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Neuroscience > Hearing, Sight And Balance > Flashcards

Hearing, Sight And Balance Flashcards

1
Q

to achieve balance and posture, the brain needs to integrate signals from which 3 key inputs?

A
  • Vestibular system
  • Proprioception (somatosensory system)
  • Vision

(As well as important connections with cerebellum and motor systems)

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2
Q

What is the vestibular system and what is it important for?

A

2 organs of equilibrium: semicircular canals and otolith organs Both found in the inner ear, or labyrinth

Vestibular system is important for balance, posture and eye movements

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3
Q

Where do you find endolymph and what is in it?

A

Membranous labaryinth
- high K
- low Na
(Opposite of plasma)

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4
Q

What is peri lymph and where is it found?

A 
Fluid in the bony labaryinth?
High Na (150mM)
Low K (7mM)
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5
Q

What do hair cells release at their synapses with the primary afferent fibres?

A

Glutamate

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6
Q

What are the sensory receptors of the vestibular system?

A

Hair cells

See page 2 of vestibular system lecture

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7
Q

What do tip links do?

A

Stereocilia are connected by tip links

Join up the lower with the higher neighbours but they do not connect horizontally - only along the axis of polarity

(40-70 actin-rich stereocilia)

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8
Q

By what mechanism are the membrane potentials of hair cells changed?

A

Bend hair cells over along axis of polarity - Stretch the tip links - open channels - ions in fluid can flow into the hair cells - pottasium and calcium. Rush into the tip - depolarisation
- slightly depolarised already - 10% of channels open already - bend the other way - close them - hyper polarised

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9
Q

What are the effects of changes in hair cell membrane potential on neural firing?

A

Depolarisation - increased neural firing (impulse frequency)

Hyperpolaristaion - decreased neural firing (impulse frequency)

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10
Q

How do hair cells respond to motion and position changes in the Semicircular ducts?

A

Ampulla - dilation of the semicircular canals -
Ampullary crest - thickened epithelium - Hairs sit on it
- cupula - gelatinous mass in ampulla - sound - pushes it to move the hair cells
(Angular acellaration)

(All hair cells in ampulla are arranged so that the axis of polarity always points in one direction, eg in horizontal semicircular duct they go towards utricle.
Therefore, because hair cells only respond along their axis, each ampulla responds with depolarization in one direction, and hyperpolarization in the other.)
(Pairing between ant. and post. On opposite sides - 3D)

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11
Q

How do hair cells respond to motion and position changes in the utricle?

A

Macula

Otoliths - ear rocks - calcium crystals on otolithic membrane on a gelatinous mass - directly on top of macula

(Muscles of support, muscles of the necks, eye movement)

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12
Q

How is the utricle macula able to detect static position etc?
What type of acellaration can it detect?

A

When upright, macula roughly horizontal, so otoliths rest directly on it. If head is tilted, gravity acts on heavy otolithic mass so it sags in direction of tilt and bends hair cells. So movement is NOT REQUIRED.
This means static head position can be detected.

Macula can also detect linear acceleration (frequent fluctuations lead to motion sickness).

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13
Q

Do the cilia of utricle macular hair cells all face in the same direction?
What does this mean?

A
  • No
    …they all point towards a curving landmark – the striola

This means the utricle can respond to tilt or linear acceleration in many directions. Tilt in any direction will depolarize some cells and hyperpolarize others – leads to complex signal to brain with accurate measure of head position

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14
Q

How do hair cells respond to motion and position changes in the saccule?

A

Saccule essentially like utricle: also has a macula, but this is oriented vertically when person in upright position.

Therefore responds to vertically directed linear force and detects position of head in space

In the saccule, cilia also oriented in different directions, but unlike maccule, they point away from striola

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15
Q

Where is the sensory ganglion of the vestibular nerve fibres?
What are the vestibular nerve fibres like?

