Week 5 Lecture 2: Special Senses

• outermost layer of eyes - sclera and cornex
• scelera - white of eye
• cornea - transparent and allows light to enter eye
• middle layer of eyes - choroid, ciliary body, iris
• choroid - contain blood vessels that nourish inner layer of eye
• ciliary body - contain ciliary muscles
• lens - focuses light on the back of eye
• iris - two layers of smooth muscle in front of lens
• innermost layer of the eye - retina
• retina - consists of neural tissue and photorecptors
• 2 areas of retina - fovea, optic disk
• fovea - central region of retina where light from the center of visual field strikes
• optic - part of retina where optic nerve and blood vessels pass through
• accommodation - curving lens to enable eye to focus on near objects
• accommodation is controlled by parasympathetic nervous system -it happens when the ciliary muscle (in ciliary body) and zonular fibers decrease their tension
• for viewing distant objects - ciliary muscle relaxes which increases dimension of lens and tightens zonular fibers
• for viewing close-up objects - ciliary muscle contracts which curves lens and lessens tension on zonular fibers
• myopia - near sightedness (cannot see far) - lens of the eye is too strong for the length of the eyeball
• regulating amount of light entering eye - iris (smooth muscle) controls pupil size - size of pupil determines the amount of light that enters the eye
• outer radial muscle of iris causes dilation of pupil (sympathetic stimulation) and inner circular muscle of iris causes constriction of pupil (parasympathetic stimulation)
• retina is made up for rods and cones
• rods - ability to see black and white in low light conditions
• cones - colour vision - active only in relatively bright light, such as sunlight during the day
• retina has 3 layers - inner layer has ganglion cells, middle layer has neurons called bipolar cells, outer layer has rods and cones
• phototransduction - conversion of light energy into electrical signals - this process is carried out by rods and cones
• process of phototransduction - photoreceptors are depolarized which opens calcium channels, calcium energy triggers NT secretion, NT acts on bipolar cells
• rods can absorb light over the widest range of wavelengths - the absorbance spectra of the 3 cones overlap
• neural processing in retina - NT released from rods and cones, communicate with bipolar neurons, bipolar neurons synapse with ganglion cells, ganglion cells can generate action potentials
• neural pathways for vision - The ganglion cells in the retina send signals to the brain through the optic nerve. These signals cross over in the optic chiasm, so the left side of the brain processes information from the right visual field and vice versa. The signals then travel to the thalamus and on to the visual cortex in the brain for further processing. This pathway ensures that both eyes receive and process information from both visual fields.
• anatomy of ear consists of the external ear the middle ear and the inner ear
• external ear - pinna, ear canal, eardum - primary function is to gather sound waves and conduct them to eardrum which separates external and middle ears
• middle ear are 3 ossicles - stapes, incuse, malleus - whose responsibility are to amplify the sound waves
• inner ear - cochlea - contains the receptors for hearing
• sound transduction by hair cells - hairs in cochlea bend and openpotassium channels which allows for depolarization - closing of channels cause hyperpolarization
• neural pathways for sound - cochlear nerve enters brainstem and synapses with second order neuron which goes to thalamus to synapse with third-order neuron which goes to the auditory cortex
• conductive deafness - problem with external/middle ear
• sensorineural deafness - problem with inner ear
• central deafness - problem with neural pathway for sound
• anatomy of vestibular apparatus - semicircular canals, utricle, saccule
• taste - there are more than 10,000 taste buds