Group 11: Smell and Taste
E-Facilitator: Sophie


Learning outcomes:
Outline the anatomical structures associated with both smell and taste and discuss the way in which these senses are closely related physiologically.

Table of Contents

Definition of Perception

Tongue - Figures 1 & 2, Table 1
The Basic Tongue Sensations
Taste - Figures 3, 4 & 5

Nose - Smell

Sensory Pathway

Relationship between Taste and Smell - Figure 6i, 6ii & 7
The Effects of aging on Taste Perception

Additional Sites - Links

Definition of perception: "the process of using the senses to acquire information about the surrounding environment or situation"

TONGUE
The tongue is the strongest muscle in the body and is responsible for taste perception, and also aids in mastication and deglutition.
Taste buds are the sensory receptors organs for taste. There are 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 (called papillae), that give the tongue an abrasive feel. There are four major types of taste buds: filliform, foliate, 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 the side of the tongue (Marieb, 1998). The cicumvallate form an inverted V at the back of the tongue which contain 7/12 round projections called the 'sulcus terminalus' (Marieb, 1998). The function of these papillae is taste. The filliform has no or little taste function; the fungiform, circumvallate and foliate papillae all have different taste perceptions depending on their location on the tongue.
taste_picture.JPG

Figure 1- Diagram of location of tongue, papillae location (and structure) and also an individual taste bud.

taste.JPG

Figure 2- The locations on the tongue of where sweet, salty, sour, and bitter sensations occur.


Salty
NaCl
0.01 M
Sour
HCl
0.0009 M
Sweet
Sucrose
0.01 M
Bitter
Quinine
0.000008 M
Umami
Glutamate
0.0007 M
Table 1. Examples of some human thresholds

The Basic Taste Sensations
The basic taste sensations on the tongue have been highlighted in the figure above.
Umami - meaty/savoury taste due to amino acid substances (Tortora & Derrickson, 2006).
Sweet – produced by organic substances (sugars, saccharin, alcohols, some amino acids, and lead salts)
Sour – produced by acids, specifically hydrogen ions in solution
Salty – produced by metal ions (inorganic salts), and table salt (NaCl).
Bitter – produced by alkaloids (quinine, nicotine, caffeine, morphine, and strychnine), and nonalkaloid substances (aspirin).


TASTE
external image gust_receptors.gif

Figure 3- The cells situated within the pore of the taste bud.

Gustation (sensation of taste) occurs by the stimluation of food chemicals dissolved in saliva (Marieb, 1998).The sense of taste is a component of our chemcial sensation system (chemo-senses). The complicated process of tasting begins when molecules released by the substances stimulate special cells in the mouth or throat. These special sensory cells transmit messages through nerves to the brain where specific tastes are identified.

The cranial nerves carry taste information into the brain and to a part of the brain stem called the nucleus of the solitary tract. From the nucleus of the solitary tract, taste information goes to the thalamus and then to the cerebral cortex. Like information for smell, taste information also goes to the limbic system (hypothalamus and amygdala), where it is further interpeted and related to previous memories (eg. taste triggers a re-occuring memory).


Figure 3
Figure 3


Figure 4- Diagram of nerve pathway from tongue to the brain stem.


NOSE
The nose consists of bone and cartilage and situated at the centre of the face. The nasal and oral cavity assist in every day breathing, the nasal cavity contains hairs to reduce bacteria becoming inhaled into the lungs. It is closely linked to the oral cavity by the fact they complement each others function. If smell is compromised, 80% of the total taste will be lost (20% of taste is from taste buds).

SMELL
How People Sense Flavors
How People Sense Flavors


Figure 5- The Sensory Pathway of Taste and Smell.

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. The nerves are only sensory and are not motor nerves.

Sensory pathway
The physiological pathway of gustation (taste) is reasonably complex. When a food chemical (tastant) is dissolved in saliva it will come into contact with the plasma membrane of the gustatory hairs (finger-like projections) of the taste bud. As a result a receptor potential occurs, where this stimulates the exocytosis of synaptic vesicles in the gustatory receptor cells. Therefore these changes (synapse) in the gustatory receptor cells trigger nerve impulses in the first order sensory neurons (neurons underneath the taste bud). The facial nerve (VII) and the glossopharyngeal nerve (IX) are the two cranial nerves that innervate the tongue and are used for taste. The facial nerve innervates the anterior 2/3 of the tongue whilst the glossopharyngeal nerve innervates the posterior 1/3 part of the tongue (Chudler, 2007). Another cranial nerve, the vagus nerve (X) carries taste information from the back part of the mouth.
The nerve impulses travel through these nerves to the nucleus of the solitary tract in the brain stem (Chudler, 2007). The taste information goes to the thalamus and to the cerebral cortex. At this point the information enters into the higher cortical brain regions for perception and interpretation (Chudler, 2007).

