Ear Boom

Tensor Tympani and Stapedius

The tensor tympani stretches from the Eustachian tube (see below) to the malleus, whereas the stapedius runs from the posterior wall of the middle ear to the stapes. *When the ears are exposed to very loud sounds, these muscles contract reflexively to limit the vibration of the ossicles and prevent damage to our hearing receptors.

Eustachian Tube

c. The Eustachian (auditory) tube; also known as the pharyngotympanic tube i. extends about 1.5 inches; links the middle ear and pharynx -The tube is normally flattened and closed; however, it can be opened briefly by swallowing or yawning to allow the air pressure in the middle ear to equalize with the outside air pressure. *This is important because the eardrum will not vibrate freely unless the pressure on both of its surfaces is the same. 1. Differences in air pressure build up across the eardrum during rapid changes in altitude (such as in an airplane).

Cochlea

i. The cochlea ("snail shell") is a spiraling chamber within the bony labyrinth. -about half the size of a split pea -From its attachment to the vestibule at its base, the cochlea coils for about two and a half turns around a pillar of bone called the modiolus. *Running through the center of the modiolus is the cochlear nerve. -The coiled part of the membranous labyrinth within the cochlea (the cochlear duct) contains the receptors for hearing. *The endolymph-filled cochlear duct (or scala media) lies between two perilymph-filled chambers. 1. scala vestibuli 2. scala tympani

Semicircular Canals

i. The semicircular canals house mechanoreceptors for rotational acceleration of the head. -The three semicircular canals are arranged so that there is one in each dimension of space. -Each canal spans about 2/3 of a complete circle and has a bulge at one end called an ampulla. *Each ampulla contains a crista ampullaris.

External Acoustic Meatus

(meatus = a canal-like passageway); also known as the external auditory canal i. A short (about 2.5 cm) tube running from the auricle to the eardrum -The canal is lined with hairs as well as sebaceous and ceruminous glands. *These glands secrete cerumen (earwax). 1. Earwax traps dust and repels insects, keeping them out of the auditory canal. ii. Sound waves entering the external acoustic meatus hit a thin, translucent membrane called the tympanic membrane (eardrum), which forms the boundary between the outer and middle ears. - shaped like a flattened cone, the apex of which points toward the middle ear cavity - Sound waves traveling through the air cause the eardrum to vibrate. *The eardrum in turns transfers these vibrations to tiny bones in the middle ear.

Scala Vestibuli and tympani

*The scala vestibuli is continuous with the vestibule at the base of the cochlea, where it touches the oval window. *The scala tympani ends at the membrane covering the round window. 1. The scala vestibuli and scala tympani are continuous with one another at the apex of the cochlea in a region called the helicotrema ("hole in the spiral").

Bony Labyrinth

-A cavity in the temporal bone consisting of twisting channels *Three parts 1. semicircular canals 2. vestibule 3. cochlea Filled with Perilymph ("surrounding water")

Dizziness

-If you spin around, the fluid in your semicircular canals will lag behind, causing the cupula to bend in the direction opposite your spin, and bending the stereocilia, which causes the hair cells to send messages to your brain. -Over time, the fluid in the semicircular canals catches up to the movement of the body; because of friction caused by the surrounding walls, the cupula straightens out, and the hair cells no longer send messages to the brain. -When you stop spinning, the walls of the semicircular canals also stop because they are attached to your body, but the fluid continues to move forward in the direction of your original spin causing the cupula to bend forward. -Because the fluid is moving forward in the direction of the spin and again displacing the cupula, the stereocilia bend again, indicating that we are moving. *However, your eyes know you have stopped. 1. The mixed messages sent to the brain cause you to feel dizzy.

Structure of Cochlear duct

-The epithelium covering the roof of the cochlear duct secretes the endolymph. -The floor of the cochlear duct is called the basilar membrane. *This membrane supports the organ of Corti, which is where the receptor cells for hearing are found. 1. Hair cells line the epithelium in the organ of Corti, with their stereocilia embedded in a gel-like tectorial membrane. -At their base, the hair cells synapse with sensory fibers of the cochlear nerve.

