E.2 Perception of Stimuli

Sensory Receptors Organisms perceive information about their environment via sensory receptors which can detect various stimuli Sensory organs are a window to the brain. When stimulated, the sense organs send a message to the central nervous system. The nerve impulses arriving at the brain result in sensation. We actually see, smell taste and feel with our brains rather than our sense organs.

Sensory cells also send messages to certain parts of the brain that control emotion and memory. This is why we link tastes, sights, and sounds with emotions and memories.

Sensory Receptors CHEMORECEPTORS Have proteins in their membranes that can bind to a particular substance and initiate an action potential Chemoreceptors in the nose sense smell Chemoreceptors on our tongues (taste buds) detect taste Chemoreceptors in our blood vessels detect blood pH Pain receptors are a type of chemoreceptors that respond to chemicals released by damaged tissues.

Animations Smelling http://www.pennmedicine.org/encyclopedia/em_DisplayAnimation.aspx?gcid=000117&ptid=17 Tasting http://www.pennmedicine.org/encyclopedia/em_DisplayAnimation.aspx?gcid=000129&ptid=17

Thermoreceptors Detect changes in temperature Cold receptors can be found just under the skin surface Warm receptors are located deeper. The hypothalamus contains thermoreceptors to monitor blood temperature

Mechanoreceptors Detect movement Stimulated by mechanical force or pressure. Pressure receptors in your skin detect touch. Pressure rectors in your arteries detect changes in blood pressure There is a system in our ears that involves fluid filled canals and hairs that detect our body positions and movement.

Photoreceptors Detect light Include the rods and cones in our eyes.

Human Eye

Parts of the Eye CONJUCTIVA: Helps focus images, refracting light Covers the sclera Keeps the eye moist CORNEA Made of a strong, transparent layer of tissue Covers iris and pupil Helps focus images, refracting light AQUEOUS HUMOUR: Clear fluid that supports the eyeball and transmits light

PUPIL: the dark circle of the eye Actually a hole that allows light into the eye IRIS: the coloured part of the eye circular band of muscle surrounding the pupil regulates the size of the pupil In dim light, the iris opens  pupil dilates (becomes wider) to allow more light in In bright light, the iris closes  pupil contracts (becomes smaller) SCLERA: The white part of the eye The protective outer layer of the eye

LENS: convex lens that focuses light rays and directs it to a point. Your lens can change focus so that you can see an object clearly regardless of whether it is right in front of you, or far away. This is possible because it is surrounded by a circle of muscles: ciliary muscles CILIARY MUSCLE muscles that surround the lens and control the shape and therefore the focus of lens

VITREOUS HUMOUR: Clear fluid that supports the eyeball and transmits light RETINA: inner lining at the back of the eye that acts as a projection screen for light rays entering your eye Made of photoreceptors (rods and cones) ROD CELLS: photoreceptor cells of the retina that detect shapes and movement in low light and shades of grey CONE CELLS: photoreceptor cells of the retina that detect colour.

FOVEA: Area of retina where cone cells are densely packed (vision is most acute here) OPTIC NERVE: connects your eye to your brain contains nerves that will send information collected by the photoreceptors to the brain BLIND SPOT: the place where the optic nerve attaches to the retina. Therefore there are no photoreceptors here and light cannot be detected.

CHOROID: SCLERA: Vascular layer of the eye Contains blood vessels that will provide oxygen to eye cells SCLERA: The white part of the eye Protective outer layer of the eyeball

How does the eye work to focus light and detect images? Light enters the eye at the cornea Light passes through the aqueous humour to reach the pupil Light is then focused by the lens through the vitreous humour to the retina. The retina is composed of photoreceptors: cells that are sensitive to light. There are 2 types: rods, and cones.

5. The rods and cones transmit the information to nerve cells in the retina. 6. The nerve cells transmit the information to the optic nerve which takes the information to the brain to be processed. (The image formed on your retina is actually inverted but your brain will flip it and interpret it right side up!)

The RETINA

Processing Visual Stimuli When light “hits” the retina, it passes in between various neurons (the ganglions (and their axons in the optic fibre) and the sensory neurons) and then finally “hits” the rods and cones. The rods and cones will receive the stimuli (the light) and initiate and action potential in the sensory (bipolar) neurons that will be sent to the brain via the ganglion cells of the optic nerve.

The axons of the ganglion cells travel to the visual area of the cerebral cortex of the brain. The brain corrects the position of the image so that is it rights side up and not reversed

Annimations Eye/ Seeing http://www.pennmedicine.org/encyclopedia/em_DisplayAnimation.aspx?gcid=000109&ptid=17 Glaucoma http://www.pennmedicine.org/encyclopedia/em_DisplayAnimation.aspx?gcid=000060&ptid=17 Retina http://www.pennmedicine.org/encyclopedia/em_DisplayAnimation.aspx?gcid=000106&ptid=17 Blinking http://www.pennmedicine.org/encyclopedia/em_DisplayAnimation.aspx?gcid=000010&ptid=17 Cataract http://www.pennmedicine.org/encyclopedia/em_DisplayAnimation.aspx?gcid=000024&ptid=17 Cornea Injury http://www.pennmedicine.org/encyclopedia/em_DisplayAnimation.aspx?gcid=000035&ptid=17

Rods vs Cones

Contra-lateral Processing This refers to the fact that some of the nerve fibres in the optic nerve will cross before reaching the brain (optic chiasma) Info from the left side of each visual field converge at the optic chiasma and pass to the right side of the brain. Info from the right side of each visual field converge at the optic chiasma and pass to the left side of the brain.

Herman Grid Illusion A B

Herman Grid Illusion Why do you see grey blobs in the white area between the black squares that vanish when you try to look at them directly? Theory: the areas where you see grey are in your peripheral vision where there are fewer light sensitive cells than at your fovea. When you directly at the “grey” area, you are using the center of the retina, your fovea, which has a high concentration of light-sensitive cells.

Edge Enhancement The Hermann grid fools your eye because of the extreme contrast between black and white edges. You have a special mechanism for seeing edges known as edge enhancement Theory: light sensitive receptors in your eye switch off their neighbouring receptors. This makes the edges look more distinct, because of the extreme contrast between dark and light.

When you look at and intersection in the grid (such as A) there is a lot of white surrounding it compared to looking at an area such as B which is surrounded by black. Your brains receives the info that the contrast at A is less than that at B. So B is seen as a white spot, and A is seen as a grey spot.

Blind Spot is the one place on the retina of every healthy eye in which there are no photoreceptors. Since there are no photoreceptors – light cannot be detected here. There are no photoreceptors because this is where the optic nerve attaches to the retina. You do not notice your blind spot because your brain fills it in.

Find your blind spot Draw a small plus sign and a small dot on a piece of paper, at least 5 cm apart. Cover your LEFT eye, and stare at the plus sign. Slowly move away (or forward). When the black spot has disappeared, you have found your blind spot. + ●

Ear Annimations http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter19/animation__effect_of_sound_waves_on_cochlear_structures__quiz_1_.html http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter19/animation__effect_of_sound_waves_on_cochlear_structures__quiz_2_.html