Case 16- Anatomy and CNS infection Flashcards
Structures within the ear
- Pinna- cartilaginous flap, composed of ridges and hallows
- Tympanic membrane- ear drum
- Middle ear- air filled, crossed by 3 small bones collectively called the Ossicles (Malleus, incus and stapes)
- Eustachian tube- connects the ear to the nasopharynx
- Inner ear- composed of two parts, the Cochlea (concerned with hearing) and the vestibular apparatus (sense of balance)
- Cranial nerve VIII- within the Cochlea, carries information about hearing and balance
- Cochlea- sound waves are converted to electrical signals in a process of transduction
- Auditory canal- there is a cartilaginous portion and a bony portion
Tympanic membrane
1) Separates the inner ear from the outer ear
2) When the tympanic membrane vibrates the vibrations are transmitter through three small bones into the inner ear (cochlea) -> inner ear -> basal membrane -> hair cells -> nerve cells
Pinna
- Funnels sound into the auditory canal- orientation towards source, mobile in animals
- Externalisation of sounds- you can listen without pinna i.e. through headphones, sound appears inside the head. Can help us tell when sound is outside the head and in the real world
- Localisation of sounds in the vertical plane- helps us tell if it is above or below the head. This done as sound waves interact in a complex way on the ridges of the pinna. Different to when sound just goes straight through the auditory canal, the difference between how these sound waves are processed helps with localisation and externalisation
External auditory meatus or ear canal
- Canal leading down to the tympanum, about 28mm long
- Cartilaginous outer portion, bony inner portion.
- Protects sensitive machinery of the inner ear in the petrous portion of the temporal bone, as the ear canal conducts sound towards it. The walls of the ear canal secrete waxy cerumen, which captures dust and foreign bodies. Its self-cleaning but may become compacted and require removal.
- Amplifying effect on sounds owing to resonance effect in the frequencies 1000-6000 Hz or 1-6 kHz. This is in the centre of our frequency range of hearing
Eustacian tube
1) Connects the middle ear to the nasopharynx
2) Pathogens can go through the Eustacian tube and cause infection of the middle ear
3) This tube allows the air pressure in the middle ear to equilibrate with the atmospheric pressure
4) Differences in pressure will cause the tympanic membrane to bulge and cause hearing loss and pain
5) The Eustacian tube opens when chewing or opening and closing your mouth
Structure of the normal tympanic membrane
1cm in diameter and 0.1mm thick
• Pars flaccida- at the top of the tympanic membrane, doesn’t contain any fibres
• Pars tensa- below the Pars flaccida, lots of fibres run through it
• Lateral process and handle of the malleus- bone of the middle ear which almost rests on the tympanic membrane
• The Umbo- where the malleus attaches to the tympanic membrane
• Cone of light- the light which reflects from the otoscope, if the cone of light is at 5 o’clock it’s the right side if its 7 o’clock it’s the left side
• The tympanic membrane should be pinkish with a pearly appearance to it.
Acute otitis media
Infection of the middle ear due to an upper respiratory tract infection
Middle ear
1) Air filled space, need equal pressure either side of the tympanic membrane
2) Ossicles- Malleus, Incus, Stapes
3) The footplate of the stapes connects to the oval window of the Cochlea, the oval window is a membraneous portion
Why we need a middle ear
The inner ear is fluid filled and there are problems with transferring energy from one medium to another, sound waves travelling from air to water are mostly reflected. Without the middle ear there is a poor transfer of energy from the air to cochlear fluid, the middle ear is an impedence transformer.
How the middle ear overcomes the air-fluid mismatch
• The area of the tympanic membrane is greater that the area of footplate of the staples- the force being applied to the tympanic membrane will act over the much smaller area of the stapes footplate. This will cause a x18 increase in pressure as pressure=force/area. This increases the sound in the Cochlea.
• The Ossicles act as a lever system- the malleus and incus are joined at a pivot, the malleus is longer then the incus causing a lever effect. So, when the malleus moves a longer distance the incus moves a shorter distance but with a larger force. Increases the force the incus acts on the stapes. This lever ratio increases the force by 1.3 times.
Total increase in force is 25 times between the tympanum ad the stapes. The increase in pressure helps improve sound transfer
Muscles of the middle ear
1) Tensor tympani (attaches to the malleus). The tensor tympani is innervated by a branch of the Mandibular nerve.
