2. THE NATURE OF SOUND
Sound is an audible variations in air pressure, defined by:
1) frequency: Number of cycles (distance between successive compressed patches)
per second expressed in units called Hertz (Hz). Human Range is btw 20 Hz to 20,000 Hz
2) Intensity: Difference in pressure between compressed and rarefied patches of air. It
determines the loudness of the sound.
Sounds propagate at a constant speed: 343 m/sec
4. THE MIDDLE EAR
Sound Force (pressure) is amplified by the Ossicles, producing greater pressure at oval window
(smaller surface) than tympanic membrane, in order to move more efficiently the fluid inside the
cochela
The Attenuation Reflex: response where onset of loud sound causes tensor tympani and
stapedius muscle contraction. It’s used to adapt ear to loud sounds, or understand speech better
in noisy environment (more attenuation of low sounds)
5. THE INNER EAR
Perilymph: Fluid in scala vestibuli and scala tympani
Endolymph: Fluid in scala media
Endolymph has an electric potential 80 mV more positive than perilymph (Endocochlear potential)
6. THE INNER EAR
Basilar Membrane is wider at apex, stiffness decreases from base to apex
7. THE INNER EAR
Pressure at oval window, pushes perilymph into scala vestibuli, round window membrane bulges
out. Endolymph movement bends basilar membrane near base, wave moves towards apex
8. THE INNER EAR
The Organ of Corti and Associated Structures. Here the mechanical energy of the
sound is transformed in electrical signal by the auditory receptor cells (hair cells).
Each hair cells has around 100 stereocilia.
Rods of corti provide structural support. Hair cells form synapses with bipolar neurons
that have their body in the spiral ganglion. Their axons form the auditory nerve
9. THE INNER EAR
Transduction by Hair Cells
When sound arrives, basilar membrane moves. According to the movement, stereocilia
bends on one or the other direction: i.e. Basilar membrane upward, reticular lamina up
and stereocilia bends outward
11. INFORMATION ABOUT THE SOUND
Information About Sound Intensity is encoded in 2 ways:
Firing rates of neurons and number of active neurons
Stimulus Frequency
Frequency sensitivity: in Basilar membrane is Highest at base, lowest at
cochlea apex. This coding is kept separate along the auditory pathways
(tonotopy)
Phase Locking is another way to code for frequency
Consistent firing of cell at same sound wave phase. Only for frequency below
4kHz
12. SOUND LOCALIZATION: HORIZONTAL PLANE
Interaural time delay: Time taken for Interaural intensity difference: Sound at
sound to reach from ear to ear high frequency from one side of ear
Sound Sound
Sound shadow waves
waves Sound
waves
Sound
waves Sound
shadow
Sound
shadow
Duplex theory of sound localization:
Interaural time delay: 20-2000 Hz
Interaural intensity difference: 2000-20000 Hz
13. SOUND LOCALIZATION: VERTICAL PLANE
pinna
Path 2, direct sound
Path 2, reflected sound
Path 2, direct sound
Path 2, reflected sound
Path 3, direct sound
Path 3, reflected sound
Based on reflections from the pinna
14. THE AUDITORY CORTEX: BA 41
Axons leaving MGN project to auditory cortex via
internal capsule in an array called Acoustic
Radiation
Primary auditory cortex
Secondary auditory cortex
15. THE VESTIBULAR SYSTEM
Importance of Vestibular System
Balance, equilibrium, posture, head position, eye movement
The Vestibular Labyrinth
16. THE VESTIBULAR SYSTEM
The Otolith Organs (saccule and utricle). Detect force of gravity (linear acceleration)
and tilts (change of angle) of the head.
Saccule is vertically oriented and utricle horizontally oriented
Crystals of calcium carbonate
Bending of the hairs
toward kinocilium:
depolarization
17. THE VESTIBULAR SYSTEM
The Semicircular Canals. Detect rotation of the head and angular acceleration
Crista: Sheet of cells where hair cells
of semicircular canals clustered
Ampulla: Bulge along canal, contains
crista
Cilia: Project into gelatinous cupula
Kinocili oriented in same direction so
all excited or inhibited together
Filled with endolymph
Three semicircular canals on one
side helps sense all possible head-
rotation angles
Each Canal paired with another on
opposite side of head
Rotation causes excitation on one
side, inhibition on the other
endolymph
19. VESTIBULO-OCULAR REFLEX (VOR)
Motion of the head
Function: Line of sight fixed on Motion of the eyes
visual target
Mechanism: Senses rotations of
head, commands compensatory
movement of eyes in opposite
direction.
Connections from semicircular
canals, to vestibular nucleus, to
cranial nerve nuclei excite
extraocular muscles
