Chapter 53 and 54 final.pptx

UNIT 10 Chapter 53: The Sense of Hearing Slides by Thomas H. Adair, PhD Copyright © 2021 by Saunders, an imprint of Elsevier Inc. The Tympanic Membrane and the Ossicular System • Tympanic membrane functions to transmit vibrations in the air to the cochlea (inner ear) • Amplifies the signal beca...

UNIT 10 Chapter 53: The Sense of Hearing Slides by Thomas H. Adair, PhD Copyright © 2021 by Saunders, an imprint of Elsevier Inc. The Tympanic Membrane and the Ossicular System • Tympanic membrane functions to transmit vibrations in the air to the cochlea (inner ear) • Amplifies the signal because the area of the tympanic membrane is 17 times larger than the oval window. • Tympanic membrane connected to the ossicles: malleus, incus, stapes Figure 53-1 Attenuation of Sound by Muscle Contraction • Two muscles attach to the ossicles – Stapedius (stapes) – Tensor tympani (malleus) • Stapedius is smallest skeletal muscle in body (1 mm long) Attenuation reflex (aka, stapedius reflex, acoustic reflex, auditory reflex): a loud noise initiates reflex contraction, causing ossicular system to develop rigidity. Both muscles involved. attenuates vibration going to cochlea. Can reduce sound transmission by 30-40 decibels. Serves to protect cochlea and dampens low frequency sounds i.e., your own voice (~1000 Hz) or the voice of others. (Humming when you don't want to hear someone else works through the stapedius reflex; this can be a 20-decibel reduction in sound transmission to cochlea) Takes 40-80 msec to activate… • • • • • Cochlea • Encased in bone • Hearing loss: conduction and/or neurological • system of three coiled tubes separated by membranes into the scala tympani, scala media, scala vestibuli • Sound waves cause back and forth movement of the tympanic membrane which moves the stapes back and forth. • This causes displacement of fluid in the cochlea and induces vibration in the basilar membrane. • Organ of Corti lies on surface of basilar membrane; contains hair cells which are electromechanically sensitive. Figure 53-3 Figure 53-1 Organ of Corti • • • • • Receptor organ that generates nerve impulses. Contains rows of hair cells that have stereocilia. Hair cells are the receptor organs that generate APs in response to sound vibrations. The tectorial membrane lies above the stereocilia of the hair cells. Movement of the basilar membrane causes the stereocilia of the hair cells to shear back and Figure 53-7 Nerve Impulse Origination • • Inner hair cells Stereocilia, when bent in one direction cause hair cells to depolarize; when bent in opposite direction hyperpolarize. – this is what begins neural transduction of auditory signal. ~90% auditory signals are transmitted by inner hair cells. – 3-4 X as many outer hair cells than inner hair cells. – outer hair cells may control the sensitivity of inner hair cells for different sound pitches. Figure 53-7 Inner and outer hair cells outer hair cells may control the sensitivity of the inner hair cells for different sound pitches. auditory signals are transmitted by the inner hair cells. Kandel, Schwartz and Jessell 4th edition 2000, McGraw Hill fig 30-5 Outer hair cells adjust sensitivity there are nerve fibers running from the brain stem to the vicinity of the outer hair cells, may function to adjust sensitivity by acting on these cells. Kandel, Schwartz and Jessell 4th edition 2000, McGraw Hill fig 30-10 Structural components of Cochlea • • • • Basilar membrane contains ~30,000 fibers which project from the bony center of the cochlea, the modiolus. Fibers are stiff reed-like structures fixed to the modiolus and embedded in the loose basilar membrane. Because they are stiff and free at one end they can vibrate like a musical reed. the length of the fibers increases and the diameter of the fibers decrease from the base at the oval window to the helicotrema, overall stiffness decreases 100 X, Short, stiff, high frequency Long, limber, Low frequency Figure 53-4 The round window serves to decompress acoustic energy that enters the cochlea via stapes movement against the oval window. Determination of Sound Frequency and Amplitude • • Place principle