Special senses

Pathway of Sound Waves

1) Sound waves vibrate the tympanic membrane 2) Auditory ossicles vibrate. Pressure is amplified 3) Pressure waves created by the stapes pushing on the oval window move through fluid in the scala vestibuli 4a) Sounds with frequencies below hearing travel through the helicotrema and do not excite hair cells 4b) Sounds in the hearing range go through the cochlear duct, vibrating the basilar membrane and deflecting hairs on inner hair cells

smell transduction

15.22 odor binds to receptor > G-proteins activated > G protein activats adenylate cyclase > adenylate cyclase converts ATP to cAMP > cAMP opens a cation channel allowing Na+ and Ca++ influx and causing depolarization

vision

70% of body's sensory receptors are in eye Half of cerebral cortex is involved in visual processing Small sphere; only one-sixth of surface visible Most of eye enclosed and protected by fat cushion and bony orbit Consists of accessory structures and the eyeball

Constriction of the pupils

Accommodation pupillary reflex involves constriction of pupils to prevent most divergent light rays from entering eye Mediated by parasympathetic nervous system

Olfactory Pathology

Anosmias: olfactory disorders; most result from: Head injuries that tear olfactory nerves Aftereffects of nasal cavity inflammation Neurological disorders, such as Parkinson's disease Olfactory hallucinations Usually caused by temporal lobe epilepsy that involves olfactory cortex Some people have olfactory auras prior to epileptic seizures

basilar membrane

Basilar membrane changes along its length: Fibers near oval window are short and stiff Resonate with high-frequency waves Fibers near cochlear apex are longer, floppier Resonate with lower-frequency waves So basilar membrane mechanically processes sound even before signals reach receptors

Activating receptors of crista ampullaris

Bending hairs in cristae causes depolarization Rapid impulses reach brain at faster rate Bending of hairs in opposite direction causes hyperpolarizations Fewer impulses reach brain Thus brain is informed of head rotations

The lens

Biconvex, transparent, flexible, and avascular Changes shape to precisely focus light on retina Two regions: Lens epithelium: anterior region of cuboidal cells that differentiate into lens fiber cells Lens fibers: form bulk of lens and are filled with transparent protein Lens fibers are continually added, so lens becomes more dense, convex, and less elastic with age

Physiology of Taste: Activation of taste receptors

Binding of food chemical (tastant) depolarizes cell membrane of gustatory epithelial cell membrane, causing release of neurotransmitter Neurotransmitter binds to dendrite of sensory neuron and initiates a generator potential that lead to action potentials Different gustatory cells have different thresholds for activation Bitter receptors are most sensitive All adapt in 3-5 seconds, with complete adaptation in 1-5 minutes

Depth perception

Both eyes view same image from slightly different angles Visual cortex fuses these slightly different images, resulting in a three-dimensional image, which leads to depth perception Requires input from both eyes

olfactory adaptation

Ca2+ influx causes decreased response to a sustained stimulus,

Accommodation of the lenses

Changing lens shape to increase refraction

Near point of vision

Closest point on which the eye can focus

Lenses

Convex lenses bend light passing through it, so that rays converge at focal point Image formed at focal point is upside-down and reversed from left to right Concave lenses disperse light, preventing light from being focused The human lens is a double convex lens

Focusing Light on the Retina

Cornea and lens focus light precisely on retina at this distance Ciliary muscles are completely relaxed in distance vision, which causes a pull on ciliary zonule; as a result, lenses are stretched flat

Activating receptors of crista ampullaris

Cristae respond to changes in velocity of rotational movements of head Inertia in ampullary cupula causes endolymph in semicircular ducts to move in direction opposite body's rotation, causing hair cells to bend

Rods

Dim light, peripheral vision receptors More numerous and more sensitive to light than cones No color vision or sharp images Numbers greatest at periphery

