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Official notes

The ear contains the organs for both balance and hearing. It contains 3 parts:

1. Outer (External) Ear

  • This consists of the auricle and the external acoustic meatus.
  • The auricle is the cartilaginous structure on the lateral side of the head. It is designed to channel the sound waves into the external acoustic meatus.
  • The external acoustic meatus (external auditory canal) is a curved tube that extends from the auricle to the tympanic membrane (eardrum). It is approximately 2.5 cms in length and directed downwards, forwards and medially. Its outer one-third is formed by cartilage and the inner two- thirds is formed by bone. It is lined by hairy skin that contains glands that secrete cerumen (earwax) which help to trap foreign objects.
  • The tympanic membrane (ear drum) is semitransparent and separates the external and middle ear. Its outer (lateral) surface is covered by skin and its inner (medial) surface by mucous membrane. The lateral surface is concave. The superior part is more flaccid than the rest. The handle of the first ossicle (the malleus) is attached to the medial surface of the membrane. Because of the tension in the lower part of the membrane a cone of light can normally be seen in the antero-inferior quadrant of the membrane when viewed with an auriscope.

2. Middle Ear

  • This is an air-filled cavity that is located within the petrous part of the temporal bone. It is a space approximately 15 mm high and 2-6 mm in width. Its lateral wall is formed by the tympanic membrane. It is divided into two parts - the epitympanic recess (above the level of the tympanic membrane) and the tympanic cavity proper. The epitympanic recess communicates with mastoid antrum and air cells via an opening in the posterior wall called the aditus.
  • The auditory tube passes forward from the anterior wall of the middle ear to the pharynx. It enables equalisation of air pressure in the middle ear with external air pressure. It is normally closed but opens when we swallow or yawn, enabling air to pass from the pharynx to the middle ear (eg. popping sensation experienced when we change altitude).
  • The medial wall of the middle ear cavity contains two membrane covered openings – the oval window (fenestra vestibuli) and the round window (fenestra cochleae), on the other side of which is the internal (inner) ear.

The ossicles are three tiny bones that form a chain across the middle ear cavity from its lateral to its medial wall. The malleus (hammer) is attached to the internal surface of the tympanic membrane. It articulates (via a tiny synovial joint) with the incus (anvil), which in turn articulates with the stapes (stirrup). The stapes covers the oval window.

  • The middle ear contains two tiny muscles which act to prevent excessive vibrations of the ossicles:
    • (i) tensor tympani m. – arises from a canal in the anterior wall of the middle ear and inserts on to the malleus.
    • (ii) stapedius m. arises a from a small projection of bone (pyramid) on the posterior wall and inserts on to the stapes.
  • The internal carotid a. and the internal jugular v. are related inferiorly to the middle ear cavity.
  • An important relation of the middle ear cavity is the facial nerve. This nerve arises from the brainstem and enters the internal acoustic meatus and then the facial canal. The facial canal passes along the medial wall before turning, to pass inferiorly down the posterior wall before leaving the skull via the stylomastoid foramen. As it passes near the middle ear it is susceptible to injury from middle ear infections and surgery.

3. Internal (inner) Ear

  • The internal ear is filled with fluid and is divided into bony and membranous labyrinths.
  • The Bony Labyrinth is formed by a complicated network of interconnected spaces within the petrous part of the temporal bone and is filled with a fluid called perilymph. It consists of a central area called the vestibule, 3 semicircular canals and a spiral tube called the cochlea, which resembles a snail shell in appearance.
  • The Membranous Labyrinth is a closed system of interconnected ducts (floating) inside the bony labyrinth. It is filled with a fluid called endolymph, which differs from perilymph in its composition. The membranous labyrinth consists of 3 semicircular ducts (within the semicircular canals), a utricle and saccule (within the vestibule) and the cochlear duct (within the cochlea). Specialised receptors for hearing and balance are located within the membranous labyrinth.

