Special Senses: Vision and Hearing PDF

Special Senses: Vision and Hearing Compiled by: Samuel Adetunji ONASANWO (Ph.D.) Department of Physiology, College of Medicine, University of Ibadan, Nigeria. VISION The Eye Conjunctiva Covers the inner...

Special Senses: Vision and Hearing Compiled by: Samuel Adetunji ONASANWO (Ph.D.) Department of Physiology, College of Medicine, University of Ibadan, Nigeria. VISION The Eye Conjunctiva Covers the inner surface of the eyelids and the anterior surface of the eye. Membrane which produces mucous that lubricates the eye and prevents dryness. Protects the eye. 1. Fibrous Tunic  Fibrous Tunic Sclera Functions: Cornea Functions: Protects eye Transparent window Shapes eye for light entry Anchors eye muscles Refracts light 2. Vascular Tunic  Vascular Tunic Choroid Functions: Provides nutrients to all eye tunics. Absorbs light preventing reflecting & scattering of light within the eye. Ciliary Body Functions: Ciliary processes and secrete aqueous humor. Suspensory ligaments hold lens in place. Ciliary muscles pull on the ligaments to change the thickness of the lens. Iris Functions: Constricts or dilates to adjust the amount of light entering the eye. Vascular Tunic Ciliary Muscles Ciliary Processes  Aqueous Humor Helps support the eye internally due to the intraocular pressure it produces inside the eye Supplies nutrients & oxygen to the cornea, lens and portions of the retina Carries away metabolic wastes from the cornea, lens and portions of the retina The iris constricts or dilates to adjust size of the pupil. The pupil allows light to enter the posterior segment of the eye.  Vitreous Humor Transmits light within the posterior segment. Supports the lens posteriorly. Holds the retina in place. Contributes to intraocular pressure. 3. Sensory Tunic  Retina Pigmented Layer Neural Layer Absorbs light Contains photoreceptors Carries out phagocytosis (rods and cones) for Stores Vitamin A visual perception Contains bipolar cells & ganglion cells for visual impulse transmission  Retina Fovea Centralis Other areas of Retina Contains only closely Contain only rods packed cones Provide night, dim light Provides acute color & peripheral vision vision in bright light Shades of grey only Macula Lutea Optic Disc Contains more widely Contains no receptors spaced cones Blind spot Called yellow spot ✓ Photoreceptors: - Rods and cones convert light into electrical energy as optical sensors. - Rods are light-sensitive night vision sensors - scotopic vision - Cones have better acuity than rods but poorer light sensitivity. - Cones provide color and photopic vision. - Rods outnumber cones 20:1 in the human eye. ✓ Intraocular fluid: - Maintains eyeball shape - Has two types; Aqueous humor (protein-free clear fluid, nourishes the cornea and Iris) and Vitreous humor (Clear gelatinous fluid, between the lens and retina). ✓ Lacrimal apparatus: - Lacrimal glands under the upper eyelid produce tears. - Tears from the upper orbital lacrimal gland moisturize and clean the cornea  Retina Optic Disc Photoreceptors  NEURAL FUNCTION OF THE RETINA - Retina layers comprises cones for colour vision and rods for black- and-white and night vision, is light-sensitive and sends information from retinal neurons to optic nerve fibers and the cerebral cortex when activated. - The retina has 10 layers including : Photoreceptor layer, Pigment epithelium (the outermost layer) and ganglionic layer among others. - Pigment cells feature tentacle-like structures that encircle the outer rods and cones in the second layer. - These mechanisms prevent transverse scatter of light between photoreceptors. - They also serve a mechanical function in maintaining contact between layers - provide nutrients and remove waste from the photoreceptors; phagocytose cells (Rods & Cones) which are continuously shed; reconvert metabolized visual pigment into a form that can be reused after it is transported back to the photoreceptors. THE DIFFERENT NEURONAL CELL TYPES ✓ The photoreceptors-rods and cones: Send signals to the outer plexiform layer, where they sync with bipolar cells and horizontal cells. ✓ Horizontal cells: Carry rod and cone impulses horizontally in the outer plexiform layer to bipolar cells. ✓ Bipolar cells: Send impulses vertically from rods, cones, and horizontal cells to the inner plexiform layer, where they sync with ganglion and amacrine cells. ✓ Amacrine cells: Transfer impulses directly from bipolar cells to ganglion cells or horizontally inside the inner plexiform layer from bipolar cell axons to ganglion cell dendrites or other amacrine cells. ✓ Ganglion cells: Provide retinal impulses to the brain through the optic nerve. ✓ Interplexiform cell: This cell sends retrograde impulses from the inner