Lecture 23 - Anatomy and Physiology of the Ear PDF

Anatomy & Physiology of the Ear • Kelly Roballo, DMV, PhD Learning Objectives 1. 2. 3. 4. 5. 6. Describe the functions of the outer, middle and inner ear. Identify in order the structures over which sound is transmitted to the auditory receptors. Diagram the auditory pathway including all centra...

Anatomy & Physiology of the Ear • Kelly Roballo, DMV, PhD Learning Objectives 1. 2. 3. 4. 5. 6. Describe the functions of the outer, middle and inner ear. Identify in order the structures over which sound is transmitted to the auditory receptors. Diagram the auditory pathway including all central connections. Describe the four clinical signs of vestibular system dysfunction. Identify the tympanic membrane function. Describe the mechanics used by hair cells to transmit and interpretate the sounds • Reading: • Basic Clinical Neuroscience: Chapter 12 What is a sound? https://www.youtube.com/watch?v=4lm75v4Ndlg ANATOMY OF THE EAR Sounds waves in air Sound Transmission From External to Middle Ear • Sound wave enters external auditory canal • Causes tympanic membrane (eardrum) at end of canal to vibrate Amplifications from Outer to Inner Ear Within the 4 cm or so occupied by the outer and middle ears, three distinct physical principles operate to magnify weak vibrations in air so that they can establish pressure waves in a liquid: 1) The organ pipe resonance of the ear canal may increase the air pressure 10 times; 2) The mechanical advantages of the bone lever system may nearly triple it (3 times); 3) The pin-pointing arrangement of the eardrum and the oval window may provide another 30 fold increase. The result of these three mechanisms is an amplification of a sound wave by more than 800 times before it sets the liquid of the inner ear in motion. Middle Ear Middle Ear Inner Ear Summary 1. Pinna reflects sound & affects ability to locate sounds 2. Outer & middle ear turns air vibration into bone vibration 3. Inner ear turns bone vibration into fluid vibration 4. Cochlea turns fluid vibration into tissue movement 5. Different parts of the cochlea resonate to different frequencies of sound 6. The basilar membrane of the cochlea contains ciliated, mechanosensitive, receptor cells (which also resonate) Next… 7. THE RECEPTOR CELLS RELEASE NEUROTRANSMITTER ON 1° AUDITORY NEURONS 8. 1° AUDITORY NEURONS OF THE SPIRAL GANGLION PROJECT IN THE 8TH CRANIAL NERVE TO THE MEDULLA 9. THE PLACE CODING IS PRESERVED THROUGHOUT THE CNS (TONOTOPY) 10. HIGH & LOW FREQUENCY SOUNDS ARE LOCALIZED IN THE HORIZONTAL PLANE BY INTERAURAL INTENSITY & TIMING DIFFERENCES, RESPECTIVELY 11. SOUND IS LOCALIZED IN THE VERTICAL PLANE, AND BACK/FRONT AMBIGUITY IS RESOLVED BY SPECTRAL DIFFERENCES How the Cochlea Functions The pressure waves in the cochlea exert energy along a route that begins at the oval window and ends abruptly at the membrane-covered round window, where pressure is dissipated. The pressure applied to the oval window at the stapes is transmitted to all parts of the cochlea. How the Cochlea Functions § One of the two walls of the cochlear duct is the vibrating basilar membrane which functions to separate sounds by frequency. § The membrane is narrow and taunt at the end near the stapes and is wider and more pliant at the other § Hydraulic pressure waves in the cochlea induce a wave-like ripple in the basilar membrane which travels from the taunt to lose end § High tones produce greatest crests where the membrane is tight (where the stapes is), low tones where the wall is slack. How the Cochlea Functions § The position of this crest is important because it determines which nerve fibers will send signals to the brain. § High frequency tones cause the crest to occur at the base of the cochlea and the lower frequencies towards the apex § Apart from airborne sounds, the basilar membrane also picks up vibrations in the skull from such sources as teeth clicking Waves Along the Basilar Membrane of the Cochlea AUDITORY AND VESTIBULAR TRANSDUCTION Sensation in both the auditory and vestibular systems begins with the inner ear, and both use a highly specialized kind of receptor called the hair cell Hair cells are mechanoreceptors that are specialized to detect minuscule movement along one particular axis n The hair bundle consists of one large kinocilium, and 50 to 150 stereocilia n The stereocilia are 0.2 to 0.8 μm in diameter and 4 to 10 μm in height n Within the bundle, stereocilia are connected one to the next, but they can slide with respect to each other as the bundle is deflected side to side n The appropriate stimulus for a hair cell is the bending of its hairs n Bending the hair bundle toward the kinocilium excites the cell and causes a depolarizing receptor potential (K+ channels) n Bending the hair bundle away from the kinocilium hyperpolarizes the cell Hair Cells n Hair cells are contained within membranous labyrinth n The auditory portion of the labyrinth is the spiraling cochlea, which detects rapid vibrations (sound) transmitted to it from the surrounding air n The vestibular portion contains two structures: the otolith organs, which detect gravity and head tilt, and the semicircular canals, which are sensitive to head rotation Cochlea & Organ of Corti How The Organ of Corti Functions § The fluids filling and surrounding it act as shock absorbers, as do the springy