SQ #2-19

Hearing loss due to prolonged exposure to loud noise is considered______________

sensory or nerve loss.

Compare and contrast sensorineural hearing loss with conductive hearing loss including A) the regions of the ear where the problem lies, and B) the type of damage that may occur in each.

Both sensorineural and conductive hearing loss can be due to conditions within the ear, however the exact region that is affected differs, but both types of hearing loss prevent sound waves from being interpreted by the brain as sound. Conductive hearing affects the outer and/or middle regions whereas sensorineural hearing loss occurs due to problems within the inner ear, or nerve or brain damage. Sensorineural and conductive hearing loss are both conditions that affect the quality of the auditory function. Conductive loss is associated with problems that prevent the transmission of sound waves into the inner ear structures. This could be due to a variety of reasons including wax build-up, middle ear bone arthritis, or the formation of scar tissue due to an ear drum injury. This can result in the sound being quieter but can often be corrected with medical attention. Sensorineural loss typically occurs when nerve impulses are not properly transmitted to the brain. This can be due to nerve damage, brain tumors or injury, genetic or birth problems, or damage to the hair cells in the Organ of Corti due to exposure to prolonged sounds. In the latter cause, hair cells lose their extensions, stereocilia, that stimulate the nerve impulse via pressure against the tectorial membrane. Similar to conductive hearing, sensorineural hearing loss can affect the ability to hear quiet sounds but also can affect the quality of the sounds. These sensorineural hearing problems many times are irreversible.

T/F: The eustachian (auditory) tube connects the inner ear directly to the nasopharynx.

False-it connects the middle ear to the nasopharynx.

Describe the location and role of the Eustachian tube.

The Eustachian tube, or auditory tube, is part of the middle ear which links the middle ear to the nasopharynx. This structure's function is to equalize air pressure in the middle ear and the outside environment. Internal air pressure is equalized because of this tube, which aids in the transmission of sound by allowing the tympanic membrane to vibrate appropriately. Normally in a closed state, this tube can be opened when yawning or swallowing, which allows the passage of air in or out of the tube. The Eustachian tube is utilized many times during sudden increases in elevation, such as when flying, and causes the ears to 'pop' when plugging the nose and blowing, which restores the pressure and muffled sound back to normalcy. This connection between the middle ear and nasopharynx also explains why sinus problems or the common cold can lead to ear infections.

The gross (visible to the naked eye) structures of the outer, middle and inner ear include: A) pinna (auricle), B) external acoustic canal, C) tympanic membrane, D) malleus, E) incus, F) stapes, G) oval window, H) round window, I) cochlea, J) cochlear nerve, K) semicircular canals, L) vestibule M) vestibular nerve. List each of the structures as part of the outer, middle, or inner ear. For each of the inner ear structures, state whether it is involved in the sense of hearing or in the sense of balance.

The outer ear structures include the pinna or auricle and the external acoustic meatus, which are both involved in the process of hearing. The middle ear structures include the tympanic membrane, malleus, incus, stapes and oval window which also assist in the hearing function. The round window, cochlea, and cochlear nerve are inner ear structures that help with hearing while the semicircular canals, vestibule, and vestibular nerve are inner ear structures that have roles in the process of balancing; all of which are explained in more detail below.

Describe in an orderly fashion how sound waves are captured by the auditory apparatus and transformed into nerve impulses. Your description should include the roles of the structures A- J above, plus the functions of these specific regions of the cochlea: A) vestibular canal, B) tympanic canal, C) cochlear duct, D) sensory hair cells, and E) tectorial membrane.

