Lecture 18: Cellular Basis of Hearing and Balance

Question 1

What does the auditory system do? 3

Answer 1

1 * Basically: Converts mechanical stimuli into action potentials

2 * More scientifically: a PERIPHERAL MECHANO-TRANSDUCER (COCHLEA) WHICH CONVERTS ACOUSTIC ENERGY INTO NERVE IMPULSES

3. • Encodes environmental sounds (MODALITY), location of sound (SPATIAL INFORMATION), loudness (INTENSITY) and frequency of sound (QUALITY)

Question 2

Cellular mechanisms are due to mechanically gated ion channels (5)

Answer 2

1. Tip links STRETCH and open Channels when stereocilia bend TOWARD tallest member.
2. K+ enters; hair cell DEPOLARIZES
3. Depolarization open voltage-gated Ca+2 channels.
4. Ca2+ entry causes greater release of neurotransmitter.
5. More neurotransmitter leads to higher rate of action potential.

Question 3

Direction of deflection of hair cell
determines AP firing rate

Answer 3

• Deflection TOWARDS
  stereocilia causes
  DEPOLARIZATION.
  —* Increased AP firing rate
• Deflection AWAY from
  stereocilia causes
  HYPERPOLARIZATION
  ——* Decreased AP firing rate

Question 4

How does the sound get to the mechanosensitive hair cells? (6)

Answer 4

1. Sound enters and reaches the tympanic membrane (accessory
   structure)
2. Tympanic membrane reverberates, causes oscillation of inner ear bones

– This amplifies the sound

3. Stapes bone vibrates against the oval window
4. Waveform is generated waveform within the cochlear fluid
5. Basilar membrane vibrates (contains hair cells)
6. Movement of inner hair cells results

Question 5

Cross section through the cochlea

Answer 5

SLIDE 9

Primary afferent nerve fibres conduct signal to the auditory cortex

SCALA MEDIA: high {K+)0

Basilar membrane (vibrates)

Question 6

Organ of Corti

Answer 6

~95 % of primary afferent
neurons in the cochlear nerve arise from inner hair cells

• Hair tips are stuck into
  the tectorial membrane
  and connected to the
  cochlear nerve
• Structural supporting cells
• outer hair cells
• tectorial membrane
• Inner hair cell
• basilar fiber
• spiral ganglion
• cochlear nerve

Question 7

What happens at Organ of Corti? (2)

Answer 7

• As the BASILAR MEMBRANE MOVES UP AND DOWN = SHEARING A FORCE IS DEVELOPED.
• This LEADS TO DEPOLARIZATION and REPOLARISATION of HAIR CELLS

Question 8

How do we encode the frequency of sound?

Answer 8

DIFFERENT FREQUENCY sounds STIMULATE DIFFERENT PARTS OF BASILAR MEMBRANE.

Question 9

High and low pitched sounds are processed along the coiled cochlea

Answer 9

1. LOW FREQUENCIES are SENSED at the APEX of the COCHLEA
2. HIGH frequencies are sensed at the BASE of the COCHLEA

Question 10

Why so many outer hair cells?

Answer 10

1. Outer hair cells INCREASE SENSITIVITY to sound and DISCRIMINATION of sound
2. An ELECTROMECHANICAL MOTOR PROCESS Causing AMPLIFICATION OF BASILAR MEMBRANE MOVEMENT.

Question 11

Outer hair cells increase sensitivity to sound and discrimination of
sound.

Answer 11

LEFT SIDE = DEPOLARISATION AND SHORTENING OF THE CELL

RIGHT SIDE= HYPERPOLARISATION AND LENGTHENING OF THE CEL

Question 12

An Electromechanical motor process causing amplification of basilar
membrane movement

Answer 12

1. Sound
2. Basilar membrane vibration
3. Mechanical feedback
4. Sound
5. Basilar membrane vibration
6. Amplified Basialr membrane vibrationn
7. Stimulates inner hair cell
8. Stimulates nerve fibres

Question 13

Hair cell damage caused by loud and repetitive sounds

Answer 13

• Long-term exposure to high intensity sound or sound of a particular frequency can cause damage to OUTER and sometimes INNER HAIR CELLS
• Damage to OUTER HAIR CELLS is
  the MOST COMMON form of HEARING LOSS

Question 14

Hair cell damage can cause deafness

Answer 14

OUTER IS THE MOST COMMON

Question 15

Summary: hearing system
- Transduction
- Amplification
- Sound frequencies
- Damage

