Topic A.3 Perception of Stimuli

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

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. 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

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

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

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

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

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!!

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

Contralateral processing

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

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

Outer Ear Pinna

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

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

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

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)

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