Special Senses 2025 Lecture Notes PDF

Lecture Notes on Human Physiology Physiology of Hearing, smell & taste 1  By Dr. Al-Zahraa Ahmed Sheref Lecturer of Physiology- Faculty of Medicine- Menoufia University 2  List of content...

Lecture Notes on Human Physiology Physiology of Hearing, smell & taste 1  By Dr. Al-Zahraa Ahmed Sheref Lecturer of Physiology- Faculty of Medicine- Menoufia University 2  List of content 1- Hearing …………………………………………………………6  Physics of sound……………………………………..6  Functional anatomy of ear………………………….8  Receptor of hearing (organ of corti)………………12  Central auditory pathway………………………….14  Routes of sound transmission ……………………..15  Discrimination of different aspects of sound………16  Deafness ……………………………………………..18 2- Smell sensation………………………………………………….19  Olfactory receptors…………………………………19  Properties of olfactory receptors…………………..20  Mechanism of stimulation…………………………20  Central olfactory pathway…………………………21  Biological significance of olfaction………………...22  Abnormalities of smell sensations…………………22 3- Taste sensation…………………………………………………..23  Primary taste sensations………………………………23  Taste receptors…………………………………………25  Properties of taste receptors…………………………..25.  Mechanism of stimulation……………………………..25  Taste pathway…………………………………………..26  Abnormalities of taste sensation………………………27 3  Vision and Mission Vision To be academically accredited college with local, regional and international reputation for having a leading role in the field of medical education and health provision. Mission Is committed to graduate a physician in accordance to the national academic reference standards who is able to meet the needs of local and regional market, skilled in conducting scientific research that participates in developing the profession and the provided health care and keen in continuous training and education to support the service. provided to the community and the surrounded environment in the frame of commitment to the ethics of the profession. 4  Intended Learning Outcomes (ILOs) The student should be able to: -Identify functional anatomy of ear. -Explain mechanism of hearing. -Identify different method of sound transmission. -Identify auditory pathway. -Differentiate between different types of deafness. -Discriminate between different aspect of sounds. -Identify smell receptors and their properties. -Explain mechanism of stimulation of olfactory receptors. -Identify olfactory pathway. -Define different types of olfactory disturbances. -Identify taste receptors and their properties. -Explain mechanism of stimulation of taste receptors. -Identify taste pathway. - Define different types of taste disturbances. -Describe brain speech areas. -Explain mechanism of speech. -Identify physiological basis of some speech disorders. -Describe types and mechanism of sleep. -Describe types and mechanism of memory. -Identify amnesia and its types. -Explain definition and types of learning. 5  Hearing Definition: is the sensation that detects sounds in the environment. -The ear is the organ of hearing because: 1) It contains the auditory receptors. 2) It has a specific structure which is responsible for sound conduction and amplification. 3) It has a specific neuronal pathway from the auditory receptors to the auditory cortex. -The ear is protected by bony structure (petrous part of temporal bone). Physics of Sounds Sound waves are longitudinal waves: alternating phases of condensation and rarefaction that strike the tympanic membrane. Figure 1: Sound waves. Characteristics of sound waves: 1) Frequency (cycles/sec. or Hertz "Hz"): It determines the pitch of sound. -The higher the frequency the higher the pitch of sound and vice versa. - The human ear can pick up sound frequency range from (20 to20,000 Hz), with greatest sensitivity at 1000-4000 Hz. - Female has high pitch voice than male. 6 2) Amplitude (dyne/cm2 or decibels): It determines the intensity of sound. -The higher the amplitude of sound wave the greater the sound intensity. -The amplitude of just audible sound = zero decibels, the amplitude of whisper = 40 decibels, the amplitude of normal conversation= 60 decibels, the amplitude of heavy traffic = 80 decibels. -Sound intensity above 120 decibels causes discomfort, Sound intensity above 180 decibels causes marked rise of body temperature and damage to delicate structures in the ear and brain. 