The Ear

Question 1

What is hearing?

Answer 1

Hearing is the neural perception of sound energy

Most important sense for social contact in many species

Pathological lesions in auditory system are common

Question 2

Evaluate the methods of diagnosing hearing loss

Answer 2

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

Question 3

What is the BAER test?

Answer 3

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

Question 4

What are the two types of hearing loss?

Answer 4

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

Question 5

Describe the anatomy of the outer ear

Answer 5

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

Question 6

Describe the functional anatomy of the outer ear

Answer 6

Act as a funnel to collect sound wave vibrations and direct them to the tympanic membrane (ear drum)

Question 7

Describe the anatomy of the middle ear

Answer 7

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

Question 8

Describe the functional anatomy of the middle ear

Answer 8

Contains ossicles whihc transfer tympanic membrane vibrations to the inner ear

Question 9

What is the anatomy & function eustachian tube ‘auditory tube’?

Answer 9

Connects middle ear to the pharynx

Narrow & normally closed

Function: equalise pressure on the two sides of tympanic membrane

Question 10

Describe the anatomy of the inner ear

Answer 10

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

Question 11

What are the physiological mechanisms in mammalian hearing?

Answer 11

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

Question 12

How are electrical impulses from hearing transported to the brain?

Answer 12

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

Question 13

Explain frequency discrimination within the cochlear

Answer 13

High frequency waves have maximum amplitudes near round window

Low frequency cause membrane to vibrate with maximum amplitude near tip of cochlear

Question 14

Explain the anatomy of the vestibular system

Answer 14

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

Question 15

What is the function of semi-circular canals?

Answer 15

• 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

Question 16

What is the function of the otolith organs?

Answer 16

• 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.

Question 17

How does the information from the vestibular labyrinth reach the brain?

Answer 17

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

Question 18

How does this explain the clinical relevance of the head tilt?

Answer 18

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).

Question 19

What is a nystagmus?

Answer 19

It is a normal physiological response when the back ground is consistently moving

Question 20

Explain how a nystagmus becomes pathological

Answer 20

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)

Question 21

Distinguish between central and peripheral vestibular syndrome