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Anatomy & Physiology I Lecture 14 Chapter 15: The Special Senses: A Brief Overview
The Senses
• • • • • • Special sensory receptors – Distinct, localized receptor cells in head Vision Taste Smell Hearing Equilibrium
Eyebrow Eyelid Eyelashes Site where conjunctiva merges with cornea Palpebral fissure
Figure 15.1a The eye and accessory structures.
Iris Eyelid Pupil Sclera (covered by conjunctiva) Surface anatomy of the right eye
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Figure 15.1b The eye and accessory structures.
Levator palpebrae superioris muscle Orbicularis oculi muscle Eyebrow Tarsal plate Palpebral conjunctiva Tarsal glands Cornea Palpebral fissure Eyelashes Bulbar conjunctiva Conjunctival sac Orbicularis oculi muscle Lateral view;
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some structures shown in sagittal section
Conjunctiva
• Transparent mucous membrane – Produces a lubricating mucous secretion – Palpebral conjunctiva lines eyelids – Bulbar conjunctiva covers white of eyes
Structure of the Eyeball
• Wall of eyeball contains three layers – Fibrous – Vascular – Inner • Internal cavity filled with fluids called humors
Figure 15.4a Internal structure of the eye (sagittal section).
Ora serrata Ciliary body Ciliary zonule (suspensory ligament) Cornea Iris Pupil Anterior pole Anterior segment (contains aqueous humor) Sclera Choroid Retina Macula lutea Fovea centralis Posterior pole Optic nerve Lens Scleral venous sinus Posterior segment (contains vitreous humor) Central artery and vein of the retina Optic disc (blind spot) Diagrammatic view.
The vitreous humor is illustrated only in the bottom part of the eyeball.
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Fibrous Layer
• • Outermost layer; dense avascular connective tissue Two regions: – Sclera – Cornea
Fibrous Layer
• • Sclera – – Opaque posterior region Protects, shapes eyeball; anchors extrinsic eye muscles Cornea – Bends light as it enters eye – Sodium pumps of corneal endothelium on inner face help maintain clarity of cornea – Numerous pain receptors contribute to blinking and tearing reflexes
Vascular Layer
• • Middle pigmented layer Three regions: – Choroid – Ciliary body – Iris
Vascular Layer
• Choroid region – Supplies blood to all layers of eyeball – Brown pigment absorbs light to prevent light scattering and visual confusion
Vascular Layer
• Ciliary body – Ring of tissue surrounding lens – Smooth muscle bundles (ciliary muscles) control lens shape
Vascular Layer
• Iris – Colored part of eye – Pupil—central opening that regulates amount of light entering eye
Parasympathetic +
Figure 15.5 Pupil constriction and dilation, anterior view.
Sympathetic + Sphincter pupillae muscle contracts: Pupil size decreases.
Iris (two muscles) • Sphincter pupillae • Dilator pupillae Dilator pupillae muscle contracts: Pupil size increases.
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Inner Layer (Retina)
• • Delicate two-layered membrane: Outer Pigmented layer – Absorbs light and prevents its scattering • Inner Neural layer – Transparent – Composed of three main types of neurons: • Photoreceptors, bipolar cells, ganglion cells
Pathway of light
Figure 15.6a Microscopic anatomy of the retina.
Neural layer of retina Optic disc Central artery and vein of retina Pigmented layer of retina Choroid Sclera Optic nerve Posterior aspect of the eyeball
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Ganglion cells Axons of ganglion cells
Figure 15.6b Microscopic anatomy of the retina.
Bipolar cells Photoreceptors • Rod • Cone Amacrine cell Horizontal cell Pathway of signal output Pathway of light Pigmented layer of retina Cells of the neural layer of the retina
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Photoreceptors
• • Rods – Dim light, peripheral vision receptors – More numerous, more sensitive to light than cones – No color vision or sharp images Cones – Vision receptors for bright light – High-resolution color vision
Light And Optics
• • Eyes respond to visible light – Small portion of electromagnetic spectrum – Wavelengths of 400-700 nm Light – Packets of energy (photons or quanta) that travel in wavelike fashion at high speeds – Color of light objects reflect determines color eye perceives
Figure 15.10 The electromagnetic spectrum and photoreceptor sensitivities.
10 –5 nm 10 –3 nm 1 nm 10 3 nm 10 6 nm (10 9 nm =) 1 m 10 3 m Gamma rays X rays UV Infrared Micro waves Radio waves Visible light 100 Blue cones (420 nm) Rods (500 nm) Green cones (530 nm) Red cones (560 nm) 50
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0 400 450 500 550 600 Wavelength (nm) 650 700
Refraction
• Bending of light rays – Due to change in speed when light passes from one transparent medium to another – Occurs when light meets surface of different medium at an oblique angle • Curved lens can refract light
Figure 15.12 Light is focused by a convex lens.