Membranous Labyrinth

-a continuous series of sacs and ducts that fit loosely within the bony labyrinth and more or less follow its contours *Main parts 1. semicircular ducts (one inside each semicircular canal) 2. the utricule and saccule (both in the vestibule) 3. the cochlear duct (in the cochlea) *The walls of the membranous labyrinth are composed of a thin layer of connective tissue lined by a simple squamous epithelium. Filled with endolymph ("internal water")

Equilibrium Pathway Part 1

-transmits information related to the position and movement of the head via the vestibular nerve to the brain stem *Equilibrium is the only special sense for which most information goes to the lower brain centers, which are primarily reflex centers, rather than to the "thinking" cerebral cortex. 1. reflects the fact that the response to a loss of balance (such as stumbling) must be rapid and reflexive 2. Nuclei in the medulla and cerebellum process most of the information related to equilibrium. -A minor pathway to the cerebral cortex provides conscious awareness of the position and movement of the head. In this pathway, the information travels from the above nuclei to the thalamus and then to the insula.

Macula

1. Receptor cells (known as hair cells) within each macula monitor the position of the head when the head is held still. 2. Thus, the utricle and saccule contribute to static equilibrium.

Crista Ampullaris

1. The cristae contain hair cells that measure rotational acceleration of the head, such as when a figure skater spins or a gymnast does a flip. *The stereocilia of the hair cells in each crista project upward into the cupula, a tall jelly-like mass that resembles a pointed cap.

Equilibrium Pathway Part 2

3. Appropriate motor impulses are then sent to various skeletal muscles to adjust our present position as needed. -Continuous stimulation of the stereocilia (e.g., the prolonged use of amusement park rides) can contribute to motion sickness, especially when messages reaching the brain conflict with visual information from the eyes. - The information sent by the saccule and utricle to the brain about the position of the head in space is particularly important when the eyes are closed, or if we are in the dark or under water.

Pinna or auricle

= what most people call the ear i. Most of the auricle consists of cartilage covered with skin. -The fleshy lobule (earlobe) lacks supporting cartilage. ii. Functions to gather and funnel (thereby amplifying) incoming sounds waves. -The way that sound waves bounce off the ridges and cavities of the auricle provides the brain with clues regarding whether the sounds are coming from above or below.

Mechanical energy to nerve impulses part 1

Stimulation of hair cells in the organ of Corti by sound waves -Sound vibrations travel from the eardrum through the ossicles, causing the stapes to oscillate back and forth against the oval window. -This oscillation sets up pressure waves in the perilymph of the scala vestibuli and in the endolymph of the cochlear duct. *These waves cause the basilar membrane to vibrate up and down. -The hair cells in the organ of Corti move along with the basilar membrane; however, the overlying tectorial membrane does not move. *Thus, movement of the hair cells causes their hairs to bend. 1. Bending causes the release of neurotransmitters from the hair cells that excite the cochlear nerve fibers, which carry the information to the brain.

Intro to Ear

The ear is the receptor organ for both hearing and equilibrium. a. The receptor cells within the ear (hair cells) are mechanoreceptors. i. Bending of the plasma membrane of these cells initiates action potentials that are sent to the CNS. 2. The ear has three main regions: the outer ear, middle ear, and inner ear. a. The outer and middle ears participate in hearing only. b. The inner ear functions in both hearing and equilibrium

Cupula

a tall jelly-like mass that resembles a pointed cap. 1. The basal parts of the hair cells synapse with fibers of the vestibular nerve. -Unlike the utricle and saccule, there are no otolithic crystals in the cupula. 2. Bending of the stereocilia as the endolymph within one of the semicircular canals flows over and displaces a cupula depolarizes the hair cells, thereby changing the pattern of impulses carried by the vestibular nerve to the brain (equilibrium pathway). 3. The brain uses information from the hair cells in the ampullae of the semicircular canals to maintain equilibrium through appropriate motor output to various skeletal muscles that can modify our position as needed.

Inner Ear

a. Lies within the temporal bone of the skull; consists of two main divisions i. the bony labyrinth ii. membranous labyrinth 1. The receptor cells for hearing and equilibrium are embedded in this epithelium at certain locations. iii. The labyrinths of the inner ear are filled with different fluids. -membranous labyrinth = endolymph ("internal water") -bony labyrinth = perilymph ("surrounding water")

Middle Ear

a. The middle ear is a small, air-filled space inside the temporal bone of the skull. It is shaped like a hockey puck standing on its side. i. It is bounded at one end by the tympanic membrane and at the other end by a wall of bone. -Two small holes penetrate this bony wall: the oval window and round window. b. The three smallest bones in the body (the auditory ossicles) are iii. Two small muscles are located in the middle ear.

