Sensory Systems PDF

CHAPTER 12 Sensory Systems Somatic sensations Hearing and deafness Vestibular apparatus Taste Smell Vision and vision disorders Somatic sensations: sensory neurons in the skin Neurons in skin detect: light touch, deep pressure, wrapped around hairs, high and low frequency vibrations,...

CHAPTER 12 Sensory Systems Somatic sensations Hearing and deafness Vestibular apparatus Taste Smell Vision and vision disorders Somatic sensations: sensory neurons in the skin Neurons in skin detect: light touch, deep pressure, wrapped around hairs, high and low frequency vibrations, temperature “Fast” pain receptors detect: excessive heat / excessive cold, excessive pressure (they are immediately stimulated when you hurt yourself) “Slow” pain receptors: damaged tissue releases chemical signals that bind to these receptors, and they send pain signals days / weeks after you are hurt. Somatic sensations: stretch receptors in muscles / joints Stretch receptors: Associated with joints, muscles and tendons The brain knows where body parts are in space, due to which receptors are stretched and which are not. Special senses: specialized structures in one part of the body. Hearing Balance Taste Smell Vision  Hearing With our sense of hearing, we can distinguish between sounds of different: Amplitude (loudness) Tone (frequency) (higher tones = higher frequency)  Hearing: outer and middle ears Outer ear: sound waves travel down the auditory canal, and hit the eardrum (tympanic membrane) causing it to vibrate. Middle ear: Vibrations of the typmpanic membrane cause vibrations in three small bones: malleus (hammer) incus (anvil) and stapes (stirrup). The stapes vibrates against the oval window, which is smaller than the tympanic membrane. All of the vibrational energy that hits the tympanic membrane is focused on the oval window. This amplifies the sound about 20 times. Without this, you could not hear some low-amplitude (soft) sounds.  Eustachian tubes Connect middle ear and pharynx, and equalize air pressure between middle ear and outside atmosphere. Yawning / chewing opens eustachian tubes. Ears “pop” when ascending or descending in the mountains, or in an airplane, as examples. Eustachian tubes are a different shape when people are very young, and bacteria from the pharynx can easily move up the tube and infect the middle ear. In some cases, middle ear infections can lead to deafness.  Hearing: inner Ear Function: Hair cell mechanoreceptors within the cochlea converts sound vibrations into nervous impulses, which travel to the brain via neurons and are interpreted as sound by the brain. Hearing: inner Ear Three fluid filled canals within cochlea: Vestibular canal, cochlear duct, tympanic canal Within cochlear duct is the organ of corti, which consists of: Tectorial membrane, hair cells, basilar membrane and auditory nerves  Hearing: The inner inner ear ear and hearing 1) Oval window: creates pressure waves in fluid of the vestibular canal these travel around to the tympanic canal. 2) Wave formed across basilar membrane, it vibrates the most at the point where the natural resonance of the basilar membrane = sound vibration frequency 3) Hair cells are a part of the basilar membrane. Tips of the hairs brush against the tectorial membrane, that does not vibrate. 4) The hairs of the hair cells bend, and the hair cells release neurotransmitters that stimulate sensory neurons. 5) The neurons take the message to the brain, and the brain interprets these neural impulses as sound. Structures and function of the cochlea Organ of Corti Distinguishing different frequencies (tones) Different parts of the basilar membrane have different natural resonances: Basilar membrane near oval window is stiff and narrow: high frequency noises cause vibration. Basilar membrane far from oval window is more flexible and wider: low frequency noises cause vibration. Different parts of the basilar membrane vibrate in response to different sound frequencies. The brain interprets neural signals arriving from different parts of the basilar membrane as different tones (notes). Distinguishing different amplitudes (loudness) Louder noises: cause more vigorous vibrations of the basilar membrane (more energy). More hair cells release neurotransitters more frequently, and more neurons are stimulated more frequently. The brain interprets this as a louder sound. Softer noises: cause less vigorous vibrations of the basilar membrane (less energy). Less hair cells release neurotransitters less frequently, and less neurons are stimulated less frequently. The brain interprets this as a softer sound. Deafness / hearing loss Conduction