Hearing and Balance Flashcards
What is hearing?
Perception of sound
Detection of vibrations, changes of pressure through solid, liquid or gas
What is vestibular function?
Perception of position and motion
Static gravitational orientation (position with respect to gravity)/ perception of motion in space (rotations, linear translations)
What is static gravitational orientation?
Perception of relationship to gravitational field
What is motion sense?
Perception of motion relative to environment
Rotations, linear translations
Essentially acceleration
How are the auditory and vestibular systems intimately connected?
- Receptor organs in ear (membranous labyrinth)
- Processing of information in the inner ear (bony labyrinth)
- Common nerve to brain (vestibular cochlear nerve)
What are the main components of the human ear?
- Outer
- Middle
- Inner
What makes up the outer ear?
- Pinna (the ear we see)
- Auditory canal (leads into middle ear)
- Tympanic membrane (sits at apex of auditory canal)
What makes up the middle ear?
Small air filled chamber containing 3 small bones (the ossicles)
Where is the inner ear?
Innermost part of ear
In the bony labyrinth of temporal bone
What is the membranous labyrinth?
- A continuous membrane within the bony labyrinth
- Contains fluid
What are the fluids in the labyrinth?
Bony= perilymph
Membranous= endolymph
Composition is different
What are the receptor organs within the membranous labyrinth?
- The vestibule= large central area
- The utricle and saccule= adjacent to vestibule (vestibular)
- The cochlea (duct)= on one side (auditory)
- The semi-circular canals= on other (vestibular)
What are the major components of the inner ear?
- Nerve
- Cochlea
- Utricle
- Saccule
- Anterior, posterior and horizontal canal
How does hearing begin?
Sound waves enter the auditory canal
Pressure variations in the air around us, enter and towards the tympanic membrane
Describe the process of hearing
- Tympanic membranes receives and vibrates
- Vibration transmitted to the ossicles
- Ossicles transmit vibrations to oval window of the vestibule
- Then transmitted to cochlea (fluid inside membranous labyrinth to cochlea)
- Cells in cochlea convert vibrations into electrical signals that are transmitted along axons of V111 cranial nerve
What are the bones of the ossicles?
- Malleus
- Incus
- Stapes
Where does the cranial nerve enter the CNS?
Vestibulocochlear nerve goes to brain stem effectively at medulla/ pons junction to the cochlear nuclei (ventral anteriorly, dorsal posteriorly)
-Auditory information ascends via tract in medial lemniscus to thalamus and cerebral cortex
What makes up the vestibulocochlear nerve?
Vestibular nerve (superior and inferior) Cochlear nerve
Describe the two pathways from the cochlear nuclei
- Dorsal= Concerned with the quality of sound, picking apart the tiny frequency differences which allow differentiation of similar sounds
- Ventral= Concerned with minute differences in the timing and loudness of the sound in each ear in order to localize sound
Describe the path of the axons from the ventral cochlear nerve
Axons go to superior olive then to inferior colliculus, medial geniculate body, cerebral cortex
Describe the path of the axons from the dorsal cochlear nerve
Axons go directly to inferior colliculus, medial geniculate body, cerebral cortex
Where is the primary auditory cortex?
Side of brain in temporal region
Bilaterally represented
How many turns are there in the cochlea?
2.5 turns around its axis in humans
What are the chambers of the cochlear?
-Scala vestibuli (abuts the oval window so the stapes transmits vibrations into this)
-Scala tympani (abuts the round window)
-Scala media (duct in middle surrounded by membrane)
Contain perilymph except for media which contains endolymph
What are the membranes that define the cochlear duct?
- Reissner’s membrane= separates cochlear duct from Scala vestibuli
- Basilar membrane= separates the cochlear duct from Scala tympani
Describe the role of the Basilar membrane
- Relatively stiff
- Supports the Organ of Corti (core part of auditory-sensory apparatus)
- Determines mechanical wave propagation properties of cochlea (as vibrations transmitted around fluid channels)
Where is the Organ of Corti?
Within cochlea duct in Scala Media
What is the Organ of Corti?
- Sensory epithelium
- A cellular layer on basilar membrane
- Hair cells
- Topped with hair-like structures that fit into tectorial membrane above
How is the Organ of Corti important in hearing?
- Waves in Scala fluid
- Cause movement of basilar membrane
- Hair cells produce electrical responses
- Axons form part of the V111 nerve
How does stiffness of the basilar membrane vary?
- Stiffness and width varies along the cochlea= stiffest near start at oval window
- Allows different parts of BM to respond to different frequencies
What is the purpose of coiling?
