THE EARS AND HEARING

Equilibrium and hearing are provided by a receptor complex called the inner ear - the receptors are called HAIR CELLS - the complex structure of the inner ear and arrangements of accessory structures account for abilities of hair cells to respond to different stimuli and to provide the input for 2 senses:

1. EQUILIBRIUM- informs us of the position of the body in space by monitoring gravity, linear acceleration, and rotation 2. HEARING- enables us to detect and interpret sound waves

ANATOMY OF THE EAR

The ear is organized into 3 parts: 1. External ear- visible portion of the ear, collects and directs sound waves toward the middle ear 2. Middle ear- collects and amplifies sound waves and transmits them to inner ear 3. Inner ear- contains the sensory organs for hearing and equilibrium

EXTERNAL EAR The external ear extends beyond the lateral surface of the head PARTS: 1. Auricle/Pinna- the visible part of the ear that has a flexible frame made of cartilage and covered with skin - this is the part of the ear that you can see! 2. Auditory Canal- opening in the center of the pinna

- the canal directs sound waves to the middle ear 3. Tympanic membrane (tympanum)- closes the inner end of the auditory canal - also called eardrum - because it is S shaped, you cannot see the tympanic membrane without a special instrument

4. Ceruminous Glands- the outer portion of the auditory canal has these wax-producing glands - EARWAX- helps prevent foreign objects and insects from getting into the ear - earwax also slows the growth of microorganisms in the ear canal, and reduces the chances of infection

MIDDLE EAR - Also called the TYMPANIC CAVITY - small air-filled chamber located within the spongy portion of the temporal bone - separated from auditory canal by the tympanic membrane, but communicates with upper portion of pharynx (nasopharynx) - connected to nasopharynx with EUSTACHIAN TUBE (auditory tube)

OSSICLES- 3 small bones located between the tympanic membrane and the inner wall of the middle ear

- Vibrations of sound waves from the tympanic membrane move to these bones 1. Malleus- hammer 2. Incus- anvil 3. Stapes- stirrups - these are responsible for transmitting sound vibrations across the inner ear cavity

The base of the stapes pulses against a membrane covering an opening called the OVAL WINDOW, in the inner wall of the middle ear - OVAL MEMBRANE- membrane covering oval window - the ossicles are completely formed at birth and do not change in size

2 muscles are responsible for controlling the movement of the ossicles: 1. Tensor tympani- when this muscle contracts, the handle of the malleus is pulled inward producing tension on the tympanic membrane (reducing movement) 2. Stapedius- acts in opposition to the tensor tympani, pulls on the stapes

Tension on either side of the tympanic membrane must be equal, or the membrane will not vibrate properly and hearing will be impaired EUSTACHIAN TUBE/ AUDITORY TUBE- equalizes the air pressure between the middle and outer ear - unfortunately can also allow microorganisms to travel from the nasopharynx into the tympanic cavity  INFECTION

Swallowing or yawning causes the inner edges of the tube to open, and air is allowed to enter or leave  ears “pop” - if the entrance to the Eustachian tube is inflamed because of infection, the edges may not open and the tympanic membrane will not move properly  temporary deafness or discomfort occurs

INNER EAR Contains receptors that initiate nerve impulses which the brain interprets as sound - most important part of auditory device - also contains parts concerned with balance - divided into 3 canals called the BONY LABYRINTH

MEMBRANOUS LABYRINTH- fills the Bony Labyrinth - separated from the bony wall by a fluid called PERILYMPH - the membrane itself is filled with another fluid called ENDOLYMPH The Bony Labyrinth is made up of 3 parts: Vestibule, Cochlea, & Semicircular Canals

1. Vestibule- connected to the middle ear by the oval window - acts as an entrance to the semicircular canals and the cochlea Inside the vestibule are 2 membranous sacs filled with endolymph called the UTRICLE and the SACCULE - receptors in these sacs provide sensations of gravity and linear acceleration

2. Cochlea- snail-shaped organ of the inner ear - contains the COCHLEAR DUCT of the membranous labyrinth - receptors in this duct provide the sense of hearing - the duct is located between a pair of perilymph-filled chambers

3. Semicircular Canals - three looped tubes that are 90 degrees with one another - enclose the semicircular ducts- tubular membranes in the semicircular canals - receptors in these ducts are stimulated by rotation of the head - the combination of the vestibule and the semicircular canals is called the VESTIBULAR COMPLEX

The bony labyrinth’s walls are dense bone everywhere except at 2 small areas near the base of the cochlea: Round window Oval window - both of these are covered with membranes that separate perilymph in the cochlea from air in the middle ear

- the membrane of the oval window is firmly attached to the base of the stapes - when a sound vibrates the tympanic membrane, the movements are conducted over the malleus and incus to the stapes - movement of the stapes leads to stimulation of receptors in the cochlear duct, and we hear the sound

RECEPTOR FUNCTION IN INNER EAR Receptors of the inner ear are called HAIR CELLS - each of these hair cells communicates with a sensory neuron by constantly releasing small quantities of neurotransmitter - the free surface of hair cells are covered with about 80-100 finger-like STEREOCILIA

- hair cells don’t actively move these stereocilia; when an external force pushes the stereocilia, their movement distorts the cell surface and alters the rate of neurotransmitter release - displacement of the stereocilia in one direction stimulates the hair cells (increases neurotransmitter release)

- displacement in the opposite direction inhibits hair cells (decreases neurotransmitter release)

