Download VESTIBULAR SYSTEM (Balance/Equilibrium) The vestibular

VESTIBULAR SYSTEM (Balance/Equilibrium)
The vestibular stimulus is provided by Earth’s
gravity, and head/body movement.
Located in the labyrinths of the inner ear, in two
components:
1. Vestibular sacs - gravity & head direction
2. Semicircular canals - angular acceleration
(changes in the rotation of the head, not steady
rotation)
1. Vestibular sacs (Otolith organs) - made of:
a) Utricle (“little pouch”)
b) Saccule (“little sac”)
Signaling mechanism of Vestibular sacs
Receptive organ located on the “floor” of Utricle and on
“wall” of Saccule when head is in upright position
- crystals move within gelatinous mass upon head
movement;
- crystals slightly bend cilia of hair cells also located within
gelatinous mass;
- this increases or decreases rate of action potentials in
bipolar vestibular sensory neurons.
Gelatinous mass
Otoconia: Calcium carbonate crystals
Cilia
Hair
cells
Vestibular nerve
Vestibular ganglion
2. Semicircular canals: 3 ring structures; each
filled with fluid, separated by a membrane.
Signaling mechanism of Semicircular canals
-head movement induces movement of endolymph, but
inertial resistance of endolymph slightly bends cupula
(endolymph movement is initially slower than head mvmt);
- cupula bending slightly moves the cilia of hair cells;
- this bending changes rate of action potentials in bipolar
vestibular sensory neurons;
- when head movement stops: endolymph movement
continues for slightly longer, again bending the cupula but
in reverse direction on hair cells which changes rate of APs;
- detects “acceleration” in all 3 planes
Crosssection
semicircular
canal
Ampulla
Crosssection
Perilymph
Membrane
Endolymph
Cupula
Hair
cells
Vestibular
nerve
Vestibular
Ganglion
3. Vestibular pathway to the nervous system:
- vestibular bipolar sensory cell bodies located in
vestibular ganglion, which looks like a nodule
(enlargement) on the vestibular nerve
- axons from vestibular neurons get together
with axons of the spiral ganglion (auditory) and
give rise to vestibulocochlear nerve
= VIII cranial nerve
- vestibular axons synapse within vestibular
nuclei in medulla, and in cerebellum;
- vestibular neurons send axons to cerebellum,
spinal cord, medulla and pons
- medullary responses to vestibular stimulation
involved in motion sickness (nausea, emesis)
- responsible for controlling neck muscles for
keeping head upright
- controls eye movement; compensates for
sudden head movement = Vestibulo-Ocular
Reflex (VOR)
Chap. 8- 4
Nerves out of the “vestibule” . . .
. . . and entering the brain
AUDITORY SYSTEM (Hearing)
I. AUDITORY (ACOUSTIC) STIMULUS
- waves of energy = sound waves
- propagates through: gases (air), liquids (water),
and solids (metals)
1 cycle
Intensity (loudness): measured on a logarithmic
scale (decibels, dB); very sensitive & wide range
- range: 0 - 160+ dB (>140 dB = painful)
Frequency: Normal range (humans): 20-20,000 cps
- cps = cycles per seconds = Hertz (Hz)
- frequency = pitch
- ex., women’s voice higher pitch than men’s
- range gets considerably narrower with age
Complexity: additivity of simple waves gives
rise to complex waves
- most sounds are complex waves that can be
analyzed with Fourier transform analysis into
their component simple waves.
II. EAR
A. OUTER EAR
a. Pinna (external ear)
b. Ear canal
Hammer Ossicles (middle
Anvil
Stirrup ear bones)
}
Oval
window
Auditory
nerve
Bone
Ear canal
Pinna
Cochlea
Vestibule
Tympanic Round
membrane window
Eustachian tube
(connects with throat)
B. MIDDLE EAR
a. Eardrum (tympanic membrane)
b. Ossicles
1. Hammer
2. Anvil
3. Stirrup
C. INNER EAR = COCHLEA
C. COCHLEA
a. Oval window
b. Round window
c. Endolymph
d. Cochlear duct - contains Organ of Corti
1. Tectorial membrane (top)
2. Basilar membrane (bottom)
3. Hair cells
Tectorial
membrane
Outer (rigid)
hair
cells
Deiter’s
cells
Cochlear
duct
Cilia of
hair cell
Axons of auditory nerve
Basilar membrane - mobile
Organ of Corti
Cochlear nerve
Spiral ganglion
Bone
Membrane surrounding cochlea
Slice through cochlea
Inner
hair cell
Signaling mechanism for hearing:
- sound waves produce movement of basilar membrane;
- movement of basilar membrane induce movement of cilia of
hair cells;
- cilia movement increase or decrease polarization of hair cells,
which increase or decrease neurotransmitter release onto axon
terminals of bipolar auditory neurons;
- this increases or decreases action potentials in bipolar auditory
neurons.
- it is the inner hair cells that provide the auditory signal to the
brain;
- the outer hair cells are believed to control the “tightness” of the
basilar membrane, and therefore provide some modulation of
hearing.
D. CODING OF FREQUENCY:
Pitch (frequency) perception:
1. Place coding
a. Different spots on basilar membrane vibrate to different
frequencies (Fig. A above)
b. Works for moderate to high frequencies, 100-200 to 20,000 Hz;
- near oval window (base): very high frequencies (20,000 Hz)
- near apex: moderate frequencies (100-200 Hz)
2. Rate coding
a. Frequency of sound waves over a large portion of basilar
membrane = frequency of action potentials
b. Works for low frequency sounds (below about 100-200 Hz)
Chap. 8- 8
E. CODING OF INTENSITY (LOUDNESS):
Determined by action potential frequency;
ex. soft sound = fewer AP, loud sound = more AP
F. CODING OF SOUND LOCALIZATION:
Based on:
1. Arrival time: ex., click sound generated to the
left arrives at left ear first
2. Phase difference: ex., continuous sound waves
will reach each ear at slightly different phases
of the oscillating sound waves
- these mechanisms work best with sounds of
moderate frequencies
3. Intensity difference: ex., sound generated to
the left are sensed slightly louder on the left
side
- this mechanism works best with sounds of
high frequencies
Note. Low frequencies (< 100 Hz) are nearly
impossible to localize (that’s why you need
only one “sub-woofer” in a home-theater
system)
G. AUDITORY SYSTEM PATHWAY
Spiral ganglion - contains bipolar neurons
- receive information from hair cells
- send their axons to brain via VIII nerve
(auditory component of vestibulocochear nerve)
Lateral fissure
Auditory cortex
Forebrain
Thalamus: Medial
geniculate body
Midbrain
Inferior colliculus
Dorsal
cochlear nucleus
Lateral
lemniscus
Ventral
cochlear nucleus
Medulla
Trapezoid
body
Superior
olivary
complex
Auditory nerve
(VIII nerve)
Frequency organization kept throughout auditory system all
the way to auditory cortex