deafness, sound vibrations do not get to the inner ear: Earwax Eardrum / oval window ruptured Bones in ear do not move properly (bacterial infection can lead to scar tissue on the bones) Hearing aids cause vibrations in skull  vibrations in the cochlea  hearing Deafness / hearing loss Nerve deafness: Cochlear nerves damaged Brain damaged Hairs in cochlea lost (partial deafness) Minor damage: can be repaired. Major damage: Hair cells die = permanent hearing loss (for certain notes). “Ringing” ears after loud environment: hairs have been damaged some are bent, and cause you to hear sounds that do not really exist. The ear bone reflex: muscles pull bones away from eardrum / oval window: reduces conduction of loud noises Deafness /hairs Damaged hearing in cochlea loss Vestibular apparatus Three semicircular canals and vestibule Sensing rotational movement (semicircular canals): ampulla with mechanoreceptors in cupula Sensing head position, gravity and acceleration / deceleration (vestibule): utricle and saccule with otoliths Semicircular canals detect head movement movement Semicircular canals in 3 planes: 3-D movement detected. Cupula with hair cells present at base (ampulla) of each canal. Head movement moves hair cells immediately, fluid moves next, bending cupula (friction)  bending hairs  hair cells stimulate neurons. Why do we get dizzy when we spin around? Any volunteers to demonstrate? : )  Vestibule senses head position and acceleration/deceleration Oriented vertically and horizontally Hairs embedded in gelatinous material (on bottom). Otoliths embedded in gelatinous material (on top). Tilting head  heavy otoliths pulling gelatinous material downward bending hairs  hair cells stimulate neurons. Acceleration: gelatinous material moves first, otoliths have inertia and respond slower, hair cells bent (think of an accelerating car). Deceleration: gelatinous material stops moving first, otoliths have momentum and respond slower, and hair cells are bent (think of a sudden stop in car) Vestibule  Maintaining balance Visual input Signals from joint, muscle and tendon stretch receptors Specialized structures in inner ear: vestibule Neurons sense pressure in toes When messages from these sources are conflicting, we can get motion sickness. What are typical situations when we get motion sickness?  Taste buds Taste buds on large papillae of tongue with 25 taste cells and 25 supporting cells “Hairs” on taste cells have receptor proteins to bind to certain chemicals Taste cells then stimulate sensory neurons  Taste Taste buds: chemoreceptors that bind with dissolved substances. Taste categories: sweet, salty, sour, bitter and umami (protein broth) (combinations of these 5: allow hundreds of individual tastes) When chemicals bind to taste hairs, taste cells release neurotransmitters to sensory neurons. “Salt” responds to sodium ions, “sour” responds to hydrogen ions, “sweet” respond to sugar, “bitter” respond to various chemicals, “umami” responds to amino acids  We are most sensitive to “bitter” tastes, and many bitter sensitive taste buds are at the back of the tongue. Why?  Smell Olfactory receptor cells: Each olfactory receptor cell produces a protein that can bind with odorant molecules Humans have over 300 different types of olfactory receptor cells, each with its own protein receptors. Some odorant molecules bind with more than one type of olfactory receptor cell, so we can distinguish more than 300 scents. More molecules binding = more neurons firing more frequently = stronger smell Sensitive: in some cases a single odorant molecule can lead to a neuron being stimulated. Our sense of smell adapts quickly Olfactory Receptors  “Taste” = smell + taste buds What we commonly refer to as how food “tastes” depends on taste and smell: chewed food releases chemicals, contact olfactory receptors via pharynx. Odor + tastes = many different “taste sensations” “Taste” impeded with a cold (too much mucus blocks receptors) sweet, sour, salty, bitter and umami still work  Vision Eyes focus light onto specialized photoreceptor cells of the retina: rod cells and cone cells Photoreceptors: light energy converted to nerve impulses  transmitted to brain for interpretation (primary visual cortex in occipital lobe)  Sclera and cornea Sclera = whites of eyes Muscles controlling eye movement attached to sclera Cornea = clear part in front, most bending of light before light hits the retina occurs at the cornea Astigmatism: irregularities in cornea or lens LASIK laser surgery burns away part of cornea to fix nearsightedness and astigmatism Sclera and cornea Iris and pupil Iris = a colored muscle that controls the amount