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The organ of hearing, the , has outer (external), middle, and inner (internal) sections.

The ear has two sensory functions: hearing and balance (equilibrium).

Vestibular Nerve Pinna

Cochlear Nerve Tympanic Membrane Auditory Canal

Round Auditory Window Tube

OUTER EAR

Gathers sound waves Consists of the pinna (external flap) and the auditory canal Opening of auditory canal is lined with fine hairs and glands OUTER EAR Collects sounds waves External Auditory Meatus Lined with ceruminous glands Carries sound to tympanic membrane Terminates with tympanic membrane Tympanic Membrane Vibrates in response to sound waves

MIddle EAR

Amplifies sound waves Begins at tympanic membrane () Ends at a bony wall containing two small openings covered by membranes called and Three small bones lie between the tympanic membrane and oval window called , individually they are malleus (hammer), incus (anvil), and stapes (stirrup) MIddle EAR Air-filled space in temporal bone Middle Ear Incus AUDITORY OSSICLES Malleus Stapes Vibrate in response to tympanic membrane Malleus, incus and stapes Hammer, anvil and stirrup OVAL WINDOW Auditory Opening in wall of tympanic cavity Tube Stapes vibrates against it to move fluids in inner ear MIddle EAR

Middle Ear Auditory Tube Incus Malleus Stapes Also known as the Connects middle ear to throat Helps maintain equal pressure on both sides of tympanic membrane Auditory Usually closed by valvelike flaps in Tube throat

INNER EAR Filled with fluid Semicircular canals and also the vestibule are both concerned with balance The cochlea is concerned with hearing INNER EAR Complex system of labyrinths • Osseous Labyrinth Bony canal in temporal bone Filled with • Tube within osseous labyrinth Filled with INNER EAR Three (3) parts of labyrinths: • Cochlea Functions in hearing • Semicircular canals Functions in equilibrium • Vestibule Functions in equilibrium

COCHLEA • Scala Vestibuli Upper compartment Leads from oval window to apex of spiral, Part of • Scala Tympani Lower compartment Extends from apex of the cochlea to round window, Part of bony labyrinth COCHLEA • Portion of membranous labyrinth in cochlea • Separates cochlear duct from scala vestibuli • Separates cochlear duct from scala tympani

Organ of Corti

• Group of hearing receptor cells (hair cells) • On upper surface of basilar membrane • Different frequencies of vibration move different parts of basilar membrane • Particular sound frequencies cause hairs of receptor cells to bend • Nerve impulse generated MECHANISM OF HEARING Sound waves enter the external acoustic meatus. ↓ Waves of changing pressures cause tympanic membrane to reproduce vibrations coming from sound wave source. ↓ Auditory ossicles amplify and transmit vibrations to the end of stapes. ↓ Movement of stapes at oval window transmits vibrations to perilymph in scala vestibuli. ↓ Vibrations pass through vestibular membrane and enter endolymph of cochlear duct. ↓ Different frequencies of vibration in endolymph move specific regions of basilar membrane, stimulating specific sets of receptor cells. ↓ MECHANISM OF HEARING ↓ A receptor cell depolarizes; its membrane becomes more permeable to calcium ions. ↓ In presence of calcium ions, vesicles at base of receptor cell release neurotransmitter. ↓ Neurotransmitter stimulates ends of nearby sensory neurons. ↓ Sensory impulses are triggered on fibers of the cochlear branch of the vestibulocochlear nerve. ↓ The auditory cortex of the temporal lobe interprets the sensory impulses.

SENSE OF TASTE

In mammals, taste receptors are a type of chemoreceptor located in taste buds There are at least four primary types of taste Sweet, sour, salty, and bitter A fifth taste, called umami, may exist for certain flavors of cheese, beef broth, and some seafood SENSE OF TASTE

Taste buds for each of these tastes are located throughout the tongue Certain regions may be most sensitive to particular tastes Tip of the tongue is most sensitive to sweet tastes Margins to salty and sour tastes Rear of the tongue to bitter tastes SENSE OF TASTE

Sweet → Stimulated by carbohydrates Sour →Stimulated by acids Salty → Stimulated by salts Bitter → Stimulated by many organic compounds

TASTE BUDS

• Organs of taste • Located on papillae of tongue, roof of mouth, linings of cheeks and walls of pharynx Taste Receptors

• Chemoreceptors • Taste cells → Modified epithelial cells that function as receptors • Taste hairs → Microvilli that protrude from taste cells; sensitive parts of taste cells TASTE GUSTATION

Sugar molecules enter the taste bud ↓ Sugar molecules bind to sweet receptors, specific protein molecules embedded in a taste receptor cell membrane ↓ The binding triggers a signal transduction pathway that causes some ion channels in the membrane to close and others to open. ↓ These changes in the flow of ions create a graded change in membrane potential called a receptor potential.

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