Ask everyone to cup their hands behind their ears, do they see any difference?
2 parts:Epitympanic recessTympanic CavityAllows communication between nasopharynx via EUSTACHIAN TUBE Allows communication between MASTOID ANTRUM (posterior to epitympanic recess)
Modiolus a.k.a. central pillarBasilar membrane responsible for dividing it into scalavestibuli and scala tympani
Movements of the eyes and body produced by these pathways serve to maintain balance and track the visual field during rotation.
The physiology of the ear
External Ear › Auricle/ Pinna: Flap of elastic cartilage shaped like flared end of a trumpet and covered by skin
External Auditory Canal: › Curved tube about 2.5 cm (1 in.) › Lies in the temporal bone and lead from the auricle to the eardrum › Contains a few hair and ceruminous glands (specialized sebaceous glands) Tympanic membrane: › A thin, semitransparent partition between the external auditory canal and middle ear › Consists of a connective tissue core lined with skin on the outside and mucous membrane on the inside › Covered by epidermis and lined by simple cuboidal epithelium
Small, air-filled cavity in the temporal bone that is lined by epithelium. Contains two small membrane-covered openings: › Oval window › Round window Contains the three smallest bones in the body › Malleus (hammer) › Incus (anvil) › Stapes (stirrup) Joints: synovial joints
Malleus: attached to the internal surface of the eardrum Incus: the middle bone; articulates with the head of the stapes Stapes: its base fits into the oval window (which is right above the round window)
Tensor Tympani muscle: inserts into the handle of the malleus. It reduces the vibrations of malleus that could potentially harm the tympanic membrane (i.e. loud noise) Stapedius muscle: Reduces the vibrations of stapes
An opening on the anterior wall of the middle ear Consists of both bone and hyaline cartilage Connects middle ear with the nasopharynx Functions to equalizes the pressure within middle ear and the atmospheric pressure (between tympanic cavity and nasopharynx) A route where pathogens may travel from the nose and throat to the middle ear
Called a labyrinth because of its complicated series of canals Two main divisions: › Bony Labyrinth: lined with periosteum and contains perilymph (similar to CSF) › Membranous Labyrinth: surrounded by the CSF. A series of sacs and tubes inside the bony labyrinth and having the same general form
› Bony Labyrinth: series of cavities in the temporal bone. Divided into three areas: Semicircular Canals: projects posteriorly and superiorly from the vestibule. Consists of an anterior, posterior and lateral semicircular canal. Ampulla: swollen enlargement at the end of each canal Semicircular ducts: allows communication between the utricle and the vestibule
Vestibule: contains receptors for equilibrium Oval central portion of the bony labyrinth Communicates anteriorly with the cochlea and posterosuperiorly with the SCC The membranous labyrinth in the vestibule consists of: Utricle Saccule
Cochlea: contains receptors for hearing › Anterior to the vestibule › A bony spiral canal that resembles a snail shell and makes almost three turns around a central bony core (modiolus). It is divided into three channels The partitions that separate the channels are shaped like a letter Y Scala vestibuli: channel above the bony partition and ends at the oval window Scala Tympani: channel below and ends at the round window
Cochlea: › Adjoins the wall of the vestibule (where the scala vestibuli opens) › It has two membranes: basilar membrane and vestibular membrane (which separates the cochlear duct from the scala vestibuli) › Spiral Organ of Corti: Rests on the basilar membrane and contains hair cells, which are receptors for hearing
Lodged within bony labyrinth Filled with endolymph Surrounded by perilymph
Sense of equilibrium---- provides orientation with respect to gravity Forms the inner ear along with the cochlea Consists of two parts: › Otolith Organs: utricle and saccule › Semicircular canals
The sensory structures of both the vestibular apparatus and cochlea are located within the membranous labyrinth (which is filled with a fluid called endolymph) which is located within the bony cavity in the skull, bony labyrinth. Perilymph is the fluid between the membranous labyrinth and the bone
Utricle and Saccule: provide information about linear acceleration › Refers to the changes in velocity when traveling horizontally or vertically (i.e. riding in a car) Semicircular Canals: provides a sense of rotational and angular motion It helps maintain balance when turning the head, spinning, or tumbling. › Refers to the changes in direction
Receptors for equilibrium; modified epithelial cells Named as they are because each cell contains twenty to fifty hairlike extensions › Stereocilia: processes containing filaments of protein surrounded by part of the cell membrane › Kinocilium: larger extension that has the structure of a true cilium
1. When the stereocilia are bent in the direction of the kinocilium, the cell membrane is depressed and becomes depolarized.2. The hair cell releases a synaptic transmitter, thus stimulating the dendrites of sensory neurons that are part of the vestibulocochlear nerve.3. When the stereocilia are bent in the opposite direction, the membrane of the hair cell becomes hyperpolarized, which causes the release of a less amount of synaptic transmitter.
