Scenario 27: Rodney's Hearing Loss and Harry's Scotoma Flashcards
What constitutes the vestibular system?
Integration of balance, posture, eye movements. 2 organs of equilibrium- semicircular canals and otolith organs (both found in labyrinth of inner ear)
What is the external ear formed of?
Elastic fibrocartilaginous structure with helix, lobule and tragus
What is the middle ear formed of?
Malleous, incus and stapes bones lying in tympanic cavity, Eustachian tube joins this to nasal cavity to equalise pressure
What is the inner ear formed of?
The oval window is the beginning of the inner ear. The bony labyrinth of the inner ear divides it into 3: vestibule, semicircular canals and the cochlea. There is the membraneous labyrinth lying inside the bony one. There is also a round window
Which bone is the auditory system embedded in?
Deep in the petrous part of the temporal bone
What are the fenestra vestibuli and fenestra cochlae?
Fenestra vestibuli- oval window
Fenestra cochlae- round window
What are the semicircular canals?
3 interconnected tubes: posterior, lateral and anterior bony structures with an osseous ampulla filled with many hair cells
What are the two otolith organs?
The utricle and saccule which lie in the vestibule of the inner ear
What is the ductus reuniens?
It connects the saccule to the cochlear duct
What is the composition of endolymph?
High K+ and low Na+
What is the composition of perilymph?
High Na+ and low K+
Where are the endolymph and perilymph?
Endolymph fills the membraneous labyrinth, perilymph fills the bony labyrinth. There is a voltage difference between them
What are stereocilia?
Tall, actin rich, pyramidal structures on hair cells which come to a point called the kinocillium. They form a staircase like structure where the axis going up to the tip is the axis of polarity. They are connected at the tips to one another by cadherin bridges, tip links.
What happens if a stereocilia tilts away from it’s axis of polarity?
It causes channels on tip links to close (as if pushed closer together) and receptor hyperpolarisation. This causes a decrease in firing in the afferent neurone
What happens if a stereocilia tilts towards it’s axis of polarity?
It causes channels on tip links to open (as if stretched) and receptor depolarisation due to influx of positive ions. This causes an increase in firing in the afferent neurone
How is an AP generated in the VIII cranial nerve by stereocilia?
Depolarisation by ion channel opening causes glutamate release from the hair cell to the afferent fibre of the vestibulocochlear nerve. If sufficient- AP
How do hair cells help our body know what direction we are moving?
The fluid surrounding them will either depolarise or hyperpolarise them by pushing them one way or the other. If we turn our head left, fluid in the horizontal semicircular duct flows right and because the hair cells are all highly ordered in the ampulla, it turns all the hair cells on the left side toward their axis (DP) and on the right away from their axis (HP) There are hair cells pointing in every direction so each dimension of movement is covered
What are the ampullary crests?
Where hair cells stick up into endolymph embedded in the cupula
How can static head position be detected?
When upright the macula is roughly horizontal so otoliths rest directly on it. If the head is tilted, gravity will act on the otolithic mass so it sags in the direction of tilt and bends hair cells.
How can complex signals of linear movement and head position be generated?
The cilia of hair cells in the utricle macula don’t all face in one direction, they face toward the striola (a curving landmark) any tilt or movement will depolarise some cells and hyperpolarise others so that complex signals can be generated to get an accurate measure of head position
How are the hair cells in the saccule arranged and how does this relate to function?
They are arranged pointing away from the striola so that vertically orientated force (e.g. up and down in lift) can be felt
What is the vestibulo-ocular reflex?
Projection of nuceli of extraocular nerves and to cervical spinal cord to coordinate head movements to eye movements. When you turn your head but keep your vision fixed, the eyes move the opposite way to the head but to the same degree (doll’s eyes)
What is Meniere’s disease?
Excess endolymph, distends membraneous labyrinth and causes vertigo, nausea, nystagmus, hearing loss andd tinnitus and even deafness
What is benign positional vertigo?
When otoconia become dislodged from the utricle and migrate into the semicircular ducts so that when the head moves, gravity-dependant movement of otoconia causes abnormal fluid displacement and vertigo
What are otoconia?
Small crystals of calcium carbonate in otoliths which stimulate hair movement when the head moves
What is an acoustic neuroma?