to the outer plexiform layer. NEURAL CONNECTIONS IN THE RETINA  NEURAL PATHWAY OF RODS AND CONES ✓ The circuitry for the two systems is slightly different. ✓ The neurons and nerve fibers that conduct the visual signals for cone vision are considerably larger than those that conduct the visual signals for rod vision. ✓ The signals are conducted to the brain two to five times as rapidly. For pure rod vision: There are four neurons in the direct visual pathway: (1) rods, (2) bipolar cells, (3) amacrine cells, and (4) ganglion cells, in addition, horizontal and amacrine cells provide lateral connectivity. For both rod an cone vision: Bipolar cells in the peripheral retinal circuitry connect with both rods and cones; the outputs of these bipolar cells pass both directly to ganglion cells and by way of amacrine cells.  MODULATION OF VISUAL SIGNALS ✓Visual information is modified and separated before reaching the visual cortex. ✓Information that reaches the primary visual cortex in the occipital lobe is not a replica of the visual field for several reasons: - The light rays. Once it is projected to the brain, the inverted image is interpreted as being in its correct orientation.  Modulation of Visual Signals (Cont’d) - Before the information reaches the brain, the retinal neuronal layers beyond the rods and cones reinforce selected information and suppress other information to enhance dark–bright contrast for sharpness of boundaries. - Various aspects of visual information, such as shape, color, and motion, are separated and projected in parallel pathways to different regions of the cortex. - Re-integrated by higher regions of visual cortex to reassemble the perceived picture  Modulation of Visual Signals (Cont’d) ✓ The left half of the cortex gets visual data from both eyes' right half of the visual field, whereas the right half receives input from both eyes' left half. ✓ Each optic nerve exiting the retina carries information from both halves of the retina it serves, and this nerves are re-organized at the optic chiasm, where partial crossing over of optic nerve fibers from one side to the other occurs. ✓ The reorganized bundles of fibers leaving the optic chiasm are known as optic tracts ✓ Each optic tract, in turn, delivers to the half of the brain on its same side information about the opposite half of the visual field.  Cones Are located in macula lutea but are most highly concentrated in the fovea centralis Are sensitive to bright light (daylight) situations in which light is very intense. Each cone synapses with a single bipolar cell which synapses with a single ganglion cell. The axons of ganglion cells form the optic nerve to conduct visual images to the brain. Provide acute (sharp) color images (vision). Cones Photoreceptors  Rods Most highly concentrated in the retina outside the macula lutea Many rods synapse with a single bipolar cell Many bipolar cells may synapse with a single ganglion cell which carries stimuli to brain More sensitive & function only in dim light, night and peripheral vision Images are blurry and only in shades of gray  Visual Pigments Composed of two components – Retinal - light absorbing molecule (made from Vitamin A) – Opsin (four types made from protein) Opsin combined with retinal = visual pigment OPSIN + RETINAL = Visual Pigment Depending on the type of opsin which retinal is bound to, each of the four pigments will only absorb certain wavelengths of light.  Visual Pigments: RODS Retinal + Opsin = Rhodopsin (visual purple). Absorbs light throughout entire visible light spectrum (most sensitive to green). Functions only in dark, dim light & peripheral vision. Light causes Retinal to change shape & separate from opsin causing generation of nerve impulse. Regenerate only in dark or dim light situations. (Light) RHO DOPSIN OPSIN RETINAL Impulse  Visual Pigments: Cones Retinal + Red, Green or Blue Opsin = Red, Green or Blue visual pigments Each Opsin absorbs light only in the area of the visible light spectrum it is sensitive to, i.e., red cones, green cones & blue cones Function only in bright light (daylight) Provide sharp color images (Light) Red Cone Red Opsin RETINAL Impulse Green Cone Green Opsin RETINAL Impulse  Summary Highly discriminatory color vision Cones (photopic vision) Bright llight Photosensitive receptors Black or white light (grey) Rods (scotopic vision) Poor (dim) light  Visual Field and Visual pathway ✓ Visual field of an eye, is the segment of the external world seen by the fixed eye in a fixed head (Measured by a Perimeter-Perimetry chart). ✓Essentially, the retina can be divided into; a left half and a right half which are referred to as a temporal half (next to the temple) and a nasal half ( next to the nose). ✓Rt-nasal and Lt- temporal see all to the right of the world, hence, said to see RIGHT HEMIFIELD  Defects in the Visual fields Case 1 Effect(s) -Left optic nerve damage loss of vision in the entire field of the left eye.