Vestibular sense (equilibrium)
- Related Topics:
- human eye
- chemoreception
- mechanoreception
- thermoreception
- pain
The inner ear contains parts (the nonauditory labyrinth or vestibular organ) that are sensitive to acceleration in space, rotation, and orientation in the gravitational field. Rotation is signaled by way of the semicircular canals, three bony tubes in each ear that lie embedded in the skull roughly at right angles to each other. These canals are filled with fluid called endolymph; in the ampulla of each canal are fine hairs equipped with mechanosensing stereocilia and a kinocilium that project into the cupula, a gelatinous component of the ampulla. When rotation begins, the cupula is displaced as the endolymph lags behind, causing the stereocilia to bend toward the kinocilium and thereby transmit signals to the brain. When rotation is maintained at a steady velocity, the fluid catches up, and stimulation of the hair cells no longer occurs until rotation suddenly stops, again circulating the endolymph. Whenever the hair cells are thus stimulated, one normally experiences a sensation of rotation in space. During rotation one exhibits reflex nystagmus (back-and-forth movement) of the eyes. Slow displacement of the eye occurs against the direction of rotation and serves to maintain the gaze at a fixed point in space; this is followed by a quick return to the initial eye position in the direction of the rotation. Stimulation of the hair cells in the absence of actual rotation tends to produce an apparent “swimming” of the visual field, often associated with dizziness and nausea.
Two sacs or enlargements of the vestibule (the saccule and utricle) react to steady (static) pressures (e.g., those of gravitational forces). Hair cells within these structures, similar to those of the semicircular canal, possess stereocilia and a kinocilium. They also are covered by a gelatinous cap in which are embedded small granular particles of calcium carbonate, called otoliths, that weigh against the hairs. Unusual stimulation of the vestibular receptors and semicircular canals can cause sensory distortions in visual and motor activity. The resulting discord between visual and motor responses and the external space (as aboard a ship in rough waters) often leads to nausea and disorientation (e.g., seasickness). In space flight abnormal gravitational and acceleratory forces may contribute to nausea or disequilibrium.
In some diseases (e.g., ear infections), irritation of vestibular nerve endings may cause the affected individual to be subject to falling as well as to spells of disorientation and vertigo. Similar symptoms may be induced by flushing hot and cold water into the outer opening of the ear, since the temperature changes produce currents in the endolymph of the semicircular canals. This effect is used in clinical tests for vestibular functions and in physiological experiments. Externally applied electrical currents may also stimulate the nerve endings of the vestibule. When a current is applied to the right mastoid bone (just behind the ear), nystagmus to the right tends to occur with a reflex right movement of the head; movement tends to the left for the opposite mastoid. Destruction of the labyrinth in only one ear causes vertigo and other vestibular symptoms, such as nystagmus, inaccurate pointing, and tendency to fall.
Taste (gustatory) sense
The sensory structures for taste are the taste buds, clusters of cells contained in goblet-shaped structures called papillae that open by a small pore to the mouth cavity. A single taste bud contains about 50 to 75 slender taste receptor cells, all arranged in a banana-like cluster pointed toward the gustatory pore. Taste receptor cells, which differentiate from the surrounding epithelium, are replaced by new cells in a turnover period as short as 7 to 10 days. The various types of cells in the taste bud appear to be different stages in this turnover process. Slender nerve fibres entwine among and make contact usually with many cells. Taste buds are located primarily in fungiform (mushroom-shaped), foliate, and circumvallate (walled-around) papillae of the tongue or in adjacent structures of the palate and throat. Many gustatory receptors in small papillae on the soft palate and back roof of the mouth in adults are particularly sensitive to sour and bitter tastes, whereas the tongue receptors are relatively more sensitive to sweet and salty tastes. Some loss of taste sensitivity suffered among denture wearers may occur because of mechanical interference of the dentures with taste receptors on the roof of the mouth.
Nerve supply
There is no single sensory nerve for taste. The anterior (front) two-thirds of the tongue is supplied by one nerve (the lingual nerve), the back of the tongue by another (the glossopharyngeal nerve), and the throat and larynx by certain branches of a third (the vagus nerve), all of which subserve touch, temperature, and pain sensitivity in the tongue, as well as taste. The gustatory fibres of the anterior tongue leave the lingual nerve to form the chorda tympani, a slender nerve that traverses the eardrum on the way to the brainstem. When the chorda tympani at one ear is cut or damaged (by injury to the eardrum), taste buds begin to disappear and gustatory sensitivity is lost on the anterior two-thirds of the tongue on the same side. The taste fibres from all the sensory nerves from the mouth come together in the medulla oblongata. Here and at all levels of the brain, gustatory fibres run in distinct and separate pathways, lying close to the pathways for other modalities from the tongue and mouth cavity. From the medulla, the gustatory fibres ascend by a pathway to a small cluster of cells in the thalamus and then to a taste-receiving area in the anterior cerebral cortex.
