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Anatomy and Physiology Unit 8: Special senses

Specialized receptors in the eyes, ears, tongue and nose turn light, sound, motion and chemicals into nerve signals.

Covers: The eye and vision, the ear, hearing and balance, taste, smell, sensory receptors and pathways

Study guide

The special senses are vision, hearing, balance, taste and smell, each with specialized receptor cells. In the eye, light passes through the cornea (which does most of the focusing), the pupil (an opening controlled by the iris), and the lens, which changes shape to focus on near or far objects (accommodation) using the ciliary muscles. Light reaches the retina, where photoreceptors convert it into signals. Rods work in dim light and give black-and-white vision; cones need bright light and detect color, and they're packed in the fovea, the point of sharpest vision. Signals travel through the optic nerve to the occipital lobe. The blind spot (optic disc) has no photoreceptors. The sclera is the white outer layer, and the choroid supplies blood. Common problems include myopia (nearsightedness, the image focuses in front of the retina), hyperopia (farsightedness), cataracts (a cloudy lens) and glaucoma (high pressure in the eye).

The ear has three parts. The outer ear (pinna and ear canal) funnels sound to the tympanic membrane (eardrum). The middle ear contains the ossicles (malleus, incus and stapes), which amplify vibrations and pass them to the oval window; the Eustachian tube equalizes pressure. The inner ear contains the cochlea, where hair cells in the organ of Corti convert vibrations into nerve impulses carried by the vestibulocochlear nerve to the temporal lobe. Pitch depends on which part of the cochlea vibrates. The vestibular system (the semicircular canals for rotation and the utricle and saccule for linear motion and head position) senses balance.

Taste (gustation) and smell (olfaction) are chemical senses. Taste buds on the tongue detect five basic tastes: sweet, sour, salty, bitter and umami. Olfactory receptors in the upper nasal cavity detect odor molecules, and smell signals go to the olfactory bulb and connect directly to the limbic system, linking smells with emotion and memory. Much of what we call flavor comes from smell. Sensory receptors in general include mechanoreceptors (touch, sound), photoreceptors (light), chemoreceptors (taste, smell), thermoreceptors and nociceptors (pain). Sensory adaptation is a decreasing response to a constant stimulus.

Key ideas

• The cornea and lens focus light on the retina.
• Rods handle dim light; cones handle color and detail.
• The ossicles amplify sound; the cochlea converts it to nerve signals.
• Semicircular canals sense rotation and balance.
• Taste and smell are chemical senses; smell links to emotion and memory.

Formulas and rules

Common mistakes

• Thinking the lens does all the focusing; the cornea does most of it.
• Mixing up the cochlea (hearing) with the semicircular canals (balance).

Words to know

Cornea

The clear front of the eye that does most of the focusing.

Iris

The colored muscle that controls pupil size.

Lens

Flexible structure that changes shape to focus light.

Accommodation

Changing lens shape to focus on near or far objects.

Retina

The layer of photoreceptors at the back of the eye.

Rods

Photoreceptors for dim light and black-and-white vision.

Cones

Photoreceptors for color and fine detail.

Fovea

The part of the retina with the sharpest vision.

Optic disc

The blind spot where the optic nerve leaves the eye.

Myopia

Nearsightedness; images focus in front of the retina.

Glaucoma

Increased pressure in the eye that can damage the optic nerve.

Tympanic membrane

The eardrum.

Ossicles

Three tiny middle-ear bones: malleus, incus and stapes.

Eustachian tube

Tube connecting the middle ear to the throat to equalize pressure.

Cochlea

Snail-shaped inner-ear structure for hearing.

Organ of Corti

Structure in the cochlea with hair cells that detect sound.

Semicircular canals

Inner-ear canals that sense head rotation.

Gustation

The sense of taste.

Olfaction

The sense of smell.

Sensory adaptation

Reduced response to a constant stimulus.

Sample practice questions

5 more questions like these, from level 1 to 5, are in the practice set.