Anatomy of mammalian ear

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Ch 6 Sensory Physiology Anatomy of mammalian ear Structure of the Mammalian Ear (external, middle, and inner) All parts of the ear are associated with temporal bone on the side of the skull 1.

1 Ch 6 Sensory Physiology Anatomy of mammalian ear Structure of the Mammalian Ear (external, middle, and inner) All parts of the ear are associated with temporal bone on the side of the skull 1. External (outer) Ear a. pinna (ear) or auricle 1. skin covering elastic cartilage 2. funnel-shaped to direct sound waves into external auditory meatus to increase hearing acuity 3. dogs will cock their ears towards a sound source to collect more sound waves b. external auditory meatus (ear canal) 2. Middle Ear a. tympanic membrane (eardrum, tympanum) b. tympanic cavity (middle ear cavity) air-filled cavity containing ossicles c. ear ossicles 1. 3 small bones: malleus, incus, and stapes 2. malleus is attached to the tympanum 3. stapes is attached to oval window on side of vestibule of inner ear 4. smallest bones in the body; possess synovial joint d. vibrations of the tympanum vibrate the chain of bones e. pharyngotympanic tube (Eustachian tube) connects the middle ear cavity to the upper part of pharynx (laryngopharynx) 1

3. Inner Ear: Sense of Hearing (cochlea) a. three parts: cochlea, vestibule and semicircular canals 1. vestibule a. the stapes sits over a flexible connective membrane called the oval window b.

2 3. Inner Ear: Sense of Hearing (cochlea) a. three parts: cochlea, vestibule and semicircular canals 1. vestibule a. the stapes sits over a flexible connective membrane called the oval window b. the oval window is on the side of the vestibule; it vibrates when the stapes vibrates 2. semicircular canals involved with one s sense of balance or equilibrium 3. cochlea is involved with the sense of hearing b. cochlea: system of fluid-filled tubes within the osseous labyrinth of the temporal bone; it consists of the cochlear duct, vestibular duct (scala vestibuli), helicotrema, and tympanic duct (scala tympani); they are collectively referred to as the membranous labyrinth. A labyrinth is a network of winding passageways. 1. cochlear duct (also called the scala media) a. middle compartment b. filled with endolymph (viscous fluid) c. floor of cochlear duct is the basilar membrane. Hair cells and bipolar sensory neurons make up the organ of Corti that sits on top of the basilar membrane 2. vestibular duct (scala vestibuli) a. begins at the oval window which is on the side of the vestibule and covered by the stapes b. upper compartment (above cochlear duct) c. filled with perilymph (watery fluid) 2

3 d. it is continuous at the apex of the cochlea with the scala tympani; the region at the top of the cochlea where the fluids in the scala vestibuli and the scala tympani mix is called the helicotrema 3. helicotrema part of the cochlear labyrinth where the scala vestibuli and the scala tympani meet; it is at the apex of the cochlear duct 4. scala tympani (also called tympanic duct) a. ends at round window which is on the side of the cochlea b. the scala tympani contains perilymph (watery fluid) c. the round window on side of cochlea is a flexible connective tissue membrane 1. As the stapes footplate moves into the oval window, it pushes on the perilymph which pushes on the round window membrane causing it to move out. This allows movement of the fluid within the cochlea, leading to movement of the cochlear inner hair cells and thus hearing 2. the movement of the round window back and forth in response to fluid movements of the perilymph acts dissipate the energy of the fluid pressure waves to reduce deflection waves that might cause echoes. 3. pressure waves in perilymph of vestibular duct causes fluid waves in perilymph of tympanic duct and endolymph of cochlear duct that vibrates the basilar membrane up and down 4. If the round window were to be absent or rigidly fixed (as can happen in some congenital abnormalities), the stapes footplate would be pushing incompressible fluid against the unyielding walls of the cochlea. The fluid would therefore not move to any useful degree leading to a hearing loss of about 60dB. 5. Fluids can t be compressed. If the perilymph is pushed by the vibration of the oval window, then it can t move unless it can be moved to another area (vibrate the round window). If squeeze a water balloon, then it will bulge at other end. 4. the fluid-filled tubes make up the membranous labyrinth that is found inside an osseous labyrinth carved out of the inside of the temporal bone 5. the vestibular and tympanic ducts form a continuous duct that is filled with perilymph. The cochlear duct is filled with endolymph c. basilar membrane and Organ of Corti 1. forms the floor of the endolymph-filled cochlear duct separating it from the scala tympani 2. the Organ of Corti rests on top of basilar membrane throughout its entire length a. the Organ of Corti consists of hair cells, the dendritic ends of bipolar neurons, and support cells. 1. It does not include the basilar membrane which forms the foundation on which the organ of Corti sits. 2. The cell bodies and axons of the bipolar neurons are found in the cochlear nerve. 3. The hair cells are neurons that release the neurotransmitter glutamate 3

