The Ear Flashcards
What is hearing?
Hearing is the neural perception of sound energy
Most important sense for social contact in many species Pathological lesions in auditory system are common
Evaluate the methods of diagnosing hearing loss
Unreliable:
Observation of behaviour in reaction to acoustic stimuli
Reaction of the animal to (sudden) noise outside of its visual field
Hand clapping, rattling with the feeding bowl
Reliable:
BAER
Brainstem Auditory Evoked Response
What is the BAER test?
Objective
Measures electrical activity within auditory pathways of the brain Accumulated action potential from cochlea via cochlea nerve can be measured
Sedation or anaethesia required
Can be used to determine brain health in a coma
What are the two types of hearing loss?
Conduction:
From external to middle ear
Otitis externa
Otitis media
Rupture of tympanic membrane
Sensation
Inner ear
Damaged hair cells – loud or continuous sound
Ototoxic drugs (e.g. streptomycin)
Inherited conditions – frequently associated with depigmentation
Describe the anatomy of the outer ear
Pinna
Mainly composed of elastic cartilage & skin
Very mobile (moved by several sets of muscles)
Innervated by Facial Nerve VII
External Acoustic Meatus
‘L’ shaped tube ending in the ear drum
Contains sebaceous glands that produce ear wax
Contains hairs that act as a barrier to dust & insects
Tympanic Membrane
Translucent membrane
Covered in epithelium on outside & mucous membrane on the inside
Describe the functional anatomy of the outer ear
Act as a funnel to collect sound wave vibrations and direct them to the tympanic membrane (ear drum)
Describe the anatomy of the middle ear
Ossicles Malleus, Incus & Stapes
Act as a series of levers to transmit sound wave vibrations
Stapes attached to the membrane whihc covers the oval window of the cochlear
Tympanic Membrane
Resonant cavity
Describe the functional anatomy of the middle ear
Contains ossicles whihc transfer tympanic membrane vibrations to the inner ear
What is the anatomy & function eustachian tube ‘auditory tube’?
Connects middle ear to the pharynx
Narrow & normally closed
Function: equalise pressure on the two sides of tympanic membrane
Describe the anatomy of the inner ear
Cochlear
3 fluid filled canals Upper, middle & lower - Upper & Lower filled with perilymph - Middle is filled with endolymph Higher in K+ ions & ve+ electric potential Basilar Membrane Band of tissue that runs alongside middle fluid filled chamber Sitting on top of it is the organ of corti Sensory cells (microvilli) line the membrane
Vestibular labyrinth
What are the physiological mechanisms in mammalian hearing?
All objects vibrating in air or water generate sound
Causes compression and rarefaction of molecules in the environment, creating areas of increased and decreased pressure
Distance between areas with maximal pressure = wavelength
Sound frequency = pitch Sound pressure = sound volume/amplitude
How are electrical impulses from hearing transported to the brain?
Movements affect the stretch sensitive ion channels at the tip of the sensory hairs (lining the basilar membrane)
Tip links connecting cilia open the K+ channels
This leads to depolarisation
Voltage gates Ca2+ channels open
Influx of Ca2+ causes the release of neurotransmitter by exocytosis
Explain frequency discrimination within the cochlear
High frequency waves have maximum amplitudes near round window
Low frequency cause membrane to vibrate with maximum amplitude near tip of cochlear
Explain the anatomy of the vestibular system
Located within the inner ear
Composed of vestibular labyrinth
-Semi-circular Canals
(x3) Sagittal, transverse & frontal planes
-Otolith Organs (Maculae)
(x2) Utricle which responds to horizontal information
Saccule which responds to vertical information
What is the function of semi-circular canals?
- Sense head rotation (dynamic reaction)
- Contain fluid endolymph - Bulbous expansion at the base known as Ampulla - Ampulla house crista which contain hair cells - These hair cells project into the gelatinous mass know as cupula Cupula expands across the Ampulla preventing circulation of endolymph
What is the function of the otolith organs?
- Static Information
- Sensory epithelium (macula) Contains hair cells & associated supporting cells - Overlying the hair cells is a gelatinous layer - A fibrous layer with embedded Calcium Carbonate crystals sits atop the gelatinous layer Add weight to the structure so it is more sensitive - Tilting or transitional movements of the head disrupts the hair cells and a receptor potential is generated.
How does the information from the vestibular labyrinth reach the brain?
Vestibular portion of the cranial nerve VIII (CN VIII) projects to the vestibular nuclei or the flocculonodular lobes of the cerebellum via the inferior cerebellar peduncle
From the vestibular nuclei in the medulla
Medial geniculate nucleus bilaterally (alongside auditory information) thence to cerebral cortex
Branches to nuclei of CN III, IV and VI
Reticular formation
Vestibulospinal tract
Part of the extrapyramidal system
Ipse-laterally into the cerebellum via the caudal cerebellar peduncle
How does this explain the clinical relevance of the head tilt?
Sum effect of normal function of the vestibulospinal system is to produce body lean away from that side.
Flocculonodular lobe receives input from the primary vestibular afferents and projects back to the vestibular nuclei This portion of the cerebellum governs eye movements and body equilibrium during stance and gait. Can see vestibular signs with cerebellar disease but may show paradoxical head tilt (Rare).
What is a nystagmus?
It is a normal physiological response when the back ground is consistently moving
Explain how a nystagmus becomes pathological
With CNS issue it can become pathological
Fast and slow phase
Slow towards lesion
Horizontal nystagmus: fast upwards and then drifting downwards (indicates not a
tremor)
Distinguish between central and peripheral vestibular syndrome