The Vestible

i. an egg-shaped cavity in the bony labyrinth -Suspended in the perilymph of the vestibule are the utricle, which is continuous with the semicircular ducts, and the saccule, which is continuous with the cochlear duct. *The utricle and saccule each contain a spot of sensory epithelium called a macula. -The receptor cells in the utricle and saccule also monitor straight-line changes in the speed and direction of head movements (linear acceleration). *Long, finger-like projections jut out of the top the hair cells within the utricle and saccule. 1. multiple stereocilia (long microvilli) and a single kinocilium *The tips of these stiff "hairs" are embedded in a jelly-like disc called the otolithic membrane that rests immediately overhead. 1. This membrane contains heavy crystals of calcium carbonate called otoliths ("ear stones"). *The axons of the hair cells pass into the vestibular nerve as part of the sensory input for the maintenance of equilibrium.

Nerve Deafness

i. caused by damage to the hair cells or any part of the auditory pathway to the brain -Ex. a single, explosively loud noise or repeated exposure to loud music, factory noise, or airport noise -A stroke or tumor that damages the auditory cortex can also cause this type of deafness. -A normal, gradual loss of hair cells also occurs. *You begin with about 20,000 hair cells in each ear. ii. In cases of complete deafness, cochlear implants are available. -They are placed in the temporal bone. *convert sound energy into electrical impulses that are send directly to the cochlear nerve 1. They can have up to about 2 dozen electrodes, each responding to a different frequency. 2. Such devices can enable children who were born deaf to hear well enough to learn to speak.

Conduction Deafness

i. occurs when sound vibrations cannot be conducted to the inner ear -can be caused by earwax blocking the external acoustic meatus, a ruptured eardrum, or otitis media (infection of the external acoustic meatus due to bacteria or fungi; can cause the auditory ossicles to fuse) -also, congenital deafness (genetic defect)

Movement of the head

ii. When the body is still, the otoliths in the utricle and the saccule rest on the otolithic membrane above the hair cells. -When the head bends or the body moves in the horizontal and vertical planes, the otoliths are displaced and the otolithic membrane sags, bending the stereocilia of the hair cells beneath. *If the stereocilia move towards the kinocilium, nerve impulses increase in the vestibular nerve. *If the stereocilia move away from the kinocilium, nerve impulses decrease in the vestibular nerve. - The macula of the utricle lies in a horizontal position within the ear, whereas the macula of the saccule has a vertical orientation within the ear. *Thus, the utricle is particularly sensitive to horizontal (back & forth) movements and bending of the head, while the saccule responds best to vertical (up & down) movements.

Mechanical energy to nerve impulses part 2

iv. The cochlea responds to incoming sound waves based on their frequency. -In response to low-frequency sounds, the basilar membrane near the apex of the cochlea will vibrate. -In response to high-frequency sounds, the basilar membrane near the base of the cochlea will vibrate. v. The vibrations of the basilar membrane cause the perilymph in the underlying scala tympani to vibrate. -These vibrations travel to the round window, where they push on the membrane covering that window. *In this way, the remaining energy is dissipated into the air of the middle ear. 1. Without this release, echoes would reverberate within the cochlea, disrupting our ability to perceive sound.

Auditory Ossicles

located between the tympanic membrane and oval window. i. Together, they transmit vibrations from the eardrum to a fluid in the inner ear. -malleus (hammer), incus (anvil), and stapes (stirrup) -The "handle" of the malleus is attached to the tympanic membrane, while the base of the stapes vibrates against the oval window. ii. Tiny ligaments hold the ossicles in place within the middle ear, and tiny joints link the ossicles into a chain. -The tympanic membrane is much larger than the oval window; as a result, the vibrations that are passed through the ossicles are concentrated. *The degree of amplification is about 20-fold. 1. Without our ossicles, we would only be able to hear loud sounds.

Outer Ear

pinna or auricle b. External acoustic meatus

Mechanical energy to nerve impulses part 3

vi. The nerve fibers from each region along the length of the organ of Corti lead to slightly different areas in the auditory cortex. -The pitch depends upon which region of the basilar membrane vibrates and which area of the auditory cortex is stimulated (higher frequency = higher pitch). vii. Volume = amplitude of the sound waves -Loud noises cause the fluid within the vestibular canal to exert more pressure and the basilar membrane to vibrate to a greater extent, thus bending the stereocilia more. *The resulting increased stimulation is interpreted by the brain as volume. viii. The auditory pathway -Nerve impulses pass through the cochlear nerve to nuclei in the medulla. *From there, the information is sent to an auditory center in the midbrain then to the thalamus. *Axons of the thalamic neurons then project to the primary auditory cortex in the temporal lobe, which provides conscious awareness of sound.