2) Stapedius (attaches to the pyramidal eminence which is attached to the neck of the Stapes). The stapedius muscle is innervated by a branch of the facial nerve
Controls the stiffness of the ossicular chain and sound emission into the cochlea. Provides the middle ear reflex
Muscles of the middle ear
Tensor tympani (attaches to the malleus) and stapedius (attaches to the pyramidal eminence which is attached to the neck of the Stapes), controls the stiffness of the ossicular chain and sound emission into the cochlea. The stapedius muscle is innervated by a branch of the facial nerve and the tensor tympani is innervated by a branch of the mandibular nerve. Provides the middle ear reflex
What does the middle ear provide protection against
The middle ear reflex protects against loud sounds but not impulsive sounds like a gun shot as its sounds too fast for the reflex to activate. It is also activated when speaking so sounds are not too loud
Inner ear- Cochlea structure
1) There is a bony wall of the Cochlear.
2) There are 3 compartments, the Scala vestibuli at the top, the Scala media in the middle and the Scala tympani at the bottom.
3) It is supplied by the Cochlear nerve.
4) Reissner’s membrane separates the Scala vestibuli and the Scala media
5) The Basilar membrane separates the Scala media and the Scala tympani.
6) Siting on the Basilar membrane is the Organ of Corti which is where you find the sensory hair cells of the Cochlea which are involved in the transduction of sound into electrical signals.
Fluid within the Cochlea
1) Each of the compartments are filled with a different fluid.
2) The fluid in the Scala tympani and Scala vestibuli is the Perilymph which is high in Na+ and low in K+.
3) The Scala media is filled with Endolymph which is high in K+ and low in Na+.
4) They are both extracellular fluid but Endolymph is similar in composition to intracellular fluid.
5) Endolymph is generated through Stria vascularis, which pump K+ within the compartment.
Organ of Corti
There is one row of inner hair cells and 3 rows of outer hair cells. Stereocilia are the hair like projections. In the outer hair cells the Stereocilia are embedded in the Tectorial membrane. The hair cells are associated with nerve cells. The apical membrane of the hair cells and the Stereocilia are bathed in the Endolymph. The basalateral surface of the hair cells are bathed in Perilymph. Involved in the transduction of sound into electrical signals.
What happens when sound enters the Cochlea
- It causes the basilar membrane to vibrate
- Travelling waves travel up the basilar membrane from base to apex
- Position of maximal displacement depends on sound frequency, high frequency small displacement
- Displacement of basilar membrane excites hair cells
How is pressure maintained in the middle ear
1) The Staples attaches to the oval window (membraneous portion) and the Scala vestibule
2) As the stapes vibrates there is a change in pressure which is accommodated by the bulging and flexing of the round window
3) The change in pressure causes waves in the basilar membrane
Wave displacement- sound frequency
A high frequency sound will cause a small displacement near the base of the Cochlea at the Scala media, a low frequency sound will cause a displacement near the apex of the Cochlea near the Helicotrema. So, in a combination of sounds it will be separated based on their frequencies.
Tectorial membrane
The Tectorial membrane overlies the hair cells, as the basilar membrane moves up and down the Tectorial membrane moves across the organ of Corti. This causes displacement of the Stereocillia, the mechanical movement of the Stereocilia is converted into an electrical signal.
What happens when the Stereocilia bend towards the longest one in the chain
When the Stereocilia are in the neutral position some of the K+ channels are open. When the basila membrane is pushed upwards then the Stereocilia are tilted towards the tallest in the rank, there are stretching of the ranks and maximal opening of the K+ channels increasing entry. Channels open due to the voltage gradient, its +80mV in the endolymph and within the hair cell its -40mv.This leads to depolarisation, Ca+2 entry and transmitter release within vesicles, the transmitter activates the auditory nerve fibres.
What happens when the Stereocilia are bent towards the shortest one in the chain
The Basilar mebrane moves downwards and the K+ channels close. There is no K+ entry so there is hyperpolarisation. No transmitter is released and the nerve stops firing. When the basilar membrane moves up and down there is alternating hyperpolarisation and depolarisation of the hair cell. This is how transduction occurs, only the nerve cells fire action potentals.