Point sources Focusing of two points of light.
Focal points The image is inverted—upside down and reversed.
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Vision and Distance
• • Eyes best adapted for distant vision Far point of vision – Distance beyond which no change in lens shape needed for focusing – Cornea and lens focus light precisely on retina – Ciliary muscles relaxed
Figure 15.13a Focusing for distant and close vision.
Sympathetic activation Nearly parallel rays from distant object Lens Ciliary zonule Ciliary muscle Inverted image Lens flattens for distant vision.
relaxes the ciliary muscle, tightening the ciliary zonule, and flattening the lens.
Sympathetic input
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Focusing and Short Distance
• Light from close objects (<6 m) diverges as approaches eye – Requires eye to make active adjustments using three simultaneous processes – Accommodation of lenses – Constriction of pupils – Convergence of eyeballs
Figure 15.13b Focusing for distant and close vision.
Divergent rays from close object Parasympathetic activation Inverted image Lens bulges for close vision.
Parasympathetic input contracts the ciliary muscle, loosening the ciliary zonule, allowing the lens to bulge.
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Functional Anatomy Of Photoreceptors
• Rods and cones – Modified neurons – Contain visual pigments (photopigments) – Molecules change shape as absorb light – Process light as action potentials to reach the optic nerve
Rods
• Functional characteristics – Very sensitive to light – Best suited for night vision and peripheral vision – Contain single pigment – Perceived input in gray tones only
Cones
• Functional characteristics – Need bright light for activation (have low sensitivity) – React more quickly – Have one of three pigments for colored view – Detailed, high-resolution vision • Color blindness–lack of one or more cone pigments
Chemistry Of Visual Pigments
• Retinal – Light-absorbing molecule that combines with one of four proteins (opsins) to form visual pigments – Synthesized from vitamin A • Retinal isomers – 11-cis-retinal (bent form) – All-trans-retinal (straight form)
Rhodopsin Pigment
• • Deep purple pigment of rods 11-cis-retinal + opsin = rhodopsin
Figure 15.15a Photoreceptors of the retina.
Process of bipolar cell Inner fibers Rod cell body Cone cell body Outer fiber The outer segments of rods and cones are embedded in the pigmented layer of the retina.
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Melanin granules Synaptic terminals Rod cell body Nuclei Mitochondria Connecting cilia Apical microvillus Discs containing visual pigments Discs being phagocytized Pigment cell nucleus Basal lamina (border with choroid)
Figure 15.15b Photoreceptors of the retina.
Rod discs Visual pigment consists of • Retinal • Opsin
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Rhodopsin, the visual pigment in rods, is embedded in the membrane that forms discs in the outer segment.
Phototransduction
• Light converts 11-cis retinal to All-trans-retinal – Activates a G protein signal transduction pathway – Activates Phosphodiesterase to convert cGMP to GMP – Loss of GMP closes Na/Ca channels from ions entering cell resulting in hyperpolarization • Similar process of both rods and cones
Figure 15.16 The formation and breakdown of rhodopsin.
11-cis-retinal 2H + 1 Pigment synthesis: Oxidation 11-cis-retinal, derived from vitamin A, is Vitamin A combined with opsin to form rhodopsin.
11-cis-retinal Reduction
Rhodopsin
Dark 2H + 3 Pigment regeneration: Enzymes slowly convert all-trans-retinal to its 11 cis form in cells of the pigmented layer; requires ATP.
Light Opsin and All-trans retinal
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O All-trans-retinal 2 Pigment bleaching: Light absorption by rhodopsin triggers a rapid series of steps in which retinal changes shape (11-cis to all trans) and eventually releases from opsin.
Figure 15.17 Events of phototransduction.
Slide 1 Visual pigment Light Recall from Chapter 3 that G protein signaling mechanisms are like a molecular relay race.
1 Retinal absorbs light and changes shape.
Visual pigment activates.
Light (1st Receptor G protein Enzyme messenger) 2nd messenger Phosphodiesterase (PDE) All-trans-retinal 11-cis-retinal Transducin (a G protein) cGMP-gated cation channel open in dark cGMP-gated cation channel closed in light 2 Visual pigment activates transducin (G protein).
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3 Transducin activates phosphodiesteras e (PDE).
4 PDE converts cGMP into GMP, causing cGMP levels to fall.
5 As cGMP levels fall, cGMP-gated cation channels close, resulting in hyperpolarization.
Figure 15.18 Signal transmission in the retina (1 of 2).
In the dark 1 cGMP-gated channels open, allowing cation influx.
Photoreceptor depolarizes.
2 Voltage-gated Ca channels open in synaptic terminals. 2+ 3 Neurotransmitter is released continuously.
4 Neurotransmitter causes IPSPs in bipolar cell.
Hyperpolarization results. 5 Hyperpolarization closes voltage-gated Ca 2+ channels, inhibiting neurotransmitter release. 6 No EPSPs occur in ganglion cell. 7 No action potentials occur along the optic nerve.
