Chapter 16 special senses eyes and ears PDF

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Dr. Kirifides ANAT 102 Drexel University...

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Chapter 16: special senses: eyes and ears ● The Eye ○ Six extrinsic eye muscles move the eyes in almost any direction theses muscles include ■ The superior rectus ■ The inferior rectus ■ The lateral rectus ■ The medial rectus ■ The superior oblique ■ The inferior oblique ○ The eyeball contains two tunics (coats) ■ The fibrous tunic (the cornea and sclera) ■ The vascular tunic (the choroid, ciliary body, and iris) ● Lacrimal Apparatus ○ The lacrimal gland (parasympathetic from CN VII) ○ Blinking occurs 15-20 times per minute ○ Lacrimal puncta ○ Lacrimal canaliculus ○ Lacrimal sac ○ Nasolacrimal duct ● Layers of the ○ Fibrous tunic ■ Scleera ■ Cornea ○ Vascular tunic ■ Iris ■ Ciliary body ■ Choroid ○ Retina ■ Pigmented layer ■ Neural layer ● Iris control of pupil ○ The most anterior region of the vascular tunic is the iris, which is the colored portion of the eye ○ The iris controls pupil size, or diameter and its two smooth muscle layers ■ The sphincter pupillae muscle is arranged in concentric circles around the pupil. This muscle contracts when stimulated by parasympathetic division through the oculomotor nerve (CN III) ■ The dilator pupillae muscle is organized in a radial pattern through the iris. This muscle contracts when stimulated by the sympathetic division ● Structure and organizations of the retina ○ The retina lines the posterior ¾ of the inner layer of the eyeball ○ The optic nerve (CN II) is also visible ○ The point at which the optic nerve exits the eye is the optic disc (blind spot)

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The exact center of the retina is the macula lutea. In its center is the fovea centralis (area of highest visual activity) Aqueous humor: secretion and reabsorption ○ Aqueous humor is a transparent watery fluid that circulates within the anterior cavity. It is continuously produced by the ciliary processes. The circulation of aqueous humor provides nutrients and oxygen to both the avascular cornea and the lens ○ Blood plasma is filtered by the ciliary processes and enters the posterior chamber from the aqueous humor ○ The aqueous humor circulates from the posterior chamber through the pupil and into the anterior chamber ○ Aqueous fluid drains from the anterior chamber into the scleral venous sinus (canal of Schlemm). Glaucoma results from the blockage of aqueous humor drainage Glaucoma ○ Angleclosure and open-angle glaucoma both involve the angle formed in the anterior chamber of the eye by the union of the choroid and the sclera or corneal-scleral junction (i.e. limbus). This angle is the important passageway for draining the aqueous humor ■ Angle-closure glaucoma: a direct consequence of the narrowing of this angle consists about ⅓ of all cases of glaucoma ■ Open-angle glaucoma: from about ⅔ of glaucoma cases. In the drain angles are adequate but fluid transport out of the anterior chamber is impaired ■ Congenital glaucoma: occurs only rarely and is due to hereditary factors or intrauterine infection ○ Fluid buildup in the anterior cavity causes a posterior dislocation of the lens and a substantial increase in pressure in the posterior cavity Refraction of light ○ Light rays are straight as they first enter the eye. However, the ability to see clearly requires refraction (or bending) of the light ray so that they hit on the retina- specifically at the fovea centralis, the portion of the retina that is predominantly composed of cones and provides the sharpest vision. Light rays are refracted when ■ They pass between 2 media of different densities ■ These media meet at a curved surface ○ Before light can reach the photoreceptor cells, it must pass from the air through the ■ Cornea ■ Aqueous humor ■ Lens ■ Vitreous humor ■ Cells forming the inner layers of the retina

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Both the cornea and lens play a significant role in refraction of light for vision- the cornea because of the relatively large refraction of light that occurs so light passes Lens shape in far vision and near vision ○ The lens focuses incoming light onto the retina, and its shape determines the degree of light refraction ○ The suspensory ligaments attach to the lens capsule at its periphery, where they transmit tension that enables the lens to change shape ○ The relative tension in the suspensory ligaments is altered by relaxation and contraction of the ciliary muscles in the ciliary body ■ When we view objects greater than 20 feet away, the ciliary muscles relax, and the tension on the suspensory ligaments Functional Visual Impairments ○ Emmetropia is the condition of normal vision, in which parallel rays of light are focused exactly on the retina. Any variation in the curvature of either the cornea or the lens, or in the overall shape of the eye, causes entering light rays to form an abnormal focal point. Conditions that can result include hyperopia, myopia, and astigmatism ○ People with hyperopia have trouble seeing close-up objects and so are called farsighted. The cause of hyperopia is a short eyeball, parallel light rays from objects close to the eye focus posterior to the retina ○ People with myopia have trouble seeing faraway objects and so are called nearsighted. The cause of this condition is along eyeball, parallel light rays from objects at some distance from the eye focus anterior to the retina within the vitreous body A cataract ○ A clouding of the lens in the eye which leads to a decrease in vision. Cataracts often develop slowly and can affect one or both eyes. Symptoms may include faded colors, blurry vision, halos around light, trouble with bright light, and trouble seeing at night Photoreceptor ○ Rods are longer and narrower than cones. Each eye contains more than 100 million rods, and they are primarily located in the peripheral retina. Rods are activated by dim light, such as when you are in a unlit room at night and provided no color recognition ○ Cones occur at a density of less than 10 million per eye and are concentrated in the fovea centralis. Cones are activated by high-intensity light and provide color recognition and precise visual sharpness. Thus, when you notice the fine details in a colorful picture, the cones of your retina are responsible Photopigments ○ The photopigment in cones is called photopsin. There are 3 different type of photopsin protein and types of cones: ■ Blue cones best detect wavelengths of light ar about 420 nanometers ■ Green cones maximally absorb light ar 531 nm ■ Red cones best detect light at 558 nm