4 b. the Organ of Corti is made up of around 15,000 hair cells that stimulate bipolar sensory neurons 1. stereocilia are non-motile extensions of the plasma membrane and are more similar to microvilli than cilia. 2. Stereocilia contain actin filaments inside (cilia have a 9+2 arrangement of microtubules inside). 3. stereocilia are not cilia or microvilli, but they are closely related to microvilli. c. the hairs of hair cells are stereocilia that project up towards the bottom of the tectorial membrane d. the hair cells are the receptor cells that ear uses to hear. They are sensitive to fluid movements in perilymph caused by sound vibrations picked up by ear e. the hair cells are mechanoceptors f. pressure waves in the perilymph of the tympanic duct make the basilar membrane bounce up and down which causes the stereocilia of the hair cells to press against the tectorial membrane and distort (bend) in shape. g. the dendritic ends of bipolar neurons stimulated by the hair cells form the cochlear branch of the vestibulocochlear nerve (cranial nerve VIII, also called the auditory or cochlear nerve); the cell bodies of the bipolar neurons are in the spiral ganglion of the cochlear nerve and their axons go down cochlear nerve 1. the cochlear branch joins with the vestibular branch from the semicircular canals to form the vestibulocochlear nerve (VIII) 2. the vestibulocochlear nerve travels to pons of brainstem where the sensory neurons travel into the cochlear nucleus (at junction of pons and medulla) 3. interneurons in the cochlear nucleus project via the thalamus to a. primary auditory cortex in the temporal lobe of the cerebrum (hearing) b. cerebellum (balance or equilibrium) 3. the stationary tectorial membrane extends from the basilar membrane and hangs over the hair cells of the Organ of Corti cells like a porch awning. 4. The cells of the Organ of Corti are found beneath the tectorial membrane and run the length of the tectorial membrane. 4

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Sound Pathway 1. sound waves move through air as jostling air molecules 2. sound waves are funneled by the pinna into the external auditory meatus 3. sound waves strike and vibrate the tympanum 4.

7 Sound Pathway 1. sound waves move through air as jostling air molecules 2. sound waves are funneled by the pinna into the external auditory meatus 3. sound waves strike and vibrate the tympanum 4. the tympanum is physically attached to the malleus 5. the tympanum vibrates the malleus, which vibrates the incus and the stapes 6. the stapes is physically attached to the oval window on the side of the vestibule (pistonlike action of stapes on oval window) 7. the oval window is the beginning of the membrane-bound tube called the scala vestibuli which runs through the cochlea and becomes the scala tympani at the helicotrema 8. vibrations of the oval window vibrate perilymph fluids within the scala vestibuli at the same frequency 9. fluid waves (pressure waves) of the perilymph go down the scala vestibule, around the helicotrema and then down the scala tympani where the waves strike the round window which is on the side of the cochlea as it faces the middle ear cavity 10. waves within the perilymph going down the scala tympani vibrate the basilar membrane a. the basilar membrane then goes up and down b. the upward deflections press the stereocilia of the hair cells against the tectorial membrane and cause them to bend 7

11. movement of the stereocilia (mechanical deformation) stimulates the hair cells to stimulate the bipolar neurons to generate nerve impulses along their axons a.

8 11. movement of the stereocilia (mechanical deformation) stimulates the hair cells to stimulate the bipolar neurons to generate nerve impulses along their axons a. the back and forth displacement of the stereocilia (hairs) on the hair cells (special epithelial cells) causes the opening and closing of mechanically-gated K+ channels at the tips of the stereocilia of the hair cells b. when the mechanically-gated K+ channel gates open, K+ influx into the cell from the endolymph and depolarize the membrane by increasing the positivity of the ICF or cytoplasmic side of the membrane (this is a K+ influx which isn t what occurs during an action potential). K+ AT pumps create the df gradient for K+. c. the voltage change across the membrane as the K+ come in causes voltage-gated Ca2+ channels to open 1. when the voltage-gated Ca2+ channels open, Ca2+ go into the cell and cause the hair cells to release the neurotransmitter glutamate by exocytosis 2. glutamate binds to receptors on the dendritic end of bipolar neurons and stimulates them to generate nerve impulses 3. the dendrites of the bipolar neurons leave the basilar membrane and go into the cochlear nerve where their cell bodies are found in the spiral ganglion. The axons of the bipolar neurons travel in the cochlear nerve 12. the impulses generated by the hair cells that make up the Organ of Corti go out the cochlear nerve to the vestibulocochlear nerve (VIII) and then to the pons of the brainstem a. within the pons, the bipolar neurons stimulate interneurons that send impulses to auditory centers in the temporal lobe of the cerebrum where they are interpreted as meaningful sounds (hear) b. the vestibulocochlear nerve (cranial nerve VIII) transmits sound and equilibrium (balance) information from the inner ear to the pons of the brainstem where decussation occurs and from there to the thalamus and then to the primary auditory cortex of the cerebrum (hearing) or the cerebellum (balance) 13. the energy of the fluid waves is dissipated as the waves strike the round window and it bulges slowly out then back in 8

