A3: Perception of stimuli

Lesson one: The human eye


Receptor cell: a cell which is capable of sensing stimuli from the environment. Common Receptor cells include thermoreceptors, chemoreceoptors, mechanoreceptors, and photoreceptors.

Rods and cone cells: Are photoreceptors in the human eye (rods detect more efficiently in dim light, and more black and white. cones detect more in colour, and more in bright light)

Ganglion cell: A cell which conducts electricity to the optic nerve, in the eye.

Optic nerve: A nerve which runs to the brain (to the visual processing centre, in the occipital lobe). 


Receptors are cells that can detect stimuli from the environment, creating an electrical impulse which passes along a sensory neurone. Make a table for the ´five senses´, that you would have heard of, and determine through discussion the sense organ involved, the relevant receptor cells (if you know them), and the kind of receptor cell they are.

Kinds of receptors:

  • Thermoreceptors, detect heat.
  • Photoreceptors, detect light
  • Mechanoreceptors, detect forces eg. pressure, or stretching
  • Chemoreceptors detect chemicals

Function of the human eye

Develop a table stating the function of the following parts of the eye: lens, choroid, retina, sclera, pupil, ciliary body, aqueous and vitreous humor, iris. Generate this through the video discussion. 

Most of the eye is designed merely to direct and focus light onto the retina. The retina actually detects light.

The retina

Task: Watch the video, and draw flow charts to show the pathway of light, and the pathway of an electrical impulse, in the retina.


Rods and Cone cells are different photoreceptors

Cones: Distinguish colours, function better in bright light as they are less light sensitive. Exist as either red, blue, or green cones.

Rods: Do not distinguish colours, are more light sensitive.

Discussion Q. Which photoreceptors are more useful at night, and which are more useful during the day?

The fovea - the area of the retina that contains the greatest density of rods and cones

Rhodopsin is a pigment that absorbs light, and is broken down in the process. If a light is shone in your eye, the rhodopsin is completely broken down (especially in rod cells), and this causes you temporary blindness as without rhodopsin light stimulus cannot be detected.

 Visual processing

The visual processing centre in the occipital lobe processes the information from both eyes to create an image (it needs to be flipped and cross-referenced). The brain is interpreting information – how do we know if we see the same thing?

Lesson 2: the Ear


  • Outer ear (funnels sound), Middle ear (transmits sound), Inner ear (detects sound, and head position)
  • Sound: vibrations in air or other materials
  • Cochlea: part of inner ear that contains receptor cells (mechanoreceptors)
  • Auditory nerve: the nerve which carries information from the cochlea to the brain

Introduction: Watch the video and discuss.

The human ear can be seen below:

Sound waves are captured by the outer ear, and funneled into the middle ear. In the middle ear, sound is transmited and amplified by the smallest bones in the body (malleus, incus, stapes. Stapes is roughly 3 x 2.5mm long). Sound is detected by the cochlea.  The auditory nerve transmits information related to sound perception to the brian.

Task: Make flow charts for the movement of sound vibrations in the ear through the following media:

  • Air
  • Solid
  • Liquid


Mini-task Sound represents energy. It has been popularly reported that eggs can be cooked at a rock concert. Is this true? Investigate and debate, presenting an argument, with evidence, and the source of your evidence. 

The perception of Sound

The cochlea contains fluid, that is incompressible and so vibrates according to the vibrations arriving from the middle ear. Inside the cochlea there are sensory hairs which detect the vibrations of this fluid. The hairs are specific to different frequencies or wavelength of sound vibrations (which translates to pitch), and in this way the perception of different pitches is distinguished. If the sound is louder, more cells vibrate.

The auditory receptor cells attached to the sensory hairs, convert the vibrations into a nerve impulse that is transmitted to the brain by the auditory nerve.

transverse wave diagram Elegant Transverse Wave Diagram To Label

The amplitude controls the volume. The wavelength is related to frequency, as a lower wavelengtrh allows for a higher frequency.

Why are people hearing impaired?

There are two possible causes of hearing impairment:

  • Obstruction or failure of the auditory conduction system. In other words, sounds do not reach the inner ear.
  • Lack of sensitivity of inner ear. In other words, failure of the sensory hairs in the cochlea.

If the cochlea is not working, a cochlear implant can help by replacing the job of the cochlea in the ear.

Detecting movement in the head

There is a similar system in involving fluid and sensory hairs in the semi-circular canals in the inner ear. Instead of receiving sound vibrations, they perceive the position of the head.

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