membranes which support it § It is even isolated from the normal body supply lines, for the faint pulsing of blood through capillary vessels would be detected as background noise § The capillaries nearest to the organ of Corti end at the wall of the cochlea and the nutrients on their way out are carried to and from the capillaries by the endolymph fluid that bathes the organ § The basilar membrane supports the organ which contains a mass of cells almost touching the branch endings of the auditory nerve How The Organ of Corti Functions § From these cells sprout about 23,500 fine hairs rising in orderly rows like the bristles of a very soft brush § The hairs project through the dome of the organ, their ends embedded in a thick overhanging sheet called the tectorial membrane >>> these hairs are transducers § As the basilar membrane bellies in and out, it pushes and pulls the complex of tissues above it. The hair cells of the organ of Corti ride with the basilar membrane § The hairs have their tops embedded in the tectorial membrane and their roots fixed in the hair cells, so the motion of the basilar membrane bends and twists and pull and push the hairs NEURAL PATHWAYS FOR HEARING § Arrangement of neurons along auditory pathway is tonotopic § Sound information from each ear is transmitted bilaterally through the thalamus to each auditory cortex § Receptive fields of neurons in auditory cortex are biaural • Important for sound localization • Damage to cortex of one hemisphere does not cause hearing loss but does cause problems with sound localization Sound Detection in a Horizontal Plane 2-20 kHz n Two ears are necessary for the detection of sound in a horizontal plane n Sounds must first be processed by the cochlea in each ear and then compared by neurons within the CNS to estimate horizontal direction < 2 kHz CNS Processing of Sounds n After a sound stimulus enters to the cochlea, the cochlear nerve carries an action potential to the cochlear nucleus, which receives information only from the ear on the same side n However, higher auditory centers receive input from both ears CNS Processing of Sounds n Neurons in the medial superior olivary (MSO) nucleus are each tuned to a different interaural delay. Only when action potentials from the right and left sides arrive at the MSO neuron simultaneously does the neuron fire an action potential (coincidence detection). Part II: Balance & Equilibrium Vestibular Transduction The Sense of Equilibrium Consists of Two Parts: Dynamic and Static 1. Equilibrium is a state of balance that allows us to position our body in three-dimensional space under normal gravitational conditions 2. Balance is maintained through the hair cells in the fluid filled vestibular apparatus and the hair cells semicircular canals of the inner ear 3. Gravity and acceleration provide the force that moves the cilia of hair cells which act as mechanoreceptors The Sense of Equilibrium Consists of Two Parts: Dynamic and Static • 4. Equilibrium has two components: (1) A dynamic component, sensing the rotational movement of the head, and a (2) static component that senses head displacement via linear acceleration and the associated gravitational changes • 5. Rotational movements are detected by hair cells found within the ampulla linked to the semicircular canals • 6. Gravitational changes are detected by the otolith organs found within the maculae at the base of the semicircular canals Organization of Semicircular Canals The semicircular canals are the body's balance mechanism and it is thought that it plays no part in hearing Role of the Vestibular System § § § § § § Conscious perception in space/head in space Stabilization of Gaze Postural Stability Dizziness and Vertigo Disorders of eye movement/eye control Disorders of posture /locomotion (Ataxias) OTOLITH ORGANS § The Otolith Organs include two sac-like structures called the utricle and the saccule § Each of these otolith organs contains a small sensory area of approximately 2 mm in diameter known as macula § Each macula contains several thousand vestibular hair cells. The cilia which emerge from these hair cells are embedded in a gelatinous matrix called the monolithic membrane which contains small piles of calcium carbonate crystals, called eoliths (ear stones) and have greater specific gravity than the surrounding tissue n n n The otolithic organs are a pair of relatively large chambers near the center of the labyrinth Detect the position of the head in space Each contains a sensory epithelium called the macula, which is vertically oriented within the saccule and horizontally oriented within the utricle when the head is upright. Macula of the Otolithic Organs n The macula contains the hair cells, which lie among a bed of supporting cells with their cilia projecting into a gelatinous cap. n Each hair cell synapses on the primary sensory axon of the vestibular nerve, which itself is a branch of the vestibulocochlear nerve (CN VIII) n Otoliths are particles of calcium carbonate Rotation & Gravity n Angular acceleration generated by sudden head rotations is the primary stimulus for the semicircular canals n Hairs get bended when the canal is rotated; as the canal moves, endolymph stays behind due to its inactivity n The relatively inactive endolymph exerts a force on the movable cupula. n This force bows the cupula, which bends the hairs