51 A Sensory Receptor
No Cla t fo ssr r o Us om e
64
humans and other mammals, these are located in the nose and tongue respectively. Each receptor type is basically similar: they are collections of receptor cells equipped with chemosensory microvilli or cilia. When chemicals stimulate their membranes, the cells respond by producing nerve impulses that are transmitted to the olfactory and gustatory regions of the cerebral cortex of the brain for interpretation.
PR E V ON IEW LY
Key Idea: Chemosensory receptors detect chemicals in the air or in fluid and are responsible for the sense of smell and taste. The sense organs of taste are the taste buds. Chemosensory receptors are responsible for our sense of smell (olfaction) and taste (gustation). The receptors for smell and taste both respond to chemicals, either carried in the air (smell) or dissolved in a fluid (taste). In
The organs of taste are the taste buds of the tongue. Most of the taste buds are located on raised protrusions of the tongue surface called papillae. Each bud is flask-like in shape, with a pore opening to the surface of the tongue enabling molecules and ions dissolved in saliva to reach the receptor cells inside. Each taste bud is an assembly of 50-150 taste cells. When a chemical binds to a receptor protein on the cell surface membrane of a taste cell, it results in depolarisation and generation of an action potential in the sensory nerve fibre. This information is relayed to the gustatory region of the brain. There are five basic taste sensations. Salty and sour operate through ion channels, whereas sweet, bitter, and umami (savoury) operate through membrane signalling proteins. These taste sensations are found on all areas of the tongue although some regions are more sensitive than others. Note that taste also relies heavily on smell because odours from food also stimulate olfactory receptors.
Gustatory hairs (microvilli) protruding from a taste pore
Both photos: EII
Taste
Tongue surface
Taste pore
Taste cell
SEM of the surface of the tongue
Sensory nerve fibre
SEM of one of the papillae
Your senses can be fooled
Taste is not experienced in isolation. In fact, by itself, taste only provides rather crude information about a particular food or drink. A large amount of information comes for the olfactory receptors in the nose. Even visual input has a large impact on how we taste food.
A 2001 study of 54 wine tasters found that when the tasters were presented with a white wine artificially coloured red with an odourless and tasteless food dye, they identified its taste as that of a red wine. The information from the visual receptors over-rode the input from the taste receptors. This may be explained by the fact primates developed colour vision as an ability to identify good foods to eat (e.g. ripe fruit). Another study in 2015 found that taste is affected by noise and air pressure, such as when in a plane, which may explain why airline food always tastes strange.
Airline food not so bad?
2. Explain why the taste of food changes when you have a blocked nose:
No Cla t fo ssr r o Us om e
1. Describe the basic mechanism by which chemical sense operates:
3. Describe the evidence that smell and sight (and even hearing) play a big part in how something tastes:
LINK
KNOW
50
15 CIE2 Control and Coordination 2015.indd 64
Š 2016 BIOZONE International ISBN: 978-1-927309-32-2 Photocopying Prohibited
5/01/16 12:48 pm