In this way, the frequency of action potentials in the sensory neurons that innervate the hair cells carries information about movements that cause the hair cell processes to bend.
Utricle and Saccule have a patch of specialized epithelium called a macula that consists of hair cells and supporting cells. › The hair cells project into the membranous labyrinth, with their hairs embedded in a gelatinous otolithic membrane Contains microscopic crystals of calcium carbonate, these increase the mass of the membrane, and increase the resistance to change in the movement
Utricle is more sensitive to horizontal acceleration › Otolithic membrane lags behind the hair cells › Hair cells are pushed backward Saccule is more sensitive to vertical acceleration › Causes the hairs of the saccule to be pushed upward
Semicircular duct: inner extension of the membranous labyrinth in each canal › Ampulla Crista ampullaris: elevated area of the ampulla where the sensory hair cells are located. Cupula: gelatinous membrane where the processes of the hair cells are embedded. It can be pushed in several directions because of the endolymph.
Endolymph: › Provides inertia so that the sensory processes will be bent in a direction opposite to that of the angular acceleration. Through this, it stimulates the hair cells
The Semicircular Canals: Anterior Semicircular canal: hair cells are stimulated when doing a somersault. Posterior Semicircular canal: stimulated when performing a cartwheel. Lateral Semicircular canal: stimulated when spinning around the long axis of the body.
Stimulation of hair cells in the vestibular apparatus activates sensory neurons of Vestibulocochlear nerve (CN VIII) These fibers transmit impulses to the cerebellum and to the vestibular nuclei of the medulla oblongata The vestibular nuclei then send fibers to the oculomotor center of the brain stem and to the spinal cord
During a spin, the bending of the cupula produces smooth movements of the eyes in a direction opposite to that of the head movement so that a stable visual fixation point is maintained. When the spin is abruptly stopped, the eyes continue to smoothly in the previous direction of the spin, and then are jerked rapidly back to the midline position This produces involuntary oscillations of the eyes called vestibular nystagmus.
• Loss of equilibrium as a result of spinning• May be caused by anything that alters the firing rate of one of the CN VIII compared to the other Usually due to a viral infection causing vestibular neuritis• Severe vertigo is accompanied by dizziness, pallor, sweating, nausea, and vomiting due to involvement of ANS, which is activated by vestibular input tothe brain stem
Involuntary movement of the eye resulting from abnormal stimuli to the inner ear. One of the symptoms of an inner-ear disease called Ménières disease › Early symptom: “ringing in the ears” or tinnitus Vestibular symptoms of vertigo and nystagmus accompany hearing problems in this disease
Types › Central produce one-way or two-way eye movement › Peripheral exhibits only one-way eye movement. Treatment › Botulinum toxin, the substance that causes botulism, is sometimes injected to reduce eye movement › Surgery is also necessary in some cases
Sound causes vibrations of the tympanic membrane, and they produce movements of the middle-ear ossicles, which press against a membrance called the oval window in the cochlea. Movements of the oval window produce pressure waves within the fluid of the cochlea, causing movements of the basilar membrane. › Bending of the sensory hair cells follows › Stimulation of action potentials transmitted to the brain in sensory fibers and interpreted as sound
Alternating zones of high and low pressure traveling in a medium (air or water) Are characterized by: › Frequency (Hz) cycles per second (cps) Pitch › Intensity (dB) Amplitude of the sound waves
Sound waves are funneled by the pinna (auricle) into the external auditory meatus, and these 2 form the outer ear. External auditory meatus channels the sound waves (while increasing the intensity) to the eardrum, or tympanic membrane Sound waves in the EAM produce extremely small vibrations of the tympanic membrane.
The cavity between the tympanic membrane on the outer side and the cochlea on the inner side 3 middle-ear ossicles – protection › Malleus (hammer) attached to the tympanic m. vibrations are transmitted via the malleus and incus to the stapes › Incus (anvil) › Stapes (stirrup) attached to the oval window in the cochlea vibrates in response to the vibrations of the tympanic m.