Taste buds

Each taste bud consists of 50-100 flask-shaped epithelial cells of two types: Gustatory epithelial cells: taste receptor cells have microvilli called gustatory hairs that project into taste pores, bathed in saliva Sensory dendrites coiled around gustatory epithelial cells send taste signals to brain Three types of gustatory epithelial cells Basal epithelial cells: dynamic stem cells that divide every 7-10 days

Equilibrium & the membranous labyrinth

Equilibrium is response to various movements of head that rely on input from inner ear, eyes, and stretch receptors

Myopia (nearsightedness)

Eyeball is too long, so focal point is in front of retina Corrected with a concave lens

Hyperopia (farsightedness)

Eyeball is too short, so focal point is behind retina Corrected with a convex lens

Focusing for distant vision

Eyes are best adapted for distant vision

Layers of the eye

Fibrous layer Vascular layer Inner layer Internal cavity filled with fluids called humors Lens separates internal cavity into anterior and posterior segments

aqueous humor

Found in anterior segment of the eye, is a plasma like fluid continuously formed (unlike vitreous humor) by capillaries of ciliary processes Drains via scleral venous sinus (canal of Schlemm) at sclera-cornea junction Supplies nutrients and oxygen mainly to lens and cornea but also to retina, and removes wastes

Physiology of Taste: Taste Transduction

Gustatory epithelial cell depolarization caused by: Salty taste is due to Na+ influx that directly causes depolarization Sour taste is due to H+ acting intracellularly by opening channels that allow other cations to enter Unique receptors for sweet, bitter, and umami, but all are coupled to G protein gustducin Activation causes release of stored Ca2+ that opens cation channels, causing depolarization and release of neurotransmitter ATP

Sound detection

Hearing is the reception of an air sound wave that is converted to a fluid wave that ultimately stimulates mechanosensitive cochlear hair cells that send impulses to the brain for interpretation

smell

In order to smell substance, it must be volatile Must be in gaseous state Odorant must also be able to dissolve in olfactory epithelium fluid Activation of olfactory sensory neurons Dissolved odorants bind to receptor proteins in olfactory cilium membranes Open cation channels, generating receptor potential At threshold, AP is conducted to first relay station in olfactory bulb

Vascular layer of the eye

Iris Colored part of eye that lies between cornea and lens, continuous with ciliary body Pupil: central opening that regulates amount of light entering eye Close vision and bright light cause sphincter pupillae (circular muscles) to contract and pupils to constrict; parasympathetic control Distant vision and dim light cause dilator pupillae (radial muscles) to contract and pupils to dilate; sympathetic control Changes in emotional state—pupils dilate when subject matter is appealing or requires problem-solving skills

Focusing for close vision

Light from close objects (<6 m) diverges as approaches eye Requires eye to make active adjustments using three simultaneous processes: Accommodation Near point of vision Closest point on which the eye can focus Presbyopia

Wavelength (color)

Light: packets of energy (photons or quanta) that travel in wavelike fashion at high speeds When visible light passes through spectrum, it is broken up into bands of colors (rainbow) Color that eye perceives is a reflection of that wavelength Grass is green because it absorbs all colors except green White reflects all colors, and black absorbs all colors

Orthonasal vs retronasal olfaction

MRI studies indicate that the brain processes smells going through the nose vs those through the mouth differently

Olfactory hair cells/olfactory cilia

Many tiny hair-like cilia protrude from the olfactory receptor cell's dendrite into the mucus covering the surface of the olfactory epithelium.

why dont we see upside down

Medial fibers from each eye cross over at the optic chiasma then continue on as optic tracts, which means each optic tract contains fibers from lateral (temporal) aspect of eye on same side and medial (nasal) aspect of opposite eye, and each carries information from same half of visual field

