The ear-anatomy and physiology Flashcards
The external ear consists of…
The auricle, external auditory canal and the epithelial surface of the tympanic membrane
The auricle (pinna) is composed of
fibroelastic cartilage covered by skin
Describe the external auditory meatus
ls known as the external acoustic canal, is about 2.5 cm long in adults and is divided into an outer cartilaginous portion (one third) and an inner bony portion (two thirds)
Function of EAC
The external auditory canal protects the middle and the inner ear and serves as a channel for sound transmission
Subcutaneous tissue of the cartilaginous portion of the meatus
Numerous cerumen glands, which secrete the cerumen
Where is the middle ear cleft?
It is a continuous space that begins from the nasopharyngeal orifice of the Eustachian tube and extends to the furthest mastoid air cell
Components of middle ear
It includes the tympanum middle ear cavity proper), the Eustachian tube, and the mastoid air cell system
The epithelium lining the tympanic cavity
lined with mucosa consisting of cuboidal or ciliated cylindrical epithelium.
The middle ear cavity proper is divided into three parts
- epitympanum or attic (above TM);
- mesotympanum (opposite TM);
- hypotympanum (below TM).
Definition and function of TM
Separates the external ear from the middle ear. It is oval in shape, 9–10 mm tall, 8–9 mm wide, and 0.1 mm thick
The primary function of the middle ear
Efficiently transfer sound energy from compression waves in the air to fluid–membrane waves within the cochlea
The sound energy is transmitted and amplified by
Three bones known as ossicles (malleus, incus and stapes) from the tympanic membrane to the oval window of the cochlea
The Eustachian tube has three important functions
ventilation (pressure regulation of the middle ear)
protection of the middle ear from ascending nasopharyngeal secretions and pathogens
clearance of middle ear secretions towards the nasopharynx (mucociliary transport, pumping action during closing)
The inner ear is composed of two parts
the bony labyrinth, a system of cavities within the petrous portion of the temporal bone;
and the membranous labyrinth, a system of ducts and sacs within the bony labyrinth.
The bony labyrinth
Protects delicate structures of the membranous labyrinth and is filled with a fluid called perilymph
The oval and round windows
Are openings in the bone of the labyrinth and they are closed by the stapes footplate and a membrane, respectively
Name the 5 parts of the bony labrynth
the vestibule (housing the utricle and saccule),
the cochlea,
the semicircular canals,
the vestibular aqueduct,
and the cochlear aqueduct
The vestibule location
ovoid cavity located just medial to the tympanic cavity
The location of the cochlea
is anterior to the vestibule, and the apex of the cochlea is directed anteriorly and laterally
The membranous labyrinth
It consists of a system of ducts and sacs within the bony labyrinth. The fluid inside the membranous labyrinth is called endolymph
The membranous labyrinth consists of
The endolymphatic duct and sac,
the saccule,
the utricle,
the semicircular ducts,
which are jointly termed the vestibular system
How are ions distributed in the perilymph and endolymph
The major cation in the perilymph is sodium (as in extracellular fluid), but the major cation in endolymph is potassium, which is vital for the functioning of the inner ear
The function of the vestibular system
Provides information about head motion and orientation with respect to gravity
The balance mechanism of the vestibule consists of the otolith organs whcih are callled…
the utricle and saccule
Otoconia
It is calcium carbonate crytals attached to both the medial wall of the saccule and the floor of the utricle.
The otoconia enable the vestibular system to detect tilts and movements of the head, because they respond primarily to linear acceleration forces such as gravity
The angular accelerations are detected by
three semicircular canals: the lateral, superior (anterior) and posterior
What does each canal consist of
the ampulla at one end, which contains the cupula: a gel-like bud
The cupula
has sensory hair cells which detect the flow of endolymph within the semicircular canals.
Reissner´s membrane and the basilar membrane
Divide the cochlea into three spaces: the scala media, the scala tympani and the scala vestibuli
scala media
contains endolymph and is linked to the saccule
The scala vestibuli
and vestibuli contain perilymph and communicate with one another at the apex of the cochlea through an opening called the helicotrema
The scala tympani
Is connected to the cerebrospinal fluid (CSF) of the subarachnoid space by the cochlear aqueduct
The sense organ responsive to acoustic energy is located on the basilar membrane and is called
the organ of Corti
What happens after deflection of the basilar membrane
Will result in a shearing movement between the hair cells and the tectorial membrane, resulting in lateral or medial deflection of the stereocili
This bending movement of the stereocilia
Initiates the transduction of acoustic energy into neural signals
The mechanism of hearing
The sound is produced by the vibration of molecules within the air in the form of pressure waves. Ultimately, the ear converts these pressure waves into neural action potentials, which are perceived by the brain as hearing.
How does the middle ear amplify sound
Area ratio between the tympanic membrane and the stapes footplate. The surface area ofthe tympanic membrane is 17 times larger than the surface area of the stapes footplate; thus, the vibrations of the eardrum are amplified by 17 times when it gets concentrated on the footplate.
Lever action of the ossicles – amplifies the sound by 1.2 times
Movement in the perilymph
causes vibration of the basilar and tectorial membranes of the organ of Corti. The organ of Corti rests on the basilar membrane in the whole length of the cochlea, and contains the hair cells. T
Effects of vibration causing deflection of the stereocilia of hair cells
opening potassium channels, which enter the hair cells from the surrounding endolymph, producing neural impulses. When these impulses reach the auditory centre of the cortex, through the auditory pathway, sound is perceived
A different frequency of sound vibrates different areas of the cochlea: explain
The low frequencies are represented towards the apex of the cochlea and the high frequencies are represented towards the base.
The vestibular system
The sensory apparatus of the inner ear that helps the body to maintain its postural equilibrium.
The main inputs into the balance system are
The vestibular labyrinths, visual system (eyes), and somatosensory system, especially proprioception
The main outputs from the vestibular nuclei are
Vestibulo ocular
Vestibulo spinal
The saccular maculae
in parallel vertical planes and probably respond more to forward and backward tilts of the head.
History evaluation
Patients should be questioned about symptoms of pain, hearing loss, ear discharge, tinnitus, aural fullness or pressure and disequilibrium
Especially with the symptom of hearing loss, family history can provide insight into the diagnosis
Inspection
When examining the external ear, one should look at the size, shape, and position of the auricle,
as well as for redness, swelling, ulceration, the presence of surgical scars, and congenital abnormalities such as accessory auricles, skin tags or pre-auricular sinuses.
In the external auditory canal we should look for cerumen, foreign bodies, and any exudates.
Palpation in evaluation
The mastoid process, auricle and regional lymph nodes should be palpated while looking for swelling or sensitivity to pressure
we can see three basic structures via the otoscope
the handle of malleus (stria mallearis),
the lateral process of the malleus (prominentia mallearis),
and the light reflex in the anterior inferior part of the membrane
Area that cholesteatomas are first seen
The strip at the top of the eardrum (called pars flaccida)