Physiological basis of taste
No simple relationship has been found between the chemical composition of stimuli and the quality of gustatory experience except in the case of acids. The taste qualities of inorganic salts (such as potassium bromide) are complex; epsom salt (magnesium sulfate) is commonly sensed as bitter, while table salt (sodium chloride) is typical of sodium salts, which usually yield the familiar saline taste. Sweet and bitter tastes are elicited by many different classes of chemical compound.
Theorists of taste sensitivity classically posited only four basic or primary types of human taste receptors, one for each gustatory quality: salty, sour, bitter, and sweet. Yet, recordings of sensory impulses in the taste nerves of laboratory animals show that many individual nerve fibres from the tongue are of mixed sensitivity, responding to more than one of the basic taste stimuli, such as acid plus salt or acid plus salt plus sugar. Other individual nerve fibres respond to stimuli of only one basic gustatory quality. Most numerous, however, are taste fibres subserving two basic taste sensitivities; those subserving one or three qualities are about equal in number and next most frequent; fibres that respond to all four primary stimuli are least common. Mixed sensitivity may be only partly attributed to multiple branches of taste nerve endings. In humans, tastes of sugars, synthetic sweeteners, weak salt solutions, and some unpleasant medications are blocked by gymnemic acid, a drug obtained from Gymnema bushes native to India. Among some laboratory animals, gymnemic acid blocks only the nerve response to sugar, even if the fibre mediates other taste qualities. Such a multiresponsive fibre still can transmit taste impulses (e.g., for salt or sour), so that blockage by the drug can be attributed to chemically specific sites or cells in the taste bud.
In some animals (e.g., the cat), specific taste receptors appear to be activated by water; these water receptors are inhibited by weak saline solutions. Water taste might be considered a fifth gustatory quality in addition to the basic four.
The qualities of taste
Sour
The hydrogen ions of acids (e.g., hydrochloric acid) are largely responsible for the sour taste; however, although a stimulus grows more sour as its hydrogen ion (H+) concentration increases, this factor alone does not determine sourness. Weak organic acids (e.g., the acetic acid in vinegar) taste more sour than would be predicted from their hydrogen ion concentration alone; apparently the rest of the acid molecule affects the efficiency with which hydrogen ions stimulate.
Salt
Although saltiness is often associated with water-soluble salts, most such compounds (except sodium chloride) have complex tastes such as bitter-salt or sour-salt. Salts of low molecular weight are predominantly salty, while those of higher molecular weight tend to be bitter. The salts of heavy metals such as mercury have a metallic taste, although some of the salts of lead (especially lead acetate) and beryllium are sweet. Both parts of the molecule (e.g., lead and acetate) contribute to taste quality and to stimulating efficiency. The following is a series for degree of saltiness, in decreasing order: ammonium (most salty), potassium, calcium, sodium, lithium, and magnesium salts (least salty).
Sweet
Except for some salts of lead or beryllium, sweetness is associated largely with organic compounds (such as alcohols, glycols, sugars, and sugar derivatives). Sensitivity to synthetic sweeteners (e.g., saccharin) is especially remarkable; the taste of saccharin can be detected in a dilution 700 times weaker than that required for cane sugar. The stereochemical (spatial) arrangement of atoms within a molecule may affect its taste; thus, slight changes within a sweet molecule will make it bitter or tasteless. Several theorists have proposed that the common feature for all of the sweet stimuli is the presence in the molecule of a proton acceptor, such as the OH (hydroxyl) components of carbohydrates (e.g., sugars) and many other sweet-tasting compounds. It has also been theorized that such molecules will not taste sweet unless they are of appropriate size.
Bitter
The experience of a bitter taste is elicited by many classes of chemical compounds and often is associated with sweet and other gustatory qualities. Among bitter substances are such alkaloids (often toxic) as quinine, caffeine, and strychnine. Most of these substances have extremely low taste thresholds and are detectable in very weak concentrations. The size of such molecules is theoretically held to account for whether or not they will taste bitter. An increase in molecular weight of inorganic salts or an increase in length of chains of carbon atoms in organic molecules tends to be associated with increased bitterness.
A substantial minority of people exhibit specific taste blindness, an inability to detect as bitter such chemicals as phenylthiocarbamide (PTC). Taste blindness for PTC and other carbamides appears to be hereditary (as a recessive trait), occurring in about a third of Europeans and in roughly 40 percent of the people in Western India. Taste blindness for carbamides is not correlated with insensitivity to other bitter stimuli.
