L3 - Hearing & Equilibrium PDF

Special senses Hearing & equilibrium H U M A N A N AT O M Y & P H Y S I O L O G Y – B I O L - 1 1 1 2 WIN TER 2 02 5 R E A D I N G S : C H A P T E R 1 7 , PA G E S 6 2 5 – 6 2 8 , 6 3 2  Mechanorecep...

Special senses Hearing & equilibrium H U M A N A N AT O M Y & P H Y S I O L O G Y – B I O L - 1 1 1 2 WIN TER 2 02 5 R E A D I N G S : C H A P T E R 1 7 , PA G E S 6 2 5 – 6 2 8 , 6 3 2  Mechanoreception Sense of force or displacement (i.e., mechanical stimuli) Possibly the most ancient sense ◦ Found in all organisms & probably all cells Important for: ◦ Hearing, equilibrium/balance, proprioception, senses of touch, cell volume control, regulating blood pressure (baroreceptors) Fig. 7.14, Hill 2022 Fig. 14.4; https://www.lecturio.com/concepts/arterial-pressure-regulation/; OpenStax 14.1 https://www.bio.miami.edu/dana/360/360F17_16print.html Tortora 17.7  Hair cell Epithelial sensory receptor cell Stereocilia (microvilli) ◦ Ordered in increasing height Kinocilium (cilium) ◦ Important for mediating directional arrangement of stereocilia ◦ Not present in adult cochlea ◦ Present at birth but degenerate Communicate with afferent sensory neurons through release of neurotransmitters May have efferent input ◦ E.g., outer hair cells in cochlea Hill 2022 Fig. 14.7; Sherwood et al. 2013 Fig. 6-12 OpenStax 14.1 Tortora 17.7  Tip-links & mechanically- gated channels Tip-links ◦ Connect the tops of stereocilia ◦ Critical to pull channels open ◦ Part of the “gate-spring” mechanism Movement of stereocilia stretch or release tension on tip links to open or close mechanically-gated Tip-links cation channels ◦ Positive ions (e.g., K+) move into cell via opened channels, depolarizing the cell OpenStax 14.1 Hill 2022 Fig. 14.7; Sherwood et al. 2013 Fig. 6-11 Tortora 17.7  Signal transduction in hair cell Directional displacement of hair bundle results in change in membrane potential (receptor potential) At rest – partially depolarized Movement toward tallest stereocilia ◦ Channels open → ↑ intracellular K + → depolarization → ↑ intracellular Ca2+ → ↑ neurotransmitter release Movement away from tallest stereocilia ◦ Channels close → ↓ intracellular K+ → hyperpolarization → ↓ intracellular Ca2+ → ↓ neurotransmitter release Graded neurotransmitter release from hair cell ◦ Alters action potential frequency in afferent neuron Moyes & Schulte 2015 Fig. 7.20 OpenStax 14.1 Tortora 17.7  Semicircular Hair cell applications canals Hair cells are involved in: Utricle ◦ Hearing (cochlea) Cochlea ◦ Balance & equilibrium (vestibular apparatus) Saccule How can the same hair cell respond to different stimulus types? Location & accessory structures (i.e., peripheral filtering) of hair cells determine whether they are activated by sound, or linear or angular acceleration OpenStax 14.1 Adapted from Tortora Fig. 17.20 Tortora 17.7  Anatomy of the ear Three distinct regions: External/outer ear ◦ Collects sound waves & channels them inward Middle ear ◦ Amplifies sound vibrations, conveying them to inner ear External ear Inner ear Middle ear ◦ Location of hair cells (receptors) for sensing sound & equilibrium Internal ear OpenStax 14.1 Adapted from Tortora Fig. 17.18 Tortora 17.7  Malleus Semicircular canal Temporal bone Incus Vestibulocochlear nerve: Vestibular branch External ear Cochlear branch Pinna (auricle) ◦ Collects sound waves Pinna External auditory canal (acoustic meatus) ◦ Skin contains hairs & ceruminous Cochlea glands (cerumen/earwax) Tympanic membrane (eardrum) Tympanic ◦ Thin connective tissue membrane membrane ◦ Separates outer & middle ear External auditory Stapes in ◦ Transfers sound energy to auditory Auditory tube canal oval window ossicles (to nasopharynx) OpenStax 14.1 Adapted from Tortora Fig. 17.18 Tortora 17.7  Stapes Incus in oval Malleus window Facial (VII) nerve Middle ear Air-filled, mucosa-lined cavity in temporal bone Auditory ossicles ◦ Malleus, incus, stapes External ◦ Lever system that transfers & amplifies sound auditory Round waves/vibrations canal window Attenuation reflex – tiny muscles contract in Tympanic response to prolonged loud sounds membrane ◦ Reduces movement of ossicles Stapedius ◦ Protects delicate sensory