Sensory organs (display 93)

Taste incites the consumption of nutritious foods and discourages the ingestion of potentially harmful substances. Each animal species has a different sense of taste, adapted to its physiological needs. Hence, sweetness characterizes energy-rich foods; saltiness indicates the presence of sodium and other minerals; sourness and bitterness are often associated with molecules that pose a danger to the individual; and umami is linked to the presence of proteins and certain amino acids. Taste reception often takes place in the oral cavity, which allows the quality of food to be assessed before it is ingested. 

The tongue of certain animals is a genuine sensory hub. On the surface we observe taste papillae and tactile papillae. Within the taste papillae are ovoid structures known as taste buds. The number of taste buds varies from hundreds to several hundreds of thousands according to the species. These taste buds enclose taste receptors. The microvilli of these receptors come into contact with the molecules dissolved in the saliva thanks to a small opening known as the taste pore. A chain reaction follows, causing the depolarisation of the receptor membrane and the transmission of electrical impulses to the cranial nerves. The signal is then directed to the brain’s cortex where it is processed.

Recent research has revealed that there is no such thing as a “taste map” and that reactivity to the five basic tastes is present in all areas of the tongue. What appears to differ from one area to another is the type and quantity of taste buds. The number of receptors may thus influence flavour perception.

Catfish have a remarkable sense of taste. This is the case for the yellow bullhead catfish which has around 175,000 taste buds. These are concentrated in the front part of the animal, particularly on its barbells, gills and mouth. Further taste receptors are present on the rest of its body.

These chemosensory structures allow the bullhead to navigate in murky waters and to locate potential sources of food in the vicinity.

It is interesting to note that fish that live in environments with good visibility have fewer taste buds.

A human being has about 8,000 taste buds that are regularly replaced throughout an individual’s life. A cow has about 20,000, mainly concentrated towards the back of the tongue.

Herbivores need to be able to differentiate between toxic and edible plants; this requires a high degree of taste sensitivity. Cows discern the five basic tastes, just like human beings. However, they have a higher tolerance to bitterness due to the large number of edible plants that have a bitter taste. A strong aversion to this taste would greatly limit their choice of edible vegetation.

Tactile perception is essential for the body’s awareness and for discovering the environment that surrounds it. It increases an individual's sensory range in terms of movement, diet and pain perception, and is at the basis of a multitude of social interactions.

The skin of mammals is rich in cells that are sensitive to a variety of mechanical stimuli including touch. These receptors, known as mechanoreceptors, are mainly found in areas of the body used for exploring the environment, manipulating objects or engaging in social expression, like the hands in the case of human beings. 

Mechanoreceptors are not all identical. An initial distinction can be made between receptors that adapt rapidly and that have a response concentrated at the beginning and at the end of the stimulation, and those that adapt slowly but that remain active throughout the stimulation. Immediately beneath the epidermis we find the Meissner corpuscles (rapid) and Merkel discs (slow). These mechanoreceptors perceive stimulations in a well-defined way. Deeper within the dermis we find the Pacinian corpuscles (rapid) and Ruffini corpuscles (slow) which perceive tactile stimulations over a larger but less defined surface area. Still within the skin, free nerve endings known as nociceptors principally pick up harmful mechanical, chemical or thermal stimuli. The nerve fibres surrounding hair roots also provide animals with tactile sensitivity.

As regards the neural pathway, deformation of the receptor membrane leads to the creation of electrical impulses that pass along the spinal or cranial nerves towards the brain’s cortex where tactile information is processed.

The skin sensitivity of mammals is enhanced by nerve endings around the hair follicles. These receptors become activated when the hair is bent, triggering an electrical impulse.

Many species have specialised hairs called vibrissae. Often located on the animal’s snout, they are capable of sensing the slightest touch. This exceptional sensitivity is due to the fact that the base of the hair follicle is suspended in a fluid-filled cavity that amplifies movement. 

Vibrissae are particularly developed in nocturnal animals or in animals that live in poorly lit conditions. These attributes allow them to better navigate within their close environment and to interact with other organisms. 

The star-nosed mole lives in damp underground environments of North America. A remarkably quick hunter, the mole locates small invertebrates thanks to its snout, which features 22 appendices bearing around 25,000 mechanoreceptors known as Eimer’s organs.

These tactile appendices can touch about 10 spots per second, which gives the mole a high degree of tactile sensitivity and a very good understanding of its territory. Almost half of its cerebral cortex is devoted to processing this information.

Thus, the environment in which a species lives can favour the enhanced development of a sense, but often at the expense of another. The mole’s poor eyesight is a case in point.