Hearing

  • The receptors for hearing are located within the cochlea. The cochlea is a long tube that is coiled two and three quarter times around a central bony core (modiolus) - shaped like a snail shell. A cross section of the cochlea shows it is divided into three parts with the cochlear duct in the centre. The cochlear duct is separated from the scala vestibuli by the vestibular membrane and the scala tympani by the basilar membrane. Scalae vestibuli & tympani communicate at apex of cochlear through the helicotrema.
  • Stria vascularis (in outer wall of the cochlea duct) produces endolymph.

Organ of Corti

  • is the auditory receptor organ, located within the cochlear duct.. The Organ of Corti sits on the basilar membrane and is made up of supporting cells and 2 sets of hair cells. These hair cells contain specialised stereocilia (‘hairs’) which make contact with a gelatinous membrane called the tectorial membrane. The hair cells are specialised neurons that are activated by bending of the ‘hairs’ on their surface. There are two types of hair cells:
    • (i) Inner Hair cells: a total of about 3500 cells form a single row which extends along the basilar membrane. They provide about 95% of input to auditory n. (1 inner hair cell -> auditory n. fibre). Code frequency & intensity of sound.
    • (ii) Outer Hair Cells: 15–20,000 in number, arranged in 3-5 rows. Provide only about 5% of the input to auditory n. Can change length, influencing the orientation of tectorial membrane and sensitivity of inner hair cells. When they shorten, they cause increased bending of hair cells, magnifying the input to the auditory (cochlear nerve), an effect known as the cochlear amplifier. Damage to the outer hair cells (and hence the cochlear amplifier) can result in hearing loss.

Mechanism of Sound Transduction

The auricle directs sound waves into the external acoustic meatus, causing the tympanic membrane to vibrate, resulting in movement of the malleus (which is attached to it). This in turn causes movement of the incus and then the stapes. The ossicles form a lever system that increases the amplitude of the sound waves (x20). Movement of the stapes causes movement of the perilymph in the scala vestibuli, which is then transmitted across into the cochlear duct, resulting in movement of the basilar membrane and bending of the hair cells, which then activate the fibres of the auditory (cochlear) part of the vestibulocochlear nerve. This nerve is the first stage of a pathway that transmits the auditory information via the brainstem and thalamus to the primary auditory area of the cerebral cortex, where it is consciously perceived and interpreted.

How do we distinguish different frequencies?

  • The human ear is able to hear sounds ranging from 20 to 20,000 Hz (cycles per second). The properties of the basilar membrane change progressively along its length. Near the base of the cochlear it is narrow and tight whereas it is wide (500um) and loose near the apex. *Therefore, high frequency sounds cause maximal vibration near the base and low frequency sounds cause maximal vibration near the apex, ie. the basilar membrane is tuned for different frequencies. Auditory nerve fibres arising from different parts of the basilar membrane have a specific location within the nerve (tonotopic organization). This tonotopic organization is maintained all the way to the auditory cortex, which contains a map of frequencies (low frequencies anterior and high frequencies posterior). Although we are not going into detail about the pathway to the auditory cortex, it is important to remember that one side of the cortex receives input from both ears. This means that damage to the auditory cortex of one hemisphere does NOT result in significant hearing loss.

Class notes

See labeled diagrams from lecture
The ear is located in the temporal bone of the skull (shown below in green). This is where the opening of the skull for the ear is. The opening is protected by a ring of bone. This opening is called the external acoustic meatus (ear canal). Most of the components of the ear are in the temporal bone.

  • Viewing the skull from above, we can see the temporal bone in the floor of the skull. The ear is in the petrous part of the temporal bone (very dense bone - to stop outside sound vibrations getting in, and to keep internal vibrations in) - this stops extraneous noise from reaching the inner ear without getting in through the
  • The internal acoustic meatus is an opening in the middle of the skull, transmits the vestibulocochlear nerve, facial nerve and labyrinthine artery.