apparatus by diminishing Tensor transmission of loud sound waves muscle Auditory tympani muscle tube OpenStax 14.1 Adapted from Tortora Fig. 17.19 Tortora 17.7  Stapes Incus in oval Malleus window Facial (VII) nerve Middle ear Auditory tube (pharyngotympanic or Eustachian tube) ◦ Links middle ear with nasopharynx ◦ Allows pressure equalization between middle & External external ear auditory Round canal ◦ Allows tympanic membrane to move freely as window sound waves strike it Tympanic membrane Stapedius Tensor muscle Auditory tympani muscle tube OpenStax 14.1 Adapted from Tortora Fig. 17.19 Tortora 17.7  Semicircular Temporal bone Inner ear ducts in Vestibular semicircular nerve Two main divisions: canals: Anterior Cochlear Bony labyrinth – network of Posterior nerve channels & cavities in temporal bone Lateral ◦ Semicircular canals, vestibule, & cochlea Cristae in Maculae membranous ◦ Filled with perilymph (similar to ECF) ampullae Membranous labyrinth – Utricle in Organ of membranous sacs & ducts within vestibule Corti bony labyrinth Saccule in Stapes in ◦ Utricle, saccule, semicircular ducts, & vestibule oval window Cochlear duct cochlear ducts Round window in cochlea ◦ Filled with endolymph (↑K+) OpenStax 14.1 Marieb 2016 Fig. 15.26 Tortora 17.7  Stapes Organ of Corti Sound detection in oval window Cochlea Cochlear duct Bony labyrinth with Round in cochlea perilymph window Cochlear duct Membranous labyrinth with endolymph Organ of Corti Location of hair cells OpenStax 14.1 Moyes & Shulte 2015 Fig. 7.28; Marieb 2016 Fig. 15.26 Tortora 17.7 Stapes in oval window Organ of Corti Cochlear Cochlea Round duct window in cochlea Bony spiral canal extending from anterior part of vestibule containing three chambers: Vestibular duct (scala vestibuli) ◦ Next to oval window (perilymph) Tympanic duct (scala tympani) ◦ Next to round window (perilymph) Cochlear duct (scala media) ◦ Basilar & tectorial membranes; organ of Corti (spiral organ) with hair cells (endolymph) Moyes & Shulte 2015 Fig. 7.28; Hill et al. 2004 Fig. 13.30 OpenStax 14.1 Marieb 2016 Fig. 15.26 Tortora 17.7 Inner hair cell Outer hair cell Organ of Corti Within cochlear duct & location of hair cells Inner hair cells – sound detection ◦ Release neurotransmitter in response to graded receptor potential ◦ Send signals along cochlear branch of vestibulocochlear nerve Outer hair cells – sound amplification ◦ Change shape in response to receptor potential → alters tension of the tectorial membrane ◦ Efferent innervation – triggered by loud noises to dampen response – protective mechanism OpenStax 14.1 Hill et al. 2004 Fig. 13.30; Marieb 2016 Fig. 15.27 Tortora 17.7  Properties of sound Sound ◦ Produced by a vibrating object & propagated by the molecules of the medium Frequency – pitch ◦ Number of waves that pass a given point in a given time ◦ Measured in hertz (Hz, cycles per second) © 2016 P earso n Ed ucatio n, Inc. Amplitude – loudness (sound intensity) ◦ Height of wave ◦ Measured in decibels (dB) OpenStax 14.1 Tortora 17.7  Transmitting sound to inner ear Sound entering the auditory canal causes tympanic membrane to vibrate Vibrations transmitted along ossicles to oval window Vibration of oval window causes pressure wave in fluid of inner ear (perilymph of vestibular duct) Sounds with high enough frequencies will vibrate the basilar membrane OpenStax 14.1 Sherwood et al. 2013 Fig. 6-20 Tortora 17.7  Sound transduction Displacement of basilar membrane causes hair cell activation by movement against tectorial membrane ◦ Hair cell ion channels open → K+ entry → depolarizing receptor potential ◦ Receptor potential → Ca2+ entry → neurotransmitter release ◦ Neurotransmitter activates primary afferent neurons → action potential in cochlear nerve Change in shape of outer hair cells alters tension of tectorial membrane to amplify Displacement of sound detection by inner hair cells basilar membrane OpenStax 14.1 Hill et al. 2022 Fig. 14.9; Moyes & Schulte 2015 Fig. 7.20 Tortora 17.7  Softer sound – lower amplitude Louder sound – higher amplitude Loudness detection Loudness detection – defined by amplitude of vibrations on basilar membrane Graded response ◦ Softer sounds: smaller displacement Tectorial