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Color blindness ○ Is an x-linked recessive inherited genetic trait that occurs when an individual has an absence of a deficit in one type of cone cell ○ The most common form of color blindness is the x-linked recessive trait which involves the red and green cone cells, resulting in red-green color blindness ○ Color blindness is much more common in males because it is x-linked recessive Phototransduction ○ Light waves reach the rod ○ Cis-retinal is transformed to trans-retinal ○ Trans-retinal disassociates from opsin and becomes activated. This process called the bleaching ○ Trans-retinal is converted back to cis-retinal using ATP ○ Rhodopsin is re-formed when cis-retinal associates with opsin slow process (approx. 30 s) In the light ○ Stimulation by light causes the photoreceptor cell to be hyperpolarized because of decreased entry of Na+ and Ca2+ ○ Voltage gated Ca2+ channels close, and no glutamate neurotransmitter is released by photoreceptor cell ○ The bipolar cell is no longer inhibited and thus it depolarizes ○ Bipolar cells release neurotransmitter (glutamate) ○ The glutamate neurotransmitter binds to receptors in the ganglion cell, and a nerve signal is initiated to the brain In the dark ○ Dark current causes the photoreceptor cell to be depolarized at -40mV ○ Voltage gated Ca2+ channels open, and the neurotransmitter glutamate is released from photoreceptor ○ Binding of glutamate hyperpolarizes the bipolar cell, causing inhibition ○ Therefore is no release of glutamate neurotransmitter from the bipolar cell ○ No nerve signal is generated by the ganglion cells Visual pathways ○ Each optic nerve conducts visual stimuli information ○ At the optic chiasm, 50% of axons from the optic nerve decussate ○ The optic tract on each side then contains axons from both eyes ○ Visual stimuli information is processed by the thalamus (lateral geniculate nucleus) ○ Optic radiations send signals to the visual association areas within the occipital lobe of the cerebrum ○ Visual sensory input involved in reflexes is related to nuclei within the superior colliculi of the midbrain Anatomic regions of the right ear ○ The most visible portion of the external ear is a skin-covered, elastic-cartilagesupported structure called the auricle or pinna

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The external acoustic meatus prevents large objects from entering and damaging the tympanic membrane. Deep within the canal, ceruminous glands produce cerumen, which combines with dead, sloughed skin cells to form earwax ○ The tympanic membrane, or eardrum, is a funnel-shaped membrane composed of fibrous connective tissue sandwiched between 2 epithelial sheets. ○ It serves as the boundary between the external and middle ear The middle ear ○ The middle ear contains an air-filled tympanic cavity ○ 2 membrane-covered openings are located within medial bony wall that separates the inner ear, the oval window and round window ○ Inferiorly, the auditory tube serves as a passageway that extends from the middle ear into the nasopharynx ■ This tube is normally closed. Air movement through this tube occurs as a result of chewing, yawning, and swallowing, which equalize pressure on either side of the tympanic membrane ■ Middle ear infections result when infectious agents move from the nasopharynx through the auditory tube into the middle ear Otitis media ○ An infection of the middle ear ■ It is most often experienced by young children, whose auditory tubes are horizontal, relatively short, and underdeveloped ■ Is a young child has a respiratory infection, the causative agent may spread from the pharynx through the auditory tube ■ Fluid the accumulates in the middle ear cavity, resulting in pressure, pain, and sometimes impaired hearing The inner ear ○ The bony labyrinth is structurally and functionally partitioned into three distinct regions, including the cochlea, vestibule, and semicircular canals ■ The cochlea houses a membranous labyrinth structure called the cochlear duct ■ The vestibule contains 2 sac like membranous labyrinth structures- the utricle and the saccule ■ The semicircular canals each contain a membranous labyrinth structure called the semicircular duct Structure of *** ○ The cochlea is a snail-shaped, spiral chamber within the temporal bone of the inner ear ○ The membranous labyrinth called the cochlear duct is housed within the cochlea ○ These membranes partition the bony labyrinth of the cochlea into 2 smaller chambers on either side of the cochlear duct: both are filled with perilymph ■ The scala vestibuli is the chamber adjacent to the vestibular membrane ■ The scala tympani is the chamber adjacent to the *** Sound waves through the ear ○ Sound waves are collected and funneled by the auricle of the external ear to enter the external acoustic meatus, which make the tympanic membrane vibrate