A benign tumour of the myelin forming cells of the vestibulocochlear system (vestibular Schwann cells) located at the CPA
What is pitch?
Cycles of vibrations per second is the frequency of the sound wave in Hz corresponding to pitch
What is loudness?
A higher pressure amplitude measured in dB
What is the function of the external ear?
To collect the sound waves and somewhat change them via their nobules
What is the external auditory meatus?
S shaped curve leading to tympanic membrane which is concave and lies at an oblique angle. The outer 1/3 is membraneous whilst the inner 2/3 is formed from temporal bone
Which nerve fibres supply the external auditory meatus and tympanic membrane?
Sensory fibres from vagus and trigeminal
How does sound transverse the middle ear?
Sounds cause the tympanic membrane to vibrate which is conveyed through the middle ear by the 3 ossicles then to the cochlea via the oval window
What amplifies the sound signal as it moves from the external ear to the inner ear?
The ossicles amplify the signal as does the concept that the area of the tympanic membrane is greater than that of the oval window so as the sound wave is forced through a smaller space, the pressure of it increases
Why do we need to amplify the sound signal as it goes from middle to inner ear?
The middle ear is air filled and the inner ear is fluid filled. Fluid takes more energy to vibrate than air does so the signal needs to be made greater to ensure it is not lost
How does the middle ear communicate with the pharynx?
Via the pharyngotympanic tube, the Eustachian tube, which is opened by swallowing and yawning. It opens into the nasopharynx at the level of the inferior cochae
How is the cochlea divided?
Into scala vestibuli, scala media and scala tympani
What is the location of the oval window in the cochlea?
Pushed up against the scala vestibular
What is the function of the round window?
It moves when the oval window transmits movement to the inner ear giving the walls of the cochlea yield so that the pressure can enter
How are cochlea arranged?
In cochlea tree with auditory nerve branches supplying each
What is the organ of Corti?
It is found at the basilar surface of the cochlea where the hair cells are found. There is an inner and outer layer of hair cells. The inner hair cells are the sensory receptors of the auditory system
What is the helicotrema?
Part of cochlear labyrinth where scala tympani and vestibuli meet, cochlear apex
What the scala tympani, media and vestibuli filled with?
Scala media: endolymph
Scala tympani and vestibuli: perilymph
How do the hair cells transmit sound information to the afferent nerve fibre?
The vibrations of sound pass in a wave over the basilar membrane and cause it to vibrate. This causes hair cells to move backwards and forwards with the vibrations causing an association voltage of alternating depolarisation and hyperpolarisation which is transmitted to the afferent nerve fibre
How are the difference between high and low frequency sounds picked up by the cochlea?
Because the basilar membrane is stiffer near the oval window, at the base, high frequency sounds will vibrate the membrane here, they are ‘strong’ enough and this means they don’t have to travel as far.
For the low frequency sounds, they will have maximum vibration at the apex where the membranes is wide and flexible (helicotrema). This means that the location of a hair cell along the cochlea codes for a frequency.
What is the function of outer hair cells?
Amplify basilar membrane motion and enhance frequency sensitivity by responding to electrical stimulation by changing their length generating receptor potentials in response to sound, this amplifies sounds especially weak ones
How do outer hair cells respond to depolarisation and hyperpolarisation?
DP: contract
HP: elongate
What allows outer hair cells to change their length?
Motor protein: prestin
What are otoacoustic emissions?
Sounds emitted by outer hair cells which propagate out of tympanic membrane and can be measured by clincians
How do we code for amplitude?
Louder sound- more vibrations of basilar membrane + bigger depolarisation/hyperpolarisation causing more neurotransmitter release and therefore a higher frequency of action potentials in the afferent fibre. AP frequency codes for loudness.
What is the spiral ganglion?
Bipolar neurones with one axon to hair cell and one to cochlear nerve, cell bodies wrapped up in cochlear tree
Where on the ventral and dorsal cochlear nuclei is high frequency sound found?
Dorsal parts of both nuclei
Where on the ventral and dorsal cochlear nuclei is low frequency sound found?
Ventrolateral parts of both nuclei
How is auditory information passed from sensory receptor to the auditory cortex?
Projections cross the midline to the inferior colliculus in the mid brain, then to the thalamic medial geniculate nucleus then to auditory cortex
Why is the auditory system bilateral?
