Lecture 18: Cellular Basis of Hearing and Balance Flashcards
What does the auditory system do? 3
1 * Basically: Converts mechanical stimuli into action potentials
2 * More scientifically: a PERIPHERAL MECHANO-TRANSDUCER (COCHLEA) WHICH CONVERTS ACOUSTIC ENERGY INTO NERVE IMPULSES
- Encodes environmental sounds (MODALITY), location of sound (SPATIAL INFORMATION), loudness (INTENSITY) and frequency of sound (QUALITY)
Cellular mechanisms are due to mechanically gated ion channels (5)
- Tip links STRETCH and open Channels when stereocilia bend TOWARD tallest member.
- K+ enters; hair cell DEPOLARIZES
- Depolarization open voltage-gated Ca+2 channels.
- Ca2+ entry causes greater release of neurotransmitter.
- More neurotransmitter leads to higher rate of action potential.
Direction of deflection of hair cell
determines AP firing rate
- Deflection TOWARDS
stereocilia causes
DEPOLARIZATION.
—* Increased AP firing rate - Deflection AWAY from
stereocilia causes
HYPERPOLARIZATION
——* Decreased AP firing rate
How does the sound get to the mechanosensitive hair cells? (6)
- Sound enters and reaches the tympanic membrane (accessory
structure) - Tympanic membrane reverberates, causes oscillation of inner ear bones
– This amplifies the sound
- Stapes bone vibrates against the oval window
- Waveform is generated waveform within the cochlear fluid
- Basilar membrane vibrates (contains hair cells)
- Movement of inner hair cells results
Cross section through the cochlea
SLIDE 9
Primary afferent nerve fibres conduct signal to the auditory cortex
SCALA MEDIA: high {K+)0
Basilar membrane (vibrates)
Organ of Corti
~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
What happens at Organ of Corti? (2)
- As the BASILAR MEMBRANE MOVES UP AND DOWN = SHEARING A FORCE IS DEVELOPED.
- This LEADS TO DEPOLARIZATION and REPOLARISATION of HAIR CELLS
How do we encode the frequency of sound?
DIFFERENT FREQUENCY sounds STIMULATE DIFFERENT PARTS OF BASILAR MEMBRANE.
High and low pitched sounds are processed along the coiled cochlea
- LOW FREQUENCIES are SENSED at the APEX of the COCHLEA
- HIGH frequencies are sensed at the BASE of the COCHLEA
Why so many outer hair cells?
- Outer hair cells INCREASE SENSITIVITY to sound and DISCRIMINATION of sound
- An ELECTROMECHANICAL MOTOR PROCESS Causing AMPLIFICATION OF BASILAR MEMBRANE MOVEMENT.
Outer hair cells increase sensitivity to sound and discrimination of
sound.
LEFT SIDE = DEPOLARISATION AND SHORTENING OF THE CELL
RIGHT SIDE= HYPERPOLARISATION AND LENGTHENING OF THE CEL
An Electromechanical motor process causing amplification of basilar
membrane movement
- Sound
- Basilar membrane vibration
- Mechanical feedback
- Sound
- Basilar membrane vibration
- Amplified Basialr membrane vibrationn
- Stimulates inner hair cell
- Stimulates nerve fibres
Hair cell damage caused by loud and repetitive sounds
- 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
Hair cell damage can cause deafness
OUTER IS THE MOST COMMON
Summary: hearing system
- Transduction
- Amplification
- Sound frequencies
- Damage
- Specialized MECHANO-SENSITIVHAIR CELLS PERFORM TRANSDUCTION in the COCHLEA
- ACCESSORY STRUCTURES and OUTER HAIR CELLS are important in the
AMPLIFICATION OF SOUND.
- ACCESSORY STRUCTURES and OUTER HAIR CELLS are important in the
- SOUND frequencies are SORTED according to DISTANCE along the COCHLEA
4.* Damage to outer hair cells is the most common form of hearing loss
What does the vestibular system do: 3
- 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)
Each component of the vestibular system detects a different aspect of
body motion:
- Otolith organs: horizontal and vertical linear acceleration and tilt
- Semicircular canals: angular (rotational) acceleration of the head
Otolith organs: Utricle and Saccule:
- 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
Dense otoliths –
important accessory
structures in utricle and saccule =4
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
Semicircular canals
Detect angular (rotational) acceleration of the head
-INERTIA OF ENDOLYMPH CAUSES DEFLECTION OF THE CUPULA UPON HEAD ROTATION, THUS ACTIVATING RECEPTOR HAIR CELLS.
Semicircular canals:
cellular mechanisms of hair cell stimulation
1 * Deflection TOWARDS KINOCILLA = DEPOLARISATION and INCREASED FIRING RATE
- DEFLECTION AWAY away from KINOCILLIA =
HYPERPOLARISATION and DECREASED FIRING RATE
- DEFLECTION AWAY away from KINOCILLIA =
3.* AXIS OF HAIR CELLS IS OPPOSITE ON THE LEFT AND RIGHT HAND SIDES
- Thus, upon ROTATION OF HEAD =
increased firing will be seen on one side and decreased firing on
the opposite side
- Thus, upon ROTATION OF HEAD =
- Important for detection of them direction that the head is rotating
Involvement of vestibular system in eye
movement and motor control systems
Vestibulo-ocular reflex (VOR):
Mechanism which stabilizing images on the retina during head movement – eye movement in opposite direction to head movement
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
Vestibulo-spinal reflexes: maintenance
of balance = 3
What does it include and what do they do?
- Utricle and saccule are important in maintaining balance and correct postural adjustments
- Utricle and saccule can activate
the vestibular nuclei which activate descending motor pathways that control the muscles of the trunk
- Utricle and saccule can activate
- This can bring about a correction in body position and prevent
falling thus maintaining balance
- This can bring about a correction in body position and prevent
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:
- Vestibular nerve
- Vestibular nuclei
- To cerebellum
or
- Lateral vestibulospinal tract
- excites limb extensor muscles
or 5. Medial vestibulopsinal tract
- controls neck and trunk muscles
Medial vs Lateral Vestibulospinal; tract:
LATERAL vestibulospinal tract excites LIMB EXTENSOR MUSCLES
MEDIAL vestibulopsinal tract controls NECK AND TRUNK MUSCLES
Summary: vestibular system
What, Structures, what fo they do, Vestibulospinal vs V.ocular reflex.
- 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
