normally recognised: sweet, salty, sour and bitter. The Japanese recognise a fifth category "umami", which might be described as "savoury". The distribution of different types of taste buds is non-uniform, but the domains overlap considerably and are not mutually exclusive.

Taste buds on the anterior 2/3 of the tongue are innervated by the facial nerve (VII) and those on the posterior 1/3 by the glossopharyngeal (IX).

Taste buds on the epiglottis and pharynx are innervated by the vagus nerve (X).

The taste buds are made up of taste cells and supporting cells stacked together. The taste cells die continuously and are replaced by division and maturation of basal cells which become innervated by the widowed nerve endings. The taste cells have receptor molecules in their apical membranes. The apical membrane is folded into micro-villi, greatly increasing the surface area. The micro-villi stick into an opening, the taste pore, at the tip of the barrel-shaped taste-bud. Chemicals dissolved in the saliva can become attached to the receptor molecules, and so stimulate the taste cell. Some taste cells respond best to sugars, some to salts, and some to acids (H⁺), due to differences in the nature of the receptor molecules in their membranes.

Sour and salt sensitive taste cells are directly depolarised by the inflow of positively charged H⁺ or other cations through ion specific channels. Combination of sweet or bitter chemicals with a specific receptor molecule, leads to depolarization, via a G protein, causing either reduced P_K+ or increased intracellular Ca²⁺ respectively.

Each taste bud responds preferentially to a particular class of stimulant, and produces a particular sensation when stimulated. But although being most sensitive to one type of stimulant, a taste bud may also respond weakly to another type. Thus a sweet sensitive taste bud may respond weakly to acid, and a sour sensitive bud may respond weakly to sucrose. To differentiate between a weakly sweet and a strongly sour stimulus, the brain must be able to "compare" the activity levels in the sweet and sour taste buds. This "comparison" probably occurs by inhibitory and excitatory synaptic interactions in the CNS.

Taste Pathway

Receptor cell depolarization leads to the release of neurotransmitter, which generates action potentials in the associated nerve endings. The axons, whose cell bodies lie in the sensory ganglia of the cranial nerve, enter the medulla and synapse in the region of the nucleus of the solitary tract. The nerve cells in this part of the medulla are important in mediating salivation and other gastrointestinal reflexes. Their axons also cross over, and relay via the contralateral medial lemniscus, to the thalamus, and thence to the post-central gyrus in the region of the insula. Like olfaction, the sense of taste is important in regulating appetite and to some degree, dietary intake. Loss or reduction of the ability to taste is termed ageusia or hypogeusia and is a feature of ageing.

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REY. 24/6/97