- Enhances low frequency waves as they travel along cochlea
- Highest frequency near oval window, lowest frequency near other end
Static gravitational orientation and perception of motion arise from which sensory apparatus?
- Visual= moving in relation to environment
- Somatosensory (proprioception)
- Vestibular
What two components of the vestibular system reflects rotations and linear translations?
-Semi-circular canals= rotation
-The Otoliths= linear translations
Vestibular apparatus detects angular and linear acceleration
What are the semi-circular canals?
-Three semi-circular canals (for 3D spatial world) in a non-orthogonal relationship (not at 90 degrees to each other) =Horizontal/ lateral =Anterior/ superior =Posterior/ inferior *Anterior and posterior: vertical
Describe the role of each semi-circular canal
- Horizontal= detect rotation of the head around a vertical axis
- Anterior and posterior= rotations of head in sagittal plane/ rotations of the head in frontal plane
Describe the anatomy of the semi-circular canals
- Each canal@: a continuous endolymph-filled hoop
- Special hair cells sit in a small swelling called the ampulla
- Hair cells project into gelatinous mass: the cupula
What occurs in the semi-circular canals due to movement of the head?
- Motion in the plane of the canal
- Inertia of endolymph= causes movement of fluid= movement of hairs= hair cell activity changes= electrical impulse
What is the relationship between the paired semi-circular canals (R and L)?
Movement one way
=Inhibition on one side
=Excitation on the other
Describe the anatomy of the Utricle and Saccule
- Both have a sheet of hair cells (macula)
- Macula have cilia embedded in a gelatinous mass
- Embedded in this gel= small crystals (otoliths)
- The otoliths provide inertia- movement bends hair cell
How is linear translation detected by the utricle and saccule?
Motion: the otolith-gel mass drags on the hair cells
=hair cells five information to 8th cranial nerve
How are hair cells arranged in the utricle and saccule?
-Arrayed in different directions to cover different directions of movement
=Utricle: horizontal
=Saccule: sagittal (up/down, forward/back)
Describe the vestibular pathway
- Sensory cells in labyrinth
- V111 nerve to brain stem, vestibular nuclei, cerebellum
What are the vestibular nuclei connections?
- Cerebellum (integrate motor action)
- Spinal cord (Vestibulo-spinal tract, adjust movements of body)
- Eye movement motor nuclei
- Cerebral cortex
What is the important output of the vestibular system?
Neural structures controlling eye movements
-Reflex to move while focussed on an object
Describe the Vestibulo-ocular reflex
- A critical role of the semi-circular canal system
- To keep your eyes still in space while your head moves around them
- Turning head left= left semi-circular canal excited, directly to 3rd and 6th nerves to medial and lateral rectus muscles
Example of Vestibulo-ocular reflex
- Detection of rotation
- Excitation of extraocular muscles on the other side, inhibition of extraocular muscles on one side
- Compensating eye movement
What is Vertigo?
One or more of the following
- A distortion of static gravitational orientation (feeling leaning to one side)
- An erroneous perception of movement of the sufferer (bobbing up and down/ spinning in relation to the environment)
- An erroneous perception of movement of the environment (environment spinning around you/ bobbing up and down)
What does Vertigo result from?
- Unusual stimulation of intact systems (motion sickness)
- Pathological dysfunction of those systems (vestibular apparatus disease)
What are the outputs/ targets of the vestibular nucleus?
- Cortex= spatial orientation, motion perception
- Brainstem= eye movements, vegetative effects
- Spinal cord= posture
What are the components of vertigo syndrome?
- Perceptual= vertigo, disorientation
- Oculomotor= nystagmus (jerky eye movements), ocular deviation
- Postural= ataxia, falls
- Autonomic/ vegetative= nausea, vomiting, sweating
- Secondary= anxiety, avoidance behaviour
What is BPPV?
- Benign Paroxysmal Positional Vertigo
- Bits of otoliths break of and get lodged
How does BPPV present?
-One of the commonest causes of vertigo in general practice
-Attacks of rotational vertigo
-Provoked by
=positional change
=lying down or sitting up
=turning over in bed
=looking up or bending forward
What are the causes of BPPV?
- Degenerative debris in cupola or semi-circular canal
- 40% antecedent event (head injury)
- 60% no obvious antecedent event (age)
How do we test and treat BPPV?
- Positional testing (60 degrees upright, 30 degrees below lying down)
- Particle repositioning manoeuvres= fluid to wash debris
What is the clinical relevance of eye movements?
- Nystagmus in diagnosis of ear/ brainstem disease
- Vestibulo-ocular reflex part of brain stem death criteria