EQUILIBRIUM AND HEARING PHYSIOLOGY OF HEARING EQUILIBRIUM AND HEARING

EQUILIBRIUM 2 types of equilibrium: 1. DYNAMIC- aids us in maintaining our balance when the head and body are moved suddenly - receptors are the semicircular ducts: provide info. about rotational movements of the head 2. STATIC- maintains our posture and stability when the body is still

- receptors are the utricle and saccule: provide info - receptors are the utricle and saccule: provide info. about your position with respect to gravity * if you stand with your head to the side, hair cells in these receptors report the angle involved, and whether your head tilts forward or backward * these receptors are also stimulated by sudden changes in velocity

Semicircular ducts- Rotational movement The 3 semicircular ducts: Anterior Posterior Lateral are continuous with the utricle - each duct contains a swollen area called the AMPULLA, which contains the sensory receptors

- hair cells attached to the walls of the ampulla form a raised structure called a CRISTA - the stereocilia of these hair cells are embedded in a gelatin-like substance called the CUPULA - when the head rotates, movement of the endolymph pushes against the cupula and stimulates the hair cells

Each semicircular duct responds to one of three possible rotational movements: - shaking the head “no” - nodding “yes” - tilting the head from side to side

Vestibule- Gravity and linear acceleration Hair cells of the utricle and saccule are clustered in oval MACULAE - as in the ampullae, stereocilia are embedded in a gelatinous material - the macular receptors lie under a thin layer of densely packed calcium carbonate crystals  the complex is called an OTOLITH

- when the head is upright, the crystals sit atop the macula, pushing the stereocilia downward - when the head is tilted, the crystals shift to the side, distorting the stereocilia this tells the CNS that the head is no longer level

Otolith crystals are heavy, so when the rest of the body makes a sudden movement, they lag behind Ex: Elevator - then an elevator starts downward, we know it right away because the crystals are no longer pushing so forcefully against the surfaces of the hair cells

- once the crystals catch up and the elevator reaches constant speed, we no longer feel like we are moving - when the elevator slows, the crystals press harder against the hair cells and we can feel the force of gravity increase

HEARING Receptors of the cochlear duct provide us with a sense of hearing that enables us to detect a quiet whisper, yet remain functional in a noisy room - receptors for auditory sensation are hair cells similar to those in the vestibular complex

In transferring vibrations from the tympanic membrane to the oval window, the ossicles convert sound energy in air to pressure pulses in the perilymph of the cochlea - these pulses stimulate hair cells along the cochlear spiral - FREQUENCY of sound is determined by which part of the cochlear duct is stimulated

- INTENSIY of sound is determined by how many hair cells are stimulated at that part of the cochlear duct

Cochlear Duct If we look at the cochlear duct in cross section, we can see that it lies between a pair of perilymph filled chambers: Vestibular duct Tympanic duct - the outer surfaces of these ducts are encased by the bony labyrinth everywhere except at the oval window

ORGAN OF CORTI The hair cells of the cochlear duct are found in the ORGAN OF CORTI - this structure sits above the BASILAR MEMBRANE, which separates the cochlear duct from the tympanic duct - in the organ of Corti, hair cells are arranged in a series of longitudinal rows with their stereocilia in contact with the overhanging TECTORIAL MEMBRANE

- this membrane is attached to the inner wall of the cochlear duct - when a certain portion of the basilar membrane bounces up and down, the stereocilia of the hair cells are distorted as they are pushed against the tectorial membrane - the basilar membrane moves in response to pressure waves in the perilymph

- these waves are produced when sounds arrive at the tympanic membrane

The Hearing Process Some hearing terms: CYCLES- a term used instead of waves HERTZ (Hz)- number of cycles per second, represents frequency PITCH- our sensory response to frequency; how high or low a sound is INTENSITY- amount of energy or power of a sound; volume - reported in DECIBELS

Ex: Soft whisper = 30 decibels Jet plane = 140 decibels

6 BASIC STEPS OF HEARING: 1. Sound waves arrive at tympanic membrane - waves enter the auditory canal and travel toward the tympanic membrane 2. Movement of the tympanic membrane causes displacement of the ossicles - when the membrane vibrates, so does the malleus, incus, and stapes

3. Movement of the stapes at the oval window establishes pressure waves in the perilymph of the vestibular duct - since the rest of the cochlea is surrounded by bone, pressure can only be relieved at the round window- membrane bulges outward

4. Pressure waves distort the basilar membrane on their way to the round window - the basilar membrane does not have the same structure throughout its length: near the oval window it is narrow and stiff; at the other end it is wider and flexible - the location of maximum stimulation varies with frequency

- high frequency sounds vibrate the membrane near the oval window - low frequency sounds vibrate the membrane further away from the oval window - the LOUDER the sound, the greater the movement of the membrane

5. Vibration of the basilar membrane causes vibration of hair cells against the tectorial membrane - the resulting displacement of the stereocilia stimulates sensory neurons - the number of hair cells stimulated in a given region of the organ of Corti gives information about the intensity of the sound  louder sound, more hair cells stimulated

6. Information about the region and intensity of stimulation is relayed to the CNS over the cochlear branch of the vestibulocochlear nerves - this information is carried to the medulla oblongata for distribution to other centers in the brain

AUDITORY SENSITIVITY We never use the full potential of our auditory system because body movements and our internal organs produce sounds that are tuned out by adaptation - when other environmental noises fade away, the level of adaptation drops and the system becomes more sensitive - if we relax in a quiet room, our heartbeat gets louder as the auditory system adjusts to the level of background noise