Lesson 2 Chapter 4 – Special Senses

Lesson 2 Chapter 4 – Special Senses Unit 7 Study Guide Exclude Pages 178 in-box “Seeing with the Tongue”, exclude pages 184 – up to 192 Special Senses The Eye and Vision Vision is our primary sense. About 70% of all the sensory receptors in the body are in the eyes and 40% of the cerebral cortex (the outer layer) is involved with processing visual information and making sense of what we’re seeing. First we’ll start with the accessory structures of the eye then label the muscles and so forth. Anterior cavity filled with aqueous humor 1 Lesson 2 Chapter 4 – Special Senses 2 Posterior cavity filled with vitreous humor Eyeballs has 3 layers: External Layer - Fibrous layer - Cornea - Anteriorly - Sclera (white) - Posterior 5/6 Middle Layer - Vascular layer - choroid (dark pigmented) - Posterior 5/6 - ciliary body and iris (colour) - Anterior Inner Layer - Nervous layer - Retina  Thin pigment layer  thicker neural layer consists of neurons - Photoreceptors cell - Bipolar cells - Ganglion cells Anatomy of the Eyeball We’ll start with the outermost layer: Covering the posterior 5/6th of the eye is the sclera. It’s white, opaque, made of dense connective tissue and basically forms the back of the eye ball. The remaining 1/6th is called the cornea. The cornea is still made of dense CT but it’s transparent. The next layer is called the choroid. This layer contains melanin pigment which absorbs the light and darkens the inside so light won’t bounce around inside. Immediately below that is the retina which contains all the nerves. The front part of the choroid (bunched up with fibers connecting to the lens) is a ciliary body that is composed of a ciliary muscle and processes that look like “guy wires” that help changes the shape of the lens to bend it. Anterior to the lens is Lesson 2 Chapter 4 – Special Senses 3 the iris. The eyeball is filled with fluid known as vitreous humor in the posterior section and aqueous humor in the anterior segment. Microscopic Anatomy of the Retina The rods and cones are named after their shape. The cones provide high-acuity colour vision while the rods are specifically for dim light. When light enters the iris, it passes the retina (the nerves) unaffected and goes straight to the choroid where it then bounces back onto the retina where the light hits the rods and cones. These photoreceptors pick up the light and send signals to the bipolar cells which relay to the ganglion cells that send their axons along the interior surface of the retina toward the optic nerve. • Phototransduction – retinal cells convert light stimuli into neural signals 1) outer most – rods and cones (receptor potential) 2) middle – neurons - bipolar cells (graded potential according to light intensity) 3) inner – ganglion cells (action potential)… join together to form the optic nerve… leave the retina through optic disc (blind spot) Lesson 2 Chapter 4 – Special Senses 4 Posterior Aspect of the Eyeball (Blind Spot) Where all the blood cells come together at the center of the optic nerve there are no nerve receptors. We don’t get light there so that’s our blind spot which we don’t notice that we have one cause we have two eyes that are a couple inches apart and the brain fills in that information. Optic disc is the area which neurons merge together. Blind spot since there are no photoreceptors in that area. Lesson 2 Chapter 4 – Special Senses Image formation 1. Refraction or bending of a light ray – refractive structures in the eye are cornea and lens Astigmatism – the curvature of the cornea (or lens) is uneven, so light rays are unequally refracted 2. Accommodation – – – – the ability to adjust the strength of the lens ciliary muscle is attached to the lens by suspensory ligament when ciliary muscles are relaxed – lens is flattened (for far vision) when ciliary muscles contract – assumes more spherical shape (for near vision) 5 Lesson 2 Chapter 4 – Special Senses • • 6 Myopia (nearsightedness) – eye ball is too long or the lens is too strong – near light source is focused on the retina without accommodation – far light source is focused in front of the retina and blurry Hyperopia (farsightedness) – eye ball is too short or the lens is too weak – near objects are focused behind the retina even with accommodation and blurry Lesson 2 Chapter 4 – Special Senses The term presbyopia means "old eye" and is a vision condition involving the loss of the eye's ability to focus on close objects. Presbyopia is a condition that occurs as a part of normal aging and is not considered to be an eye disease. The process occurs gradually over a number of years. Symptoms are usually noticeable by age 40-45 and continue to develop until the process stabilizes some 10-20 years later. Presbyopia occurs without regard to other eye conditions. Visual Pathways Vision is generated by photoreceptors in the retina, a layer of cells at the back of the eye. The information leaves the eye by way of the optic nerve, and there is a partial crossing of axons at the optic chiasm. After the chiasm, the axons are called the optic