Sound vibrates tympanum, vibrates ossicles, vibrate oval window, creates fluid pressure waves in perilymph of vestibular and tympanic ducts (1) pressure waves in fluids in perilymph move basilar

9 Sound vibrates tympanum, vibrates ossicles, vibrate oval window, creates fluid pressure waves in perilymph of vestibular and tympanic ducts (1) pressure waves in fluids in perilymph move basilar membrane, (2) stereocilia of hair cells in Organ of Corti move against tectorial membrane, (3) stereoocilia bend, (4) opens mechanicallygated K+ channels in stereocilia, (5) K+ efflux depolarizes membrane, (6) opens voltage-gated Ca2+ channels, (7) Ca2+ influx triggers exocytosis of glutamate, (8) stimulate bipolar neurons 9

Inner Ear: Sense of Balance or Equilibrium (Semicircular Canals) 1. Vestibular Apparatus helps a person keep their balance during movements a. found in inner ear b.

10 Inner Ear: Sense of Balance or Equilibrium (Semicircular Canals) 1. Vestibular Apparatus helps a person keep their balance during movements a. found in inner ear b. made up of 3 semicircular canals (SCC) (similar to gyroscope on airplanes) 1. each canal is filled with endolymph (watery fluid) 2. each canal oriented in a different 3-D plane (frontal, sagittal, and transverse) to create an x, y, z coordinate system for tracking fluid movements in the SCC s that occur when the head moves 3. hair cells with stereocilia are found at the base of each SCC 2. Function of Hair Cells trigger nerve impulses in sensory neurons a. special neurons that release the neurotransmitter glutamate by exocytosis b. hair cells have stereocilia (hairs) protruding from their apical ends that face the endolymph c. head movements cause the endolymph to move d. movement of the endolymph causes the stereocilia on the hair cells to bend back and forth e. movement of the hair cells back and forth opens mechanically-gated K+ channels in the tips of the stereocilia 10

11 f. K+ diffuse from the endolymph through the open channels into the cytoplasm of the hair cells (this is a K+ influx which is different than the K+ efflux that occurs during an action potential) g. the K+ influx depolarizes the inside of the plasma membrane and causes the opening of voltage-gated Ca2+ channels in the plasma membrane h. Ca2+ enter the cell and trigger the exocytosis of the neurotransmitter glutamate from secretory vesicles into the synaptic cleft. i. glutamate stimulates the dendrites of unipolar sensory neurons and the axons of the sensory neurons generate a nerve impulse j. the nerve impulse travels down the axons of the unipolar sensory neurons in the vestibular nerve that joins the cochlear nerve to become the vestibulocochlear nerve which goes to the pons of the brainstem k. the impulses on the axons of sensory neurons in stimulate the dendritic zones of interneurons in the pons l. interneurons in the pons then then send impulses to the cerebellum m. the cerebellum uses the information about the position of the head relative to the body to tell the motor centers of the cerebrum which skeletal muscles to contract to get the body under the head in order to be well-balanced. 11

Photoreception: Eyes and Vision 1. over 90% of known animal species have some light sensors 2. light-sensing organs range from simple eyespots to complex eyes a.

