The vestibular apparatus in the inner ear contains receptors that detect changes in head position and motion to maintain equilibrium. It has two functional parts - the static vestibule monitors linear acceleration and head position relative to gravity, while the dynamic semicircular canals monitor rotational head movements. The vestibule contains maculae with hair cells and otoliths that signal head position. The semicircular canals contain cristae ampullares with hair cells and a gelatinous cupula that detects angular head movements. Together they send signals to the brain for balance reflexes.

1. Equilibrium
▪ vestibular apparatus - equilibrium
receptors in the semicircular canals and
vestibule
–send signals to the brain that initiate reflexes
needed to make the simplest changes in
position as well as more complex moves

2. Equilibrium
Two Functional Parts
▪ Static Equilibrium (Vestibule) - monitor linear
acceleration and the position of the head with
respect to gravity (constant)
▪ Dynamic Equilibrium (Semicircular Canals) -
monitor changes in head rotation

3. Structure of Crista
Ampullaris

4. Static Equilibrium
 Maculae
– receptors in
the vestibule
 Report on the
position of the
head
 Send
information via
the vestibular
nerve

5. Static Equilibrium
 Anatomy of the Maculae
 Hair cells are embedded in the otolithic
membrane
 Otoliths (tiny stones made of calcium salts) float
in a gel around the hair cells
 respond to static equilibrium cues relative to
the position of the head in space
 Movements cause otoliths to bend the hair cells

6. Function of Maculae
Diagrammatic view of
part of a macula

7. Function of Maculae
When the head is tipped, the maculae are stimulated by movement of the
otoliths in the gelatinous otolithic membrane in the direction of
gravitational pull, which creates a pull on the hair cells.

8. Dynamic Equilibrium
 Crista ampullaris
– receptors in the
semicircular canals
Tuft of hair cells
Cupula (gelatinous cap)
covers the hair cells

9. Dynamic Equilibrium
 Action of angular head
movements
The cupula stimulates the
hair cells
An impulse is sent via the
vestibular nerve to the
cerebellum

10. Crista Ampullaris

12. Recap
▪ What sense do the vestibule and semicircular canals
serve?
– Balance or equilibrium.
▪ Benji is enjoying a boat ride until a storm suddenly
descends on the bay. So he is nauseated and can
barely stand up. Which equilibrium receptors – static
or dynamic – are operating furiously during such a
rough voyage?
– Dynamic receptors located in the semicircular canals (crista
ampullaris).

13. Recap
▪ What are otoliths, and what is their
role in equilibrium?
– Otoliths are tiny stones made of calcium
salts which are located in the maculae of
the vestibule.They respond to static
equilibrium cues relative to the position
of the head in space.

14. Hearing
 Spiral Organ of Corti
 Located within the cochlear duct
 Hearing receptors = hair cells on the basilar
membrane
 Gel-like tectorial membrane is capable of
bending hair cells
 Cochlear nerve attached to hair cells transmits
nerve impulses to auditory cortex on temporal
lobe – interpretation of sound or hearing occurs

15. Anatomy of the cochlea. (a) Lateral view of part of the internal ear
with a wedge-shaped section removed from the cochlea.

16. Anatomy of the cochlea. (b) Magnified cross section of one turn of the
cochlea, showing the relationship of the three scalae.This cross section has
been rotated from its position in (a). (c) Detailed structure of the spiral organ.

17. Mechanisms of Hearing
Vibrations from
sound waves move
tectorial membrane
 (pass through the
endolymph fluid
filling the
membranous
labyrinth in the
cochlear duct)

18. Mechanisms of Hearing
▪ Length of fibers spanning the basilar membrane “tunes”
specific regions to vibrate at specific frequencies
High-Pitched
Sounds
Low-Pitched
Sounds
disturb the shorter affect longer
stiffer fibers of the basilar
membrane
more floppy fibers
stimulate receptor cells
close to the oval window
activated specific hair cells
further along the cochlea

19. Mechanisms of Hearing
 An action potential starts
in the cochlear nerve (a
division of cranial nerve
VIII – vestibulocochlear
nerve)
 The signal is transmitted
to the midbrain (for
auditory reflexes and then
directed to the auditory
cortex of the temporal
lobe)

20. Mechanisms of Hearing
▪ Continued stimulation can
lead to adaptation
–(over stimulation to the brain
makes it stop interpreting the
sounds)

21. Hearing and Equilibrium Deficits
▪ Tuning fork or audiometry testing
– try to diagnose ear problems or
hearing deficits
▪ Deafness – hearing loss of any
degree-from a slight loss to a
total inability to hear sound
1. Conduction Deafness
2. Sensorineural Deafness

22. Hearing Deficits
▪ Temporary or Permanent Conduction
Deafness
– something interferes with the conduction of sound
vibrations to the fluids of the inner ear
– Causes
▪ build up of earwax
▪ fussion of ossicles (otosclerosis)
▪ ruptured eardrum
▪ otitis media (middle ear inflammation)
– still be able to hear by bone conduction (hearing aids)

23. Hearing Deficits
▪ Sensorineural Deafness
– degeneration or damage to the receptor
cells in the spiral organ of Corti, to the
cochlear nerve, or neurons of the auditory
cortex
– Causes
▪ Extended listening to excessively loud sounds
▪ Problem with nervous system structures
– Can’t hear better by either conduction
route

24. Equilibrium Deficits
▪ Nausea, dizziness, and problems in
maintaing balance
–Impulses from the vestibular apparatus
“disagree” with what we see (visual input)
–Strange eye movements (jerky or rolling)
▪ Meniere’s Syndrome

25. Equilibrium Deficits
▪ Meniere’s Syndrome
– Suspected causes
▪ Arteriosclerosis, degeneration of cranial nerveVIII,
and increased pressure of the inner ear fluids
– Progressive deafness occurs
– Nauseted and howling or ringing sounds in the
ears and vertigo (a sensation of spinning)
– So severe = cannot stand up without
discomfort
– Anti-motion sickness drugs = decrease
discomfort