auditory system, involved in the detection of sound,
the vestibular system, involved with maintaining body balance/ equilibrium.
• In auditory system - perception of sound
• In vestibular system - maintenance of balance.
The external ear (or pinna, the part you can see) serves to protect the tympanic membrane (eardrum), as well to collect and direct sound waves through the ear canal to the eardrum. About 1¼ inches long, the canal contains modified sweat glands that secrete cerumen, or earwax. Too much cerumen can block sound transmission.
AURICLE (PINNA)
Location: oval-shaped appendage on the lateral surface of the head.
Function: sound localization and amplification.
Composition: thin skin with hair follicles, sweat glands, and sebaceous glands covers supporting structure of elastic cartilage
EXTERAL AUDITORY MEATUS (CANAL)
Location: between the auricle and tympanic membrane
Composition: skin that contains hair follicles, ceruminous - modified sweat glands, and sebaceous glands covers supporting structure of elastic cartilage (lateral one-third) or bone (medial two-thirds)
TYMPANIC MEMBRANE (EARDRUM)
Location: separates the external auditory canal from the middle ear
Function: sound in the form of airwaves causes the membrane to vibrate, and these vibrations are transmitted to the attached auditory ossicles
Composition: the layers from outside to inside
• The epidermis of skin
• Radially and circularly arranged collagen fibers
• Mucous membrane covered by simple squamous epithelium
AUDITORY OSSICLES
Location: cross the space of middle ear in series and connect the tympanic membrane to the oval window
Function: help to convert sound waves (vibrations in air) to mechanical (hydraulic) vibrations in tissues and fluid-filled chambers
Composition: movable joints connect the bones:
• Malleus (hammer), attached to the tympanic membrane
• Stapes (stirrup), whose footplate fits into the oval window
• Incus (anvil), linking the malleus to the stapes
AUDITORY (EUSTACHIAN) TUBE
Location: connects the middle ear to the nasopharynx
Function: allow pressure in the middle ear to equilibrate with atmospheric pressure.
Composition: lined with ciliated pseudostratified columnar epithelium with small mass of lymphatic tissue
CRISTAE AMPULLARIS
Location: three sensory regions of the semicircular ducts in the ampullae of the semicircular canals
Function: sensors of angular movement
Composition: the ridge of epithelium that is oriented perpendicular to the long axis of the semicircular canal
Cells:
• Hair cells - nonneuronal mechnoreceptors with 50-100 stereocilia (sensory hairs) - modified microvilli. They are associated with both afferent and efferent nerve endings
o Type I hair cells - piriform in shape with a rounded base
o Type II hair cells - cylindrical in shape
• Supporting epithelial cells
• Cupula - gelatinous structure projects into the lumen and is surrounded by endolymph.
MACULAE
Location: saccule and utricle of the vestibule
Function: sensors of gravity and liner movement
Composition: the innervated sensory thickenings of the epithelium facing the endolymph
Cells:
• {#hair} Hair cells - nonneuronal mechanoreceptors with 50-100 stereocilia (sensory hairs) - modified microvilli and single true cilium - kinocilium. They are associated with both afferent and efferent nerve endings
o Type I hair cells - piriform in shape with a rounded base
o Type II hair cells - cylindrical in shape
• Supporting epithelial cells
• Otolithic membrane - gelatinous material with otoliths - 3-5mkm crystalline particles of calcium carbonate and protein
SCALA MEDIA (COCHLEA DUCT)
Location: the middle compartment in the cochlear canal
Composition: appears in transverse section as a triangular space.
• The most acute angle is attached to a bony extension of the modiolus
• The upper wall is the vestibular (Reissner's) membrane -two layers of squamous epithelial cells
• The lateral or outer wall is the stria vascularis - thick pseudostratified epithelium rich in blood vessels, the site of synthesis of endolymph
• The lower wall or floor is the basilar membrane - a dense mat of collagenic and some elastic fibers on which organ of Corti rests
ORGAN OF CORTI
Location: on the floor of scala media
Function: sound perception
Composition: complex epithelial layer
Cells:
• Inner (close to the stria vascularis) hair cells with 50-100 stereocilia and outer hair cells with 100-300 stereocilia are arranged in the rows of cells
• Inner (close to the stria vascularis) and outer phalangeal (supporting) cells - completely surround the basal portions of sensory cells, preventing them from touching the basilar membrane
• Pillar cells - form a triangular-shaped tunnel of Corti with cortilymph
• Tectorial membrane extends over the hair cells and attaches to the stereocilia. It consists of ground substance and collagen fibers
eardrum, is an air-filled cavity (tympanic cavity)
carved out of the temporal bone. It connects to the throat/nasopharynx via the Eustachian tube.
This ear-throat connection makes the ear susceptible to infection (otitis media).
The eustachian tube functions to equalize air pressure on both sides of the eardrum.
The inner ear consists of a maze of fluid-filled tubes, running through the temporal bone of the skull. The bony tubes, the bony labyrinth, are filled with a fluid called perilymph. Within this bony labyrinth is a second series of delicate cellular tubes, called the membranous labyrinth, filled with the fluid called endolymph. This membranous labyrinth contains the actual hearing cells, the hair cells of the organ of Corti. There are three major sections of the bony labyrinth:
1. The front portion is the snail-shaped cochlea, which functions in hearing.
2. The rear part, the semicircular canals, helps maintain balance.
3. Interconnecting the cochlea and the semicircular canals is the vestibule, containing the sense organs responsible for balance, the utricle and saccule.
The round window serves as a pressure valve, bulging outward as fluid pressure rises in the inner ear. Nerve impulses generated in the inner ear travel along the vestibulocochlear nerve (cranial nerve VIII), which leads to the brain. This is actually two nerves, somewhat joined together, the cochlear nerve for hearing and the vestibular nerve for equilibrium.
The base of the stapes rocks in and out against the oval window - this is the entrance for the vibrations.
The round window dissipates the pressure generated by the fluid vibrations, thus serves as the release valve: It can push out or expand as needed. The nerve impulses travel over the cochlear nerve to the auditory cortex of the brain, which interprets the impulses as sound.
The semicircular canals and vestibule function to sense movement (acceleration and deceleration) and static position.
Hearing
The most basic function of the ear is hearing. The following is the short description of the hearing process:
The first step is when the pinna collects external sounds that enter through the meatus or ear canal as sound waves. The ear drum begins to vibrate as these sound waves strikes. These vibrations pass through to the three ossicles of the middle ear (hammer, anvil and stapes) where they are amplified. As the transmission proceeds, the vibrations first hit the hammer, then the hammer pushes the anvil, and the anvil hits the stapes.
The vibrations are finally interpreted as sound in the brain after being transmitted and transformed into nerve signals by the cochlea (snail shaped component of the inner ear). This is due to the connectivity of the oval window of the inner ear to the edge of the stapes. When the stapes vibrates, they always transmit the sound vibrations to the inner ear.
Balance
The other important function of the ear is to help maintain balance. Oriented at the right angles to each other are three semicircular canals of the inner ear. Whenever the head is turned or change position, the resulting movement of fluids within these canals help the brain to identify or detect the extent of movement and positioning of the head.
In response to gravity, another part of the inner ear sends information to the brain when the head is held still in a stagnant position.
• Otitis media – also known as middle ear infection
• Otitis externa – also known as a swimmer's ear
There are other several ear disorders that we will soon have a look at. These include the following:
• Tinnitus – ringing of ear
• Vertigo – nahihilo
• Meniere's disease – nagkatubig ang tenga
• Hyperacusis – echoing of ear
• neural prebycusis – Alzheimer’s
• otitis - tuga
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