The Big Picture
Your five senses are sight, hearing, taste, smell, and touch. Your eyes, tongue, nose, and skin contain receptors connected with sight, taste, smell, and touch.
Your eyes provide sight. Your ears are involved in hearing and balance. Your tongue and nose contain receptors for taste and smell, while touch receptors are found in the skin.
Start Here If You’re Overwhelmed
Focus on these core ideas:
- The five senses are sight, hearing, taste, smell, and touch.
- The eye has three layers: the sclera, choroid, and retina.
- The ear has three main parts: the external ear, middle ear, and inner ear.
- The three middle-ear bones are the malleus, incus, and stapes.
- Taste receptors are found in taste buds, while smell uses olfactory receptors.
- The three semi-circular canals are involved in maintaining balance.
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Course Notes: Key Ideas You Need to Know
The Five Senses and Their Receptors
The five senses are:
- sight
- hearing
- taste
- smell
- touch
Organs are structures made of different tissues arranged to perform a specialized function.
Taste receptors are called taste buds. Smell receptors are found in the nose.
Touch receptors are located in the dermis of the skin. Their numbers vary throughout the body. Large numbers are found in the fingertips, toes, and lips.
The Eye and Sight
Receptors for vision are located in the eyes. The eye is easily injured.
Several structures protect the eyes:
- bones of the skull
- eyelids
- eyelashes
- tears
The eye has three layers: the sclera, choroid, and retina.
Sclera
The sclera is the white outer layer of the eye. It is made of tough connective tissue.
The anterior one sixth of this outer layer is the cornea.
Choroid
The choroid is the second layer of the eye. It includes:
- blood vessels
- the ciliary muscle
- the iris
The iris gives the eye its colour. The opening in the middle of the iris is the pupil.
Pupil size changes according to the amount of light entering the eye.
- In bright light, the pupil contracts, or narrows.
- In dim or dark places, the pupil dilates, or widens.
Retina
The retina is the inner layer of the eye. It contains receptors for vision and nerve fibres of the optic nerve.
How Light Travels Through the Eye
Light enters the eye through the cornea. The cornea is the transparent part of the eye’s outer layer.
Light rays then pass to the lens, which lies behind the pupil. The light is reflected to the retina.
The optic nerve carries the information from the retina to the brain. The brain interprets what is being seen.
Fluids Inside the Eye
The aqueous chamber is between the cornea and the lens. It contains a fluid called aqueous humor.
Aqueous humor helps the cornea keep its shape and position.
The vitreous humor is found behind the lens. The vitreous body is a gelatin-like substance that supports the retina and helps maintain the eye’s shape.
The Ear, Hearing, and Balance
The ear functions in both hearing and balance.
It has three main parts:
- external ear
- middle ear
- inner ear
External Ear
The external ear is also called the pinna or auricle.
Sound waves travel through the external ear into the auditory canal. Glands in the auditory canal produce a waxy substance called cerumen.
Cerumen catches particles that enter the ear.
The auditory canal extends about 2.5 cm (1 in.) to the eardrum. The eardrum is also called the tympanic membrane.
The eardrum separates the external ear from the middle ear.
Middle Ear
The Eustachian tube, also called the auditory tube, connects the middle ear and the throat.
Air enters this tube from the throat. This keeps pressure equal on both sides of the eardrum.
The middle ear contains three small bones called ossicles. They amplify sound received from the eardrum and transmit it to the inner ear.
The three ossicles are:
- Malleus, which looks like a hammer.
- Incus, which resembles an anvil.
- Stapes, which is shaped like a stirrup.
Inner Ear
The inner ear contains the cochlea and semi-circular canals.
The cochlea looks like a snail shell and contains fluid. This fluid carries sound waves received from the middle ear to the auditory nerve.
The auditory nerve carries the message to the brain. The brain then interprets what is being heard.
The inner ear also has three semi-circular canals. They sense the position of the head and changes in its position.
The semi-circular canals send these messages to the brain and are involved in maintaining balance.
The Tongue and Taste
Taste buds contain receptors for taste.
Taste buds are located around small structures found on:
- the upper surface of the tongue
- the soft palate
- the upper esophagus
- the epiglottis
Taste buds send sensations to the brain for interpretation.
The four basic taste sensations are:
- salty
- sweet
- sour
- bitter
Receptors for all four sensations are found throughout the tongue.
The anterior two-thirds of the tongue is controlled by the facial nerve, or cranial nerve CN VII.
The posterior third is controlled by the glossopharyngeal nerve, or CN IX.
The Nose and Smell
Smell receptors are located on the roof of the nasal cavity. These receptors are called olfactory receptors.
The sense of smell is controlled by the olfactory nerve, or CN I.
Fluid in the mucous membrane of the nose stimulates the olfactory receptors. Impulses then travel to the brain, where they are interpreted as smell.
Smell and taste are closely connected. When the nose becomes congested during a cold, foods often do not taste the same.
Fast Review
Use these points for a fast review before your quiz or exam.
- The five senses are sight, hearing, taste, smell, and touch.
- Touch receptors are found in the dermis, with large numbers in the fingertips, toes, and lips.
- The eye has three layers: sclera, choroid, and retina.
- The pupil contracts in bright light and dilates in dim or dark places.
- Light enters through the cornea, passes to the lens, reaches the retina, and information travels through the optic nerve to the brain.
- The ear has three parts: external ear, middle ear, and inner ear.
- The three ossicles are the malleus, incus, and stapes.
- The cochlea is involved in hearing, while the three semi-circular canals are involved in balance.
- The four basic taste sensations are salty, sweet, sour, and bitter.
- Smell uses olfactory receptors and the olfactory nerve, CN I.
- Taste involves the facial nerve, CN VII, and the glossopharyngeal nerve, CN IX.
- Smell and taste are closely connected.
Final Pep Talk
There are many unfamiliar structure names in this topic, so group them by sense instead of learning everything at once. Review the eye parts, ear parts, taste, smell, and touch in small groups until the names feel familiar.