Every day we consume large quantities of food and drink. How do we differentiate between the thousands of different tastes and flavour sensations created? In this review the following aims will be evaluated and discussed:
• What is the gustatory system?
• What receptors are involved in taste?
• How is the message transduced from our tongue to our brain?
Humans all possess a very complex sensory system which processes and interprets all sensory information. The sensory system is made up of three parts: sensory receptors, neural pathways and parts of the brain. There are five main sensory areas: vision (sight), auditory (hearing), olfaction (smell), somatic (touch) and gustatory (taste). The combination of these areas enables us to have the ability to determine between pleasurable and potentially lethal situations.
What is the Gustatory System?
The gustatory system is responsible for the process of taste. The system incorporates all of the mechanisms involved in perceiving taste from consumption to perception. There are four main stages in the taste mechanism:
- The recognition of chemical stimuli from food and drink molecules by the taste cell receptors predominantly found in the oral cavity.
- The activated taste receptor cell initiates a cascade of events to produce an electrochemical signal.
- The electrical signal travels along the nervous system pathway to the brain.
- The brain interprets the electrical signals from the taste receptor cells. The brain then combines this information with other sensory messages in order to perceive the flavour of the food or drink that has just been consumed.
There has been lots of research into taste transduction but there are still many areas that are not fully understood.
What Receptors are Involved in Taste?
In our mouths, we have clusters of cells called taste buds. They are elongated taste receptor cells and are located on our tongue, soft palate, upper oesophagus and epiglottis. The taste receptor cells can respond to five basic tastes from different chemical stimuli: sweet, sour, bitter, salty and umami (savoury). Each of the five basic tastes binds and stimulates a specific taste receptor. Taste receptors are also known as G protein-coupled taste receptors (GPCR). The GPCR can be subdivided into two groups the T1R and T2R families. The GPCR is located on one end of the taste bud and is the site of interaction between the taste receptor and the taste molecule. The other end is attached or ‘coupled’ to the G protein. The G protein is the location of the subsequent chemical processes that create the electrochemical signal that is relayed to the brain along the nervous system pathway.
How is the Message Transduced from Our Tongue to Our Brain?
Sweet substances bind to the G protein-coupled receptors at the cell surface. The sweet receptor is a heterodimer and is made up of two subunits T1R2 and T1R3. The two subunits are coupled to a gustducin(G) protein. Activation of the G protein by a sweet molecule binding to the receptor triggers a cascade of intracellular reactions:
- Production of the secondary messengers inositol trisphosphate (IP3) and diacylglycerol (DAG).
- The secondary messengers cause the release of Ca2+ out of intracellular stores.
- This causes an influx of Na+ ions into the cell which causes depolarisation.
- ATP is released.
- Action potentials are triggered in a nearby sensory neuron.
The brain then combines this information with other sensory information to perceive the flavour of what we have just eaten.
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Written by Charlotte Catignani, Research and Development Manager at Treatt