Physiology of Special Senses: Hearing & Balance Flashcards
The ear is divided into three major areas:
❑ External ear
❑ Middle ear
❑ Internal ear
- The external and middle ear structures are involved with hearing only and are rather simply engineered.
- The internal ear functions in both equilibrium and hearing and is extremely complex.
The external (outer) ear consists of:
- the auricle,
- the external acoustic meatus,
- eardrum.
Describe the auricle (pinna)
- Composed of elastic cartilage
- Has a Thick helix
- Dangling lobule
Describe the external acoustic meatus
- Extends from the auricle to the eardrum
- Lined with skin bearing hairs, sebaceous glands, and cereminious glans (secrete cerumen)
Describe the tympanic membrane
- Connective tissue membrane
- covered by skin and mucosa
- sound waves make it vibrate which turns sound energy to the bones of the middle ear
Describe the middle ear (tympanic cavity)
- Flanked laterally by the tympanic membrane
- Has an oval and round windown
- Has an auditory tube (normally flattened and closed but swallowing or yawing opens it - important bc vibration only occurs when pressure is the same in both ears)
What are the Three bones in the middle ear
- malleus
- incus
- stapes
What opens the pharyngotympanic tube. Why?
swallowing or yawning opens it briefly to equalize pressure in the middle ear cavity with external air pressure
What transmit the vibratory motion of the eardrum to the oval window
The ossicle
What are the muscles associated with the ossicle? Describe them
- The tensor tympani arises from the wall of the pharyngotympanic tube and inserts on the malleus.
- The stapedius runs from the posterior wall of the middle ear to the stapes.
What happens to the tensor tympani and stapedius we hear loud sounds?
the tensor tympani and stapedius contract reflexively to limit the ossicles’ vibration and minimize damage to the hearing receptors.
The internal ear has two major divisions:
- The bony labyrinth is a system of channels worming through the bone. It’s filled with perilymph
- The membranous labyrinth is a continuous series of membranous sacs and ducts contained within the bony labyrinth following its contours. It contains endolymph, which is chemically similar to K+ rich intracellular fluid.
The bony labyrinth has three parts
Vestibule, Semicircular canals, and Cochlea
Explain human hearing:
- Sounds set up vibrations in air, which Beat against the eardrum
- It pushes a chain of tiny bones and Press fluid in the internal ear against membranes
- Set up shearing forces that pull on the tiny hair cells
- Stimulate nearby neurons and Give rise to action potentials
- Travel to the brain, interprets them, and you hear
The route of sound to the inner ear follows which pathway:
- Outer ear – pinna, auditory canal, eardrum
- Middle ear – malleus, incus, and stapes to the oval window
- Inner ear – scalae vestibuli and tympani to the cochlear duct - Stimulation of the organ of Corti and Generation of impulses in the cochlear nerve
What is Step 1 of the pathway of sound waves?
Tympanic membrane.
- Sound waves entering the external acoustic meatus strike the tympanic membrane and set it vibrating at the same frequency.
- The greater the intensity, the farther the membrane is displaced in its vibratory motion.
What is Step 2 of the pathway of sound waves?
Auditory ossicles.
- The motion of the tympanic membrane is amplified and transferred to the oval window by the ossicle lever system.
** increased pressure overcomes the stiffness and inertia of cochlear fluid and sets it into wave motion.
What is Step 3 of the pathway of sound waves?
Scala vestibuli.
- As the stapes rocks back and forth against the oval window, it sets the perilymph in the scala vestibuli into a similar motion.
What is Step 4a of the pathway of sound waves?
low frequency sounds
Helicotrema path.
- Sounds of very low frequency (below 20 Hz) create pressure waves that take the complete route through the cochlea, up the scala vestibuli, around the helicotrema, and back toward the round window through the scala tympani.
- These low- frequency sounds do not activate the spiral organ and so are below the range of hearing.
What is Step 4b of the pathway of sound waves?
high frequency sounds
Basilar membrane path.
- pressure waves are transmitted through the cochlear duct, into the perilymph of the scala tympani and vibrates the basilar membrane.
- This vibration activates hair cells through the basilar membrane, causing action potentials to be sent to the brain.
Sound waves make what vibrate.
the basilar membrane
Describe inner hair cells
- sensory cells
- nerves extending from them send acoustic information to the brain
Describe outer hair cells
- motile
- amplify and modify the movement of the basilar membrane.
- improve hearing sensitivity
Describe stereocilia
- protrude into the K+ endolymph
- the longest are enmeshed in the overlying stiff gel-like tectorial membrane.
What happens when there is Bending cilia of inner hair cells
- Opens mechanically gated ion channels, Ca and K of inner hair cells
- Causes a graded potential and the release of a neurotransmitter (probably glutamate)
Outer hair cell motility serves two functions:
- Increases the responsiveness of the inner hair cells by amplifying the motion of the basilar membrane (to increases the cochlea’s ability to distinguish small differences in frequency)
- help protect the inner hair cells from damage.
Describe the auditory pathway
- AP in the cochlear receptors pass through the spiral ganglion & the afferent fibers of the cochlear nerve to the cochlear nuclei of the medulla
- neurons project to the superior olivary nucleus.
- axons ascend in the lateral lemniscus to the inferior colliculus (midbrain), which projects to the medial geniculate nucleus of the thalamus
- Axons of the thalamic neurons then project to the primary auditory cortex
The equilibrium receptors of the vestibular apparatus can be divided into two functional arms:
- The receptors in the vestibule monitor linear acceleration and the position of the head with respect to gravity. Because gravity is constant, called static equilibrium.
- The semicircular canals monitor changes in head rotation, called dynamic equilibrium.
Describe the macula
- Sensory receptor organ
- Monitor the position of the head in space
- Play a key role in controlling posture
- Respond to changes in straight-line speed and direction, but not rotation
Describe the utricle
- the macula is horizontal, and the hairs are vertically oriented when the head is upright.
- the utricular maculae respond to horizontal movements and tilting the head, because
Describe the saccule
- the macula is nearly vertical, and the hairs protrude horizontally into the otolith membrane.
- The saccular maculae respond best to vertical movements, such as the sudden acceleration of an elevator.
What happens to hairs cells when they bend toward the kinocilium or away from the kinocilium
-
Toward - the hair cells depolarize, stepping up their pace of NT release, and more impulses travel up the vestibular nerve to the
brain. - Away - the receptors hyperpolarize and release less NT, generating fewer impulses.
Where are The cell bodies of the sensory neurons located
superior and inferior vestibular ganglia
The receptor for rotational acceleration, called the …
crista ampullaris
Each crista ampullaris is composed of:
- Supporting cells
- Hair cells whose structure and function are the same as the hair cells of the cochlea and maculae.
- Ampullary cupula, the gelled mass which resembles a pointed cap.
Describe The Equilibrium Pathway to the Brain
- The transmission sequence begins when the hair cells in the vestibular apparatus are activated.
- Impulses travel initially to one of two destinations: the vestibular nuclei in the brain stem or the cerebellum.
- the information is integrated and then send commands to brain stem motor centers that control the extrinsic eye muscles and reflex movements of the neck, limb, and trunk muscles
*
What is the vestibular nuclei
- major integrative center for balance,
- receive inputs from the visual and somatic receptors, particularly from proprioceptors in neck muscles that report on the position of the head.
