Lecture 10 Physiology of Hearing PDF

Functions of the Ear The ears have two functions: 1. Hearing: Detection of sound by the cochlea. 2. Equilibrium: maintain body balance by the vestibule & semicircular canals  Parts of the Ear The ear is composed of three main regions: External ear (Outer ear) Mi...

Functions of the Ear The ears have two functions: 1. Hearing: Detection of sound by the cochlea. 2. Equilibrium: maintain body balance by the vestibule & semicircular canals  Parts of the Ear The ear is composed of three main regions: External ear (Outer ear) Middle ear: Both the outer and the middle ear transmit sound waves to inner ear, amplifying sound energy Inner ear Consists of 2 sensory systems: I. Cochlea II. Vestibular apparatus The External Ear Plays a role in Sound localization Amplifies sound (approx. by 5-6 dB) Pinna: collects sound waves and channels them down the ear canal Ceruminous (wax) glands produce ear wax (traps foreign particles, preventing them from entering the ear) Ends at the tympanic membrane (eardrum): It vibrates when sound waves hit it and transmits the sound waves to the middle ear. The Middle Ear It contains three Ossicles (smallest bones) – Malleus (hammer) – Incus (anvil) – Stapes (stirrup) attached to the oval window Bones are connected to each other and transmit sound waves from the tympanic membrane to inner ear Amplify sound waves to 22 times The auditory tube (Eustachian tube) connecting the middle ear with the throat (nasopharynx): helps equalize air pressure on both sides of the tympanic membrane during yawning or swallowing. The auditory tube  The Middle Ear Con.. Muscles of the Middle Ear Two muscles in the middle ear: I. Tensor tympani: Contraction of this muscle is a reflex triggered by the loud sounds, pulling the tympanic membrane inwards, restricting its freedom of movement Protects ear from loud noise or trauma II. Stapedius: Smallest muscle in the body Its contractions are part of a reflex initiated by loud sounds Contraction pulls the stapes posteriorly, tilting its footplate Dampening the ossicular chain vibration and limiting the potential damage caused by the loud noise  The Inner Ear Most complex portion of the ear A maze of bony chambers within the temporal bone It is separated from the middle ear by a membrane called the oval window Includes sense organs for hearing and balance Consists of three parts. 1. Semicircular canals –for balance Anterior 2. Vestibule – for balance 3. Cochlea – for hearing Semicircular Posterior Facial canals nerve Lateral Vestibular nerve Cochlear nerve Cochlea Vestibule  1. Hearing (Audition) Hearing is the transduction of sound waves into a electrical signals. Spiral organ of Corti is housed within the cochlear duct and contains hair cells (hearing receptors) Hair cells convert sound waves into nerve impulses which are transmitted via the auditory nerves to the brain for perception and interpretation as sound. The cochlea encodes auditory stimuli for frequencies between 20 and 20,000 Hz, which is the range of sound that human ears can detect.  Mechanism of Hearing Sound waves in the ear canal strike the tympanic membrane (eardrum) and become vibrations. The sound wave energy is transferred to the three bones of the middle ear (malleus, incus & staples), which vibrate. Vibrations of the staples against the oval window are converted into fluid waves within the vestibular duct. Fluid waves push on the membranes of the cochlear duct, thereby activating the sensory hair cell receptors. Fluid waves energy transfers across the cochlear duct and into the tympanic duct (scala tympani) and dissipated at the round window. Activating the sensory hair cell receptors The inner hair cells are the cells that “hear” Hair cells are mechanoreceptors sandwiched between the basilar membrane and the gel-like tectorial membrane Vibrations in the basilar membrane causes the receptor cells to become stimulated as their hairs (stereocilia) are bent by movement of the gel-like tectorial membrane Transform the mechanical forces of sound (cochlear fluid vibration) into the electrical impulses of hearing (action potentials)  Mechanism of Hearing Con… The stereocilia of each hair cell are organized into graded heights The tips of stereocilia are linked to the adjacent stereocilia by Tip links Upward movement of basilar membrane bends the bundle of stereocilia toward the tallest stereocilia, stretching the tip links Stretched tip links open the cation channels to which they are attached Endolymph has a higher concentration