Stapedius muscle › Attaches to the neck of the stapes › Increases protective function › Helps prevent nerve damage within the cochlea in very loud sounds as it contracts and dampens the movements of the stapes against the oval window
Auditory (eustachian) tube › A passageway leading from the middle ear to the nasopharynx › Is usually collapsed; to prevent debris and infectious agents from traveling from the oral cavity to the middle ear. › Tensor tympani muscle Must contract to open the auditory tube Occurs during swallowing, yawning, sneezing “popping” sensation in swallowing when driving up to a higher altitude The auditory canal opening allows air to move from the region of higher pressure (middle ear) to the region of lower pressure (nasopharynx)
• cochlea which serves as the bodys microphone, converting sound pressure impulses from the outer ear into electrical impulses which are passed on to the brain via the auditory nerve.• The inner ear structure called the cochlea is a snail-shell like structure.
The pressure changes in the cochleacaused by sound entering the ear travel down the fluid filled tympanic(scala tympani) and vestibular canals(scala vestibuli) which are filled with a fluid called perilymph. This perilymph is almost identical to spinal fluid and differs significantly from the endolymph which fills the cochlear duct(scala media) and surrounds the sensitive organ of Corti.
• Receptor organ of hearing• It contains four rows ofhair cells which protrude from its surface. Above them is the tectoral membrane which can move in response to pressure variations in the fluid- filled tympanic and vestibularcanals. There are some 16,000 - 20,000 of the hair cells distributed along the basilar membrane which follows the spiral of the cochlea.
The place along the basilar membrane where maximum excitation of the hair cells occurs determines the perception of pitch according to the place theory. The perception of loudness is also connected with this organ.
Tiny relative movements of the layers of the membrane are sufficient to trigger the hair cells. Like other nerve cells, their response to stimulus is to send a tiny voltage pulse called an "action potential" down the associated nerve fiber (axon). These impulses travel to the auditory areas of the brain for processing.
Sensory neurons in the vestibulocochlear nerve (VIII) synapse with neurons in the medulla oblongata that projects to the inferior colliculus of the midbrain. Neurons in this area project to the thalamus thats sends axons to the auditory cortex of temporal lobe. Neurons in different regions of basilar membrane stimulate neurons in corresponding areas in auditory cortex.
Each area of the auditory cortex thus represents a different part of the basilar membrane and a different pitch.
The cochlea acts like a frequency analyzer, in different frequencies (pitches) of sound stimulate different sensory neurons that project to different places in the auditory cortex The analysis is based on which hair cells activate the sensory neurons It is related to the position of the hair cells on the basilar membrane. This is known as the PLACE THEORY OF PITCH.
Since the different sensory neurons project to different places in the auditory cortex, the organization of this cortex is said to be tonotopic. tone frequencies are transmitted separately along specific parts of the structure.
Able to recognize that a given sound frequency (such as 400 Hz) is the same regardless of whether it is played by violin or piano In harmonics, can vary, depending on their amplitudes. However, if the fundamental frequency is the same, the pitch is recognized being the same on the different instruments
Conduction Deafness › Transmission of sound waves through the middle ear to the oval window is impaired Sensorineural or Perceptive Deafness › Transmission of nerve impulses anywhere from the cochlea to the auditory cortex is impaired
Caused by middle–ear damage from otitis media or otosclerosis Impairs hearing at all sound frequencies Can be helped by Hearing Aids Device that amplify sounds and conduct the sound waves through bone to the inner ear.
Result from a wide variety of pathological processes and from exposure to extremeley loud sounds Unfortunately, the hair cells in the inner ears cannot regenerate once destroyed. Impairs the ability to hear some pitches more than others. This may be due to pathological processes or to changes that occur during aging.
Can be corrected by Cochlear Implants It consists of elctrodes threaded into the cochlea, a receiver implanted in the temporal bone, and an external microphone, processor and transmitter.
Age-related hearing impairment Begins after age 20 when the ability to hear high frequencies (18000-20000 Hz) diminishes Men are affected to greater degree than women, but the progression is variable Deficits may gradually extend to 4000- 8000 Hz range
Impairment can be detected by Audiometry A technique in which threshold intensity of different pitches is determined. The ability to hear speech is particularly affected by hearing loss in the higher frequencies