3) Wave form: It determines the quality of sound (timbre of sound). -Pure tone: simple sine waves. Musical sounds: sound waves that have repeating pattern. Noise: Aperiodic non-repeating vibrations. Figure 2: Characteristics of sound waves 7  Functional Anatomy of the Ear *Each ear consists of three parts: 1- External ear. 2- Middle ear. 3- Inner ear. Figure 3: Anatomy of the ear. 1- External ear consists of: a- Auricle (ear pinna): collect sound from exterior and direct it to external auditory canal. b- External auditory canal (meatus): It has many functions: i. Sound conduction from the auricle to the tympanic membrane. ii. Presence of hair and wax, prevent entry of large objects and insects into the ear. iii. Keeps the air inside moist and warm for proper function of the tympanic membrane. iv. Condensation of sound waves applied to the tympanic membrane. c- Tympanic membrane (ear drum): -it is conical in shape, 1 cm in diameter, 0.1 mm in thickness. - Elastic and tense in nature. 8  - Properties and Functions: i. Resonator: the amplitude of its vibration is directly proportional to the intensity of sound. ii. Highly damped: its vibration stops when the applied sound stops so, it prevents unnecessary sound prolongation. iii. Aperiodic: takes up the characteristics of the vibrations applied to it as it has no natural frequency. 2- Middle ear consists of: a- Three bony ossicles (Malleus, incus, and stapes): -The handle of malleus is attached to back of the tympanic membrane, its head articulates to the incus. The incus articulates with the head of the stapes, its foot plate attached to the walls of the oval window of the cochlea by the annular ligament. - Functions: i.Sound transmission: vibration of tympanic membrane causes vibration of ossicles and vibration of fluid in the inner ear. ii.Sound amplification: by 2 mechanisms: 1-They act as a lever system: amplify sound 1.3 times. 2-Concentration of sound waves: the surface area of tympanic membrane is 16 times that of oval window causing amplification of sound waves by about 16 times. -So, the sound picked up by the large tympanic membrane and applied to the small oval window is amplified by about 20 times (16 x 1.3). b- Two muscles (Tensor tympani supplied by trigeminal nerve an stapedius muscle supplied by facial nerve): 9 -Functions: i. protective function: by their contraction, they dampen the transmission of sound vibration to the inner ear, so protect delicate structure in the inner ear from damage on exposure to loud sound as they decrease its intensity. ii. Decreased intensity of vocalization and chewing sounds. c- Air: filling the middle ear chamber, entered from Eustachian tube that connects the middle ear with the naso-pharynx. *The Eustachian tube is normally closed and open during swallowing and yawning. If it is open all the time the noises of breathing and talking will interfere with hearing. *In common cold: blocking of Eustachian tube creating negative pressure inside middle ear, sucking of tympanic membrane and reduction of auditory acuity. *During descent in airplane: the atmospheric pressure increases so, the person should swallow frequently to keep his Eustachian tube open to equalize pressure on both sides of the tympanic membrane preventing its rupture. - Functions of air: i. Equalization of pressure on both sides of the tympanic membrane. ii. Air renewal in the middle ear. iii. Drainage of middle ear secretions. 3- Inner ear consists of: a- Bony labyrinth: Series of tubules in the petrous part of temporal bone. b- Membranous labyrinth: inside bony labyrinth and is formed of 10  Auditory part (cochlea): responsible for hearing.  Non Auditory part (vestibular apparatus): responsible for equilibrium. *Cochlea: Is a coiled tube (about 2.5 turns), which is coiled around a bony core (modiolus). -It is divided into 3 chambers (scalae) by 2 membranes: Reissner's (vestibular) and basilar membranes. a) Scala vestibule (upper scala): contains perilymph. b) Scala media (middle scala): contains endolymph. c) Scala tympani (lower scala): contains perilymph. -The upper ends of scala vestibule and scala tympani communicate with each other at the apex of the cochlea. -The lower end of scala vestibule ends at the oval window that is closed by the foot plate of stapes. -The lower end of scala tympani ends at the round window that is closed by the secondary tympanic membrane. Figure (4): Diagrammatic view of an unfolded cochlea and its relation to middle ear structure. 