Convergence of the eyeballs

Medial rotation of eyeballs causes convergence of eyes toward object being viewed Controlled by somatic motor neuron innervation on medial rectus muscles

vascular layer of eyeball

Middle pigmented layer of eye, also called uvea Three regions: choroid, ciliary body, and iris Choroid region Posterior portion of uvea Supplies blood to all layers of eyeball Brown pigment absorbs light to prevent scattering of light, which would cause visual confusion Ciliary body Anteriorly, choroid becomes ciliary body Thickened ring of tissue surrounding lens Consists of smooth muscle bundles, ciliary muscles, that control shape of lens Capillaries of ciliary processes secrete fluid for anterior segment of eyeball Ciliary zonule (suspensory ligament) extends from ciliary processes to lens Holds lens in position

Sound transduction: Excitation of inner hair cells

Movement of basilar membrane deflects hairs of inner hair cells Cochlear hair cells have microvilli that contain many stereocilia (hairs) that bend at their base

ear

Nerve fibers coiled around hair cells of outer row are efferent neurons that convey messages from brain to ear Neural impulses from cochlear bipolar cells reach auditory cortex via following pathway: Spiral ganglion > Cochlear nuclei (medulla) > Inferior colliculus (midbrain auditory reflex center > thalamus > primary auditory cortex

perception of visual images

Optic tract >thalamus > visual cortex,

Fibrous layer of the eye

Outermost layer; dense avascular connective tissue Two regions: Sclera Opaque posterior region Protects and shapes eyeball Anchors extrinsic eye muscles Posteriorly, where optic nerve exits, sclera is continuous with dura mater of brain Cornea Transparent anterior one-sixth of fibrous layer Forms clear window that lets light enter and bends light as it enters eye Epithelium covers both surfaces Outer surface protects from abrasions Inner layer, corneal endothelium, contains sodium pumps that help maintain clarity of cornea Numerous pain receptors contribute to blinking and tearing reflexes

Path of light through the eye

Pathway of light entering eye: cornea, aqueous humor, lens, vitreous humor, entire neural layer of retina, and finally photoreceptors Light is refracted three times along path: (1) entering cornea, (2) entering lens, and (3) leaving lens Majority of refractory power is in cornea; however, it is constant and cannot change focus Lens is able to adjust its curvature to allow for fine focusing Can focus for distant vision and for close vision

Photoreceptor shape determines function

Photoreceptors are vulnerable to damage Degenerate if retina detached Destroyed by intense light Vision is maintained because outer segment is renewed every 24 hours Tips fragment off and are phagocytized

outer Pigmented layer of the retina

Pigmented layer of the retina Single-cell-thick lining next to choroid Extends anteriorly, covering ciliary body and iris Functions: Absorbs light and prevents its scattering Phagocytizes photoreceptor cell fragments Stores vitamin A

Semicircular canals & dynamic equilibrium

Receptor for rotational acceleration is crista ampullaris (crista) Small elevation in ampulla of each semicircular canal Cristae are excited by acceleration and deceleration of head Major stimuli are rotational (angular) movements, such as twirling of the body Semicircular canals are located in all three planes of space, so cristae can pick up on all rotational movements of head

Refraction

Refraction: bending of light rays Due to change in speed of light when it passes from one transparent medium to another and path of light is at an oblique angle Example: from liquid to air Lenses of eyes can also refract light because they are curved on both sides Convex: thicker in center than at edges Concave: thicker at edges than in cen

Inner layer of the eye (retina)

Retina originates as an outpocketing of brain Contains: Millions of photoreceptor cells that transduce light energy Neurons Glial cells Delicate two-layered membrane: Outer pigmented layer Inner neural layer

Rods and cones

Rhodopsin is so sensitive that bleaching occurs even in starlight In bright light, bleaching occurs so fast that rods are virtually nonfunctional Cones respond to bright light So, activation of rods and cones depends on: Light adaptation Dark adaptation