12 Photoreception: Eyes and Vision 1. over 90% of known animal species have some light sensors 2. light-sensing organs range from simple eyespots to complex eyes a. Planaria (flatworm) each of the 2 eyespots consists of less than 100 photoceptor cells lining a cup-like pit; allows Planaria to move away from light (under rocks in freshwater streams during daylight) b. complex eyes mammals and birds c. birds have the largest eyes relative to body size; they also have more photoceptor cells hence a sharp-eyed hawk can spot a mouse in a field from hundreds of meters away 3. the main function of the eye is to focus light on the photoceptor cells of the retina. The photoceptor cells then transduce light into nerve impulses that travel to centers in the cerebrum to form images Anatomy of the mammalian eye 3 Layers or tunics of the Mammalian Eye (fibrous, vascular, retina) 1. Fibrous tunic (layer) a. sclera 1. dense regular connective tissue that is rich in white-colored collagen fibers (structurally similar to tendons); very tough 2. white part of eye 3. about 1 in diameter (humans) b. cornea 1. transparent and avascular 12

13 2. consists of several layers of thin epithelial cells with some collagen fibers between the cells 3. curvature of cornea helps to focus light through lens to the retina; light bends as it goes through the cornea a. the cornea provides most of the eye s focusing power b. the curvature of the cornea is fixed, hence the fine-tuning of focus is done by the lens which can change shape to fine tune focus depending on the object s distance from the eye 4. densely innervated with pain receptors 2. Vascular tunic (middle layer) a. Choroid 1. highly pigmented with melanin; chocolate brown in color 2. vascular; blood vessels provide nutrients to the retina (photoceptor cells) b. ciliary body made up of ciliary muscles and ciliary processes 1. anterior extension of the choroid 2. aqueous humor a. blood vessels in the ciliary processes of the ciliary body continuously secrete a watery fluid called the aqueous humor that fills up the anterior cavity (between lens and cornea) b. supplies nutrients to the avascular lens and cornea 3. ciliary muscles control the tension on the suspensory ligaments; ciliary muscles are smooth muscle cells that make up most of the ciliary body c. iris 1. comes off the ciliary body 2. pigmented smooth muscle cells give color to the eye 3. contraction and relaxation of smooth muscle within the iris controls the diameter (aperture) of the pupil 4. the diameter of the pupil controls how much light reflecting off an object enters the eye and strikes the retina to form an image d. pupil 1. hole in the middle of the iris 2. it is not a physical structure 3. Pupil diameter 1/α amount of ambient light (dark > incr dia; bright > decr dia) e. lens 1. transparent and avascular; no nerves 2. made up mostly of thin epithelial cells (The lens itself lacks nerves, blood vessels, or connective tissue) 3. biconvex 4. lens can change shape to focus light on the retina to sharpen image 5. held in place by suspensory ligaments coming off the ciliary body that attach to the margin of the lens 3. Retina (inner tunic) pigmented epithelium and neural layer a. contains light-sensitive photoceptor cells (rods and cones) b. vitreous humor 13

14 1. clear, gel-like fluid (similar to jelly) that fills up the posterior cavity (space between the retina and lens) 2. formed during fetal life; we are born with all the vitreous humor that we will ever have c. Tapetum lucidum 1. reflective layer beneath the photoceptor cells that helps some animals see better in dim light 2. highly developed in crepuscular (twilight) and nocturnal (active at night) animals 3. tapetum reflects light that has passed through the retina back through it a second time to stimulate the photoceptor cells a second time 4. the cat eye is much more sensitive to light than the human eye 5. the tapetum accounts for the eye shine in animals that one sees when light is shown on their eyes in the dark Neural Structure of the retina (3 layers of neurons) 1. 3 layers of neurons in the retina: rods and cones, bipolar cells and ganglion cells a. photoceptor cells: rods and cones 1. about million rods (20x more rods than cones) 2. about 5-6 million cones b. bipolar cells c. ganglion cells (about 1 million ganglion cells in the retina) 14

15 2. Photoceptor cells (special neurons) a. Rods (active in dim light, inactive in bright light) 1. contain a photopigment called rhodopsin (visual purple); rhodopsin requires Vitamin A for its synthesis 2. rods allow for indistinct grayscale image (black and white) formation in dim light; images lack sharp detail 3. rods are specialized for night vision; in dim light the rods produce an grayscale (black and white) image that is colorless and softly focused (lacks sharp details) c. cones (inactive in dim light, active in moderate to bright light) 1. cones allow for detailed color vision in moderate to bright light 2. cones are specialized for day vision and in well-lit rooms 3. fovea capitis (0.4 mm) in middle of the macula lutea of retina has the densest concentration of cones and forms the sharpest part of a color image d. photoceptor cells tonically secrete glutamate in the dark that inhibits any activity on the bipolar neurons e. light interacts with photopigments in rods and cones which causes them to stimulate bipolar cells 3. bipolar cells (bipolar neurons) glutamate is the neurotransmitter released by these cells 4. ganglion cells (ganglion neurons) a. axons of ganglion cells go across the surface of the retina and come together at the optic disk to form the optic nerve b. optic nerve goes through the optic canal in the sphenoid bone and exit on either side of the sella turcica c. optic nerves take impulses on axons of ganglion cells to the optic tract then to the thalamus of the diencephalon where axons synapse with interneurons that go to visual centers (primary visual cortex) in the occipital lobe of the cerebrum where the impulses are used to create images Photopigments 1. photopigments in rods and cones interact with light and undergo chemical changes a. dim light excites a photopigment called rhodopsin in rods b. moderate to bright light excites photopigments called scotopsins in cones; different scotopsins are sensitive to red, green and blue (RGB) wavelengths of the visibile spectrum that allow one to see in color; cones operate similar to an RGB color system (e.g., RGB monitor) 1. scotopsins of trichromatic RGB eyes (3-color vision) of primates are sensitive to red, blue, and green (RGB) wavelengths of light 2. 3 types of cone cells and each of them have their own form of scotopsin a. long (L) cones cones that are most sensitive to wavelengths of light around 560 nm give the perception of red b. middle (M) cones most sensitive to green wavelengths of light (around 530 nm) c. short (S) cones most sensitive to blue wavelengths of light (around 420 nm) 15