Answer 15

1. • Specialized MECHANO-SENSITIVHAIR CELLS PERFORM TRANSDUCTION in the COCHLEA
2. • ACCESSORY STRUCTURES and OUTER HAIR CELLS are important in the
     AMPLIFICATION OF SOUND.
3. • SOUND frequencies are SORTED according to DISTANCE along the COCHLEA

4.* Damage to outer hair cells is the most common form of hearing loss

Question 16

What does the vestibular system do: 3

Answer 16

• Provides information about body position in gravitational field
• Provides information about motion of the body
• Maintenance of balance, postural control, progress of movements (together with visual and proprioceptive input)

Question 17

Each component of the vestibular system detects a different aspect of
body motion:

Answer 17

• Otolith organs: horizontal and vertical linear acceleration and tilt
• Semicircular canals: angular (rotational) acceleration of the head

Question 18

Otolith organs: Utricle and Saccule:

Answer 18

• Utricle is oriented horizontally
• Saccule is oriented vertically
• Each is curved and has groups

of hair cells of opposite
orientation – all angles of motion are detected

Question 19

Dense otoliths –

important accessory
structures in utricle and saccule =4

Answer 19

1 * Dense and heavy otolith CRYSTALS are
on TOP of the GELATINOUS CAP

2 * The INERTIA of OTOLITHS ACTIVATE HAIR CELLS

3 * INERTIA CAUSES DIFFERENT ORIENTATION OF THE CAP AND BASE

4* MECHANOSENSITIVE HAIR CELLS DETECT CHANGE IN ORIENTATION

Question 20

Semicircular canals

Answer 20

Detect angular (rotational) acceleration of the head

-INERTIA OF ENDOLYMPH CAUSES DEFLECTION OF THE CUPULA UPON HEAD ROTATION, THUS ACTIVATING RECEPTOR HAIR CELLS.

Question 21

Semicircular canals:

cellular mechanisms of hair cell stimulation

Answer 21

1 * Deflection TOWARDS KINOCILLA = DEPOLARISATION and INCREASED FIRING RATE

2. • DEFLECTION AWAY away from KINOCILLIA =
     HYPERPOLARISATION and DECREASED FIRING RATE

3.* AXIS OF HAIR CELLS IS OPPOSITE ON THE LEFT AND RIGHT HAND SIDES

4. • Thus, upon ROTATION OF HEAD =
     increased firing will be seen on one side and decreased firing on
     the opposite side
5. • Important for detection of them direction that the head is rotating

Question 22

Involvement of vestibular system in eye
movement and motor control systems

Answer 22

Vestibulo-ocular reflex (VOR):

Mechanism which stabilizing images on the retina during head movement – eye movement in opposite direction to head movement

Question 23

Vestibulo-spinal reflexes: maintenance of balance

Answer 23

Coordination of head and neck movement with the trunk and body, with the
goal of maintaining the head in an upright position

Question 24

Vestibulo-spinal reflexes: maintenance
of balance = 3

What does it include and what do they do?

Answer 24

1. • Utricle and saccule are important in maintaining balance and correct postural adjustments
2. • Utricle and saccule can activate
     the vestibular nuclei which activate descending motor pathways that control the muscles of the trunk
3. • This can bring about a correction in body position and prevent
     falling thus maintaining balance

Question 25

Vestibulo-spinal reflexes: maintenance of balance

Coordination of head and neck movement with the trunk and body, with the
goal of maintaining the head in an upright position

PATHWAY:

Answer 25

1. Vestibular nerve
2. Vestibular nuclei

• 3. To cerebellum

or

• 4. Lateral vestibulospinal tract
• excites limb extensor muscles

or 5. Medial vestibulopsinal tract

• controls neck and trunk muscles

Question 26

Medial vs Lateral Vestibulospinal; tract:

Answer 26

LATERAL vestibulospinal tract excites LIMB EXTENSOR MUSCLES

MEDIAL vestibulopsinal tract controls NECK AND TRUNK MUSCLES

Question 27

Summary: vestibular system

What, Structures, what fo they do, Vestibulospinal vs V.ocular reflex.

Answer 27

• Vestibular system provides information about the motion of the body
• Two main structures are involved:
  otolith organs (utricle and saccule)
  and the semicircular canals
• These structures use similar mechanisms to detect changes in body
  position
• Vestibulo-ocular reflex: stabilizes images on the retina during movement of the head
• Vestibulo-spinal reflex: helps in maintenance of balance and correct
  postural adjustments to maintain balance