Chapter 18 Study Guide Part 2 (Hearing & Equilibrium)

External Ear

The part of the ear that collects and directs sound waves to the eardrum. It consists of the auricle, external auditory canal, and tympanic membrane. 

Auricle (Pinna)

The structure of the external ear that is the outer funnel that collects sound waves and brings it to your ear. It leads to the external auditory canal and surrounds it. Protects the opening passage way and provides directional sensitivity to the ear by directing sound inward to the tympanic membrane.

External Auditory Canal (Meatus)

The structure of the external ear that is the tube that leads into the ear. It is shaped like the letter “S", it twists a little. Leads inward through temporal bone. It is about one inch long.

Tympanic Membrane

• The structure of the external ear that is the eardrum. It is a thin membrane that separates the outer ear from the middle ear. It is delicate and the auricle protects it. Made of 3 layers. It is oval or cone shaped, with the point of the cone pointing inwards.

• It vibrates with sound waves. It needs to stay intact so that when sound waves travel through the external acoustic meatus, it will cause vibrations to occur on the membrane which can then be transmitted into your middle ear.

• The malleus bone from the middle ear is visible in the tympanic membrane because it pushes up right against it.

Ceruminous Glands

• Modified sweat glands that secrete cerumen (ear wax) to help trap debris that might get into the canal, before reaching the eardrum, or tympanic membrane.

• Cerumen naturally makes its way out, it slows the growth of microorganisms and reduce chances of infection.

• Impacted Cerumen: Shoving objects into ear is just pushing cerumen against tympanic membrane. This can cause the loss of hearing. 

Middle Ear (Tympanic Cavity)

• The part of the ear that amplifies sound waves and transmits them to an appropriate portion of the internal ear. It is inside the temporal bone and is filled with air and ossicles (bones). Consists of auditory ossicles, oval window, round window, and auditory tube. 

• Middle Ear Vibration Transmission

  • Vibration from tympanic membrane vibrates malleus, which then vibrates incus, and then vibrates stapes. Then it vibrates thin membrane under stapes called the oval window, which will then send vibration to inner ear to process sound wave.

Auditory Ossicles

• Some of the smallest bones (3) in the body that are located in the middle ear. They are covered by mucous membranes.

• Each bones is attached to the wall of the cavity by a series of ligaments. They are connected to each other which allows them to transmit vibrations from the tympanic membrane across middle ear to the inner ear. Passing vibration from one to the other.

• 3 bones:

  • Malleus = Hammer shape, what you can actually see through tympanic membrane. 

  • Incus = Anvil shape

  • Stapes = Stirup shape

• Order: Malleus > Incus > Stapes

Oval Window

The structure of the middle ear that sits between the stapes and inner ear (under stapes). It is more superior and in the cochlear wall. It receives vibrations from the stapes.

Round Window

The structure of the middle ear that is smaller than the oval window and is more inferior. It is thin and is covered by the secondary tympanic membrane. It helps to relieve pressure in the inner ear. Separates the perilymph of cochlear chambers from air-filled middle ear.

Auditory Tube

The structure of the middle ear that allows air from the tympanic cavity to get outside by going through you nose and mouth. Normal hearing depends on air pressure being equal inside middle ear and outside of head. Sometimes when you’re sick, this structure will swell shut and make it hard to equalize the pressure. It leads down to the nasopharynx. Helps equalize pressure in middle ear with external, atmospheric pressure. 

Internal Ear (Labyrinth)

The part of the ear that contains sensory organs for equilibrium (balance) and hearing. It is made up of a system of connecting tubes. Consists of 3 main parts, the cochlea, vestibule, and semicircular canals. All 3 of these organs connect to the cranial nerve, vestibulocochlear nerve. 

• Osseous Labyrinth = The bony outer layer filled with perilymph (flows between bony and membranous labyrinths, resembles CSF).

• Membranous Labyrinth = The inner layer made of epithelial tissue and is filled with endolymph.

• Structure: Bone, fluid, membrane, fluid.

Labyrinth (Internal Ear) Layers

Cochlea

The organ of the internal ear that is used for hearing. It is a long tube that is wrapped up like snail shell. Contains cochlear duct, scala chambers, and spiral organ.

Cochlear Duct (scala media)

A spiral shaped tube in the cochlea of the inner ear that receives sound waves and transmit those into the cochlear nerve. It is the slender, elongated portion of the membranous labyrinth. contains endolymph and spiral organ

• The fluid filled chambers of the cochlea in the inner ear.

• Scala Vestibule = Scala chamber in the superior side. Leads to the oval window to end of spiral organ. Contains perilymph.

• Vestibular Membrane = Separates scala vestibuli from cochlear duct.

• Basilar Membrane = Separates scala tympani from cochlear duct. Supports spiral organ.