(Blindness of the respective eye) Case2 -left optic tract damage loss of the right halves of the visual fields of both eye (TR + NL ) Case 3 -lesion of the optic chiasma loss of vision in the both temporal fields Case 4 -lesion of the left optic radiations complete blindness for detection of visual patterns (homonymous hemianopia)  Consequences along the visual system: Damage to the visual system before the chiasm is fairly similar to closing one eye Damage to the pathway after the chiasm will cause partial damage to both eyes, and only one hemifield  Vision-Clinical tests 1. Visual acuity This is the ability of the eye to distinguish between two closely spaced lines The measure of acuity is t minimum distance between two points, which are clearly seen as separate points, at a standard distance from the observer.  Assessment: Use Snellen’s chart: By calculation; Visual acuity= Actual distance the letter can be read = d Normal distance the letter can be read = D d=6metres from the chart, where subject stands The subject reads from the top down to the bottom of the chart where letters on chart are in reducing sizes and equivalent to maximum appropriate distances possible to read them. Usually, they are: 60,36,34,18,12,9 and 6m  Cont’d Top letter readable has visual acuity: 6/60 Last letter readale has visual acuity: 6/6  Pupillary Light Reflex Is a reflex that controls the diameter of the pupil, in response to the intensity (luminance) of light that falls on the retina. Greater intensity causes the pupil to become smaller (allowing less light in) whereas lower intensity light causes the pupil to become larger (allowing more light in)  Mechanism Retinal ganglion cells convey information from photoreceptors to the optic nerve via the optic disc. Optic nerve is the afferent limb of the reflex, sensing and conveying the incoming light to the pretectal nucleus (upper midbrain) and bypassing LGN and the primary visual cortex  Mechanism (cont’d) Visceral parasympathetic fibres in the oculomotor nerve from accessory nucleus of Edinger-Westphal (close to Oculomotor complex) constitute the efferent limb of the reflex. These fibres synapse at ciliary ganglion before innervating sphinter pupillae to drive it to constrict.  Accommodation of the eye -Accommodation is the process by which the vertebrate eye changes optical power to maintain a clear image or focus on an object as its distance varies. Young eyes can focus from infinity to 6.5 cm -When looking at a distant object, the ciliary muscle is relaxed, which causes the suspensory ligaments attached to the lens to be taut, flattening the lens and reducing its refractive power. - When looking at a near object, the ciliary muscle contracts, releasing the tension on the suspensory ligaments, allowing the lens to become more convex and increasing its refractive power. -This change in the shape of the lens allows it to focus light from near objects onto the retina, producing a clear image. Ciliary muscle contraction reduces zonular tension, resulting in a 15-dioptre change in eye focal power. - The process of accommodation is controlled by the autonomic nervous system (ANS), specifically the parasympathetic nervous system, which stimulates the ciliary muscle to contract during near vision. Part of the accommodation-vergence reflex, can be controlled consciously  Strong Light weak light Receptors retinal photoreceptors Response - Circular / sphincter -Radial / dilator muscle contracts muscle contracts -Circular / sphincter muscle - Radial / dilator muscle relaxes relaxes -Pupil diameter increases - Pupil diameter - more light enters Why constriction (miosis) in both eyes? From the Pretectal nucleus, axons run bilaterally to both left- and right- accessory Edinger-Westphal nuclei of the Oculomotor complex, thus the efferents leave these nuclei for both eyes to manifest constriction (miosis).  Lens Refracts (bends) light Focuses precise image on the retina (fovea) through accommodation (changing thickness)  Myopia (Nearsighted) Eyeball too long Distant objects focused in front of retina Image striking retina is blurred Correction: Concave lens or laser surgery to slightly flatten the cornea  Hyperopia (Farsighted) Eyeball too short, lens too thin or too stiff. Nearby objects are focused behind retina. Image striking the fovea is blurred. Correction: Convex lens  Astigmatism Irregular Curvature in parts of the cornea or lens Causes blurry image This may be corrected by specially ground lenses which compensate for the irregularity or laser surgery.  