of basilar membrane → less activation membrane of hair cell → lower action potential frequency in afferent neuron Inner hair cells ◦ Louder sounds: larger displacement of basilar membrane → more activation of hair cell → greater action potential frequency in afferent Displacement of neuron basilar membrane OpenStax 14.1 Hill et al. 2022 Fig. 14.9; Moyes & Schulte 2015 Fig. 7.20 Tortora 17.7  Frequency detection Frequency detection (i.e., pitch discrimination) – based on structure of basilar membrane ◦ Proximal end – narrow & stiff – vibrates at high frequencies ◦ Distal end – wide & flexible – vibrates at low frequencies Place coding ◦ Neurons from each part of the basilar membrane synapse with neurons in particular areas in the auditory cortex Proximal Distal ◦ Therefore, specific areas of auditory cortex end end respond to particular frequencies Sherwood et al. 2013 Fig. 6-20 OpenStax 14.1 Tortora 17.7  Consolidate your knowledge Which sound(s) would: Sound A Sound B Cause the greatest displacement of the basilar membrane? Be detected at the proximal end of the basilar membrane? Cause the least amount of Sound C Sound D displacement at the distal end of the basilar membrane?  Medial geniculate nucleus of thalamus Primary auditory cortex in temporal lobe Auditory pathway Inferior colliculus Cochlear branch of Lateral lemniscus vestibulocochlear nerve (VIII) Superior olivary nucleus (pons- Midbrain medulla junction) Medulla (cochlear nuclei) Cochlear nuclei Midbrain (inferior colliculus) Medulla Vibrations Vestibulocochlear Thalamus nerve (medial geniculate nucleus) Vibrations Spiral ganglion of cochlear nerve Primary auditory cortex in Bipolar cell temporal lobe Organ of Corti OpenStax 14.1 Marieb 2016 Fig. 15.33 Tortora 17.7  Vestibular Cochlear nerve nerve Semicircular Equilibrium ducts in semicircular Sense of equilibrium canals ◦ Responds to head movement, visual information, & proprioceptors (stretch receptors in muscles & Cristae in tendons) membranous ◦ Used to initiate reflexes that maintain position & ampullae coordinate complex movements Utricle in Vestibular apparatus of inner ear vestibule ◦ Linear acceleration (static equilibrium) ◦ Maculae of saccule & utricle of the vestibule Saccule in vestibule Stapes in ◦ Angular acceleration (dynamic equilibrium) oval window Maculae ◦ Cristae of ampullae of semicircular ducts of Round window semicircular canals OpenStax 14.1 Tortora Fig. 17.20 Tortora 17.8  Linear acceleration Saccule & utricle – membranous sacs within the vestibule ◦ House equilibrium receptor regions – maculae (location of hair cells) Otoliths – mineralized concretions of Ca2CO3 suspended in a gelatinous matrix Bending of stereocilia provides direction of gravity ◦ Utricle – horizonal plane ◦ Forward & backward movements (e.g., head tilting) ◦ Saccule – vertical plane ◦ Up & down movements (e.g., jumping up & down) OpenStax 14.1 Moyes & Schulte 2015 Fig. 7.24 Tortora 17.8  Linear acceleration At rest – hair cells release neurotransmitter (NT) continuously Changes in acceleration along horizontal/vertical plane modify NT release & action potential (AP) frequency ◦ No information about constant speed provided Example: macula of utricle ◦ Bending toward kinocilium ◦ Depolarization → ↑ NT release → ↑ AP frequency ◦ Bending away from kinocilium ◦ Hyperpolarization → ↓ NT release → ↓ AP frequency OpenStax 14.1 Moyes & Schulte 2015 Fig. 7.25 Tortora 17.8  Angular acceleration Ampulla Three perpendicular semicircular canals, each with a semicircular duct ◦ Detects changes in acceleration in 3 planes Ampulla – dilated portion of each duct Crista – small elevation in ampulla ◦ Location of hair cells & supporting cells Cupula – gelatinous mass covering crista Detect rotational movement (e.g., shake head) Also plays a role in keeping eyes oriented on a single point when head is moving OpenStax 14.1 Moyes & Schulte 2015 Fig. 7.25 Tortora 17.8  duct Angular acceleration Fluid (endolymph) within the semicircular duct moves when the duct head is turned & pushes on cupula, stimulating hair cells duct Direction of movement determines the frequency of action potentials OpenStax 14.1 Moyes & Schulte 2015 Fig. 7.26 Tortora 17.8  Input: Information