determines the frequency of sound perceived. – different frequencies of sound will cause the basilar membrane to oscillate at different positions. – position along the basilar membrane where hair cells are being stimulated determines the pitch of the sound being perceived. Amplitude is determined by how much the basilar membrane is displaced. Figure 53-5 Decibel Unit of Sound • Unit of sound • Sound intensity is expressed in terms of the logarithm of their actual intensity because of the wide range in sound intensity. • A 10-fold increase in sound energy is 1 bel • 0.1 bel is a decibel • 1-decibel is an increase in sound energy of 1.26 times • Ears can barely distinguish a 1-decibel change in sound intensity. Decibel values for various sounds Does the attenuation reflex protect against a shotgun blast? END UNIT 10 Chapter 54: The Chemical Senses—Taste and Smell Slides by Thomas H. Adair, PhD Copyright © 2021 by Saunders, an imprint of Elsevier Inc. Taste and Smell • Allows one to separate undesirable or lethal foods from those that are nutritious. • Recognize the proximity of other individuals or animals. • Tied to primitive emotional and behavioral functions of the nervous system. Because the tongue can only indicate texture and differentiate between sweet, sour, bitter, salty, and umami, most of what is perceived as the sense of taste is actually derived from smell. Taste Perception • • • • • Sour – caused by H+ from acids (citric acid, acetic acid) – Threshold for citric acid: 2 mM Salty – caused by ionized salts, mainly sodium – Threshold for NaCl: 10 mM Sweet – many chemicals mostly organic compounds – Threshold for sucrose: 20 mM Umami (Japanese, meaning delicious) – Glutamate – Threshold for MSG: <10 mM Bitter – long chain organic substances containing nitrogen – Alkaloids (quinine, strychnine, nicotine, etc.) – Threshold for quinine: 0.008 mM – Threshold for strychnine: 0.0001 mM – (survival mechanism since many poisons are bitter) Babies and young children are exceptionally sensitive to the bitter However, every part of the tongue includes receptors for every basic taste. Location of Taste Buds • Found on three types of papillae of the tongue. – Circumvallate papillae form a V on posterior surface of tongue. (50% of taste buds) – Foliate papillae are located along lateral surfaces of tongue. (25% of taste buds) – Fungiform papillae located over flat surface of tongue. (25% of taste buds) Filiform papillae are the most numerous of the lingual papillae. They are fine, small, cone-shaped papillae covering most of the dorsum of the tongue. They are responsible for giving the tongue its texture and are responsible for the sensation of TOUCH. • Extraglossal taste buds – on the tonsillar pillars, palate, epiglottis, and proximal esophagus. Taste Buds Taste bud • Each taste bud contains ~100 taste receptor cells • Taste receptor cells create a taste pore, where tastants can more efficiently interact with the microvilli (where the taste receptors are located) • Taste receptor cells are depolarized by tastants, creating action potentials in afferent neurons • Afferent neurons that project into the CNS (CNs VII, IX, X) make synaptic contacts with many taste receptor cells • Basal cells are undifferentiated cells that give rise to taste receptor cells every ~12 days Gustatory (taste)Transduction Mechanisms • Ionotropic: tastant ion channels: (salt, sour) • Metabotropic: Gprotein coupled receptors: (sweet, umami, and bitter) • Voltage regulated Na+, K+, and Ca++ channels mediate transmitter release (serotonin, GABA, ATP), which activates afferent neurons. Both ionotropic and metabotropic Stevia herb - steviol glycosides interact with TRPM5 channel, producing a taste sensation 30-100x sweeter compared to sugar. Steviol glycosides can also amplify umami and bitter tastes at higher concentrations. Transmission of Taste Sensations • • • • • Anterior 2/3 of tongue through facial nerve (VII). Posterior 1/3 of tongue through glossopharyngeal nerve (IX). Posterior aspects of the mouth through vagus nerve (X). From solitary nucleus to thalamus. From thalamus to cortex. Figure 54-2 Adaptation of Taste • Taste sensations adapt