Structure of the ear

  • External ear: the auricle/pinna of the ear. This is a cartilaginous structure with overlying skin. The cartilage is elastic and the skin is very thin (not much sc tissue below the skin). The human ear is not good at localising sound, but if you use wax to fill up the nooks and crannies in the external ear, then you're less able to localise sound
  • Lobule: the fleshy bit that hangs below
  • The curled edge is the helix
  • External acoustic canal: the bony external acoustic meatus and the cartilaginous extension of it. All of this canal is covered by skin, designed to repel fungal or bacterial invaders. It produces its own bacteriocidal substance: cerumen (full of fatty acids which are toxic to bacteria and fungi). If you get water in there, you may interfere with the ability of this to do its work (swimmer's ear).
  • This is a self-cleaning oven (don't need to shove things in there. If you have a lot of earwax, get it flushed out from time to time - active sebaceous glands). Don't stick anything in your ear that's smaller than your elbow (i.e. nothing should go in your ear)
  • Tympanic membrane at the inside limit of the external acoustic meatus
  • Beyond that is the middle ear, with auditory ossicles that transmit vibrations to the inner ear (labyrinth - cochlear components for hearing and vestibular components for balance).
  • There is also a tube downwards (eustachian or pharyngotympanic tube tube) to the nasal cavity (equalise pressure, drain secretions)

Middle ear

  1. Epitympanic recess: area above the tympanic membrane
  2. Tympanic cavity proper: the lower part
  • There are 3 ossicles arranged in a chain across the middle ear:
    • Malleus (malleus)
    • Incus (anvil)
    • Stapes (stirrup; 2 feet and a foot plate)
  • These bones are connected by synovial joints
    • These have a cavity and fluid filled space
    • Can get osteoarthritis between the middle ear ossicles (this is one contributing factor to losing hearing as you age)
  • The tympanic membrane should be a complete membrane. Ruptures result in fluid being drained to the external ear - allowing it to repair again (can be helpful).
  • Should see a long handle of the malleus
  • The tympanic membrane is a cone (shallow), which has a tense part (down the bottom - pars tensa - should see a reflection of oroscope light when you look at the tympanic membrane. This cone of light tells you that the tympanic membrane is tense at that point. If it's not tense, it may indicate a rupture)

Eustachian tube

  • Upper part of the pharyngotympanic tube is bony, and below it is cartilage (making it a cartilaginous tube
    • Because of this connection, there is a possibility for bacteria/viruses to ascend from the throat into the middle ear (if you suck snot backwards, you can fire it into the auditory tube)

Split temporal bone showing middle ear

  • Note the density of the bone
  • A lot of the components of the temporal bone have air sacs in them, to resonate sound, and these can harbour infection that spreads
    • These are mastoid air cells
  • Put finger behind your ear, you will feel a mastoid process. This is not solid, but has mastoid air cells that pass into the mastoid antrum, and goes into the middle ear.
    • Mastoiditis: pus from the nasopharynx goes up eustachean tube and gets into mastoid air cells - very painful

Ossicles

  • The footplate of the stapes sits over the oval window
  • Beneath this is the round window
  • Superiorly is the opening to the mastoid air cells
  • Inferiorly there is the auditory tube
  • Auditory ossicles: transmit vibrations to the internal ear, but also provides an amplification of sound (because of mechanical advantage and relative areas).

Small muscles of the middle ear

  • Two muscles in the middle ear attach to the malleus (tensor tympani - by trigeminal nerve) and to the stapes (stapedius - facial nerve)
  • These two muscles are responsible for an acoustic reflex to dampen vibrations. This is for protection against excessive repetition of certain vibrations. This has a protective role, except that loud explosions/gunshots do not provide enough time for reflexes to carry out their job -- most important for long-standing loud sounds
    • Good for cocktail parties

Inner ear

  • Made of cochlea (hearing) and vestibule (balance)
  • Outer component of labyrinth: bony (interior of the inner ear); floating within that is a membranous component of the labyrinth
  • The membranous structure appears blue in the picture, and is suspended to the pink-coloured bony labyrinth, by a few small ligaments
    • There are 2 types of fluid in the labyrinth - in the bony labyrinth is perilymph and within in membranous labyrinth is endolymph
  • Spiral structure = cochlea (looks like a snail shell. it has 2.75 turns as it spirals in
  • Above the cochlea is a region that is vestibular in function - it has the 3 semicircular canals orientated at right angles to each other
    • Detect rotational movement of the head
  • Beneath the semicircular canals is the vestibule proper, with the horizontal saccule
    • Detect linear acceleration and the direction of gravity