about the body’s position in space comes from three main sources and is fed into two major processing areas in the central nervous system. Vestibular Visual Somatic receptors Equilibrium pathway (skin, muscle receptors receptors and joints) Vestibular branch of Vestibular vestibulocochlear nerve (VIII) Cerebellum nuclei (brain stem) Central nervous system processing Cerebellum Vestibular nuclei in Why? pons & medulla Oculomotor control Spinal motor control (cranial nerve nuclei (cranial nerve XI nuclei III, IV, VI) and vestibulospinal tracts) (eye movements) (neck, limb, and trunk movements) Output: Responses by the central nervous system provide fast reflexive control of the muscles serving the eyes, neck, limbs, and trunk. Marieb 2016 Fig. 15.36 OpenStax 14.1 Tortora 17.8  Input: Information about the body’s position in space comes from three main sources and is fed into two major processing areas in the central nervous system. Vestibular Visual Somatic receptors Equilibrium pathway (skin, muscle receptors receptors and joints) Balance & body orientation depend on inputs from the visual Vestibular system, proprioceptors, & the Cerebellum nuclei (brain stem) inner ear Central nervous ◦ E.g., difficult to keep balance with system processing eyes closed ◦ E.g., motion sickness – signals from the vestibular & visual systems in Oculomotor control Spinal motor control conflict (cranial nerve nuclei (cranial nerve XI nuclei III, IV, VI) and vestibulospinal tracts) (eye movements) (neck, limb, and trunk movements) Output: Responses by the central nervous system provide fast reflexive control of the muscles serving the eyes, neck, limbs, and trunk. Marieb 2016 Fig. 15.36 OpenStax 14.1 Tortora 17.8 Summary of the inner ear Bony labyrinth Membranous Receptor region Function labyrinth Cochlea Cochlear duct Organ of Corti Hearing Vestibule Utricle & saccule Macula Equilibrium – head position relative to gravity & linear acceleration Semicircular canals Semicircular ducts Cristae of ampulla Equilibrium – angular (rotational) acceleration Check your knowledge Explain how hair cells can generate a graded potential resulting in a graded release of neurotransmitter. How is action potential frequency in the afferent neuron affected by this graded response? Describe the structure & general function of the outer & middle ears. The eardrum is also know as the __________________ membrane. What portion of the ear allows for a balance in pressure between the middle ear & the atmosphere to allow this membrane to freely move? What is the function of the auditory ossicles? What is the purpose of the attenuation reflex? Describe the structure & general function of the inner ear. Compare & contrast the bony & membranous labyrinths of the inner ear. Check your knowledge Describe the sound conduction pathway to the fluids of the inner ear. Describe sound transduction. Explain why the tympanic membrane can amplify the sounds transferred to the oval window. Explain how we are able to differentiate pitch and loudness. Describe the pathway of impulses traveling from the cochlea to the auditory cortex. Explain how the balance organs of the semicircular canals and the vestibule help maintain equilibrium. Explain how both acceleration and deceleration can be sensed by the same hair cells in the utricle and saccule. How are these changes in acceleration communicated to the brain? Why is information from the vestibular apparatus communicated to the cerebellum? Check your knowledge True OR false – all sensory receptor hair cells in the adult ear have a kinocilium that is joined to stereocilia by tip links True OR false – the cochlear duct is filled with endolymph, which is high in potassium ions, whereas the cochlea is filled with perilymph that is low in potassium. True OR false – the organ of Corti contains both inner & outer hair cells, where the inner hair cells are involved in sound amplification & outer hair cells are involved in sound detection. True OR false – the maculae of the semicircular ducts are involved in detection of linear acceleration. True OR false – only changes in acceleration can be sensed by the hair cells in the vestibular apparatus, not constant speeds.