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Ca 2+ Na + Ca 2+ Photoreceptor cell (rod) Bipolar Cell Ganglion cell Slide 1
Figure 15.18 Signal transmission in the retina. (2 of 2).
Below, we look at a tiny column of retina.
The outer segment of the rod, closest to the back of the eye and farthest from the incoming light, is at the top.
In the light Photoreceptor cell (rod)
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Light Bipolar Cell Ganglion cell Light Ca 2+ 1 cGMP-gated channels close, so cation influx stops. Photoreceptor hyperpolarizes.
2 Voltage-gated Ca channels close in synaptic terminals. 2+ 3 No neurotransmitter is released. 4 Lack of IPSPs in bipolar cell results in depolarization. 5 Depolarization opens voltage-gated Ca 2+ channels; neurotransmitter is released. 6 EPSPs occur in ganglion cell. 7 Action potentials propagate along the optic nerve. Slide 1
Adapting to Bright Light
• Move from darkness into bright light – Both rods and cones strongly stimulated – Pupils constrict to lessen entering light – Large amounts of pigments broken down instantaneously, producing glare
Adapting to Darkness
• Move from bright light into darkness – Cones stop functioning in low-intensity light – Rod pigments bleached; system turned off – Rhodopsin accumulates in dark – Pupils dilate to allow more light in – Increased light allows for improved vision in dark settings
Visual Processing
• • Thalamus – Process for depth perception, cone input emphasized, contrast sharpened Primary visual cortex (striate cortex) – Neurons respond to dark and bright edges, and object orientation – Provide form, color, motion inputs to visual association areas
Depth Perception
• Both eyes view same image from slightly different angles • Three-dimensional results from cortical fusion of slightly different images • Requires input from both eyes
Figure 15.19 Visual pathway to the brain and visual fields, inferior view.
Both eyes Fixation point Supra chiasmatic nucleus Pretectal nucleus Right eye Left eye Optic nerve Lateral geniculate nucleus of thalamus Superior Occipital colliculus lobe (primary visual cortex) The visual fields of the two eyes overlap considerably.
Note that fibers from the lateral portion of each retinal field do not cross at the optic chiasma.
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Optic chiasma Optic tract Lateral geniculate nucleus Superior colliculus (sectioned) Uncrossed (ipsilateral) fiber Crossed (contralateral) fiber Optic radiation Corpus callosum Photograph of human brain, with the right side dissected to reveal internal structures.
The Sense of Smell
• Olfactory epithelium in roof of nasal cavity – Contains olfactory sensory neurons – Bundles of nonmyelinated axons of olfactory receptor cells form olfactory nerve (CN I)
Olfactory Receptors
• Humans can distinguish ~10,000 odors – ~400 "smell" genes active only in nose – Each encodes unique receptor protein – Protein responds to one or more odors – Each odor binds to several different receptors
Smell Transduction
• • • • Odorant binds to receptor activates G protein G protein activation cAMP synthesis cAMP activates Na+ and Ca2+ channels ion influx depolarizes cell
Figure 15.21 Olfactory transduction process. Slide 1 1 Odorant binds to its receptor.
Odorant Adenylate cyclase G protein (G olf ) cAMP cAMP Open cAMP-gated cation channel Receptor GDP 2 Receptor activates G protein (G olf ).
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3 G protein activates adenylate cyclase.
4 Adenylate cyclase converts ATP to cAMP.
5 cAMP opens a cation channel, allowing Na + and Ca 2+ influx and causing depolarization.
Taste Senses
• Receptor organs are taste buds – Most of 10,000 taste buds on tongue papillae • There are five basic taste sensations – Sweet—sugars, saccharin, alcohol, some amino acids, some lead salts – – Sour—hydrogen ions in solution Salty—metal ions (inorganic salts) – – Bitter—alkaloids such as quinine and nicotine; aspirin Umami—amino acids glutamate and aspartate
Activating Taste Receptors
• Binding of food chemical depolarizes taste cell membrane – neurotransmitter release – Initiates a generator potential that elicits an action potential • Different thresholds for activation – Bitter receptors most sensitive
Tasteduction
• Gustatory epithelial cell depolarization caused by – Salty taste due to Na + influx (directly causes depolarization) – Sour taste due to H + (by opening cation channels) – Unique receptors for sweet, bitter, and umami coupled to G protein activation • neurotransmitter ATP release
Role of Taste
• • • • Triggers reflexes involved in digestion Increase secretion of saliva into mouth Increase secretion of gastric juice into stomach May initiate protective reactions – Gagging – Reflexive vomiting
Figure 15.23 The gustatory pathway.