Cararact Clouding of lens due to aging, diabetes mellitus, heavy smoking, frequent exposure to intense sunlight or congenital factors Treatment: Lens Implant  Conjunctivitis Inflammation of the conjunctiva by: Bacteria, fungi or viruses Trauma  Glaucoma Most common cause of blindness. Increasing intraocular pressure compresses retina, optic nerve & blood vessels. Late symptoms include blurred vision & halos around bright objects Canal of Schlemn Glaucoma  Color Blindness Congenital lack of one or more cone types Deficit or absence of red or green cones most common Sex-linked trait Most common in males What numbers can you see in each of these?  Night Blindness Impaired vision at night or in dim light situations Rhodopsin deficiency affecting rods Most common cause - prolonged Vitamin A deficiency Rods degenerate  Macular Degeneration Most common cause of vision loss after 65. Progressive deterioration of macula causing loss of central vision Dry Form - due to accumulation of pigments in macula due to reduced phagocytosis of cone debris by pigmented layer Wet Form - due to invasion of macula with new blood vessels from choroid causing scarring & retinal detachment  HEARING & BALANCING Middle Ear Middle Ear  Inner Ear Inner ear also referred to as the “labyrinth” because of its shape. It lies deep within the temporal bone. The bony labyrinth is a system of channels in the Vestibule bone which are filled with fluid similar to cerebral spinal fluid. The inner ear has 3 parts - The semicircular canals (1, 7 & 8 above) - The cochlea (5 & 6 above) - The vestibule which is the central area enclosed by the square.  Vestibule Suspended in the fluid inside the bony labyrinth of the inner ear is a continuous system of membranous sacks and ducts. The vestibule contains a membranous sack called the Utricle, which continues into the semicircular canals forming the membranous labyrinth and has sensory receptors which are associated with perception of dynamic equilibrium. Utricle Another membranous sack in the vestibule, the Saccule, continues into the cochlea forming the cochlear duct Saccule which houses the receptors that are sensitive to sound. Vibrations of the oval window (# 10), are conducted through the fluid in the bony labyrinth and membranous labyrinth throughout the inner ear.  Maculae Utricle and Saccule contain receptors called a macula Monitors position of head in space Responds to straight-line changes in speed & direction Receptors for static equilibrium  Macula Receptor for Static Equilibrium  Inner Ear Semicircular Canals At the base of each semicircular canal is an expanded area, the ampulla, which contains receptors for rotational movements. Semicircular Canals  Semicircular Canals Christa ampularis - receptor for dynamic equilibrium Responds to rotational (angular) movements Changes in rotatory velocity movements Hence, fluid in the canal pushes against the jelly-like cupula of the Christa ampularis in which hair cells are embedded. Depression of the cupula by moving fluid causes the hair cells to send information to the brain about rotational movements and velocity changes.  Semicircular Canals Dynamic Equilibrium This, in concert with information from the maculae and visual stimuli enables us to maintain a sense of balance and orientation.  Inner Ear As indicated previously, vibrations of the stapes against the Oval Window oval window transmit sound waves into the fluids of the inner ear. Inner Ear From the vestibule, the waves pass Cochlea through the fluid into the cochlea where the receptors for sound are located. *  Cochlea In this illustration of the inner ear, the cochlea has been unrolled to show the relationship between the structures we have been discussing more clearly. Vibrations of the stapes are transmitted through the oval window, into the fluids of the vestibule, and on into the upper scala vestibuli duct inside the cochlea. Depending on their frequency, the sound waves eventually pass through the cochlear duct and the spiral organ of Corti * which sends impulses to the brain. Having passed through the cochlear duct into the scala tympani, the sound waves travel on to the round window which acts as a relief valve, Spiral Organ of Corti allowing the vibrations to dissipate into the air filled middle ear. *  Spiral Organ of Corti Receptor organ of hearing Different frequencies of vibrations (compression waves) in cochlea stimulate different areas of Organ of Corti (Vetstib. N – Sensory; Trigem. N – Motor) Interpreted as differences in pitch  Inner Ear Round Window As indicated previously, * the round window * serves as the relief valve which allows the vibrations (compression waves) within the cochlea to be released into the middle ear. *  Cochlea Oval Window Round Window THANK YOU