The labyrinth

  • Membranous labyrinth is blue colour, outside it is the bony labyrinth, a pink colour (in this diagram)
  • A component of the bony labyrinth coils around the membranous labyrinth in the cochlea
  • Membranous = ducts; Bony = canals
  • At the base of each semicircular duct is an ampulla (with the sensory parts)
  • Beneath these ampullae are the utricle (horizontal) and saccule (vertical) - these detect linear acceleration as well as the directionality of gravity
  • The vestibular apparatus has a connection through fine channels with the cochlea
    • Menier's disease - bouts of vertigo as well as auditory problems (they're connected)
  • There is also a communication with the cranial cavity (to the endolymphatic sac and duct) - this acts as a pressure relief system for the inner ear.

Membranous labyrinth

  • The inner ear is a neural structure - designed to sense things. You need sensory elements both for the cochlea and for the vestibular apparatus
  • The deeper blue is the endolymph-filled membranous labyrinth. The lighter areas are where there are sensory structures
  • The inner ear works by detection the motion of gelatinous structures in a fluid environment over sensory apparatuses
    • This works for both sound, for rotation, and gravity orientation
  • The semicircular ducts detect movement of fluid around the interior when you rotate your head (because of the inertia of the fluid in your head) - movement of the fluid is detected and perceived as rotation of the head. The fluid tends to stay still as the canals are moved, resulting in relative movement between the fluid and the canals

Cochlea

  • The cochlea is a long tube coiled 2.75 times around a bony core (the modiolus)
  • The cochlear duct spirals up around the cochlea (blue structure)
  • Clumps of
  • Scala tympani (inferiorly to cochlear duct)
  • Scala vestibuli (superiorly to cochlear duct)
  • The important sensory structure for the cochlea is the organ of corti
    • Sits in the centre of our three chambers (scala media or cochlear duct). It consists of hair cells
    • A single row of inner hair cells and 3 rows of outer hair cells
    • Sitting above these hair cells is the tectorial membrane
    • These hair cells have stereocilia
    • The outer hair cell processes are embedded in the tectorial membrane. The inner hair cell processes don't quite make it
    • Both these types of cell have nerves connecting them
  • In the histological picture, it's all greatly shrunken (due to dehydration
  • Note also the spiral ganglion, getting the information from the inner and outer hair cells from the nerve fibres
  • The inner hair cells are designed to fire off action potential when their hair cell processes are bent. When there is fluid movement and tilting of the membrane the organ of corti sits on. The hairs are linked by tip links, and when these little hairs are tilted, it opens ion channels and a message is passed to the brain that vibration has been experienced.
  • The inner and outer hair cells are different in function
    • Inner hair cells are exclusively sensory
    • Outer hair cells are amplifiers - when there is movement of tectorial membrane, the processes are bent. This results in the contraction of the outer hair cell, resulting in more fluid movement, stimulating the inner hair cell
      • These are the cochlear amplifiers
      • If you play music to them, they contract (responding to vibration)

How do we hear?

  • Footplate of stapes pushes against the inner ear, so pressure waves are transmitted down the cochlear duct, and produce movement of the organ of corti. The membrane the organ of corti sits on is physically different at different parts of the organ of corti (different frequencies are selected)
    • The membrane is very narrow and stiff at the base of the cochlear duct, and wide and floppy at the apex.
      • Therefore detects high frequency sounds at the base and low frequency sounds at the apex
  • You need two windows so there is actual movement of the pressure wave in the inner ear

Ascending auditory pathways

  • What happens to the sound information?
  • It goes in, along the pathways of the spiral gangion cells, to the brainstem, to the midbrain, thalamus and to the cerebral cortex.
  • Information from your left ear goes to both left and right hemispheres (if you damage your left hemisphere, you only lose the localisation of sound in the left ear.

Images