of light entering the eye Pupil = the opening that allows light in. Pupils are smaller in bright sun, larger in dark. Iris and pupil  Lens and ciliary muscle Lens: made of layers of transparent proteins, focuses light on retina Adjusts shape for close or far away objects Close objects: light needs to be bent more  ciliary muscles contract  lens gets thicker Far away objects: light needs to be bent less  ciliary muscles relax  lens gets thinner Lens and ciliary muscle  Focusing Near object Far object Nearsightedness and far sightedness “Nearsighted” because: 1) Eyes are longer than normal, or 2) Ciliary muscles will not relax to look at far objects This inability to relax can be caused by doing too much prolonged close-up work “Farsighted” because: 1) Eye are shorter than normal, or 2) Ciliary muscles will not contract enough to look at close objects Usually both are caused by improper eye shape, that often occurs during puberty  Nearsightedness and farsightedness Normal Nearsighted (too long) Farsighted (too short) Near object  less thickening  lens needs to get necessary thicker, but can’t Far object  lens needs to get  doesn’t need to get thinner, but can’t as thin  Lens problems Starting at about 40 years old, the lens starts to lose its flexibility, so focusing becomes harder. The lens usually loses all flexibility by age 70. Aging can cause cataracts too, when the proteins of the lens progressively lose their transparency. Lens transplants would be needed to replace the cloudy lenses.  Light through the eye: review 1) Light through cornea, which bends it (non-adjustable) then passes through aqueous humor. 2) Light passes through pupil formed by the iris (opens or closes to control amount of light reaching retina) 3) Light passes through lens, which is flexible (adjustable) focuses light on the retina. 4) Light passes through jelly-like vitreous humor before striking retina  Retina Retina contains photoreceptor cells: light converted to nerve impulses. 4 types of photoreceptors: Rods (black and white) Cones most sensitive to either: yellow, green or blue/violet Fovea on retina: look directly at object = light focused here, high amounts of cones, no rods. Clear, color vision at fovea “Blind spot” on retina: optic nerve goes to brain Retina, fovea and “blind spot” Structure of the Retina Figure 12.16 Slide 12.13A Cone cells: color vision but not as sensitive to light. Fewer cones cells / ganglion cell = clearer vision. In fovea as little as 5 cone cells / ganglion cell = clearest vision. Rod cells: only black and white, but more sensitive to light (300 times more sensitive). Many rod cells / ganglion cell = blurrier vision. During night time, you only see in black, white and gray: only rods are sensitive to faint light. Night vision is blurrier too. Seeing colors Cones are “best” at absorbing either yellow, green or blue/violet light. “Mixed” colors = brain interprets ratios of the 3 cone types stimulated in an area. Seeing colors  Rod cells: several hundred rod cells / ganglion cell Cone cells: several dozen cone cells / ganglion cell Cone cells in fovea: as few as five cone cells / ganglion cell Structure of the Retina Figure 12.16 Slide 12.13A Disorders affecting the retina Retinal detachment: due to blow to head / eye, this is an emergency! Blurred vision, stars and lack of peripheral vision results Glaucoma: Fluid in aqueous humor no longer drains properly  pressure on capillaries supplying retina. Retinal and nerve cell death can result. Color blindness: Certain cone cells are present in low amounts, or are missing completely (sex-linked genetic disorder) so people cannot distinguish between certain colors. Aqueous humor Fun with the senses lab Stare at the dot for 30 seconds, then record the colors of the “after image” you see on the white screen !! Cells associated with yellow, blue and green cone cells. By staring at something these cells become fatigued, but other 2 types are not. White = combination of all 3 colors. When blue cells are fatigued: white – blue = yellow/orange When yellow cells fatigued: white – yellow = navy blue When green cells fatigued: white – green = pink Binocular vision and depth perception Binocular vision Each eye receives slightly different images (look at something up close through one eye, then the other rapidly). This helps us determine how far away it is At the side of the head: sound waves hit one ear, then the next Above and behind the head: sound waves hit both ears, at the same time Blind spot and optic nerve Figure 12.16 Slide 12.13A  Chiras DD Human biology, Health, Homeostasis, and the environment,Jones and Bartlett, Sudbury, Mass, 2002