11 *Receptor of Hearing (Organ of Corti) -It is located on the basilar membrane. Structure: -It is arranged in 4 rows: 3 rows of outer hair cells and 1 row of inner hair cells separated by the tunnel of corti. -The rows of hair cells are covered by a thin viscous and elastic tectorial membrane in which the tips of the outer hairs are embedded not the inner hair cells. -Inner hair cells are the receptors of hearing and receive 90- 95% of the afferent fibers that form the cochlear division of the vestibulo-cochlear nerve. -Outer hair cells are motile cells that have little direct receptive function, but they improve hearing by influencing the vibration patterns of the basilar membrane by un known mechanism. *N.B. hair cells are specialized epithelial cells not a neuron (can`t generate action potential). Figure (5): organ of corti. 12 *Mechanism of Stimulation of Auditory Receptors A. Endocochlear potential: - It is a potential difference of +80 mV between the endolymph and perilymph with positivity inside scala media and negativity outside. -The scala media is filled with endolymph which is similar in composition to ICF (rich in K+ and poor in Na+). There is continuous secretion of K+ ions into scala media by the stria vascularis. -The scalae vestibuli and tympani are filled with perilymph, it`s composition is similar to ECF (rich in Na+and poor in K+). -There are tight junctions between hair cells and adjacent supporting cells, preventing endolymph from reaching the bases of cells. -The basilar membrane is relatively permeable to perilymph in the scala tympani thus, the bases of hair cells and the tunnel of Corti are bathed with perilymph. -The tops of the hair cells are bathed with endolymph while the bases are bathed with perilymph. The hair cells have a negative intracellular potential of -70 mV with respect to perilymph (at the base). And -150 mV with respect to endolymph (at the hair border). This high potential difference at the hair border of the cell is believed to increase their sensitivity and to respond to slight movement of hairs. Mechanism of stimulation: -Vibration of oval window causes vibration of perilymph in scala vestibule, movement of Reissner's and basilar membrane, shearing force between tectorial membrane and reticular lamina, bending of hair of the inner hair cells creating potential changes in inner hair cells (receptor potential). 13 - Upward movement of basilar membrane ……opening of K+ channels ….entry of K+ into hair cells cause depolarization of hair cells leading to opening of Ca+2 channels and release of synaptic transmitter that depolarize afferent fibers in contact with the hair cells and increase frequency of discharge along the afferent auditory fibers. Central Auditory Pathway 1) Nerve fibers from organ of corti pass to the spiral ganglion (the first order neuron). 2) The axons of cells of the spiral ganglion form the cochlear division of the vestibulocochlear nerve that end around dorsal and ventral cochlear nuclei in upper medulla (second order neuron). 3) Fibers of second order neuron pass to the opposite side through trapezoid body to the superior olivary nucleus (third order neuron). 4) Fibers of the third order neuron ascend forming lateral lemniscus and some of the fibers terminate in nucleus of the lateral lemniscus and the other fibers end in the inferior colliculus of the midbrain (fourth order neuron). 5) Axons of the fourth order neurons end in medial geniculate body (MGB) in the thalamus (fifth order neuron). 6) Axons of the fifth order neuron form auditory radiation that reach auditory cortex in the superior portion of the temporal lobe. Functions of the auditory cortex: A-Primary auditory cortex (Brod mann's area 41): located in the superior portion of the temporal lobe 14 1-Conscious perception of sounds (pitch, amplitude, and pattern of sound). 