tract. The optic tract wraps 7 Lesson 2 Chapter 4 – Special Senses around the midbrain to get to the lateral geniculate nucleus (LGN), where all the axons must synapse. From there, the LGN axons fan out through the deep white matter of the brain as the optic radiations, which will ultimately travel to primary visual cortex, at the back of the brain. • The left half of the visual cortex receives right half of the visual field of both eyes • The right half of the cortex receives left half of the visual field of both eyes Visual field: The field of vision of view that can be seen without moving the head. [Peripheral vision] Lateral field: temporal vision and nasal vision. Information about the world enters both eyes with a great deal of overlap. Try closing one eye, and you will find that your range of vision in the remaining eye is mainly limited by your nose. The 8 Lesson 2 Chapter 4 – Special Senses 9 image projected onto your retina can be cut down the middle, with the fovea defining the center. Now you have essentially two halves of the retina, a left half and a right half. Generally, the halves are referred to as a temporal half (next to your temple) and a nasal half (next to your nose). [Note] Specifically, fibers from the nasal retinas [medial optic nerves] cross over at the optic chiasm - whereas the temporal retinas [lateral optic nerves], already positioned to see the opposite side of the world, do not cross. In other words the medial optic fibers cross over, whereas lateral optic fibers do not. Visual field deficits • Optic nerve injury [A] Same sided blindness • Optic chiasm [B] Bi-temporal hemianopia • Optic tract [C] Homonymous hemianopia • Macular degeneration (doughnut) Vision loss in the center of your field of vision. Lesson 2 Chapter 4 – Special Senses • Cataracts 10 (aging, DM, sun exposure, trauma) Hearing 3 ossicles Mallcus, incus, stapes [axial skeleton] Vestibule Ultricle and saccule Vestibular Apparatus Utricle and saccule + Semi-circular canals Endolymph Perilymph Watery fluid in the membranous labyrinth of the ear. Watery fluid between the membranous & bony labyrinth of ear Lesson 2 Chapter 4 – Special Senses 11 The perylimph which fills the space within the bony labyrinth surrounding the membranous labyrinth, is similar, but not identical in composition to extracellular fluids of the body, such as cerebrospinal fluid. The concentration of sodium ions in the perylimph is high (about 150 milliequivalents per litter) and that of potassium ions is low (about 5 milliequivalents per litter). Endolymph is a totally unique extracellular fluid, which ion composition is unlike anywhere in the body. Potassium ions major in endolymph and virtually no sodium. Because endolymph is unlike any other, the processes which are maintained and regulated cannot be extrapolated by other fluid system. The total ion content of endolymph is significantly higher than that of perilymph. The Middle Ear The middle ear consists of an air-filled space between the tympanic membrane and the inner ear that contains three tiny bones linked together that connect the tympanic membrane to an opening into the inner ear. It also contains tiny ligaments and muscles that support and adjust tension of the bony chain. The Inner Ear The inner ear is comprised of two functionally separate sections: The "VESTIBULAR" or balance part and the cochlea, which is the hearing part. These two parts are interconnected and each serves its own vital function. Eustachian Tube Lesson 2 Chapter 4 – Special Senses 12 The Eustachian Tube ensures that the ears pressure remain at near standard ambient air pressure. It also drains any accumulated secretions, infection, or debris from the middle ear space. It is positioned from the middle ear to the pharynx. Hearing Mechanism Sound waves enter the outer ear and travel through the external auditory canal until they reach the tympanic membrane, causing the membrane and the attached chain of auditory ossicles to vibrate. The motion of the stapes against the oval window sets up waves in the fluids of the cochlea, causing the basilar membrane to vibrate. This stimulates the sensory cells of the organ of Corti, atop the basilar membrane, to send nerve impulses to the brain. - Cochlea duct is between 2 chambers (2 ducts): Vestibular duct and Tympanic duct - Oval window – at the base of vestibular duct - Round window – at the base of tympanic duct - Organ of Corti – sensory receptors for hearing [sitting on basilar membrane] - Tectorial membrane – on top of hairs Lesson 2 Chapter 4 – Special Senses Sound waves synapse with the 8th cranial nerve to the brain stem and medial geniculate nucleus of thalamus. Hearing  Pitch (tone) of sound  Depends on frequency of air waves  Intensity (loudness)  Depends on amplitude of air waves  Timbre (quality)  Determined by overtones 13 Lesson 2 Chapter 4 – Special Senses 14 Hearing Loss Qualified as mild, moderate, severe, or profound. – Conductive hearing loss (Traveling) • – Sensorineural hearing loss (Receptor potentials) • – Stimuli are not adequately transmitted Disorders that affect the