Learning outcomes:
Outline the anatomical structures associated with both smell and taste and discuss the way in which
these senses are closely related physiologically.
TONGUE
The tongue is the strongest muscle in the body. It is responsible for taste perception,
and aids in mastication and deglutition. There are about 10, 000 taste buds on most of
the tongue and some are scattered on the soft palate, inner surface of cheeks, the
pharynx and epiglottis (Marieb, 1998). They are peg like projection of tongue mucosa
which, are called papillae, giving the tongue an abrasive feel.
Taste buds are the sensory receptors organs for taste. There are three major types:
filliform, fungiform (mushroom shaped) and circumallate (round). Two of these types
house most of the taste buds so fungiform are scattered over the entire tongue surface
and are most abundant at the tip of the tongue and along side of the tongue (Marieb,
1998). The cicrcumallate form an inverted V at the back of the tongue which contain
7 to 12 round projections called the 'sulcus terminalus' (Marieb, 1998). The filliform
has no or little taste function; the fungiform, and circumvallate papillae all have
different taste perceptions depending on their location on the tongue.
Figure 1. The Tongue
TASTE
The basic taste sensations on the tongue have been highlighted in the figure above.
Taste sensations can be grouped into four basic qualities; sweet, sour, salty and bitter.
Sweet – located at the tip of the tongue and produced by organic substances (sugars,
saccharin, alcohols, some amino acids, and lead salts)
Sour – located at the sides of the tongue and produced by acids, specifically hydrogen
ions in solution
Salty – located at the tip of the tongue and produced by metal ions (inorganic salts),
and table salt (sodium chloride)
Bitter – located at the back of the tongue (near its root) and produced by alkaloids
(quinine, nicotine, caffeine, morphine, and strychnine), and nonalkaloid substances
(aspirin).
These taste buds however are not fixed. Most taste buds are able to respond to two,
three or all four taste qualities.
Figure 2. Pore of the taste bud and the cells situated within.
Taste belongs to our chemical sensing system, or the chemo-senses. The complicated
process of tasting begins when molecules released by the substances are dissolved in
saliva, diffuse into the taste pore and are stimulated by the gustatory cells within the
pores. These special sensory cells transmit messages through nerves to the brain
where specific tastes are identified. The afferent nerve fibers consist of the facial
nerve (VII), the chorda tympani and the lingual branch of the glossopharyngeal nerve
(IX). These nerves carry taste information into the brain and to a part of the brain
stem called the nucleus of the solitary tract of the medulla. From the nucleus of the
solitary tract, taste information goes to the thalamus and then to the gustatory cortex
in the parietal lobes. Like information for smell, taste information also goes to the
limbic system (hypothalamus and amygdala).
The receptor for taste is called a gustation, and is classified as a chemoreceptor which
responds to food chemicals dissolved in saliva (Marieb, 1998). The food chemicals
bind to the gustatory cell membranes inducing a depolarizing action potential.
NOSE
The nose consists of bone and cartilage and situated at the centre of the face. The
nasal cavity is closely linked to the oral cavity by the fact they complement each
others function. If smell is compromised as a result taste is altered as 80 percent of
what we perceive as "taste" is actually smell. The nasal and oral cavity assist in daily
breathing, the nasal cavity contains hairs to stop bacteria being placed in the lungs.
SMELL
Figure 3.
The olfactory nerve is situated in the nasal cavity and is the major contributor to smell
perception. The "olfactory nerves" are the first pair of the cranial nerves and are
located in the upper nasal cavity. They are associated with the sense of smell. They
contain only sensory nerves.
The main olfactory bulb has a multi-layered cellular architecture. In order from the
surface to the centre of the bulb the layers are glomerular layer, external plexiform
layer, mitral cell layer, internal plexiform layer and granule cell layer. The glomerular
layer receives direct input from olfactory nerves, made up of the axons from
approximately ten million olfactory receptor nerves in the olfactory mucosa, a region
of the nasal cavity. The ends of the axons cluster in spherical structures known as
glomeruli such that each glomerulus receives input primarily from olfactory receptor
neurons that express the same olfactory receptor. Glomeruli are also permeated by
dendrites from neurons called mitral cells, which in turn output to the olfactory cells.
Numerous interneuron types exist in the olfactory bulb including periglomerular cells
which synapse within and between glomeruli, and granule cells which synapse with
mitral cells.
RELATION BETWEEN SMELL AND TASTE PHYSIOLOGICALLY
Approximately 80-90% of what we perceive as taste is due to the sense of smell.
When having a cold or a stuffed up nose food tastes dull food. At first we may not be
able to tell the specific flavour of a candy, just perhaps a sensation of sweetness or
sourness. If students are patient, some may notice that as the candy dissolves they can
identify the specific taste. This is due to some scent molecules travel up to the
olfactory organ through the oropharynx, the passage at the back of the throat and to
the nose. Since we can only taste four different true taste, it is actually smell that
allows the individual to experience their favourite flavours. (NOTE TO JEN USE IN
POSTER)
References:
The bit used for relationship between smell and taste: by
Karen Kalumuck, Your Sense of Taste: Relationship between taste and smell,
http://www.exploratorium.edu/snacks/your_sense_of_taste/index.html
Marieb EN 1998 Human Anatomy & Physiology 4th Ed Benjamin Cummings, Amsterdam pages
537-541