Olfactory receptor specificity

Smells may contain 100s of different odorants Humans have ~400 "smell" genes active in nose Each encodes a unique receptor protein Protein responds to one or more odors Each odor binds to several different receptors Each receptor has one type of receptor protein Pain and temperature receptors are also in nasal cavities Respond to irritants, such as ammonia, or can "smell" hot or cold (chili peppers, menthol)

Basic Taste Sensations

Sweet—sugars, saccharin, alcohol, some amino acids, some lead salts Sour—hydrogen ions in solution Salty—metal ions (inorganic salts); sodium chloride tastes saltiest Perception of salty flavor can be adjusted over time - persons on low Na diet have an increased sensitivity to salty foods Bitter—alkaloids such as quinine and nicotine, caffeine, and nonalkaloids such as aspirin Unable to distinguish between many different bitter flavors Umami—amino acids glutamate and aspartate; example: beef (meat) or cheese taste, and monosodium glutamate

Location and Structure of Taste Buds

Taste buds: sensory organs for taste Most of 10,000 taste buds are located on tongue in papillae, peglike projections of tongue mucosa Fungiform papillae: tops of these mushroom-shaped structures house most taste buds; scattered across tongue Foliate papillae: on side walls of tongue Vallate papillae: largest taste buds with 8-12 forming "V" at back of tongue Few on soft palate, cheeks, pharynx, epiglottis

Influence of Other Sensations on Taste

Taste is 80% smell If nose is blocked, foods taste bland Mouth also contains thermoreceptors, mechanoreceptors, and nociceptors Temperature and texture enhance or detract from taste Spicy hot foods can excite pain receptors in mouth, which some people experience as pleasure Example: hot chili peppers

Inner ear: Membranous & Bony labyrinth

The bony labyrinth is bony outer wall of the inner ear in the temporal bone. It consists of three parts: the vestibule, semicircular canals, and cochlea. These are cavities hollowed out of the substance of the bone, and lined by periosteum. They contain a clear fluid, the perilymph, in which the membranous labyrinth is situated.

The tapetum lucidum

The tapetum lucidum is a biologic reflector system that is a common feature in the eyes of vertebrates. It normally functions to provide the light-sensitive retinal cells with a second opportunity for photon-photoreceptor stimulation, thereby enhancing visual sensitivity at low light levels.

Olfactory stem cell = olfactory basal cells

These are the cells that are capable of differentiating into either supporting or olfactory cells.

Physiology of Taste

To be able to taste a chemical, it must: Be dissolved in saliva Diffuse into taste pore Contact gustatory hairs

vitreous humor

Transmits light Supports posterior surface of lens Holds neural layer of retina firmly against pigmented layer Contributes to intraocular pressure Vitreous humor forms in embryo and lasts whole lifetime

Inner neural layer of the retina

Transparent layer that runs anteriorly to margin of ciliary body Anterior end has serrated edges called ora serrata Composed of three main types of neurons: Photoreceptors, bipolar cells, ganglion cells Signals spread from photoreceptors bipolar cells ganglion cells Ganglion cell axons exit eye as optic nerve Optic disc: Site where optic nerve leaves eye; lacks photoreceptors, so referred to as blind spot Retina has quarter-billion photoreceptors that are one of two types: Rods Cones

Important roles of Taste

Triggering reflexes involved in digestion, such as: Increased secretion of saliva into mouth Increased secretion of gastric juice into stomach May initiate protective reactions, such as: Gagging Reflexive vomiting

Gustatory Pathway

Two main cranial nerve pairs carry taste impulses from tongue to brain: Facial nerve (VII) carries impulses from anterior two-thirds of tongue Glossopharyngeal (X) carries impulses from posterior one-third and pharynx Vagus nerve transmits from epiglottis and lower pharynx Fibers synapse in the solitary nucleus of the medulla, then travel to thalamus, and then to gustatory cortex in the insula Hypothalamus and limbic system are involved; allow us to determine appreciation of taste

Astigmatism

Unequal curvatures in different parts of cornea or lens Corrected with cylindrically ground lenses or laser procedures

