Option E2 Perception of Stimuli

Assessment StatementsE.2.1 Outline the diversity of stimuli that can be

detected by human sensory receptors.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

E.2.1 Outline the diversity of stimuli that can be detected by human sensory receptors.

Mechanoreceptors

Chemoreceptors

Thermoreceptors

Photoreceptors

What do each of these receptors respond to?

MechanoreceptorsProvide information to the CNS on:touchpressurevibrationstretch

ChemoreceptorProvides information on:MoleculesSolutese.g. blood pH/CO2 levels

Thermoreceptor

Feeds back information on:TemperatureHot/Cold

skin

Photoreceptors

Provides information on:LightVisible wavelength400-700 nm

rods and cones

retinar

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

E.2.3 Annotate a diagram of the retina

Ganglion

Amacrine neuronBipolar Cells

Rods

Cones

Horizontal Cells

Optic nerve

● 3 rods are connected to 1 bipolar cell and 1 ganglion cell.

● When interpreted in the brain it is not possible to tell which of the receptors was activated.

● This reduces the detail in this region of the field of vision.

● This is typical of the arrangement of rods and other neurons in areas outside of the fovea.

Convergence

● 1 cone synapses with 1 bipolar and 1 ganglion.● The impulse traveling along the axon of the

ganglion neuron can be mapped to a precise region of the retina and therefore the field of vision.

● This provides detailed visual information (resolution).

● This arrangement is typical of the fovea where there are 1.6 X105 mm-2 cone cells.

● Other species, particularly birds that have far greater densities of photoreceptors and indeed more than one fovea.

Distribution of rods and cones

E.2.4 Compare rod and cone cellsSimilaritiesBoth cells are photoreceptorsBoth are found in the retina

Differences

Photoreceptors (rod and cone cells) in the retina convert light into nerve impulses

The impulses pass to bipolar cells, which relay the signal to the optic nerve (via ganglion cells)

E.2.5 Explain the processing of visual stimuli

Edge enhancement is a ‘pre-central nervous system’ processing of information on the retina itself. This processing is not carried out by part of the brain but by the organisation of the retinal cells.

Contralateral processing is the way in which the brain collects and integrates information from the eyes to create the perception of seeing.

Edge Enhancement

● The edges of the squareappear to glow brightly

● This is edge enhancement - a result of retinal processing● Provides greater contrast at

the edges of objects● Gives greater detail of the visual

environment

Edge EnhancementSignals from rods and cones follow both vertical and lateral pathways

Photoreceptors stimulate opposing bipolar cells but inhibit adjacent bipolar cells (lateral inhibition)

This makes light spots lighter and dark spots darker, with the contrast greatest at the edges (edge enhancement)

Visual Processing

Contralateral ProcessingWhen stimuli is processed on the opposite side of

where it was detected

Information from the left half of the visual field is detected by the right half of the retina in both eyes and is processed by the right hemisphere

Information from the right half of the visual field is detected by the left half of the retina in both eyes and is processed by the left hemisphere

• At the optic chiasma, information from both eyes may swap so that the left or right visual field is processed together

• The optic nerves that swap sides are transmitting signals contralaterally, while the optic nerves that do not swap are transmitting signals ipsilaterally (same side)

• Impulses continue to the thalamus where the optical information is processed before an image forms in the visual cortex

E.2.6 Label a diagram of the ear

Pinna

Auditory canal

Ear drum

Ossicles

Semicircular canals

Auditory nerve

Cochlea

Eustachian tube

Oval and round windows

E.2.7 Explain how sound is perceived

Sound travels as pressure waves in the air which push the membrane of the eardrum, causing it to vibrate

The degree of vibration will vary according to the frequency and amplitude of the sound wavesThe ear drum pushes on the bones of the middle ear

(ossicles) which magnify the vibrations up to 20 timesThe ossicles push against the oval window, displacing fluid

within the cochleaMovement of the cochlear fluid affects the position of cilia on

sensory hair cellsCilia on hair cells vary in length and each resonates to a different frequency of soundActivation of the hair cells generates nerve impulses which

are transmitted via the auditory nerve to the brain The kinetic motion of the cochlear fluid is dissipated by the

movement of the round window

1. List two groups of sensory receptors, giving the stimulus each perceives (2)

Mechanoreceptors — pressure;Chemoreceptors — chemical substances/pH;Thermoreceptors — temperature;Photoreceptors — light;Mechanoreceptors/proprioceptors —

stretching/pressure;Hydroreceptors — humidity;

2. Explain the role of receptors, sensory neurons and motor neurons in the response of animals to stimuli (3)

receptors detect stimuli;transmit information regarding stimuli to the CNS;via sensory neurons;central nervous system sends impulse to effector;via motor neuron;

3. Identify structures I to IV (2)

I. cornea;II. lens;III. vitreous humour;IV. choroid;Two correct for [1] and four correct for [2].

4. Outline contralateral processing of visual stimuli (3)both retinas receive information/stimuli from left and right

fields of vision;left and right optic nerves cross in optic chiasma;neurons from both eyes carrying impulses from left field of

view go to righthemisphere / vice versa / right field of vision is processed in

left side of brain / vice versa;neurones from the optic nerve synapse (in the lateral

geniculate nucleus) with neurones to the (primary) visual cortex;

allowing brain to have perception of depth, distances and sizes;

Accept any of these points made on an annotated diagram.

5. Compare rods and cones (3)

6. Identify structures A to D (4)

A: pinna;B: eardrum;C: stapes / bones of the middle ear;D: semicircular canals;

Award [2] for 4 correct answers, [1] for three

correct answers, [0] for two or one correct answer(s)

7.Explain how sound is perceived by the ear (6)

eardrum moved by sound waves;eardrum/tympanic membrane causes movement of the

malleus/bones of the middle ear/ossicles;bones of the middle ear/malleus, incus and stapes/hammer,

anvil and stirrup amplify/magnify movement;bones of the middle ear/stapes push on the oval window;causing movement of fluid/vibration within the cochlea/inner

ear;hair cells are mechanoreceptors;which release a chemical neurotransmitter when stimulated;sounds/vibrations are transformed into nerve impulses/action

potentials;carried by auditory nerve to brain;round window releases pressure so fluid in cochlea can vibrate;