Neurobiology of the ear Flashcards
(39 cards)
What are the three anatomical sections of the ear?
What are their boundaries?
Outer - until tympanic membrane
Middle - until oval window
Inner - all bony labyrinth and nerves
What are the ossicles within the middle ear?
The malleus
The incus
The stapes - which has a base on the oval window
What are the external features of the ear?
Note external ear is also called auricle or pinna
What nerve innervates the ear?
The vestibulocochlear nerve CN8
Splits into cochlear nerve - for the cochlear- hearing
The vestibular nerve - vestibular system - balance
What are the important features of the bony labrynth?
Semicircular canals
Vestibule
Oval window
Round window
Cochlear
What is the important of the round window in the bony larynthe of the ear?
Release pressure from within the cochlear.
What makes up the cross section of the cochlea?
A scala vestibuli
A scale media
A scale tympani
These are hollow fluid filled spaces
The organ of corti seperates the scala media and scale tympania
What makes up the organ of Corti?
Found between the scala media and the scala tympani
The basilar membrane, with overlying support cells and hair cells (with sterocillia).
All overlayed by the heavy tectorial membrane
Inner hair cells link to afferent axons to the cochlear nerve
Outer hair cells link to efferent axons to the cochlear nerve.
What fluid is found within the cochlear?
Endolymph - within the scala media
Perilymph within the scale vestibui and the scala tympani
Constrat the function of inner and outer hair cells
Inner - afferent nerve fibres - project depolarisation patterns to detect sound to brain
Outer - efferent nerve fibres - increase sensitivity or focus on sound, aka frequency specific.
How is sound transmitted from the external environment to the inner ear?
Pinna amplifies sound waves and projects down external auditory canal
Vibrates the tympanic membrane
Causes vibration of the ossicles in the middle ear, of these the stapes bounces on the oval window of the bony labrynthe
Sound waves are amplified and pass into the endolymph and perilymph within the cochlear, this fluid moves due to sound waves in the scala vestibuli, scala media and scala tympania
What is the function of the organ of Corti?
Compute sound waves into electrical information
Structure in the inner ear.
How does the organ of Corti process sound waves?
Sound waves in scala media cause movement of the basilar membrane
Results in movement of overlying hair cells stereocillia relevant to overlying tectorial membrane
This movement opens ion channels which leads to depolarisation
How does movement of the stereocillia on hair cells lead to ion movement?
The kinocilium is the tallest stereocillia on the hair cells, height is progressive across the cell.
Deviation towards this pulls top links connecting sterocillia together - results in opening of K+ channels - trigger increased rates of depolarisation
Neutral position - norm rate of depolarisation (resting state)
Deviate away - decreased rate of depolarisation (hyperpolarised state)
What is meant by a topographic mapping of frequency in the ear?
The basillar membrane runs the width from the base to the apex of the cochlear
The base - high frequency sounds
The apex - low frequency sounds.
How is K+ influx in hair cells converted into sound perception?
(identical mechanism for stereocilia in the vestibular apparatus)
K+ influx from scala media (with very high K+ conc) through open channels causes depolarisation of hair cells.
Opening of voltage gated Ca2+ channels - which causes glutamate containing vesicles to fuse with the cell membrane, diffuse across and bind to receptor causing AP in the the afferent nerve fibres - travel to spiral/cochlear ganglion and up cochlear nerve.
What is the central circuity of sound perception?
Afferent projects a pattern of action potential to arrive through auditory (Cochlear) branch of CNVIII
Terminate and synapse on the dorsal/ventral cochlear nuclei at the junction between the pons and medulla.
Cochlear neurons project and synapse to the superior olive both ispi and contralateral to locate the sound.
Travel up the lateral leminiscus pathway to synapse in the inferior colliculus
Project and synapse in the Medial geniculate nucleus of the thalamus
Project and synapse in the auditory cortex. A1
What is the purpose of the inferior colliculus?
The reaction centre for the auditory stimuli.
Co-ordinates motor reflexes in response to sound
Where is the tectum located?
What is its function?
The roof over the cerebral aqueduct between 3rd and 4th ventricle
Contains the superior and inferior colliculus.
Gives rise to descending tectospinal/tectobulbar tracts (dedicated in pons).
What is the relevant anatomical location of the primary and secondary auditory cortex?
A1 - superior temporal gyrus of temporal lobe, deep to A2
A2 - temporal lobe, rostral and lateral superior temporal gyrus BA42 - includes wernicks area posteriorly (BA22). Also contributions from BA39 and BA40
What is wernicks aphasia?
Lesion in wernicks
Difficulty understanding language /language processing - speak words at normal rate, rhythm,pitch but words do not make sense in sentences or identifying objects
Difficulty following verbal or written instruction.
What is Brocas aphasia?
Difficulty producing speech - may repeat words and phrases, but have difficulty repeating phrases to you, may only face few/one word at a time.
Have very limited language.
What is the role of the Arcuate fasiculus?
Connects Wernicks and Broccas area - important to engage and understand conversation.
What is the topographic map on the auditory cortex?
Arranged by sound frequency matching that of the basillar membrane in the cochlear.
