1. Topic A.3 Perception of Stimuli

2. Sensory Receptors
Receptors in the nervous system the are sensitive to changes in the environment around the cells.
Humans have the 4 following types of specialized receptors
Mechanoreceptors
Respond to mechanical forces and movements
Chemoreceptors
Respond to chemical substances
Thermoreceptors
Respond to temperature changes
Photoreceptors
Respond to light

3. Smell
Detection of chemicals in the air is carried out by the Olfactory receptors.
These cells have cilia that project into the nasal cavity and their membrane contains odorant receptor molecules which assist in the sense of smell
Different organisms have differing amounts of odorant receptors
Mice have over a thousand different receptors which allows them to have a very keen sense of smell
Humans are severely lacking in odorant receptors which provides an imprecise sense of smell

5. Eye Functions
1. sclera = opaque (usually white), fibrous, protective layer of the eye containing collagen and elastic fibers
2. cornea = transparent front part of the eye that covers the iris, pupil, and anterior chamber,providing most of an eye's optical power
3. conjunctiva = membrane that covers the sclera (white part of the eye) and lines the inside of the eyelids
4. eyelid - outside flesh covering of the eye. Protects from damage
5. choroid = vascular layer of the eye lying between the retina and the sclera. The choroid provides oxygen and nourishment to the outer layers of the retina

6. 6. aqueous humor = thick, watery substance in the eye
7. pupil = variable-sized, black circular opening in the center of the iris that regulates the amount of light that enters the eye
8. lens = transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to focus on the retina
9. iris = colored part of the eye
10. vitreous humour = the clear aqueous solution that fills the space between the lens and the retina
11. fovea = responsible for sharp central vision
12. optic nerve = transmits visual information from the retina to the brain
13. blind spot = the specific region of the retina where the optic nerve and blood vessels pass through to connect to the back of the eye
14. retina = thin layer of neural cells that lines the back of the eyeball

7. Photoreceptors
Light enters the eye through the cornea and the lens which focuses it onto the retina.
The retina is a thin layer of light-sensitive tissue at the back of the eye.
The retina involves two types of photoreceptors
Rods and Cones
Many nocturnal animals only have rods and cannot distinguish color

8. Rods and Cones Rods are very sensitive to light. But not color
When exposed to bright light “bleaching” of rods can occur
Cones are able to absorb different ranges of wavelengths of light

9. Cones Continued There are three types of cones in the human eye.
Red, Green, Blue
The relative stimulation of each of the three cone types allows for interpretation of color
Color vision is only in bright light and tends to fade in dim light

11. Bipolar cells Rods and cones synapse with neurons called bipolar cells
Bipolar cells send the electrical impulses from the rods and cones to the Ganglion cells

12. Neurons and action potentials
If a rod or cone cell is not stimulated by light they will depolarize and release an INHIBITORY Neurotransmitter onto a bipolar cell.
This causes that bipolar cell to become hyperpolarized and not transmit impulses to its associated retinal ganglion cell.
When light is absorbed by a rod or cone cell it becomes hyperpolarized and stops sending that inhibitory neurotransmitter to the bipolar cell
This then allows the bipolar cell to depolarize, activating the adjacent ganglion cell.
Groups of rods send signals to the brain via a single bipolar cell
Grainy Picture
Cones send signals to the brain via their own individual bipolar cell
High Definition vision!!

13. Ganglion Cells
Ganglion cells have cell bodies in the retina and synapse with bipolar cells
Ganglion cells pass across the front of the retina to form the “blind spot”
The “Blind spot” is actually where the ganglion cells combine to form the Optic Nerve

14. Contralateral processing

15. The Human Ear Transmit pressure waves into auditory sounds
Brain interprets the information from the receptors

17. Parts & Functions of the Ear
Structure
Function
Pinna
Collects sound waves
Eardrum
(tympanic membrane)
Transmits sound waves to the inner ear bones
Bones of the middle ear
(hammer, anvil, stirrup)
Transmit and amplify (by up to 20 times) the sound waves received from the eardrum
Also known as the malleus, incus and stapes
Oval window
Receives pressure from middle ear bones (stirrup) and transmits pressure waves into the middle ear
Round window
Absorbs pressure waves after they travel through the cochlea, preventing the waves from moving backwards
Semicircular canals
Help maintain balance by sensing position; hair cells
Auditory nerve
Made up of neurons that are activated in the cochlea by pressure waves bending hairs in the inner ear; sends messages to brain
Cochlea
Part of ear that contains hairs that are attached to neurons and activated by pressure waves; transforms the pressure waves into neural impulses

18. Outer Ear
Pinna

19. Middle Ear
Eardrum
3 smallest bones in the body (malleus, Incus and Stapes)

20. Inner Ear Does both auditory and balance
Cochlea and Vestibula or semicircular canals

21. Transmission of Sound
Pinna collects sound waves and passes them to the eardrum
The eardrum vibrates
Vibration of the eardrum puts pressure on the ear bones
Ear bones absorb the vibrations and amplify them
Vibrations reach the stirrup and act like a piston to convert it to mechanical energy
Mechanical energy is passed on to the cochlea via the oval and round windows
As it moves through the cochlea it causes the hair cells to bend

22. Transmission continued
Hair cells are varying heights and are in a jelly-like substance
Larger sound waves vibrate more hair cells
The more hair cells activated (bent) the louder the sound
The more frequent the sound waves the higher the pitch (frequency)