Sense Organs and Homeostatic Mechanisms

Lessons

Senior 3 · Biology

Sense Organs and Homeostatic Mechanisms

5 lessons

Lesson 3 of 5

The ear, and the senses of smell and taste

25 min

A drum in your head

On Heroes' Day a school band marches past, and you feel the big drum in your chest before you can tell which street it is in. The drum's skin vibrates, the air vibrates, and a few metres away a small, tight membrane in each of your ears, your eardrum, vibrates too. From that tiny movement your brain works out the rhythm, the loudness and even the direction of the band. The same ear also tells you which way up you are. This lesson looks at the ear, and then at two senses that detect chemicals: smell and taste.

Definition

Sound: vibrations that travel through a medium such as air, water or bone. The pitch depends on how many vibrations there are each second (frequency); the loudness on their size.

Ossicles: the three tiny bones of the middle ear: the hammer (malleus), anvil (incus) and stirrup (stapes).

Cochlea: the coiled, fluid-filled tube of the inner ear that contains the receptor cells for hearing (sensory hair cells).

Semicircular canals: three fluid-filled loops of the inner ear, at right angles to each other, that detect movements of the head and help with balance.

Taste bud: a small group of receptor cells, mainly on the tongue, that detect chemicals dissolved in saliva.

Figure 1: the parts of the human ear

Figure 1: simplified; not to scale. The middle ear is filled with air; the cochlea and semicircular canals are filled with fluid.

How you hear

The pinna collects sound waves and directs them down the ear canal.
The waves make the eardrum vibrate.
The ossicles pass the vibrations across the middle ear to the oval window. Because the eardrum is much bigger than the oval window, and the bones act as levers, the pressure of the vibrations is increased (amplified).
The oval window makes the fluid in the cochlea vibrate. The round window bulges out and in to let the fluid move.
The vibrating fluid bends the sensory hair cells in the cochlea, which start nerve impulses.
The auditory nerve carries the impulses to the brain, which interprets them as sound.

Balance and the Eustachian tube

When you turn or tilt your head, the fluid in the semicircular canals lags behind and bends receptor cells, which send impulses to the brain. Because the three canals are at right angles, movement in any direction is detected. Other parts of the inner ear detect which way gravity pulls, so you know which way up you are even with your eyes shut. The cerebellum uses this information to keep you balanced (Unit 9).

The Eustachian tube joins the middle ear to the throat. It opens when you swallow or yawn, so the air pressure on both sides of the eardrum stays equal. Your ears "pop" when it opens, for example when a bus climbs from the Nyabarongo valley up to Kigali.

Caring for your ears

Loud sound damages the hair cells, and they do not grow back. The World Health Organization's safe-listening standard allows about 80 decibels for at most 40 hours a week for adults, and 75 decibels for children. Keep earphones at a low volume and take breaks.
Do not push cotton buds, pens or matchsticks into the ear canal: they push wax deeper and can tear the eardrum.
Get ear infections treated, especially in children: untreated infections can damage hearing.

Smell

In the roof of the nasal cavity is a small patch of olfactory epithelium containing smell receptors. Molecules in the air dissolve in the mucus there and bind to receptor cells, which send impulses along the olfactory nerve to the brain. Receptors adapt: after a few minutes in a room you stop noticing its smell. Smell warns you of smoke, rotten food and leaking cooking gas.

Taste

Taste buds lie mainly on the tongue, in the sides of the small bumps (papillae) on its surface. A substance can be tasted only when it has dissolved in saliva. There are five basic tastes: sweet, salty, sour, bitter and umami (savoury, as in meat broth). The old "tongue map", with each taste in its own zone, is wrong: every part of the tongue that has taste buds can detect all five. What we call the flavour of food is taste and smell together, which is why food seems tasteless when you have a cold.

Worked example 1: why three small bones?

Typical values: the eardrum has an area of about 55 mm², the oval window about 3.2 mm². Assume the ossicles pass the same force from one to the other. By how many times is the pressure increased?

Pressure $= \frac{\text{force}}{\text{area}}$ . With the same force, pressure is inversely proportional to area.
Ratio $= \frac{55\ \text{mm}^2}{3.2\ \text{mm}^2} \approx 17$ .
So the pressure at the oval window is about 17 times greater. The lever action of the bones adds a little more. This amplification is needed because it is much harder to make a liquid (the fluid of the cochlea) vibrate than air.