• Scala Tympani = Inferior compartment of cochlea. Goes from round window to apex or tip of cochlea. Tip of cochlea is where two scala chambers meet. Contains perilymph.

Scala Chambers

Spiral Organ

• The structure of the cochlea that contains hearing receptors that will talk to the branch of cochlear nerve.

• Contains hair cells that have tiny rows of stereocilia  that extend upwards. They are attached to neurons that run through cochlear nerve. Arranged in 2 longitudinal rows. 

• Contains tectorial membrane which is firmly attached to the inner wall of the cochlear duct.

• Sound wave going through this structure:

  • Sound wave goes through this structure and causes the tectorial membrane to vibrate, which will rub against stereocilia (hair cells), which will trigger nerve impulses to go down vestibulocochlear nerve to your brain and let you know that you head a sound.

1. Sound wave enters the external auditory canal.

2. Vibrates the tympanic membrane.

3. Which then vibrates malleus > incus > stapes, stapes will vibrate oval window.

4. Oval window transmits vibration into the scala vestibule to the upper level of the cochlear organ. Vibrations move through length of scala vestibule (perilymph). It goes all the way around to the tip of the cochlea , then goes to scala tympani, and excess vibrations go out the round window.

5. Along the way, it will vibrate vestibular and basilar membranes which will cause the tectorial membrane to start vibrating and rub against hair cells (bend), which will then trigger what you hear as a sound.

• High pitch sounds vs. Low pitch sounds

  • Where the hair cells wiggle along the spiral organ determines the type of sound you hear.

  • Hair cells close to oval window, near beginning of cochlea = Hears lower sounds

  • Hair cells closer to tip of cochlea = Hears high pitch sounds

Physiology of Hearing

Cochlear Implant

A fix for people when hearing loss is beyond the repair of hearing aid, which just like an earphone that takes noise and makes it louder in the canal. This fix directly transmits sound waves in the form of electrical signals directly into the cochlea to stimulate auditory nerve. However, it is difficult to differentiate between high and low pitches, so it sounds like a robot. This bypasses the middle ear. 

Vestibule

The organ of the inner ear used for static equilibrium. Senses your body position when you are not moving. It is how you sense the position of your head, whether its upright or not. Consists of 2 expanded sacs of membranous labyrinth, utricle (closer to semicircular canals) and saccule (closer to cochlea), and each chamber contains macula (little sensory organs that help sense static equilibrium).

• Contains hair cells. Tiny microvilli extend off of them. They sense service sensor receptors for the neurons that will go down into the nerves.

• Microvilli project upwards into gelatinous material called the otolithic membrane which contain little grains called otoliths (grains of calcium carbonate, provides weight).

• This helps stimulate autonomic nervous system. Its how you know you’re tilting your head so you can maintain balance.

• Physiology:

  • When you tilt your head, the otolithic membrane will slide one way or the other, depending on the way you tilt. This causes the otoliths to slide the same way, bending the microvilli and stimulating a nerve impulse to let you know that you’ve tilt your head.

  • Example of tilting your head down: Otoliths will slide forward, dragging the otolithic membrane downwards, bending microvilli, trigger nerve impulse, then you know you’ve tilted you head down/forward.

Macula of Utricle (Physiology of Static Equilibrium)

Semicircular Canals

The organ of the internal ear that is used for dynamic equilibrium, which is how you know when your body is moving. It is how you maintain balance when your head and body are moved or rotated. Senses motion and maintain balance when you are moving. Relies on 3 of them: anterior, posterior, and lateral that are continuous with vestibule. Each of them lie at a right angle to each other along the anatomical plane. Contain ampullae with cupula and crista ampullaris.

Ampulla

The organs within the semicircular canals that sense dynamic equilibrium. They are extensions at the ends of the semicircular canals. Crista ampullaris are located inside of them. Expanded region that contains sensory receptors. Contain cupula, hair cells (releases neurotransmitters), and crista ampullaris.

Cupula

The gelatinous material in the ampulla of the semicircular canal. It floats above the receptor surface, nearly filling ampulla. It acts as the sensory receptor for rotational movements of the head. 

Crista Ampullaris

The sensory organ in the ampulla of the semicircular canals that detects rotational movement of the head. It has hair cells embedded in the gelatinous material called cupula.

• When you rotate your head, it will cause the endolymph inside the semicircular canals to swirl around which will then jiggle the cupula and wiggle the hair cells inside the ampullae, which will then tell you that your head has been rotated.

• Motion Sickness: When sensory receptors are sending conflicting information regarding your body position or your movement.

• Vertigo: When sensory receptors in your head are telling you that you are moving but your skin and muscles are not.

Physiology of Dynamic Equilibrium