2-Sound localization which depends on: -The difference in time between the arrival of the stimulus in the two ears. -The sound is louder on the side closer to the source of the sound. B-Auditory association areas (Brodmann's area 21 & 22): located just adjacent to primary auditory area. 1-Interpretation of auditory experience. 2- Responsible for auditory short term memory. *The ear is bilaterally represented in the 2 cerebral hemispheres because of the crossing fibers at the following levels. a) Trapezoid body (medulla). b) Lateral lemniscus (pons). c) Inferior colliculus (mid brain). So, Removal of one auditory cortex cause slight affection of auditory acuity. -Collateral fibers from auditory pathway pass directly to reticular activating system (RAS) of the brain stem activating all cerebral cortex and so, sound is one of the arousal stimuli. Routes of sound transmission: 1- Ossicular route (most efficient route- 20 times magnification): Sound waves are collected by the auricle → external auditory canal → vibration of tympanic membrane → vibration of ossicles → vibration of fluid in the cochlea → stimulation of auditory receptors (hair cells) → afferent fibers of vestibule-cochlear nerve → auditory pathway to the auditor cortex in the temporal lobe. 15  2- Bone route (less efficient): Vibration of the bones of the skull → transmission of some vibrations to the fluid in the cochlea → stimulation of auditory receptors (hair cells) → afferent fibers of vestibule-cochlear nerve → auditory pathway to the auditory cortex in the temporal lobe. 3- Air route (not efficient): Vibration of air in the middle ear → transmission of some vibrations to the fluid in the cochlea → the same pathway till reach auditory cortex. Mechanism of discrimination of different aspects of Sound The auditory cortex can differentiate different sound pitch, amplitude, and pattern. 1-Sound pitch (frequency): “Place theory” -Different sound frequencies induce vibrations in the different parts of the basilar membrane and the brain interprets impulses arriving from different parts as having different frequencies. -The basilar membrane at the base of the cochlea is very narrow and stiff, needs high frequency sounds to vibrate it. It is wider and lax at the apex of the cochlea, needs low frequency sounds to vibrate it. Evidences: a- Animals exposed to intense sounds of certain frequencies for long time become deaf to those frequencies. b- Experimental removal of certain parts of the basilar membrane cause deafness to certain frequencies. Against the theory: The basilar membrane is not stiff atthe base or lax at the apex. So, the theory is correct, but its explanation is false. 16  “Travelling wave theory” *Movement of foot plate of stapes produces a series of travelling waves in the perilymph. -High-pitched sounds produce waves that reach maximum height near the base of cochlea. -Low-pitched sounds produce waves that reach maximum height near the apex of cochlea. -The travelling waves cause displacement of Reissner's and basilar membrane. -The site of maximal displacement of the basilar membrane depends on the frequency of the sound. *The cerebral cortex interprets impulses coming from different parts of basilar membrane as difference in frequency. 2-Sound intensity (amplitude): -Loud sound causes more displacement of the basilar membrane>>>> greater shearing force>>>>> greater amplitude of receptor potential and greater number of firing hair cells >>>>>increased firing rate of impulses at sensory nerve endings. -The cortex interprets differences in frequency of impulses as differences in sound intensity. -Stimulation of outer hair cells inform cortex that sound is loud. 3-Sound pattern(amplitude): Recognition of tonal pattern: shift from one frequency to another. 17  DEAFNESS *Definition: Rise of the threshold of hearing or decreased auditory acuity. *Types: 1-Conductive deafness: the conduction of vibrations to the basilar membrane is impaired. -Causes in the external ear: Wax, Foreign bodies, Fungus infection (otomycosis). -Causes in the middle ear: i.Inflammation (otitis media). ii.Otosclerosis (fusion of the foot plate of stapes to the bones of the oval window). iii.Blockage of the Eustachian tube: inflammation of Eustachian tube due to spread of infection from the pharynx, blocking of the tube, absorption of air in the middle ear ceating negative pressure in the middle ear that sucking the tympanic membrane and decrease its vibration. 