A

Internal acoustic meatus

Fast, myelinated (efferent a and afferents)

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16
Q

Where does the 8th cranial nerves enter the brainstem and have its nuclei?

A

Enter brain stem at CPA

nuclei near floor of fourth ventricle - near cerrebellar peduncle

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17
Q

What is the role of the ascending and descending tracts from vestibular nuclei?

A

Project to nuclei of extra-oculomotor nerves and to cervical spinal cord to co-ordinate head movements with eye movements -this is the vestibulo-ocular reflex “small rotation of the head is
accompanied by movement of the eyes through the same angle but in opposite direction” – ‘doll’s eyes’

Project to cerebellum and lower spinal levels to co-ordinate extensor and flexor muscles to maintain balance and posture

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18
Q

What do you know about Ménière’s Disease?

A

too much endolymph and distention of membranous labyrinth =>

attacks of severe vertigo, nausea, nystagmus, hearing loss and tinnitus,

plus eventual permanent progressive hearing loss.

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19
Q

What do you know about Benign Positional Vertigo?

A

otoconia dislodged from utricle and migrate into semicircular ducts (often posterior one).

When head moves, gravity-dependent movement of otoconia causes abnormal fluid displacement in the affected semicircular duct and resultant vertigo

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20
Q

What is acoustic neuroma?

A

Benign tumour of the myelin forming cells of the 8th cranial nerve - classically at the CPA
- balance and hearing problems

21
Q

What level of sound pressure can damage the sensory apparatus of the ear?

A

Sound pressures > 100dB can damage sensory apparatus of ear

22
Q

What is the frequency range of human hearing?

A

20-20,000 Hz

23
Q

Important points about the external ear?

A

Elastic fibrocartilaginous structure

Main features are helix, lobule and tragus

Helps collect sound waves and discriminate direction of sound

24
Q

What is the external auditory meatus and what is its nerves supply?

A

S-shaped curve, ~ 2.5 cm long, leading to tympanic membrane or ear drum which is concave and lies at an oblique angle

  • Outer 1/3rd cartilaginous
  • Inner 2/3rd temporal bone

Meatus and tympanic membrane are supplied by sensory fibres from vagus and trigeminal

25
Q

In otitis media what happens to the light reflex in auroscopy of the tympanic membrane?

A

It is lost

26
Q

What is the Eustachian tube/auditory tube and when does it open and where?

A

Middle ear communicates with pharynx via pharyngotympanic tube (= Eustachian tube)

  • opened by swallowing and yawning
  • opens into nasopharynx at level of inferior concha
27
Q

What is the sensory organ in the cochlear?

A

Organ of corti - hair cells - sits on the basilar membrane

- hair attached also to the tectorial membrane

28
Q

What are the sensory receptors of the auditory system and how do they work?

A

Inner hair cells
- bend along axis of polarity - K enters

  • Cycles of depolarisation and hyperpolaristaion as the hair oscillates back and forth
  • Amplitude can be coded for by the amount of depolarisation that you get
29
Q

How does frequency coding/tonotopy work?

A

See page 3 of auditory system lecture

30
Q

What are the roles of outer hair cells?

A

Outer hair cells amplify BM motion and enhance frequency sensitivity
(Role not completely understood but of you loose them you become deaf)

31
Q

How do outer hair cells work?

A

Outer hair cells responds to electrical stimulation by changing its length – unique property, due to special motor protein ‘prestin’.

  • Response is voltage dependent: depolarization => contraction, hyperpolarization => elongation
  • In vivo, generate receptor potentials in response to sound
  • OHC motility is thought to contribute to basilar membrane motility and thereby amplify signal
  • OHCs also emit sounds – otoacoustic emissions. These propagate in reverse, back through middle ear to move tympanic membrane. These are useful to clinicians as hearing tests, especially for infants
32
Q

How are auditory signals traduced to afferent fibres and what are these fibres?