Anatomy of a macula

Utricle maculae are horizontal with vertical hairs Respond to change along a horizontal plane, such as tilting head Forward/backward movements stimulate utricle Saccule maculae are vertical with horizontal hairs Respond to change along a vertical plane Up/down movements stimulate saccule (Example: acceleration of an elevator)

Cones

Vision receptors for bright light High-resolution color vision Macula lutea area at posterior pole lateral to blind spot Contains mostly cones Fovea centralis: tiny pit in center of macula lutea that contains all cones, so is region with best visual acuity Eye movement allows us to focus in on object so that fovea can pick it up

Overview: Light and Optics

Wavelength and color Electromagnetic radiation: all energy waves, from long radio waves to short X rays; visible light occupies a small portion in the middle of the spectrum Light has wavelengths between 400 and 700 nm Eyes respond only to visible light

Dark adaptation

When moving from bright light into darkness, we see blackness because: Cones stop functioning in low-intensity light Bright light bleached rod pigments, so they are still turned off Pupils dilate Rhodopsin accumulates in dark, so retinal sensitivity starts to increase Transducin returns to outer segments Sensitivity increases within 20-30 minutes

Light adaptation

When moving from darkness into bright light we see glare because: Both rods and cones are strongly stimulated Large amounts of pigments are broken down instantaneously, producing glare Pupils constrict Visual acuity improves over 5-10 minutes as: Rod system turns off Retinal sensitivity decreases Cones and neurons rapidly adapt

Mitral cells in the olfactory bulb

are second-order neurons. output of the mitral cells forms the olfactory tract, which projects to the cortex.

Olfactory epithelium

bipolar neurons Bundles of nonmyelinated axons of olfactory receptor cells gather in fascicles that make up filaments of olfactory nerve (cranial nerve I) Olfactory neurons, unlike other neurons, have stem cells that give rise to new neurons every 30-60 days

Far point of vision

distance beyond which no change in lens shape is needed for focusing

Vestibular apparatus

equilibrium receptors in semicircular canals and vestibule Vestibular receptors monitor static equilibrium Semicircular canal receptors monitor dynamic equilibrium

sound

is a pressure disturbance (alternating areas of high and low pressure) produced by a vibrating object and propagated by molecules of the medium (air)

Retinal

key light-absorbing molecule that combines with one of four proteins (opsins) to form visual pigments Synthesized from vitamin A Four opsins are rhodopsin (found in rods only), and three found in cones: green, blue, red (depending on wavelength of light they absorb) Cone wavelengths do overlap, so same wavelength may trigger more than one cone, enabling us to see variety of hues of colors

Presbyopia

loss of accommodation over age 50

Color-blindness

often a result of a malfunctioning cone that causes wavelengths to overlap even more, resulting in poor color discrimination. The EnChroma glasses use a filter to cut out these overlapping wavelengths, allowing for a clearer distinction between colors, especially red and green.

Maculae

sensory receptor organs that monitor static equilibrium One organ located in each saccule wall and one in each utricle wall Monitor the position of head in space Play a key role in control of posture Respond to linear acceleration forces, but not rotation

Tympanic membrane

sound waves enter external acoustic meatus and strike tympanic membrane, causing it to vibrate The higher the intensity, the more vibration

Basilar membrane path

sounds in hearing range go through cochlear duct, vibrating basilar membrane at specific location, according to frequency of sound

Scala vestibuli

stapes rocks back and forth on oval window with each vibration, causing wave motions in perilymph Wave ends at round window, causing it to bulge outward into middle ear cavity

Supporting cells

surround and cushion olfactory receptor cells

Auditory ossicles

transfer vibration of eardrum to oval window Tympanic membrane is about 20x larger than oval window, so vibration transferred to oval window is amplified about 20x

Helicotrema path

waves with frequencies below threshold of hearing travel through helicotrema and scali tympani to round window