Taste buds and the receptors of the skin – General Principles of Reception and Response in Animals (Senior 6 Biology) | Ganzaa

General Principles of Reception and Response in Animals

Lessons

Senior 6 · Biology

General Principles of Reception and Response in Animals

5 lessons

Lesson 5 of 5

Taste buds and the receptors of the skin

25 min

Bitter warnings and sensitive fingertips

A child who tastes a crushed leaf of a bitter plant spits it out at once; the same child will happily eat a ripe mango. Taste is an ancient safety check: it rewards food that gives energy and minerals and rejects much of what could poison us. The skin, meanwhile, is the body's largest sense organ, with several kinds of receptor packed far more densely in the fingertips and lips than on the back. This lesson studies the chemoreceptors of the tongue and the mechanoreceptors, thermoreceptors and nociceptors of the skin, and ends the unit.

Definition

Taste bud: a small, flask-shaped group of 50 to 150 taste receptor cells with supporting and basal cells, opening at the surface through a taste pore.

Papillae: small projections on the upper surface of the tongue. Most taste buds lie on the fungiform, foliate and circumvallate papillae; the numerous filiform papillae have none.

Gustation: the sense of taste. Olfaction: the sense of smell.

Two-point threshold: the smallest distance between two points touching the skin at which they are felt as two separate points.

Structure of a taste bud

How a taste is detected. Chemicals in food dissolve in saliva, enter the taste pore and bind to receptor proteins, or pass through ion channels, in the membranes of the microvilli. The receptor cell depolarises (a generator potential) and releases neurotransmitter onto a sensory neurone, which carries impulses to the brain. Taste receptor cells are secondary receptors, and they live only a week or two before basal cells replace them.

Basic taste	Typical stimulus	How the receptor cell is depolarised	Likely value to the animal
Salty	Na⁺ ions	Na⁺ enters through ion channels	Finding sodium, needed for nerves and water balance
Sour	H⁺ ions (acids)	H⁺ enters, or blocks K⁺ channels	Detecting unripe or spoiled food
Sweet	Sugars	Binding to a membrane receptor protein starts a second-messenger cascade	Finding energy-rich food
Bitter	Many plant poisons, such as alkaloids	Receptor protein and second messenger	Avoiding poisons
Umami	Glutamate (an amino acid)	Receptor protein and second messenger	Finding protein-rich food

Every part of the tongue that has taste buds can detect all five tastes; the old "tongue map" with separate areas is wrong. Much of what we call the flavour of food is really smell: odour molecules from food in the mouth reach the olfactory receptors in the roof of the nasal cavity. That is why food seems tasteless when you have a blocked nose.

Worked example 1: why are we so sensitive to bitterness?

Illustrative detection thresholds for one person: sucrose (sweet) $10\,\mathrm{mmol\,dm^{-3}}$ ; quinine (bitter) $0.008\,\mathrm{mmol\,dm^{-3}}$ . (a) How many times more sensitive is this person to quinine than to sucrose? (b) Suggest an explanation.

(a) $\frac{10}{0.008} = 1\,250$ . Quinine is detected at a concentration about 1 250 times lower.
(b) Many bitter substances are plant poisons. An animal that detects a poison at a very low concentration rejects the food before it swallows a harmful dose, so very sensitive bitter receptors give a strong survival advantage. Sugar is harmless, and is only worth seeking when there is plenty of it, so a high threshold costs little.

Receptors in the skin

Receptor	Position	Stimulus	Adaptation
Meissner's corpuscle	Dermal papillae, just below the epidermis; many in fingertips, lips and palms	Light touch and slow vibration	Rapid (phasic)
Merkel disc	Base of the epidermis	Steady light pressure; edges and texture	Slow (tonic)
Pacinian corpuscle	Deep dermis and fat layer	Deep pressure and fast vibration	Very rapid (phasic)
Ruffini ending	Dermis	Stretching of the skin	Slow (tonic)
Hair follicle receptor	Wrapped round the base of each hair	Movement of the hair	Rapid (phasic)
Free nerve endings	Dermis, reaching into the epidermis	Heat, cold (thermoreceptors) and damage (nociceptors)	Mostly slow

Why fingertips are so sensitive. Each sensory neurone collects from a patch of skin, its receptive field. In the fingertips, receptors are packed densely and the receptive fields are small, so two points only 2 to 3 mm apart stimulate different neurones and are felt as two. On the back, receptive fields are large and two points must be several centimetres apart to be told apart. The brain also gives more cortex to the fingertips and lips than to the back.

