Perception of stimuliOption E.2

Assessment statements• E.2.1 Outline the diversity of stimuli that can be

detected by human sensory receptors, including mechanoreceptors, chemoreceptors, thermoreceptors and photoreceptors.

• E.2.2 Label a diagram of the structure of the human eye.

• E.2.3 Annotate a diagram of the retina to show the cell types and the direction in which light moves.

• E.2.4 Compare rod and cone cells.• E.2.5 Explain the processing of visual stimuli,

including edge enhancement and contralateral processing.

• E.2.6 Label a diagram of the ear.• E.2.7 Explain how sound is perceived by the ear,

including the roles of the eardrum, bones of the middle ear, oval and round windows, and the hair cells of the cochlea.

Sensory receptors and diversity of stimuli• We have learned to link certain

tastes, sights and sounds with emotions

• Sensory cells send messages to certain parts of the brain that control emotion and memory

• Nerve impulses arriving at the brain results in sensation

Mechanoreceptors• Stimulated by mechanical force or

some type of pressure• Sense of touch is due to pressure

receptors• Pressure receptors are also found

in:– Arteries (detect change in blood

pressure)– Lungs (stretch receptors respond to

the degree of lung inflation)– Arms and legs (proprioceptors which

tell us position and help maintain posture)

– Inner ear (pressure receptors sensitive to waves of fluid moving over them)

Chemoreceptors• Respond to chemical substances• Allows us to taste and smell• Found in blood vessels and

monitor pH• Pain receptors respond to

chemicals released by damaged tissues

Thermoreceptors• Respond to change in temperature

Photoreceptors• Respond to light energy• Found in our eyes• Rod cells respond to dim light

resulting in black and white vision• Cone cells respond to bright light

and give us color vision

TOK• To what extent are we dependent

on technology for our knowledge of biology?

• http://physics.ucsd.edu/neurophysics/publications/New_Scientist.pdf

Structure of the human eye

pupil

Suspensory ligament

Ciliary muscle

Posterior chamber

fovea

Functions of parts of the eyePart Function

Iris Regulates the size of the pupilPupil Admits lightRetina Contains receptors for visionAqueous humour Transmits light rays and supports the eyeballVitreous humour Transmits light rays and supports the eyeballRods Allow black and white vision in dim lightCones Allow color vision in bright lightFovea An area of densely packed cone cells where vision is most

acuteLens Focuses the light raysSclera Protects and supports the eyeballCornea Focusing begins hereChoroid Absorbs stray lightConjunctiva Covers the sclera and cornea and keeps eye moistOptic nerve Transmits impulses to the brainEye lid Protects the eye

Cow eye dissection• http://www.exploratorium.edu/lear

ning_studio/cow_eye/

The retina• Vision begins when

light enters the eye and is focused on the photoreceptor cells of the retina

• Both rods and cones synapse with their own bipolar neurons

• Each bipolar neuron synapses with a ganglion cell

• Axons of the ganglion cell make up the optic nerve which carries the message of vision to the brain

Annotation of retina diagram• Rods are photoreceptor

cells which are sensitive to light and function well in dim light; synapse with a bipolar neuron

• Cones are photoreceptor cells which are activated by bright light; synapse with a bipolar neuron

• Bipolar neurons are cells in the retina which carry impulses from a rod or a cone to a ganglion cell of the optic nerve; called bipolar b/c they each have two processes extending from the cell body

• Ganglion cells are the cell bodies of the optic nerve; synapse with the bipolar neurons and send the impulses to the brain

Comparison of rods and conesRods ConesCells are more sensitive to light and function well in dim light

Cells are less sensitive to light and function well in bright light

Only one type of rod is found in the retina. It can absorb all wavelengths of visible light.

Three types of cones are found in the retina. One type is sensitive to red light, one type to blue light and one type to green light.

The impulses from a group of rod cells pass to a single nerve fiber in the optic nerve.

The impulse from a single cone cells passes to a single nerve fiber in the optic nerve.

Color fun: http://www.colourtherapyhealing.com/colour/colour_fun/

Processing visual stimuli• When we look at an object, light

rays pass through the pupil and are focused by the cornea, lens and the humours

• Image focused on the retina is upside down and reversed from left to right

• The brain must correct the position of the image so that it is right side up and not reversed

• It must also coordinate the images coming from the left and right eye

Edge enhancement• Complex

structure of vision is exposed by studying illusions

• Why do you see some grey blobs in the white area between the black squares?

• Theory is that the areas where you see grey are in your peripheral vision, where there are fewer light-sensitive cells than in the center of your retina

• It demonstrates that you have a special mechanism for seeing edges – it is called edge enhancement

• Theorized that light-sensitive receptors in your eye switch off their neighboring receptors making the edges look more distinct

Contralateral processing• Opposite side processing is due to

the optic chiasma

• Nerve fibers bringing information from the right half of each visual field converge at the optic chiasma and pass to the left side of the brain

• Nerve fibers bringing information from the left half of each visual field converge at the optic chiasma and pass to the right half of the brain

• Information eventually ends in the visual cortex of the brain and information is shared to form a complete visual image

• Image received by visual cortex is both inverted and reversed

• Brain must correct in the cerebral cortex

• Process of two sides of the brain working together can be illustrated by the abnormal perceptions of patients with brain lesions

• Patients with right side brain lesions, when looking at an object from above, does not recognize that object and will deny what it is

• Patients with left brain lesions can describe the function of the object but cannot come up with the name of the object

• It takes both sides of the brain working together to have correct “vision” which is able to recognize an object and understand what it is

Vision 2020• Joint initiative of the World Health

Organization and the International Agency for the Prevention of Blindness whose goal it is to eliminate avoidable blindness worldwide by 2020

• Aim is to give every one in the world the right to sight

Videos• How the eye works• Laser surgery

Structure of the ear

pinna Oval windowRound window

How sound is perceived by the ear• Outer ear catches sound waves

(vibrations of air molecules)• Sound waves travel down

auditory/ear canal and cause the tympanic membrane (eardrum) to move back and forth slightly

• Bones of the ear – malleus, incus, and stapes – receive vibrations from the tympanic membrane and multiply them approximately 20 times

• Stapes strikes the oval window causing it to vibrate

• Vibration is passed to the fluid of the cochlea which causes hair cells within the cochlea to vibrate

• These receptors release a chemical message across a synapse to the sensory neuron of the auditory nerve and is carried to the brain

• The wave in the fluid of the cochlea dissapates as it reaches the round window

• Loud noises cause the fluid to vibrate to a higher degree and the hair cells bend even more

• Brain interprets this as higher volume

• Pitch is a function of sound wave frequency– Short, high-frequency waves produce

high-pitched sound– Long, low-frequency waves produce

low-pitched sound• Sound which is sensed by the

brain is processes in the auditory area of the cerbral cortex

Videos• Sense of hearing• Cochlear implant