Worked example 2: taste or smell?

Ten blindfolded students are given small pieces of pineapple, mango, banana, passion fruit and tree tomato, once with the nose pinched and once with it open (illustrative results). With the nose pinched they name 18 of the 50 pieces correctly; with it open, 46 of 50.

Percentages: $\frac{18}{50} \times 100 = 36\%$ ; $\frac{46}{50} \times 100 = 92\%$ .
Explain: with the nose pinched, air carrying the fruits' smell molecules cannot reach the olfactory epithelium, so the students rely on taste alone: the fruits are all sweet and a little sour, so they are hard to tell apart.
Conclude: most of what we call the flavour of a fruit comes from smell.
Fair test: pieces should be the same size and temperature, and the order should be random. Safety: check first that no one is allergic to any of the fruits, and use clean food prepared in a kitchen, never anything from the laboratory.

Warning

Common mistakes. (1) Saying the eardrum sends impulses to the brain: the receptors are the hair cells in the cochlea. (2) Putting the semicircular canals in charge of hearing: they are for balance. (3) Writing that each part of the tongue detects only one taste: all five can be detected all over. (4) Forgetting that substances must dissolve (in mucus or saliva) to be smelled or tasted.

The science of hearing, Douglas L. Oliver, TED-Ed. Watch for the path of a sound from the eardrum through the ossicles to the hair cells of the cochlea; compare it with Figure 1.

Tip

Memory hook: "HAS" goes in order. Hammer, Anvil, Stirrup: from the eardrum inwards to the oval window, in alphabetical order.

✦

Practice: the ear, nose and tongue

List, in order, the structures that sound passes through from the pinna to the auditory nerve.
What is the function of (a) the ossicles, (b) the Eustachian tube, (c) the semicircular canals?
Why are there three semicircular canals, at right angles to each other?
Explain why your ears may "pop" in a bus going up a steep hill.
Why can you not taste a dry sugar crystal on a tongue that has just been dried with a clean tissue?
An eardrum of area 60 mm² passes vibrations to an oval window of area 3.0 mm². By what factor is the pressure increased, if the force is the same?
Explain why loud music through earphones can cause permanent deafness.
A person with a heavy cold says food "has no taste". Explain.

Show answersHide answers

Pinna → ear canal → eardrum → hammer → anvil → stirrup → oval window → fluid of the cochlea → hair cells → auditory nerve.
(a) Pass vibrations from the eardrum to the oval window and amplify them; (b) keep the air pressure equal on both sides of the eardrum; (c) detect movements of the head, for balance.
So that movement of the head in any direction (any plane) is detected.
The air pressure outside falls as the bus climbs; when you swallow, the Eustachian tube opens and equalises the pressure on both sides of the eardrum, which moves with a pop.
Taste receptors detect only substances dissolved in saliva; the dry crystal cannot dissolve.
$\frac{60}{3.0} = 20$ times.
Very loud vibrations damage the sensory hair cells in the cochlea, and these do not grow back.
Mucus blocks air from reaching the smell receptors, and flavour is mostly smell, so only the basic tastes are left.

Key points

Pinna → ear canal → eardrum → hammer, anvil, stirrup → oval window → cochlea (hair cells) → auditory nerve.
The ossicles amplify the vibrations; the Eustachian tube equalises pressure; the semicircular canals detect head movement.
Smell receptors in the roof of the nose and taste buds on the tongue detect dissolved chemicals; flavour is taste plus smell.
Loud sound permanently damages the cochlea's hair cells.

Sources and credits

World Health Organization, Deafness and hearing loss (fact sheet) and the WHO–ITU safe-listening standard, for the safe-listening levels.
Video: The science of hearing, Douglas L. Oliver, TED-Ed, embedded from YouTube.

Written by Ganzaa: the text, Figure 1, the worked examples (typical values; Worked example 2 is illustrative) and the practice questions.

1A drum in your head

2Figure 1: the parts of the human ear

3How you hear

4Balance and the Eustachian tube

5Caring for your ears

6Smell

7Taste