Worked example 2: a two-point threshold investigation

Students touched a blindfolded partner with two blunt pins at decreasing distances and recorded the smallest distance felt as two points (illustrative class means): fingertip 2.5 mm; palm 10 mm; forearm 35 mm; back 50 mm.

Compare: the threshold on the back is $\frac{50}{2.5} = 20$ times that on the fingertip.
Explain: the fingertip has a high density of touch receptors (Meissner's corpuscles and Merkel discs) and small receptive fields, so two nearby points stimulate separate sensory neurones. On the back, one large receptive field can contain both points, which then feel like one.
Evaluate the method: the partner must not see the pins; one-point and two-point touches should be mixed at random, so that guessing can be detected; and each site should be tested several times and a mean taken.

Warning

Common mistakes

"The tip of the tongue tastes sweet and the back tastes bitter." All taste buds respond to all five basic tastes.
"Taste buds are the bumps on the tongue." The bumps are papillae; taste buds are microscopic and lie in the walls of most papillae.
"Hot chilli is a taste." The burning of chilli comes from capsaicin stimulating heat- and pain-sensitive free nerve endings, not taste receptors.

Alila Medical Media: "Taste: Anatomy and Physiology, Animation". Watch for the structure of a taste bud and how each basic taste depolarises the receptor cells.

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Practice: taste and skin receptors

Name the five basic tastes.
Describe the route by which a dissolved chemical in food produces impulses in a sensory neurone from a taste bud.
Explain why taste receptor cells are described as secondary receptors.
Name the skin receptor that detects (a) deep pressure, (b) heat, (c) movement of a hair.
A person's two-point threshold is 3 mm on the lip and 42 mm on the thigh. Calculate the ratio, and explain the difference.
Explain why food seems to have little flavour when a person has a cold with a blocked nose.
Suggest why it is an advantage for the receptors in a fingertip to include both phasic and tonic types.
Evaluate the claim: "Bitter taste is useless to humans because we cook our food."

Show answersHide answers

Sweet, sour, salty, bitter and umami.
The chemical dissolves in saliva, enters the taste pore and binds to receptor proteins (or passes through ion channels) on the microvilli; the receptor cell depolarises and releases neurotransmitter across a synapse; the sensory neurone is depolarised to threshold and carries impulses to the brain.
They are separate cells, not neurone endings; they pass their signal across a synapse to a sensory neurone.
(a) Pacinian corpuscle. (b) Free nerve endings (warm thermoreceptors). (c) Hair follicle receptor.
$\frac{42}{3} = 14$ . The lip has many more touch receptors per unit area and smaller receptive fields, and a larger area of sensory cortex.
Much of flavour is smell. Mucus stops odour molecules from food reaching the olfactory receptors in the nose, so only the five basic tastes remain.
Phasic receptors (Meissner's, Pacinian) signal the moment of contact, slipping and vibration, which helps grip; tonic receptors (Merkel discs) keep signalling steady pressure and shape while an object is held.
The claim is weak. Cooking destroys some poisons but not all; many medicines, spoiled foods and wild plants are bitter, so the warning is still useful. On the other hand, bitterness also makes some healthy vegetables less liked, so its value today is smaller than in the past.

Key points

Taste buds lie mainly in the papillae; dissolved chemicals bind to the microvilli of taste receptor cells, which depolarise and stimulate sensory neurones.
The five basic tastes are sweet, sour, salty, bitter and umami; flavour also depends on smell.
Skin receptors: Meissner's corpuscles (light touch), Merkel discs (steady touch), Pacinian corpuscles (pressure, vibration), Ruffini endings (stretch), hair follicle receptors (hair movement), free nerve endings (heat, cold, pain).
Touch acuity depends on receptor density and receptive field size, measured as the two-point threshold.

1Bitter warnings and sensitive fingertips

2Structure of a taste bud

3Receptors in the skin

Bitter warnings and sensitive fingertips

Structure of a taste bud

Receptors in the skin