inner ear, auditory nerve, or auditory pathways of the brain Mixed hearing loss • Osteosclerosis • Ear drum rupture • Ototoxic drugs (gentamycin, anti-TB, aminoglycosides) Major Components in Ear Outer Ear: The outer ear or external ear is the visible portion of the ear, which serves as a protective organ for the eardrum. It collects and guides sound waves into the middle ear. The outer ear consists of the following two parts. - Ear Flap (Pinna) - The sound waves enter the ear via the ear flap or pinna. Ear Canal (Meatus) - The ear canal is about 2 cm in length. It amplifies the sound waves and channelizes them to the middle ear. Sweat glands are present in this canal, which secrete earwax. Middle Ear: The middle ear, located between the outer ear and the inner ear, perceives sound waves from the outer ear in the form of pressure waves. The middle ear is an air-filled cavity and consists of the following parts. - - Eardrum (Tympanic membrane) - The eardrum is a thin membrane that acts as a partition between the outer ear and the middle ear. It vibrates as soon as it receives sound waves, and transforms the sound energy into mechanical energy. Hammer (Malleus) - It is a tiny bone, located next to the eardrum. Since it lies adjacent to the eardrum, the vibrations from the eardrum cause the hammer to vibrate. Anvil (Incus) - Anvil is another tiny bone next to hammer; it vibrates in response to the vibration of hammer. Stirrup (Stapes) - Similar to hammer and anvil, stirrup is a tiny bone in the middle ear. Eventually, it also vibrates and passes the compressional waves to the inner ear. Lesson 2 Chapter 4 – Special Senses 15 Inner Ear (Labyrinth): The inner ear, as the name suggests, is the innermost portion of the ear. It is filled with a water-like substance and comprises both hearing and balancing organs. The inner ear comprises the following parts. - - Cochlea - The cochlea or the spiral tube is a rolled structure that can stretch to about 3 cm. The membrane lining of cochlea consists of numerous nerve cells. These hairlike nerve cells respond differently to various frequencies of vibrations, which ultimately lead to generation of electrical impulses. Semicircular Canals - These are fluid-filled loops, attached to the cochlea and help in maintaining the balance. Auditory Nerve - The electrical impulses, generated by the nerve cells, are then passed to the brain. Chemical Senses Taste and smell • Receptors are chemoreceptors Taste Perception • Influenced by information derived from other receptors, especially odour • Temperature and texture of food influence taste • Psychological experiences associated with past experiences with food influence taste • How cortex accomplishes perceptual processing of taste sensation is currently unknown Lesson 2 Chapter 4 – Special Senses 16 Congenital deafblindness In the past, rubella (German measles) was the leading cause of congenital deafblindness. Rubella is a highly infectious virus that can seriously damage a pregnant woman's unborn baby. In particular, the baby's eyes, ears and heart can be damaged. However, since routine rubella vaccinations were introduced in 1988, and due to the success of the MMR vaccine, the number of babies affected by rubella has decreased significantly. Problems associated with premature birth are now a more common cause of deafblindness. Smoking and excessive alcohol consumption are both major risk factors for premature birth, so you should avoid both if you are pregnant. Acquired deafblindness Acquired deafblindness is where a person is born without a hearing or sight impairment, or a single sensory impairment, and loses both senses or part or all of their other sense later. This can be due to age, illness or injury. Sometimes, a child born deaf can gradually begin to lose their sight. This can be caused by a condition called Usher syndrome, where the retina (the light-sensitive layer of cells that line the back of the eye) gradually deteriorates. As a person gets older, their hearing will often start to deteriorate as the cells inside their ear become damaged and are unable to repair themselves. A similar process can also happen with a person's sight. Over time, it can gradually deteriorate as the cells at the centre of the retina start to break down. This process is often referred to agerelated macular degeneration. There are a number of other eye conditions that can cause sight loss if they are not identified and treated at an early stage. Some of the more common ones include:  cataracts – where cloudy patches develop in the lens of the eye, making a person's vision misty or blurred; cataracts can usually be removed with cataract surgery  glaucoma – where the tiny drainage tubes in the eye become blocked, causing pressure to build-up inside the eye that can damage the optic nerve (the nerve that transmits images from your eye to your brain)  retinopathy – a number of eye disorders that damage the tiny blood vessels of the retina; diabetic retinopathy is a common complication of diabetes where the cells of the retina are damaged by high blood sugar levels