Sensory Receptors and their Roles in Sensory Evaluation

Sensory receptors are detectors (or senses), which indicate about the physical and chemical changes in our environment, Psychologists recognize 22 special senses, out of which 5 are primary senses and perceive stimuli. These senses are sight, hearing, touch, smell, and taste. Other senses include: temperature, pain, visceral, hunger, thirst, fatigue, sex (drive) and equilibrium.

In human beings at least three different senses respond to specific chemical stimuli:taste, smell and common chemical or pain sense. The sense organs consist of sensory cells or group of cells, which respond to stimuli and transmit an impulse via the nervous system to the brain. Human nervous system constitute of chemical senses, somatic senses and auditory. The type of stimulus, corresponding receptors and human experience exhibited by these senses is shown in Table

i. Taste Receptors

The tongue serves as the major organ of taste. The raised portions on the tongue,known as papillae, are considered to be sites of taste buds, the receptors of taste.The tongue is insensitive to taste in the regions where there are no papillae.Gustatory (taste) sensibility is mainly confined to the tip and edges of the tongue and absent in middle of the tongue.Four kinds of papillae are found on the human tongue as shown in Fig. These are Foliate, Circumvallate, Fungi form, and Filli form.

 

                                     Front View of Tongue Showing Differert Papillae and The Regions Where

BasicTastes are Perceived

Filli form papillae, evenly distributed on the anterior 2/3 of the tongue are most numerous but have no taste buds. Fungi form papillae, large and round and mushroom-like in appearance are large in number at the tip and sides of the tongue. Foliate papillae are found on the posterior one third of the tongue, usually in folds on the sides. These are also not well developed in man and have little function. The circumvallate papillae present in form of “V-shape” on the back of the tongue are large and easily visible.

Human taste buds are located on moist surfaces within the oral cavity and pharynx. A few non-papillae associated taste buds may also be found in such locations as the Soft palate, Pharynx and Larynx. These taste buds are simply embedded within the epithelium of the mucous membrane. The group of epithelium cells, some of which are supporting cells and other taste cells are innervated (stimulated) by sensory nerve endings. When a taste bud is exposed to the taste stimulus,strong taste signals are transmitted by taste nerves into the central nervous system and finally taste reflexes are integrated into the brain stem directly into superior and inferior nuclei. Based on transmission of these impulses to the sub maxillary, sublingual and parotid glands, excretion of saliva is controlled during ingestion of food.

Taste perception: Taste buds differ somewhat in their response to stimuli. The four basic taste solutions are perceived by different regions of the tongue (Fig.).

• Sour taste may be noted chiefly along the sides of the tongue
•  Saltiness along the sides and tip
•  Sweet taste generally at the tip
•  Bitter taste perceived at the base of the tongue.

 ii. Odour/ Smell Receptors

The sense of smell is far more sensitive than the sense of taste. An average person  requires relatively concentrated solution to perceive taste sensation. In contrast an odoriferous substance, e.g. mercaptan may be diluted to the extent of 0.43 x 10-15 mg/litre of air and yet be recognized as such by the sense of smell.

The nose equipped with olfactory nerves, is the special organ of smell. The olfactory receptors are located mainly in the uppermost region (roof) of the nasal cavity and are lined with so-called olfactory epithelium, which is yellowish in colour with pink respiratory epitherial cells around it. The surface of the olfactory epithelium is coated with a layer of mucous. Embedded in the mucous layer is a mat of fine hair-like appendages of the olfactory cells called cilica. The cilia are excited by molecules of odorous substances. The olfactory area, about 2.5 cm size, is reached chiefly by eddy currents rather than the direct passage of air during inhalation. The odorous substance must, therefore, be sniffed or whiffed rather slowly but strongly. The molecules of odorous substances also reach the olfactory area from the mouth during mastication of the food. Therefore, during the tasting of dairy products the judge/ evaluator actually is sensing the odour and taste of the product at the same time. The overall physiological concept of odour perception is schematically shown in Fig.

 

Physiological concept of odour perception

Odour Perception: In order to properly perceive the odour of a substance, the following aspects are important and should be kept in mind.

i) The prospective odorous substance/chemical must be volatile. The volatility of a substance depends on its molecular weight and molecular bonding properties.The upper limit of a ‘smellability’ is usually a molecular weight of about 300, although a compound of molecular weight, 394 has odour. This means that substances with heavier molecular weight, such as proteins, fats, starches and sugars are too heavy to be airborne. The breakdown of these heavy molecular weight compounds produces low molecular weight substances, which impart desirable and undesirable flavours in dairy products. The volatility of chemical substances also increases with the increase in temperature.

ii) The odour-laden air must reach the olfactory receptors. The substance must, therefore, be strongly sniffed.

iii) Odorous substances must be adsorbed or adhered to the chemoreceptors sites in the nose. To be absorbed, the volatile substance must be soluble to some extent in the receptor cell membrane, which consists of lipids, proteins and water.

iv) The evaluator should have normal smelling capacity. People with obstructed nasal passage, brain lesions or injured olfactory nerve are unable to smell, either partially or of have a particular substance or group of substances. The situation is called as “anosmia”.