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The vibration of the tympanic membrane causes movement of the auditory ossicles, which makes the oval window vibrate. Sounds transmitted across the middle ear are amplified 20-fold ○ Vibration of the oval window causes pressure waves in the perilymph within the scala vestibuli which then deform the vestibular membrane resulting in pressure waves in the endolymph within the cochlear duct ○ Hair cells in the spiral organ of this region are distorted, initiating nerve signals in the cochlear branch of the vestibulocochlear nerve ○ Simultaneously, the pressure wave vibrations within the cochlear duct are transmitted to the perilymph within the scala tympani, and are absorbed at the round window Inner hair cells ○ The inner hair cells contain ion channels at the tips of their stereocilia ○ The ion channel K+ is attached to its taller neighboring stereocilia by a tip link process ○ Open ion channels allow K+ to move into the inner hair cell Sound wave interpretation at the basilar membrane ○ Sound waves are interpreted at specific sites along the basilar membrane of the spiral organ ○ High-frequency sounds generate pressure waves that cause the basilar membrane to displace closest to the base of the cochlea ○ Medium-frequency sounds generate pressure waves that cause the basilar membrane to displace near the center of the cochlea ○ Low-frequency sounds generate pressure waves that cause the basilar membrane to displace near the helicotrema Central nervous system pathways for hearing ○ Stimulation of the organ of corti by movement of the basilar membrane ○ Secondary sensory neurons relay signals to the inferior colliculus or superior olive ○ Neurons relay signals from inferior colliculus to the thalamus ○ Nerve signals are relayed from the thalamus to the primary auditory cortex of the temporal lobe Static equilibrium and dynamic equilibrium ○ Maculae detect both the orientation of the head when the body is stationary and the linear acceleration of the head ○ The maculae are located within the walls of the saccule and utricle ○ The apical surface of the hair cells are covered by a gelatinous layer call the otolithic membrane ○ Each individual hair cell has numerous microvilli called stereocilia and a single long kinocilium How maculae detect head position and linear acceleration of the head ○ When the head is in an upright position hair cells and stereocilia are parallel and supported by the otolithic membrane ○ Tilting the head causes the otolithic membrane to move slightly, causing the sterocilia to bend and alter the frequency of propagated nerve signals

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When the stereocilia are bent toward the kinocilium, the hair cell hyperpolarizes and thus nerve signals propagation decreases in frequency Ampulla ○ Angular acceleration is detected by the sensory receptors housed at the base of each semicircular canal in a region called the ampulla ○ The ampulla contains an elevated region, called the crista ampullaris which is covered by an epithelium of hair cells and supporting cells ○ Both the stereocilia and kinocilia are embedded into an overlying gelatinous done called cupula Functions of the crista ampullaris ○ The crista ampullaris in each semicircular duct detects rotational movements of the head ■ When the head first rotates, inertia causes endolymph to lag. This endolymph pushes against the cupula, causing bending of the stereocilia ■ Stereocilia bending results in altered neurotransmitter release from the hair cells and simultaneous stimulation of the sensory neuron ■ Bending of the stereocilia in the directions of the kinocilia results in depolarization of the hair cells and increased frequency of nerve signals, whereas bending in the opposite direction results in hyperpolarization of the hair cells and decreased frequency of nerve signals ■ Interestingly, these sensory receptors respond primarily to changes in velocity- meaning they respond to rotational acceleration or deceleration Equilibrium **** ○ When the stereocilia of either the maculae of the crista ampullaris distort, nerve signals are initiated through the vestibular branch of the vestibulocochlear nerve ○ The sensory axons of the vestibular branch terminate at either the medulla oblongata or cerebellum ■ The paired vestibular nuclei within the medulla oblongata integrate the stimuli from the vestibular apparatus to reflexively control movements of the eye and to control skeletal muscle contraction for maintaining balance ■ The cerebellum integrates the sensory input from the vestibular apparatus to coordinate skeletal muscle to maintain balance ○ Nerve signals are also sent from the vestibular nuclei and cerebellum to the thalamus and eventually the cerebral cortex for our awareness of body position Motion sickness ○ Motion sickness is a sense of nausea, mild disorientation, and dizziness that some of us have felt while flying in an airplane or riding in a car ○ It develops when a person is subject to acceleration and directional changes, but there is limited or discrepant visual contact with the outside horizon. In this situation, the vestibular complex of the inner ear is sending nerve signals to the brain that conflict with the visual reference. The eyes inform the brain we are standing still in an airplane or a ship’s cabin, but the inner ear is relaying something completely different ○ Motion sickness may be alleviated by seeking a place of less movement and establishing the visual reference. Some people drink a carbonated beverage or

eat soda crackers, although the reason this lessens the symptoms is not clear. Antihistamines are effective in reducing symptoms, and a number of nonprescription oral or transdermal preparations are available as well, including dimenhydrinate, meclizine, and cyclizine...