A

Voltage-gated calcium channels sense changes in membrane voltage and adjust rate of glutamate release from IHC, so afferent AP firing rate reflects amplitude of BM deflection

Anatomical arrangement of auditory nerve fibres follows that of the BM, preserving tonotopy: ‘rate-to-place code’

Type 1 neurones - hair cells only connect to one fibre.
One fibre can connect to c. 20 hair cells
Type 2 neurones - not really understood

33
Q

Where are the sensory ganglia of the auditory afferents?

A

Spiral ganglion in the Modiolus of the cochlear

34
Q

What type of signals go to which cochlear brain stem nuclei?

A

Low freq- - ventral nuclei

High freq. - dorsal nuclei

35
Q

Central connections of the cochlear nuclei?

A

Tonotopic frequency maps exist in cochlear nuclei, superior olivary nucleus, inferior colliculus, the ventral division of medial geniculate body and some parts of auditory cortex

Associated with the auditory cortex on the left side of the brain are Wernicke’s speech sensory area and Broca’s speech motor area.

Bilateral connectivity allow distance and directional sensitivity

Tectospinal connections from inferior colliculus control auditory reflexes (e.g. turning head towards sound)

Superior olivary nucleus is source of efferent fibres that project back to the cochlear hair cells……..feedback inhibition to refine sound perception

36
Q

Common causes of conductive deafness?

A 
Earwax
damage to ear-drum
otosclerosis of the middle ear
trauma
middle ear infections
genetic defects
37
Q

Common causes of sensorineural deafness?

A 
Cochlea –
infection
trauma
noise
age
ototoxic drugs
genetic defects (myosins, gap junction mutations etc)
tumours.
38
Q

Common causes of central deafness?

A 
Vascular accident
trauma
MS
infection
tumour
neonatal distress
39
Q

What is tinnitus?

A

This is a constant sensation of tone(s) – high pitch, low pitch, buzzing, etc which can persist for many years. It is caused by spontaneous activity in hair cells, spiral ganglion cells or neurons of the CNS. It can indicate damage to hair cells (e.g after excessive noise), but is often of unknown aetiology, and is difficult to treat.

40
Q

In the retina, what is the vertical pathway of neurones?

A

Photoreceptors - bipolar cells - retinal ganglion cells

41
Q

In the retina, what are the lateral neuronal connections and what do they do?

A

Horizontal cells and amacrine cells modulate vertical pathways.

42
Q

What are the synaptic laminae of the retina?

A

OPL- outer plexiform layer

IPL- inner plexiform layer

43
Q

What are the photoreceptors cells of the retina?

A

rods (r) mediate vision in dim (scotopic) light

cones (c) (S,M,L) mediate bright light, colour vision

(Outer segment: specialised for phototransduction
Inner: specialised for synaptic transmission)

44
Q

What is the electrical state of photoreceptors in the light and in the dark and in which of these is glutamate released?

A

In the dark photoreceptors are depolarized and release glutamate in a tonic fashion.

In light conditions photoreceptors are hyperpolarized and glutamate release stops

45
Q

Way is found in the outer plexiform layer of the retina?

A

synapses between photoreceptors, horizontal cells and bipolar cells

46
Q

Way do horizontal cells do in the retina?

A

Receive glutamatergic input from photoreceptors.

Make GABAergic inputs onto photoreceptors laterally.

Modulate photoreceptor signals according to ambient light levels (feedback)-signalling becomes less sensitive in dim light

Shape centre-surround organization of bipolar cell receptive fields.

Colour coding of bipolar cell responsesl

47
Q

What are the roles of the bipolar cells of the retina?

A

Relay information vertically from OPL to IPL

Broadly divided into cone bipolars and rod bipolars.

ON and OFF pathways are generated at the photoreceptor-bipolar cell synapse

48
Q

What is contained in the inner plexiform layer of the retina?

A

Contains synapses between bipolar cells, amacrine cells and RGCs.

Neuroscience (16 decks)

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