2. once stimulated by light, the rods and cones stimulate bipolar cells (neurons) to release a neurotransmitter that then stimulates ganglion cells 3.

16 2. once stimulated by light, the rods and cones stimulate bipolar cells (neurons) to release a neurotransmitter that then stimulates ganglion cells 3. the axons of ganglion neurons run across the surface of the retina and converge at the optic disk to form the optic nerve. 4. the optic nerves (one from each eye) carry the axons of the ganglion neurons through the optic chiasma to the optic tracts 5. the optic tracts travel to the thalamus 6. the ganglion neurons synapse with interneurons within the thalamus (lenticulate nucleus) of the diencephalon 7. the interneurons then transmit the image information from the thalamus to the primary visual cortex of the occipital lobe of the cerebrum where the image forms 16

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The ratios of stimulation of the 3 cone types are shown for 3 sample colors (red, green, yellow)

18 RGB color scheme. Sensitivity of the different types of cones to different wavelengths. Trichromatic eyes of primates. The ratios of stimulation of the 3 cone types are shown for 3 sample colors (red, green, yellow) Electromagnetic spectrum and visible light 1. visible light is a mix of wavelengths from violet-blue-cyan-green-yellow-orange-red Electromagnetic spectrum Visible spectrum 18

19 Transduction by Sensory Cells 1. Transduction conversion of one form of energy to another a. an external stimulus like touch, pressure, pain, temperature change is converted to nerve impulses by sensory cells b. transduction is associated with opening and closing of ion channels 2. 2 types of ion channels a. leak channels always open b. gated channels open/close in response to specific stimuli 3. 4 types of gated channels a. Mechanically Gated 1. stimulated by stretch; gates open when the sensory cell is stretched or deformed 2. opening of channels generates impulse on sensory cells b. chemically-gated 1. stimulated by chemicals (ligands) that bind to receptors 2. ligand-receptor binding opens ion channels and generates impulses on sensory cells c. voltage-gated channels d. thermally gated channels Perception 1. sensory information is perceived if it reaches the brain 2. perception animal s interpretation of the external world is created by the brain from a pattern of nerve impulses coming from sensory cells Sense of Hearing: Sound waves 1. hearing is the perception of sound waves through gas, solid, or liquid by the brain 2. sound waves travel through a medium as molecules are jostled against one another 3. many animals can detect sound a. fishes detect sound through lateral line system b. insects, amphibians and some reptiles hear with use of tympanic membrane (eardrum) on body surface that vibrates when struck by sound waves 4. sound travels fast a m/sec in water (6000 mph) b. 340 m/sec in air (760 mph) 5. Two main properties of a sound a. intensity (loudness) expressed in decibels (db) b. Pitch or tone frequency of the vibrations (cycles per second or Hertz) 1. humans can perceive sounds with frequencies from 20 to 20,000 Hz 2. dogs can hear sounds from 20 to 40,000 Hz (or even up to 60 khz). Higher frequency sound waves above 20 khz are called ultrasounds. Humans cannot hear them 3. dog whistles generate ultrasound frequencies with in a range of khz (humans only hear a quiet hissing sound) 19

Taste buds Gustatory cells extend taste hairs through a narrow taste pore

Taste buds Gustatory cells extend taste hairs through a narrow taste pore The Special Senses Objectives Describe the sensory organs of smell, and olfaction. Identify the accessory and internal structures of the eye, and explain their function. Explain how light stimulates the