Hearing and Balance Mechanoreceptors

Vocab

Optic - Eye
Otic - Ear

Ear

Outer Ear (external)
Middle ear (ossicles) for hearing
Inner ear (labyrinth) for hearing and equilibrium

Middle & Inner Ear

Tympanic Membrane - Vibrates in response to sound waves

Malleus (hammer)- Attached to tympanic membrane and vibrates as a result of changes to membrane

Incus (Anvil)- Atached to malleus vibrates in response to changes in malleus

Stapes (Stirrup) - Attached to incus and vibrates in response to cahnges in incus

Oval Window - Membrane attached to stapes; moves fluid within cochlea in response to vibrations from stapes

Cochlea- Contains receptors that react to changes in sound waves

Semicircular Canals - Fluid filled structures that respond to changes in body position

Hearing

Vibrations enter oval window
Vibrations cause disturbance in perilymph -> endolymph -> basilar membrane -> tectorial membrane -> hair cells

Different hair cells respond to different frequencies

Highest frequency is detected close to oval window

Lowest frequency detected further from oval window

Aging and Hearing

As people get older they lose the ability to hear higher sound frequencies.
This is the reason that fire alarms and other alarms are made with low frequencies for old people to hear them

Mechanoreceptors

Mechanics = Movement
Respond to changes in position to send signals
Movement of fluid surrounding the receptors causes stereocilia to move and triggers an action potential

Transmitting and Interpreting Sound

1. Sound in the external acoustic meatus hit the tympanic membrane (eardrum) causing it to vibrate

2. Pressure is equalized by the pharyngotympanic tube which is about 45mm long

3. The eardrum causes the ossicles in the air-filled middle ear to move

4. The ossicles articulate to form a lever system that amplifies and transmits the vibratory motion of the TM to fluids of the inner ear cochlea via an oval window

5. Vibration of the Stirrup at the oval window causes waves to travel in the fluid-filled cochlea

6. Sensory hair cells are stimulated

7. The stereocilia send impulses to the cochlear nerve (Vestibulocochlear nerve)

Auditory Pathway In the Brain

1. Cochlear Nerve
2. Medulla
3.Pons-Medulla Junction
4. Midbrain
5. Thalamus
6. Temporal lobe of the cerebrum

Skeletal muscles in the middle Ear

When loud muscles contract, limit vibration and dampen the noise

Inner Ear

1. Cochlea - Hearing (filled with fluids)
2. Vestibule - equilibrium
3. Semicirular canals - equilibruim

Equilibrium

Semicircular Canals: Determine kinetic equlibrium ; detects rotational movement of the body

Movement causes the fluids to displace

Vestibule Determine static equilibrium

Boyd's positionrelative to gravity; detects linear acceleration, not rotational acceleration

Equilibrium Pathway

Vestibular and sensory systems work to maintain balance
Vestibular detects rotational and linear motion
VESTIBULAR NERVE TO THE BRAIN STEM
When your head move the fluid in the vestibule and semicircular canals move
Movement of fluid causees the hair cells to move
Movement of hair sends an electrical impulse to the vestibular nerve
Transmits the signal to the cerebellum, brain stem and spinal cord

Vestibule contain utricle and saccule

Each contain a macula
Senses static equilibrium and linear acceleration of the head
Tips of hairs embedded in otolithic membrane
Vestibular nerve brances off Vestibulocochlear nerve

Semicircular Canals

Each of the 3 lies in one of the 3 planes of space
Sense rotational acceleration of the head
Duct with ampulla housing a small crest: crista ampulla
Hairs project into jellylike cupula & basilar cells synapse with fibers of vestibular nerve

Vestibular Disorders

Vertigo- A sensation of spinning. Caused by an inner ear problem
Nystagmus - The eyes move up and down, side to side, or in a circle. You can not control eye movements. No known cause. May be caused by many things including cataracts, stroke, head injury, inner ear problems. More common in older people