The sensory receptors for special senses are localized rather than widely distributed, and they, like all sensory receptors, are specialized to respond to only certain types of stimuli. There are three different kinds of sensory receptors for the special senses. Taste and olfactory receptors are chemoreceptors, which are sensitive to chemical substances. Sensory receptors for hearing and equilibrium are mechanoreceptors, which are sensitive to vibrations formed by sound waves and movement of the head. Sensory receptors for vision are photoreceptors, which are sensitive to light energy.
The chemoreceptors for taste are located in specialized microscopic organs called taste buds. Most taste buds are located on the tongue in small, raised structures called lingual papillae, though some can be found in areas such as the soft palate, pharynx, and esophagus.
A taste bud consists of a bulblike arrangement of rapidly adapting taste receptors, called gustatory epithelial cells, located within the epithelium of the lingual papillae. The taste bud possesses an opening called a taste pore. Taste receptors have hair like projections called gustatory hairs that extend through the pore and are exposed to chemicals on the tongue. Sensory axons leading to the brain are connected to the opposite end of the taste receptors. In order to activate the taste receptors, a substance must be dissolved in a liquid such as saliva.
There are five confirmed basic tastes that can be detected by the tongue: sweet, sour, salty, bitter, and umami (savory). The receptors for each basic taste are located across the tongue surface, which disproves the earlier belief that the basic tastes were mapped to specific regions of the tongue. It is probable that other substances, such as fats and Ca2+i will be added as basic tastes in the near future as a result of ongoing taste research. It has been suggested that water is also a basic taste; however, not enough experimental data has been produced to support this claim. The many flavor sensations of food result from the stimulation of one or more taste receptors and, more importantly, the activation of olfactory receptors.
The pathway of nerve impulses from taste receptors to the brain depends on where the taste receptors are located. Nerve impulses created by taste receptors on the anterior two-thirds of the tongue are carried by the facial nerve (CN VII), while those created on the posterior one-third travel over the glossopharyngeal nerve (CN IX). Nerve impulses created at the base of the tongue are carried by the vagus nerve (CN X). These cranial nerves carry the nerve impulses to the medulla oblongata, from which the nerve impulses travel to the thalamus and on to the taste areas in the parietal lobes of the cerebrum.
The olfactory receptors are located in the superior portion of the nasal cavity, including the superior nasal conchae and nasal septum. The olfactory receptors, also called olfactory sensory neurons, are surrounded by the supporting epithelial cells of the olfactory epithelium. The distal ends of the olfactory receptors are covered with cilia that project into the nasal cavity, where they can contact airborne molecules. Chemicals in inhaled air are in a gaseous state and must dissolve in the mucus layer covering the olfactory epithelium in order to stimulate nerve impulse formation. The nerve impulses are carried by axons of the olfactory receptors, which form the olfactory nerves (CN I), to the olfactory bulbs. Here they synapse with neurons that form the olfactory tract and relay the nerve impulses to the olfactory areas deep within the temporal lobes and at the bases of the frontal lobes of the cerebrum.
It is common for a person to sniff the air when trying to detect faint odors. This is because the olfactory receptors are located superior to the usual path of inhaled air and additional force is needed to send larger amounts of air over the olfactory epithelium. Like taste receptors, olfactory receptors rapidly adapt to a particular stimulus.
The human olfactory epithelium possesses approximately 350 functional types of olfactory receptors. However, the average person can distinguish between 2,000 and 4,000 different odors. The ability to detect so many types of odors largely depends upon how the temporal lobes process the nerve impulses from various combinations of olfactory receptors. Studies have shown that women can detect, discern, and identify a wider range of odors than men. It is also possible with training to enhance your olfactory ability and potentially discern up to 10,000 different odors, an ability important for those in the wine industry. The decrease in odor detection that occurs with age, which is why the elderly tend to use more cologne and perfume, is a result of receptor loss and desensitization rather than temporal lobe dysfunction. Research suggests that the olfactory epithelium is capable of detecting human pheromones. Human pheromones, which have been found in apocrine sweat and vaginal secretions, have been shown to have influence over reproductive functions. For example, pheromones from one female have been shown to lengthen or shorten the menstrual cycle of exposed females. The olfactory epithelium is also highly regenerative owing to its direct exposure to the external environment. On average, an olfactory receptor lives only approximately 60 days before being replaced.
Disorders of Taste And Smell
Ageusia is a loss of taste function, meaning there is no perception of the five basic tastes, and is rare. Hypogeusia, or a reduced ability to taste, is more common and can be caused by zinc deficiency and chemotherapy. Dysgeusia, which is a distortion or impaired perception of taste, can be caused by taste bud distortion, pregnancy, diabetes, allergy medications like albuterol, zinc deficiency, and chemotherapy.
Anosmia is the inability to detect odor. The loss can be for one odor or all odors. It may also be permanent or temporary depending upon the cause. Typical causes are inflammation of the nasal mucosa, blockage of the nasal pathways, damage to the olfactory nerve, or head trauma leading to temporal lobe damage. Hyposmia is a decrease in the ability to detect odors. Hyposmia is common with advanced age due to a decrease in olfactory epithelium regeneration or smoking.
Dysosmia is distorted sense of smell. Parosmia, a type of dysosmia, occurs when an individual has altered smell perception, meaning that something normally pleasant is perceived as being unpleasant. Phantosmia occurs when an individual perceives an odor that is not present. These phantom smells can be clinical signs of migraine, mood disorders, schizophrenia, or epilepsy.