Humans have a very keen sense of smell: we can detect thousands of different smells. This ability relies on the presence of special sensory receptors in the upper part of the nose. When stimulated by odour molecules, these receptors send signals along nerves to the brain for processing. Sometimes odour molecules do not reach the sensory area, but sniffing will help get them there.
Smell receptors are specialized nerve cells. Each bears many tiny cilia (hairs), which project into the space in the upper part of the nose. A nerve fiber extends from the other end of each cell. This joins other fibres to form the olfactory nerves, which carry signals to the brain.
The cilia can detect tiny amounts of substances in the air, though molecules of those substances must first be absorbed by the mucus layer. There they interact with the cilia to trigger nerve impulses.
Taste
- We can taste substances in food and drink thanks to the 10,000 or so taste buds located on structures, called papillae, on the surface of our tongues. These receptors send signals along nerves to the brain for interpretation. Four main tastes – sweet, salty, sour, and bitter – are detected by the taste buds in four areas of the tongue. The senses of taste and smell combine to analyse flavours.
Papillae are tiny protrusions on the surface of the tongue. The fungiform papillae and some other types of papillae contain taste buds. The smaller, more numerous, filiform papillae do not contain taste buds but give the tongue a rough surface, which helps it move food around the mouth.
Hairs emerge from each receptor cell. Food and drink molecules must dissolve in saliva before they can interact with these hairs and trigger signals to the brain.
Hearing
Our ears allow us to detect sounds, which pass through the air as waves of varying pressure. On reaching the ear, the waves travel through several structures to the cochlea in the inner ear. There, receptor cells produce signals that go to the brain. The human ear can detect sounds over a very wide range of pitch and loudness, from the high-pitched squeaks of a mouse to the roar of a passenger jet.
The outer ear channels sound waves into the ear canal. These sound waves cause the eardrum, a thin membrane at the end of the ear canal, to vibrate. The vibrations are transmitted via three tiny bones in the middle ear to the cochlea in the inner ear.
Inside the cochlea, sound vibrations make these sensory hairs move, which triggers signals in attached receptor cells. The signals pass to the brain, which works out the pitch and loudness of the sound.
Balance
Balance is an internal sense and relies on sensory receptors that monitor the position of the head and body. Whether we are still or moving, balance is essential for maintaining our posture and stopping us falling over. The vestibule and semicircular canals of the inner ear provide information on the position and movements of the head. Combined with signals from the eyes, this helps us balance.
Turning movements of the head are picked up by sensory hair cells embedded in structures called cupulae in the semicircular canals. Tilting movements of the head, and its position, are monitored by hair cells within structures in the vestibule.
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