Though the midline is crossed, there are also ipslateral connections and other crossings so the brain can understand what is coming into both ears at the same time
How is sound fine tuned by efferent fibres?
Superior olivary nucleus has efferent fibres which project back towards the cochlear hair cells for fine tuning of sound
What are some conductive causes of deafness?
Earwax buildup, eardrum damage, otosclerosis of middle ear, trauma, middle ear infection, genetic
What are some sensorineural causes of deafness?
Cochlea infection, trauma, noise damage, old age, ototoxic drugs, genetic, tumours
What are some central causes of deafness?
MS, vascular incident, trauma, infection, tumour, neonatal disease
What is a reflex?
A stereotyped response to a stimulus not subject to conscious control
What are autonomic reflexes?
E.g. pupillary light reflex, mediated by autonomic nervous system and activated by cardiac/smooth muscle and glands
What are somatic reflexes?
Mediated by somatic nervous system activated by stimulation of skeletal muscle
What are the 5 components of a reflex?
1) receptor 2) sensory neurone afferent to CNS 3) interpretation centre 4) motor neurone efferent from CNS 5) effector organ/gland
What is a monosynaptic reflex?
1 synapse, 1 quick twitch e.g. control muscles in legs to maintain upright posture
Why are polysynaptic reflexes slower?
Synaptic delay of several synapse
What are proprioceptors?
Receptors located in skeletal muscle at tendon-muscle junction, joints and ligaments. Carry info on joint location to CNS along sensory afferents
How do muscle spindles lie in relation to extrafusal fibres fibres?
In parallel with extrafusal fibres, lie embedded in muscle (intrafusal)
What are the proprioceptors in muscles and tendons?
Muscle spindles and Golgi tendon bodies
How do Golgi tendon bodies lie in relation to extrafusal fibres?
Lie with musclotendon fibres in series with extrafusal fibres between collagen fibres
Why do muscle spindles have a contractile element?
Due to presence of y motor neurones
What is the function of muscle spindles?
Monitor muscle length and rate of changing of length to prevent overstretching (safety device)
What do the tonically active sensory neurones do?
Measure the tone of the muscle fibres
What is alpha-gamma coactivation?
The concept that alpha motor neurones fire to contract extrafusal fibres and gamma motor neurones are coactivated to fire with alphas to contract intrafusal fibres simutaneously to ensure both fibres are the same length
What will prevent overstretching of muscles past limits e.g. by holding a load which is too large?
Inhibitory interneurones prevent overtensing e.g. by dropping load
What are cross cord reflexes?
E.g. when we flex one limb we need to stabilise the other to prevent falling over by extension
What are spinal pattern generators?
Rhythm generators built into circuit to allow coordinated movement e.g. of stepping, timing of flexion and extension
What are central pattern generators?
Rhythm generators for more complex movements such as turning (need intersegmental control from body)
When can a reflex be overridden?
By descending pathways from higher levels e.g. if you’re holding your nan’s best plate, you won’t drop it, even if red hot
What are Renshaw cells?
Inhibitory interneurons which regulate spinal motor neurones by receiving excitatory collateral from alpha neurone and sending an inhibitory axon back
What is the supraspinal reflex?
Labyrinthine righting reflex/ vestibular reflex. Stimulation of semicircular canals when you lean off balance, motor response in neck and limbs to maintain upright posture,
What can a weak or absence reflex response indicate?
Damage to nerves outside the spinal cord, peripheral neuropathy, damage to motor neurones- MND, damage to NMJ- myasthenia gravis, or muscle disease- myopathy
What can an excessive response in a reflex indicate?
Spinal cord damage above the level controlling hyperactive response, higher CNS damage
What can an asymmetric response indicate?
Early onset of progressive disease, localised nerve damage e.g. trauma
How can reflex tests be useful for testing for a spinal cord injury?
Determine area of injury: if the injury is above the reflex the reflex will be unaffected, if below then the reflex will be absent
What is Babinski’s sign?
In anyone older than 2 years, if the lateral sole of the foot is stroke then the big toe should flex (curl down), if there is an upper motor lesion or if it’s a neonate the big toe will extend (spring up)
How is cerebral palsy characterised?
Random, uncontrolled movements due to failed development of higher control, spectra of disorder retain primitive reflexes. Affects sensory perception of movemetn.
