AP Biology - Plants and Human Body Systems

OpenStax Biology for AP — Chapter 27: Sensory Systems — AP BIOLOGY Revision Notes & Topical Questions

5 min read 8 parts Lesson 8 of 15 Unit quiz

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Lesson summary

OpenStax Biology for AP — Chapter 27: Sensory Systems — AP BIOLOGY notes: Describe sensory transduction, receptor types, the eye, hearing and balance, and receptor adaptation.Sensory transduction = conversion of a stimulus into an electrical signal by a sensory receptor.The stimu…

Perception
Humans detect five tastes
Receptor potential
Photoreceptor
Chemoreceptor
Mechanoreceptor

Part 1 of 6From stimulus to signal

Sensory transduction

Sensory transduction conversion of a stimulus into an electrical signal by a sensory receptor.

The stimulus changes receptor membrane permeability, producing a graded receptor potential.

If the potential reaches threshold, action potentials pass to the central nervous system.

Stronger stimuli give a higher frequency of impulses, not larger impulses.

Perception

Perception the brain's interpretation of a sensation.

Part 2 of 6Types of receptor

Receptor	Stimulus	Example
Photoreceptor	Light	Rods and cones in the retina
Mechanoreceptor	Touch, pressure, sound, balance	Hair cells of the cochlea
Chemoreceptor	Chemicals	Taste buds, smell receptors, CO₂ sensors
Thermoreceptor	Temperature	Skin receptors
Pain receptor	Tissue damage	Free nerve endings

Part 3 of 6Taste and smell

Taste and smell

Humans detect five tastes

Humans detect five tastes sweet, sour, bitter, salty and umami — each with its own receptor type.

Olfactory receptors each respond to one odorant, and odorants dissolve in nasal mucus.

Olfactory signals pass to the olfactory bulb and cortex, whereas other senses are routed through the thalamus.

Part 4 of 6The eye

Light is focused by the cornea and lens onto the retina.

Rods work in dim light and give black and white vision; cones work in bright light and give colour vision.

Cones come in three forms (L, M and S) sensitive to different wavelengths and are concentrated in the fovea.
Pigments change shape when they absorb light.

The iris controls the amount of light: in bright light the pupil constricts, and in fight or flight the sympathetic division dilates it.

Signals leave the eye along ganglion cell axons in the optic nerves.

Part 5 of 6Hearing and balance

Hearing and balance

Sound waves reach the pinna, ear canal and tympanum, whose vibrations pass to the middle ear bones.

The stapes transfers energy to the oval window and the fluid-filled cochlea, where the basilar membrane flexes and bends hair cell stereocilia.

The utricle and saccule detect head orientation, and the semicircular canals detect rotation, using hair cells.

Part 6 of 6Adaptation and integration

Adaptation and integration

With a constant stimulus the response declines, so the nervous system responds mainly to change.

The brain integrates information from receptors and produces a response.

Chemoreceptors in the brainstem and arteries respond to blood CO₂ and pH, which sets the breathing rate through the medulla oblongata.

Key terms

Term	Meaning
Sensory transduction	Conversion of a stimulus into an electrical signal.
Receptor potential	Graded change in membrane potential in a receptor.
Photoreceptor	Receptor that detects light.
Chemoreceptor	Receptor that detects chemicals.
Mechanoreceptor	Receptor that detects touch, pressure, sound or balance.
Sensory adaptation	Decline in response to a constant stimulus.

Avoid these mistakes

Saying stronger stimuli produce bigger action potentials — they produce more frequent ones

Mixing up rods and cones — rods work in dim light and cones give colour

Treating all receptors as detecting light — receptors are specific to their stimulus

Forgetting that receptors adapt to constant stimuli

Last updated: 18 October 2026