Relation between Taste and Smell
Approximately 80-90% of what we taste is due to the sense of smell. When food is consumed the gustatory cells (taste buds) respond to taste, whilst odours from the food pass up through the nasal cavity where smell is detected. The smell and taste signals both pass into the same area of the brain (hypothalamus and orbito-frontal area), where they are interpreted and selectively distinguished together. Smell (olfacton) receptors are far more sensitive (thousands of times) more than gustatory senses (taste buds), and assist greatly with the interpretation of taste of foods. Therefore without smell (e.g having a blocked nose) the taste of certain foods would be far weaker (or absent) in strength.

Figure 2A
Figure 2A



Figure 6 (i)- Diagram of olfactory mucosa and olfactory bulb location.

Figure 2B
Figure 2B


Figure 6 (ii)- The different nerve pathways of the olfactory sensory nerves.


smell_taste.jpg


Figure 7- Diagram of both the histology of tongue and nasal area.

The effects of aging on taste perception
The normal aging process of a human contributes largely to a gradual loss of sensory perceptions, especially with gustation. This loss of gustation generally starts to change from the age of 50 years and onwards. This may be due to tongue cancers, smoking, nasal congestion, dentures, poor oral hygiene, nutritional deficiencies (Vitamin A, Niacin, and Zinc), and slower epithelial turnover rate/decreased number of taste buds can all affect gustation. Often the olfactory bulb withers as a person ages and this can greatly effect olfaction, and ultimately affect gustation. Usually a person of an older age can still identify different tastes (sweet, sour, salty, bitter, umami), but the strength of the taste is very weak. Therefore often people of an older age, whilst being unaware; increase the concentration of flavours (e.g. salt) in their food in order to enjoy their food. There are various effects which may arise from the partial (hypogeusia) or complete (ageusia) loss of taste. The most common and obvious effect is the loss of taste which ultimately affects the taste of food consumed. This loss of taste can result in a decrease or change in appetite of which weight loss is more common (or possible anorexia), and deperession can occur. Other medical conditions such as heart disease, diabetes, digestive disorders, stroke, and hypertension may arise due to a change in dietary habits (e.g. increased salt intake). The loss of taste can also make an individual more susceptible to the ingestion of poisons and hazardous chemicals (as are unable to taste poison). It has been found in limited studies conducted that taking vitamin A and B3 (niacin) supplements can assist in restoring taste to a certain degree.


Links
Smell: [[http://www.innerbody.com/image/nerv11.html**|http://www.innerbody.com/image/nerv11.html ]]
Taste: http://www.innerbody.com/image/nerv12.html


References:
Bailey R 2007 Physiology of Taste Viewed 20th September 2007 <http://biology.about.com/library/organs/blpathodigest2.htm>
Bowen R 2006 Physiology of Taste USA Viewed 30th August 2007 <http://www.vivo.colostate.edu/hbooks/pathphys/digestion/pregastric/taste.html>
Bromley SM 2000 Smell and Taste Disorders: A Primary Care Approach University of Pennsylvania Smell and Taste Center, Philadelphia, Pennsylvania Viewed 20th September <http://www.aafp.org/afp/20000115/427.html>
Calvert JV 2002 The Mystery of the Senses University of Denver, USA Viewed 28th August 2007
< http://www.cerebromente.org.br/n16/mente/senses1.html >
Chudler EH 2007 That’s Tasty, National Centre for Research Resources, Viewed 28 September 2007
< http://faculty.washington.edu/chudler/tasty.html >
Encarta World English Dictionary 2007 Definition of Perception Bloomsbury Publishing Viewed 15th September
< http://encarta.msn.com/dictionary_1861724920/perception.html>
Jacewicz M 2007 Smell and Taste Disorders, Merck & Co. Viewed 22nd September 2007 < www.merck.com/mmhe/print/sec06/ch097/ch097a.html >
Jacob T 2007 Taste (Gustation), Viewed 15th September 2007 < http://www.cf.ac.uk/biosi/staff/jacob/teaching/sensory/taste.html>
Kalumumuck K 2006 Your Sense of Taste: Relationship between taste and smell
<
http://www.exploratorium.edu/snacks/your_sense_of_taste/index.html>
Mann MD 2007 Sensory Receptors II: Ch 4b Viewed 12th September 2007 < www.tastescience.com/abouttaste2.html>
Marieb EN 1998 Human Anatomy and Physiology 4th Ed. Benjamin/Cummings Science Publishing, California pg 537-538
Tortora GJ & Derrickson B 2006 Principles of Anatomy and Physiology 11th Ed John Wiley & Sons Inc. USA, pg 574-584