Gustatory cortex (in insula) Thalamic nucleus (ventral posteromedial nucleus) Pons Facial nerve (VII) Glossopharyngeal nerve (IX) Solitary nucleus in medulla oblongata Vagus nerve (X)
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Influences on Taste
• • Taste is 80% smell Thermoreceptors, mechanoreceptors, nociceptors in mouth also influence tastes – Temperature
The Ear: Hearing and Balance
• Three major areas of ear – External (outer) ear – hearing only – Middle ear (tympanic cavity) – hearing only – Internal (inner) ear – hearing and equilibrium
Figure 15.24a Structure of the ear.
External ear Middle ear Internal ear (labyrinth) Auricle (pinna) Helix Lobule External acoustic meatus The three regions of the ear
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Tympanic membrane Pharyngotympanic (auditory) tube
External Ear
• External ear – Funnels sound waves into auditory canal • External acoustic meatus (auditory canal) – Transmits sound waves to eardrum • Tympanic membrane (eardrum) – Boundary between external and middle ears – Connective tissue membrane that vibrates in response to sound – Transfers sound energy to bones of middle ear
Middle Ear (Tympanic Cavity)
• A small, air-filled, mucosa-lined cavity in temporal bone • Flanked laterally by eardrum • Flanked medially by bony wall containing oval (vestibular) and round (cochlear) windows
Figure 15.24b Structure of the ear.
Oval window (deep to stapes) Entrance to mastoid antrum in the epitympanic recess Auditory ossicles Malleus (hammer) Incus (anvil) Stapes (stirrup) Tympanic membrane Round window Middle and internal ear
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Semicircular canals Vestibule Vestibular nerve Cochlear nerve Cochlea Pharyngotympanic (auditory) tube
Ear Ossicles
• Three small bones in tympanic cavity: the malleus, incus, and stapes – Suspended by ligaments and joined by synovial joints – Transmit vibratory motion of eardrum to oval window
Internal Ear
• Two primary divisions – Semicircular canals – Cochlea
Semicircular Canals
• Three canals (anterior, lateral, and posterior) that each define ⅔ circle – Lie in three planes of space (x, y and z) – Receptors respond to angular (rotational) movements of the head – Work in tandem with eyes and muscles for coordination, balance, positioning, and movement
The Cochlea
• • A spiral, conical, bony chamber Transmits sound waves via hair cells to the cochlear branch of CN VIII
Figure 15.27a Anatomy of the cochlea.
Helicotrema at apex Modiolus Cochlear nerve, division of the vestibulocochlear nerve (VIII) Spiral ganglion Osseous spiral lamina Vestibular membrane Cochlear duct (scala media)
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Properties of Sound
• Sound is – Pressure disturbance (alternating areas of high and low pressure) produced by vibrating object • Sound wave – Moves outward in all directions – Illustrated as an S-shaped curve or sine wave
Figure 15.28 Sound: Source and propagation.
Wavelength Area of high pressure (compressed molecules) Area of low pressure (rarefaction) Crest Trough Distance Amplitude A struck tuning fork alternately compresses and rarefies the air molecules around it, creating alternate zones of high and low pressure.
Sound waves radiate outward in all directions.
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Figure 15.29 Frequency and amplitude of sound waves.
High frequency (short wavelength) = high pitch Low frequency (long wavelength) = low pitch 0.01
Time (s) 0.02
Frequency is perceived as pitch.
High amplitude = loud Low amplitude = soft 0.03
0.01
Time (s) 0.02
0.03
Transmission of Sound
• • • • Sound waves vibrate tympanic membrane Ossicles vibrate and amplify pressure within internal ear Cochlear fluid set into wave motion Wave vibration activates action potential
Figure 15.30a Pathway of sound waves and resonance of the basilar membrane.
Auditory ossicles Malleus Incus Stapes Cochlear nerve Oval window Scala vestibuli Helicotrema 4a 2 3 Slide 1 4b Scala tympani Cochlear duct Basilar membrane 1 Tympanic membrane Round window Route of sound waves through the ear 1 Sound waves vibrate the tympanic membrane.
2 Auditory ossicles vibrate. Pressure is amplified.
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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.
Figure 15.32 The auditory pathway.
Medial geniculate nucleus of thalamus Primary auditory cortex in temporal lobe Inferior colliculus Lateral lemniscus Superior olivary nucleus (pons medulla junction) Cochlear nuclei
Vibrations Vibrations
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Midbrain Medulla Vestibulocochlear nerve Spiral ganglion of cochlear nerve Bipolar cell Spiral organ
Auditory Processing
• Pitch – impulses from specific hair cells in different positions along membrane • Loudness – by increased numbers of action potentials that result when hair cells experience larger deflections • Localization of sound – relative intensity and timing of sound waves reaching both ears
End
• No lab • Student Presentations