of K+ therefore entry of K+ inside the cell causes Depolarization (receptor potential) This in turn opens voltage gated Ca2+ channels and causes neurotransmitter release at the basal end of the hair cell, eliciting an action potential in the Cochlear nerve  Mechanism of Hearing Cont… Movement of basilar membrane in opposite direction bends the hair bundle away from the tallest stereocilium loosening the tip links – closing all channels causing Hyperpolarization Hair cells do not undergo action potentials Action potential in the Cochlear nerve propagates to the auditory cortex for sound perception through vestibulocochlear nerve  Auditory pathway Schematic diagram of auditory pathway  Auditory pathway The nerve impulse traveling along the vestibulocochlear nerve synapses with neurons in the cochlear nuclei of the medulla. The cochlear nuclei receives information only from one side of the body (ipsilateral) From the cochlear nucleus, some fibres synapse at the ipsilateral superior olivary nuclei, however, most fibers decussate to the contralateral superior olivary nuclei (information from both ears travels bilaterally). Hence, supranuclear lesion in one side (i.e. above the level of the cochlear nucleus) do not lead to major hearing impairment. The superior olivary nucleus begins the process of interpreting and combing input from the cochlear nucleus (for example sound localization through estimating time difference and intensity between each ear) Fibers from superior olivary nuclei projects upwards through the lateral lemniscus to reach the inferior colliculus nucleus, where all fibres carrying auditory information converge. Axons carrying the auditory information from the inferior colliculus project to the medial geniculate nucleus of the thalamus which then projects that information to the auditory cortex in the temporal lobe of the cerebral where they are interpreted as hearing. 2. Physiology of Balance: Vestibular Apparatus The vestibular apparatus consists of two structures of the bony labyrinth of the inner ear and the structures of the membranous labyrinth contained within them: I. The vestibule which responsible for monitoring static equilibrium (linear acceleration) II. The semicircular canals which responsible for monitoring the dynamic equilibrium (angular acceleration)  Vestibule: Utricle and Saccule Static equilibrium: changes in position of the head in space with respect to the pull of gravity when the body is not moving (static) for example forward or backward tilting of the head or riding elevator Maculae (static equilibrium receptors) are located within the membranous sacs of the Vestibule (utricle and saccule). Each macula is a patch of hair cells with stereocilia embedded in the otolithic membrane (a viscous gel contains calcium crystals called otoliths [otoconia]). Hence, utricle and saccule are sometimes called the otolithic organs. While tilting the head, otoliths roll in response to the pull of gravity creating a pull on the otolithic membrane and bending of macula hair cells. The hair cells become activated and send impulses along the vestibulocochlear nerve to the brain stem and cerebellum informing it of the head position in space. Static Equilibrium  Semicircular ducts Provide the sense of dynamic equilibrium in which there is angular or rotatory movements of the head. The semicircular ducts are three ring-like extensions of the vestibule. One is oriented in the horizontal plane, whereas the other two are oriented in the vertical plane. The base of each semicircular ducts connects to an swollen regions known as the ampulla. The ampulla contain the crista ampullaris which are the receptors for dynamic equilibrium. The crista ampullaris is a cone-shaped structure covered in hair cells that are impeded in a gel- like structure called the cupula. Upon head rotation, the fluid (endolymph) within the semicircular duct lags and deflects the cupula against the hair cells of the crista ampullaris. The hair cells thus respond by stimulating the vestibulocochlear nerve to report of the head movements within three-dimensional space. Dynamic Equilibrium and Crista Ampullaris Summary of the Structures of Internal Ear Bony Labyrinth Membranous Labyrinth Function Receptor region Semicircular Semicircular ducts Equilibrium: rotational Crista ampullaris canals (angular) acceleration Vestibule Utricle and saccule Equilibrium: head position Macula relative to gravity, linear acceleration Cochlea Cochlear duct Hearing Spiral organ (scala media) (organ of Corti)