A N P of Auditory System Flashcards
Peripheral Auditory System
Outer, Middle, Inner Ears, 8th Cranial Nerve (Vestibulocochlear)
Outer Ear Primary Structures
The Pinna (Auricle) and External Auditory Meatus (Ear Canal)
Pinna
External Ear, made of cartilage covered with skin.
Has characteristic folds
Resonant frequency at 1500 Hz
Same side localization
First 1/3 of external auditory canal is made of…
Cartilage, continuous with that of pinna
Inner 2/3 of Ex. Aud. Canal is…
the osseous/ bony portion.
Course through part of the temporal bone of skull
Osseocartilaginous Junction
Where the cartilaginous and osseous portions of ear canal meet
External Auditory Meatus
Direct Sound to Ear Drum
Situate eardrum deep to protect it from trauma
Often have a curve to enhance protection
Produces cerumen
Resonator tube, between 2,700 - 3,400 Hz, Increases by 10-20 dB
Cerumen
Help repel foreign bodies from entering the ear
Tympanic Membrane
Eardrum, Separates the outer and middle ear
Made of 3 Layers
1. Skin that is found in bony portion of ear canal
2. Fibrous connective tissue, allows to vibrate
3. Mucous Membrane
Can be divided into 4 quadrants
Semitransparent
Pars Tensa
Stiff, lots of fibrous tissue
Largest surface area of ear drum
Vibrates maximally
Pars Flaccida
Superior part of ear drum, no fibrous tissue
Vibrates minimally
Manubrium
Handle of the Malleus,
1 o’clock in the left ear 11 o’clock in the right ear
Otoscope
Allows one to see in ear canal
A healthy ear will reflect back a cone of light in anterior/inferior quadrant
Middle Ear Space
Air-filled space lined with mucous membrane
Irregularly shaped cavity
Tegmen Tympani
On the top
Thin layer of bone that separates the middle ear cavity from the brain
Fundus Tympani
On the floor
Thin plate of bone that separates the middle ear from the jugular bulb
Mastoid
Posteriorly Pneumatized bone (bone containing air pockets)
Ossicles/Ossicular Chain
The Malleus, Incus, and Stapes
Eustachian Tube
Leads to nasopharynx
Normally closed, opens when yawning, chewing, swallow
Keeps pressure in middle ear space at atmospheric
The middle ear is also an…
impedance-matching device
3 ways middle ear enhances sound intensity
- Lots of pressure on “big” eardrum, all transferred to tiny stapes, boost of 23 dB
- Tympanic Membrane is curved so more movement on curved aspects less near manubrium
- The ossicular chain is like a lever so it naturally increases sound pressure, provides boost of 2.4 dB
Middle Ear Muscles
Stapedius Muscle and Tensor Tympani Muscle
Both muscles respond reflexively and bilaterally
Stapedius Muscle
Originates in the posterior (mastoid) wall of the middle ear attaches to the neck of the stapes bone.
Contracts to protect inner ear when encounters loud noises
Tensor Tympani
Attaches to the manubrium of the malleus and contracts in response to nonauditory stimulation ie. air in ears of eyes
Acoustic Reflex Arc
When both stapedius muscles contract in response to loud sounds
2 Kinds of Reflex Pathways
Ipsilateral (Same-side) and Contralateral (opposite side) acoustic reflex pathways
Not fully understood in humans
2 pathways for each type
The 2 types differ from one another, and the 2 same differ from one another?!??!