rapidly. • Adaptation of taste buds themselves accounts for only about 50% of the adaptation. • Central adaptation must occur but the mechanism for this is not known; this is unusual because adaptation for most sensory systems occurs at the receptor. Ageusia – the loss of taste sensation Often confused with anosmia True ageusia, the complete which is a loss of the sense loss of taste, is relatively rare of smell compared to hypogeusia – a partial loss of taste – and dysgeusia – a distortion or alteration Causes of taste. • Neurological damage: Neurological disorders such as Bell’s palsy, Familial dysautonomia, and Multiple sclerosis • Problems with the endocrine system: Cushing's syndrome, hypothyroidism and diabetes mellitus • Medicinal side-effects: antirheumatic drugs, ACE inhibitors, and others • Other causes: Local damage and inflammation, radiation therapy, tobacco use, and denture use. Other known causes include loss of taste sensitivity from aging, anxiety disorder, cancer, renal failure and liver failure. Smell • Least understood of all senses. • Poorly developed in humans. • Olfactory membrane located on the superior part of each nostril. • Contains olfactory cells which contain cilia. • On the cilia are odorant-binding protein receptors. • Binding of chemical odorant to receptor induces the G-protein transduced formation of cAMP which opens sodium channels. Olfaction (aka, Smell) Because the tongue can only sense texture and differentiate between sweet, sour, bitter, salty, and umami, most of what is perceived as the sense of actually derived •taste Leastisunderstood of all from senses. •smell. Olfactory membrane located on the superior part of each nostril. • Olfaction involves CN I which are olfactory receptor neurons (ORNs). They are true neurons, which differentiates them from many other sensory system receptor types. Olfactory receptor neurons (ORNs) • ORNs are located in the olfactory epithelium. • The olfactory membrane has a surface area of about 5 cm2. Figure 54-3 Composite from figure 53-03 and Kandel, Schwartz and Jessell 4th edition 2000, McGraw Hill fig 32-1 • ORNs project axons to the olfactory bulb via the cribriform plate A closer look… Olfactory receptor neurons (ORNs) • Bowman’s glands produce mucus (and odorant binding proteins) that help trap odorants • Basal cells are undifferentiated cells that give rise to ORNs every ~45 days • Cilia on ORNs contain receptors, and are the site of transduction Aka, Sustentacular cell Transduction Mechanism • Odorant-binding receptors are located on cilia. • ~1000 different genes code for ORNs, only about 400 genes encode working ORNs. • ORNs can bind to a variety of odorants, with varying affinities. The differences in affinities causes differences in activation patterns resulting in unique odorant Binding of odorant to receptors induces Gprofiles. protein activation of adenyl cyclase, which causes formation of cAMP. The cAMP opens sodium channels causing depolarization. Figure 54-4 Anosmia is the lack of smell • Can be due to traumatic injury, or from disruption to nasal epithelium or central pathways – Traumatic injury can sheer ORN axons in the cribriform plate – Infection (e.g., rhinitis) can block odorants and cause edema – Tumors (e.g., olfactory groove meningiomas) – Smoking decreases sensitivity to odorants – Associated with normal aging and neurodevelopmental disorders (e.g., Alzheimer’s and Parkinson diseases) • Most disruptions to “taste” are from olfactory issues, due to “flavor” being a perception that involves both senses • Constant regeneration of ORNs allows for many forms of anosmia to be temporary How does COVID-19 decrease taste and small? Why does COVID-19 affect smell and taste? While the precise cause of smell dysfunction is not entirely understood, the mostly likely cause is damage to the cells that support and assist the olfactory neurons, called sustentacular cells. These cells can regenerate from stem cells, which may explain why smell recovers quickly in most cases. How long does the loss of taste and smell last? Approximately 90% of those affected can expect improvement within four weeks. Unfortunately, some will