The process of auditory transduction begins in the outer ear with air as the medium. The pinna, or auricle, is a concave structure that captures the soundwaves and directs it into the external auditory meatus. The soundwaves travel down this acoustic canal where it resonates to the tympanic membrane, which is also known as the ear drum. In addition to providing a passageway for soundwaves, the canal also consists of glands which produce ear wax that assist in the preservation of the elasticity of the tympanic membrane, which is the start of the middle ear. The tympanic membrane relays the alternating compressions and decompressions into vibrations of specific patterns, transforming the medium into a mechanical or solid mechanism. High frequency sound waves generate fast vibrations where low frequency waves generate slower vibrations. These patterns are then transmitted into the ossicular chain which includes the malleus (hammer), incus (anvil) and stapes (stir-up), which also provide a protective function along with the muscles associated with these bones by muffling the vibrations from the tympanic membrane. The is known as the tympanic reflex. The tympanic membrane also provides some protection back to these small bones within the ear as foreign matter is not able to pass through it easily. The vibrations within the ossicular chain travel from the malleus to the anvil and then to the stapes which causes the footplate of the stapes to push back and forth against the oval window. The oval window is a flexible membrane that then creates waves in the cochlear fluid located in the perilymph of the inner ear, which is now a liquid or fluid medium, so the vibrations need to be strong to continue. This happens because the tympanic membrane is much larger than the oval window, so there is a greater force per unit area to overcome this change in medium. These waves within the perilymph travel to the vestibular canal/scala vestibuli in the cochlea, where they exert pressure on the vestibular membrane pressure onto the vestibular membrane, which then increases the pressure within the vestibular canal of the cochlear duct in the fluid called endolymph. This pressure then causes further pressure on the basilar membrane of the tympanic canal/scala tympani, which causes movement, or pressure, and forces the round window, also known as the secondary tympanic membrane, to bulge outwards, reversing the pressure. This reverse pressure causes the sensory hair cells within the Organ of Corti to stimulate impulses along the hair cells that are pushed closer to the stationary tectorial membrane, which opens the hair cell's transmembrane protein channels, allowing the inflow of K ions. The signal is generated in the fibers at the base of the hair cells, the axons of which collectively constitute the cochlear nerve. The pressure of the basilar membrane pushing the hair cells against the stationary tectorial membrane causes a stimulation of the channels and an inflow of K+ ions. The strong electrochemical gradient between the endolymph (+80mv) and the hair cell cytoplasm (-40mv) that creates potential energy which enables the hair cell to do its work. Depending on the frequency of the soundwave, different hair cells will be stimulated. The sounds we hear are directly related to which hair cells are stimulated along the tectorial membrane at that given time. Each hair cell contains a transmembrane protein at the tip which provides the stimulation to open its mechanically-gated channels. These hair cells then bend, causing the tip links between other hair cells and bend the stereocilia and open their ion channels. Because of the strong electrochemical gradient, potassium flows into each stimulated hair cell..., which causes it to be depolarized. The hair cells then release action potential neurotransmitters (now a neural medium) that excites the sensory dendrite of the postsynaptic cell and generates a signal in the cochlear nerve, which then sends the signal to the cerebral cortex of the brain, where they are interpreted as distinct sounds. When the basilar membrane returns to the lower starting position, the tip link is straightened and hair cell ion channels close, causing the hair cell to briefly become hyperpolarized.

Contrast the functions of the vestibule and the semicircular canals with respect to their roles in sensing position and balance.

The semicircular canals sense only angular acceleration, or rotational movement, of the head. The vestibule senses both the stationary position of the head and linear acceleration. The vestibule gathers information about the position of the head relative to the rest of the body and sends that information to the brain so the brain is able to coordinate what the head should be doing. The vestibule also detects changes in the velocity of the movement in linear acceleration. The vestibular nerve cells gather this information when they are bent by the gelatinous otolithic membrane that envelops the cells stereocilia and cilium. This bending indicates the head's position or change in linear velocityEach one of the three semicircular canals occupies different positions and senses different types of rotational movements, or change in the rate of rotational motion. Depending on which position the head is in, one of the three semicircular ducts will be stimulated to detect the motion. There are gelatinous caps, called cupula, that are located at each ampulla, where the semicircular ducts open into the utricle. These caps envelop the stereocilia and cilium of the sensory hair cells that are stimulated when the cupula bends due to the turning of the head. This stimulation will stop if it continues for longer periods. The vestibule is divided into two chambers called the utricle and saccule. The gelatinous otolithic membrane within these chambers also envelops the stereocilia hair structures that are stimulated as the otolithic membrane bends when the head is in motion. The brain combines this information with the sensory information from the eyes to determine the complete picture. Because of inertia, when the head first starts moving or first stops, the otolithic membrane of the utricle has a slight delay compared to the rest of the body's tissues and stimulates the stereocilia and cilium of the hair cells during linear acceleration. This pattern helps the brain distinguish the position of the head and its linear movement. This is similar to the delay of the otolithic membrane in the saccule when detecting vertical acceleration.

T/F: The vestibular and tympanic canals actually connect to form one continuous channel.

True