2-Sensorineural deafness (nerve deafness): Causes: a- Damage of the organ of Corti: due to prolonged exposure to loud sounds as in airports or noisy factories. b- Injury of cochlear nerve or central auditory pathway: e.g. head injuries, tumors, or by drugs as streptomycin. *Differentiation between two types of deafness: -Two tests are used: a- Weber test. B- Rinne` test. 18  noeaasneS llemS saololmlrsleoeclml -fla rrAeetrln ceyl earrh elahart yr tca rrAeetrln nanelera scyec ryah yr tca lrrA rA tca rrha rael tca haethn. earrh -reec rrAeetrln nanelera ceh hheerltyrl earrh eru rahlrerehtye ryra elrlarytrl tcet Arln tca rrAeetrln laeaetrl rahlrr. yrtalhealhau eatsaar tcaha earrh ela mh tr ah nyrryrr laeaetrl earrh. -en nhehh scyec yh elruheau en tca crsner dcan ela er alau lreruh scyec ele ehht hrual tca eeher renyre rA nanelera. -erAeetrln ceyl earrh ela eyerrel earrh scyec ce a: m.dcrlt tcyer uarulyta sytc ateeruau aruh eerrau olfactory rods. From these rod nanelera eyrye elreaet tr tca hhlAeea rA nhehh. a.dca etrrs of the olfactory receptor neurons pierce cribriform palate of ethmoid and enter olfactory bulb. 19 *slmcellee smosmaololmlrsleoeclml : m.dcan ela ecanrlaeaetrlh. a.aeytcaryhn eru yaherru rrrn tr hhehterea tcet erna yr errteet sytc rrAeetrln dissolved in the thin mucous layer covering it. 3.dcayl harhyty ytn tr rrAeetrln htynhry yh cylc. n.ner uyhtyrlhyhc eatsaar ahhh tr nhhh uyAAalart rurlh eht tcayl uyhelynyretyrr ersal rA uyAAalareah yr tca yrtarhytn rA ern ly ar rlual yh errl. So, smell sensation is concerned with detecting presence or absence of odors with discrimination of their intensities. 5.dcan ela nrualetarn eueetyrl laeaetrlhe uha tr -etrlhncerlah yr tca laea. -Central phenomenon: strong inhibitory signals from CNS suppress transmission of signals through olfactory bulb. *nleorallemSsmosmaololmlrsleceptors: The stimulus for smell must be: - srretyra tr laeec tca laeaetrlh. - Slightly water soluble, so it can pass through mucous layer to reach to receptors. - Slightly lipid soluble, to pass through membrane of olfactory hair cells. s*feonlSe osmos leorallemSsmosmaololmlrsleoeclmls: -The odorant substance, on coming in contact with olfactory surface, it diffuses into the mucous covering the cilia, then binds with odorant binding protein (OBP) that concentrate the odorant and transfer it to the receptors. 20 - nrneyretyrr rA rurlert sytc e laeaetrl elrtayr tcet elrtlhua tclrhlc tca eyryeln - nanelera raeu tr : a- Activation of G-protein complex that activate adenyl cyclase enzyme and increase CAMP formation leading to opening of many Na+ channels. b- Activation of phospholipase C, also opens Na+ channels. (a)&(b) cause depolarization of the olfactory neuron (receptor potential), then transmission of this potential into CNS through olfactory nerve. B.N:Irritant odors stimulate pain free nerve endings causing lacrimation, sneezing, and bronchospasm. Central olfactory pathway: rrAeetrln ceyl earrh hnreeha sytc tca uarulyta er tca rrAeetrln ehreb tca etrrh rA tca - rA tca nytler eru thAtau earrh tr Arln rrAeetrln lrrnalhry ftrrh rA nytler eru thAtau earrh Arln rrAeetrln tleetb scyec artal tca eleyr eatsaar- etr laeec tca nyueleyr eru ealaelhn tcar uy yuau yrtr tsr eetcsenh -Medial olfactory area: hypothalamus& primitive portion of limbic system. Functions: i-Primitive responses to olfaction (licking lips and salivation) caused by smell the food. ii.Primitive emotional drives associated with smell b-lateral olfactory area: fibers take 2 pathways to: -pyriform cortex and amygdaloid nucleus then to all parts of limbic system especially hippocampus (less old pathway): it concerned with.y. oaelryrl tr ryra rl uyhryra ealteyr Arruh 21.uyAAalart Arruh yy. cace yrler laherrhah tr iii.Olfactory reflexes. -Dorsomedial nucleus of thalamus (newer system): concerned with odor analysis. *Biological significance of smell: m- It is important in food selection. 2- Together with other senses, it determines flavor of food. 3-There is relationship between smell and sexual function. Abnormalities of olfaction: -Anosmia: absence of the smell sensation. -Hyposmia: diminished olfactory sensitivity as in old age. -Dysosmia: distorted sense of smell. a-loss os certain smells. b-olfactory hallucinations. c-Hypersensitivity for both smell and taste. 