12.1 Essential ideas

12.1.3 Perception of stimuli

  • Living organisms are able to detect changes in the environment.
  • The eye and the ear are two important sensory organs.

Sensory receptors detect changes in the environment

Mechanoreceptors (sensitive to touch, changes in air/liquid pressure, or gravity)

  • Touch receptors in the dermis of skin
  • Sensory hair cells in the cochlea and semi-circular canals (see below)
  • Baroreceptors in the aorta and carotid arteries
  • Stretch receptors in the muscles

Chemoreceptors (sensitive to concentration of chemicals or pH)

  • Taste buds of the tongue
  • Osmoregulatory centre of the hypothalamus
  • Olfactory epithelium of the nose
  • Chemoreceptor cells of the carotid artery

Thermoreceptors (heat sensitive)

  • Temperature centre of the hypothalamus
  • Nerve endings in the dermis of skin

Photoreceptors (light sensitive)

  • Rod and cone cells of the retina (see below)

Rods and cones differ in sensitivity to light intensities and wavelength

Figure 12.1.3a - Absorption spectra for rods and conesFigure 12.1.3a - Absorption spectra for rods and cones





High (respond to lower intensities of light)

Low (respond to higher intensities of light)


Broad range of wavelengths (1 type)

Wavelength ranges corresponding to blue, red and green light (three different types)

Type of vision

Black and white



Skill: Annotate a diagram of the retina

Figure 12.1.3b - Structure of the retinaFigure 12.1.3b - Structure of the retina

  • Download, print and annotate the diagram of the retina. Check your answers by comparing them to Figure 12.1.3b.
  • Be sure to include the:
    • direction that light travels through the retina
    • functions of rods, cones, bipolar cells, ganglion cells.

Bipolar cells send the impulses from rods and cones to ganglion cells

  • Bipolar cells are neurons that synapse with rods and cones at one end and ganglion cells at the other.
  • Rods are connected to bipolar cells in groups, whereas each cone is connected to a single bipolar cell.

Ganglion cells send messages to the brain via the optic nerve

  • The cell bodies of ganglion cells are in the retina, and the axons of ganglion cells extend into the optic nerve.

Skill: Label a diagram of the human eye

Figure 12.1.3c - Structure of the human eye (with eyelid) in cross-sectionFigure 12.1.3c - Structure of the human eye (with eyelid) in cross-section

The information from the right field of vision from both eyes is sent to the left part of the visual cortex and vice versa

  • This is called contralateral processing: each eye receives a flat and slightly different image of the field of view.
  • The visual cortex in each hemisphere superimposes the images from each eye, creating depth perception, or stereoscopic vision.

Figure 12.1.3d - Contralateral processing in the human eyeFigure 12.1.3d - Contralateral processing in the human eye

  • The optic chiasma is the point in the brain where the neurons carrying signals from the opposite field of view switch hemispheres.

Skill: Label a diagram of the human ear

Figure 12.1.3e - Structure of the human earFigure 12.1.3e - Structure of the human ear

Outer ear

• Pinna

• Auditory canal

Middle ear

• Ear drum

• Bones of the middle ear (malleus, incus, stapes)

• Round window

• Oval window

• Eustachian tube

Inner ear (fluid-filled)

• Semi-circular canals

• Cochlea

• Auditory nerve


Structures of the middle ear transmit and amplify sound

  • Sound waves cause air pressure changes.
  • The ear drum vibrates when hit by a sound wave. The membrane extends and pushes against the first of three bones in the middle ear.
  • The first bone hits the second, the second hits the third. The three bones act as levers to amplify the sound.
  • The last bone strikes the oval window. By this time, the sound has been amplified by about twenty times.

Sensory hairs of the cochlea detect sounds at specific wavelengths

  • When struck, the oval window compresses liquid in the cochlea.
  • A portion of the cochlear membrane is made of hair cells that are sensitive to the resulting pressure changes.
  • Different regions of this membrane are stimulated by specific wavelengths.

Impulses caused by sound perception are transmitted to the brain via the auditory nerve

  • Nervous input from the hair cells of the cochlea converge in the auditory nerve.
  • The auditory nerve connects to the auditory cortex in the temporal lobe of the brain.
  • Information from the cochlear hair cells allows the auditory cortex to perceive sound waves as different pitches.

Figure 12.1.3f - Structural detail of the inner ear

Figure 12.1.3f - Structural detail of the inner ear

Hair cells in the semicircular canals detect head movements

  • Hair cells located in fluid-filled semi-circular canals are oriented on three different planes, each perpendicular to the others.
  • Head movements are detected by the relative amounts of stimulation of hair cells in the ampulla of each canal.

Figure 12.1.3g – Sensory receptors are cells or neuronsFigure 12.1.3g – Sensory receptors are cells or neurons

Key questions

  • Name types of sensory receptors.
  • Distinguish between rods and cones.
  • Explain the roles of photoreceptors in vision and mechanoreceptors in hearing.

Figure 12.1.3h – Blind spot test

Try it! 

Cover your right eye. Keep looking at the black dot as you move closer to the screen. The plus sign disappears when you reach a certain distance. You have just found the blind spot in your left eye!

Food for thought

  • The blind spots of the left and right eyes are at slightly different distances. Discuss the significance of this fact.
  • The cochlea is about the size of a pea. The structures in Figure 12.1.3e are not to scale.

12.1.3iFigure 12.1.3i – Keep spinning


Why does your head keep ‘spinning’ after getting off a playground roundabout? Why does the effect worsen with age?

Concept help

  • The Eustachian tube is not involved in hearing. It is connected to the throat and its role is to equalise pressure in the middle ear.
  • Pitch is determined by the frequency of a sound. Wavelength and frequency are related properties of waves.
  • The flexible round window allows fluid to move in the cochlea.

Course link

Balance is maintained by the autonomic nervous system and coordinated by the medulla. Review the parts of the brain in 12.1.2.