Both pathways contain a series of synapses that travel from beginning (ventral cochlear nucleus) to end (Stapedius Muscle)
Ipsilateral Synapses
The 8th nerve
the ventral cochlear nucleus
the ipsilateral facial (7th) nerve nucleus
Contralateral Synapses
Ipsilateral Superior Olivary Complex
Contralateral facial nerve nucleus
7th facial nerve
Contralateral Stapedius Muscle
Inner Ear
Vestibular System and the Cochlea
Vestibular System
Deals with balance
Directly beyond the oval window is the vestibule which is filled with perilymph
Membranous Sacs Located in the Vestibule
Utricle and Saccule
Surrounded by perilymph but filled with endolymph
Semicircular Canals
Arise from the utricle
Contain endolymph, surrounded by perilymph
Each canal contains an enlarged area called an ampulla
Ampulla
Enlarged area in each semicircular canal
Ampullae contain cristae
Cristae
Sense organs for balance
….respond through fluid movement to changes in the body that require maintenance of balance
Utricle, saccule, and the semicircular canals
The Cochlea
Contains the sense organ for hearing, the organ of Corti
Snail Shaped Organ, carved into the temporal bone
Scala, three primary chambers
3 Types of Scala
Scala Vestibuli - Perilymph
Scala Media - Endolymph
Scala Tympani - Perilymph
Scala Vestibuli
Lies immediately beyond the oval window, close to vestibule
Scala Tympani
The lowermost chamber, terminates at the round window
Helicotrema
Located at the top of the cochlea
The perilymphs “meet” here
Scala Media
Most important, is in the middle, also known as Cochlea Duct
CONTAINS ENDOLYMPH
Reissner’s Membrane
Separates Media from Vestibuli
Baslar Membrane
Separates Media from Tympani
Tonotopic
Different frequencies result in different points of displacement of the basilar membrane
High Freq. close to base, Low Freq. closer to apex
Organ of Corti
Contains rows of outer and inner hair cells that differ in shape and function
Tectorial Membrane
Overhangs organ of Corti
Some of the cilia (attached to outer hair cells) embedded into this membrane
Hair Cells
Approx 12,000 outer hair cells configured in three rows throughout the cochlea
Approx. 3,500 inner hair cells, single row, do not come into contact with the tectorial membrane
About 30,000 between both ears
Differentiation between inner/outer hair cells
Inner hair cells are afferent, they carry sound info to the brain
Outer hair cells are efferent, they carry messages from the brain to cochlea
Movement of fluid in Scala Media
Stapes rocks back and forth, moving fluid in scala media
Causes basilar and tectorial membranes to move in different directions
Causes shearing which triggers an electrochemical reaction that transmits info to auditory neurons attached to the hair cells
Modiolus
A central bony pillar in the cochlea from which auditory nerve fibers course through
Cell bodies of these neurons cluster and form the spiral ganglion which end up forming the cochlear branch of the auditory nerve?
Auditory Nerve
The cochlear branch joins the vestibular branch of this nerve
Coursing through the internal auditory canal
Tonotopic characteristic of the cochlea is also present in the 8th nerve
Central Nervous System
Brainstem, spinal cord, and the brain
Central Auditory System
involves those brainstem and brain structures responsible for directing signals from the peripheral auditory system to the cerebral cortex
Auditory Nuclei
referred to as relay stations
the site of a series of synapses
this neuronal communication allows the central auditory system to make its contribution to auditory perception
Cochlear Nuclei
Beginning of the central auditory system
Bundles of nerves located on the brainstem at the junction of the pons and the medulla
Believed that all fibers from the 8th cranial nerve terminate at the cochlear nuclei
Maintains tonotopic nature
Nerve Fibers either stay ipsilateral or go contralateral after this point
Predominance of…
Contralateral fibers in the auditory system
Stimulation of one ear better represented on the cortex of the opposite side
Importance of Contralateral Fibers
- Redundancy created by multiple overlapping pathways allows for more accurate perception of auditory information
- Ipsilateral fibers dominant peripheral auditory system
- More complex auditory tasks are processed using more central pathways
Central Auditory Pathways
Cochlear Nucleus Superior Olivary Complex Lateral Lemniscus Inferior Colliculus Medial Geniculate Body Internal Capsule Auditory Cortex Corpus Callosum
Superior Olivary Complex
Processes time and intensity cues, contribute to localization abilities
First point where a signal delivered to one ear is represented on both sides of the central auditory system
First instance of binaural representation of a monaurallay presented signal
TONOTOPICITY IS MAINTAINED
Lateral Lemniscus
Considered by some to be the primary brainstem auditory pathway
Receives nerve fibers from the cochlear nuclei and the ipsilateral and contralateral portions of the superior olivary complex
TONOTOPICITY IS MAINTAINED
Redundancy contributes to our ability to perceive speech that is presented in poor conditions
Inferior Colliculus
Located in the midbrain
Largest of the auditory structures of the brainstem ad contains neurons that are particularly sensitive to binaural stimulation.
TONOTOPICITY
Receives input from most of the fibers of the lateral lemniscus and lower auditory areas
Medial Geniculate Body
Last subcortical auditory relay station
Located in the thalamus
Receives most of its fibers from the ipsilateral inferior colliculus and some from the lateral lemniscus
Contains neurons that are sensitive to binaural stimulation
TONOTOPIC
Reticular Formation
A diffusely organized area comprised of nuclei and tracts that form the central portion of the brainstem
Interacts with the auditory system through its connections to the spinal cord and cerebrum
Prepares the cortex to respond to incoming auditory information and may have a role in selective attention
Cerebral Cortex
Final auditory area in the process of auditory perception
Auditory reception are located in the temporal lobes of Heschl’s Gyrus
This area is capable of decoding info about frequency, intensity, and time