22  Taste sensation Figure (6): primary taste sensation s*smrlscleollrsll les eS llemS -s 1:nteelsll les- -Most of substances that cause sweet taste are organic chemicals sugars (sucrose, glucose, …. ). -Felt by tip of the tongue. 2:nlalsll les- eh aryeytau en yrrynau herthb neyrrn en tca eetyrrh oieybrA tca herth- -Felt by the dorsum of tongue anteriorly. 3:nmrlsll le- eh eehhau en eeyuh. dca uallaa rA hrhlrahh yh elrerltyrrerrn tr uy yrr - concentration. -Felt by edges of the tongue anteriorly. 23 4:Nellelsll les- ehrtt eehhau en ern hyrlra rA ecanyeer elarth. dsr erehhah rA hhehtereah eehha eyttal - harhetyrr a- Long chain organic substance that contain nitrogen (nicotine, caffeine). b- Alkaloids. -Felt by the back of the tongue. ssssssssssssssssssssssssssssssssssssssssssssssssssssss**sl lestra -Are oval structures present in: a- Mucosa of palate, epiglottis, pharynx b-In lateral wall of fungiform papillae (present in tip&edges of tongue). c-Vallate papillae arranged in V-shape area on the back of tongue. -Small conical filliform papillae cover the dorsum of the tongue but don’t contain taste buds. -Each taste bud formed of the following cells: i.Sustenticular cells. ii.Gustatory hair cells (taste receptors). iii.Basal cells (arise from epithelial cells surrounding the taste buds. They differentiate into new receptor cells, as old receptors are continuously replaced. Specificity of taste buds: -Each taste bud usually respond to one of the 4 primary taste stimuli, this occurs with adequate stimulus (substance in low concentration). - If sensory nerve innervate taste bud is cut, this taste bud degenerate and disappear. 24 s**sllntlrsmosll les eS llemS: -laeaetrlh olhhtetrln ceyl earrhb elreaet yrtr tehta erla orearyrl yr dca ceylh rA tehta-.otca aeytcaryer hhlAeea rA tehta ehuh, the bases of receptor cells are surrounded by sensory nerve fibers, its mother cell present in ganglion along course of 7 th, 9th, and 10th cranial nerves *slmcellee smosll lesleoeclml : 1- They are chemoreceptors. 2- The threshold concentrations of stimulating substances vary for different substances. 3- Poor discrimination difference in the intensity of taste stimulus. 4- They are moderately adapting receptors due to: -Central mechanism and receptor adaptation. *feonlSe osmos leorallemSsmosll lesleoeclml : Substances dissolved in oral fluid combine with specific protein receptors on the hairs of gustatory cells>>>>>>generation of receptor potential in taste receptor 25  neuron>>>>> generation of action potentials that travel along sensory nerve endings surrounding these cells. --dca nelrythua rA laeaetrl ertartyer yh elrerltyrrer tr rrlelytcn rA tca htynhretyrl -.hhehterea erreartletyrr --dehta eyruyrl elrtayr odcsb erreartleta eru tlerherlt tcet tehta elruheyrl-.nrraehra tr tcayl heaeyAye laeaetrlh. *The generated receptor potential differs according to stimulating substances: 1-Salt: depolarization of receptor cell occurs by influx of Na+ ions. 2-Acids: depolarization of receptor cell occurs by influx of H+ ions. 3-Sweet: depolarization of receptor cell occurs by closure K+ channels, decreasing its conductance. 4-Bitter: depolarization of receptor cell occurs by increase release of Ca+2 ions. *yeSlllasll lescllntlr: slmosll lestra scle eSlseS:s - Anterior 2/3 of tongue>>>>>>chorda tympani (branch of facial nerve). - Posterior 1/3 of tongue>>>>>>> glossopharyngeal nerve. - Pharynx& epiglottis >>>>>>>> vagus nerve. *First order neuron (in ganglion along course of 7th, 9th, and 10th cranial nerves). --Sensory fibers from these 3nerves unite in medulla to form tractus solitaris that end around nucleus of tractus solitaris (Second order neuron). 26 --Axons of second order neuron cross to opposite side and join medial leminscus end in specific nuclei in thalamus VPNT (Third order neuron). -- Axons of third order neuron >>>>>>thalamic radiation>>>>taste area in cerebral cortex in post central gyrus. *Nemamaeolas eaSeoeolSoesmosll les eS llemS- -m-uare ealhrrh tr haraet Arru-.-adrlatcal sytc rtcal harhah uatalnyra tca Are rl rA tca Arru- *Abnormalities of taste sensation: Ageusia: absence taste sensation. Hypogeusia: decrease taste sensietivity. Dysgeusia: distorted taste sensation 27