Human Ear Anatomy PDF

**Anatomy of the Ear** The ear is divided into three anatomical regions... - External ear - Middle ear - Internal ear **The External Ear** **[Auricle (pinna):]** surrounds and protects external acoustic meatus (EAM). - Provides directional sensitivity. **[Tympanic membrane (eardrum):...

**Anatomy of the Ear** The ear is divided into three anatomical regions... - External ear - Middle ear - Internal ear **The External Ear** **[Auricle (pinna):]** surrounds and protects external acoustic meatus (EAM). - Provides directional sensitivity. **[Tympanic membrane (eardrum):]** thin, semitransparent sheet at the end of EAM that separates the external ear from the middle ear. **[Ceruminous glands:]** integumentary gland along EAM that secrete **[cerumen]** (ear wax). - Helps keep out foreign objects and insects. - Slows growth of microorganisms. **The Middle Ear** **[Middle ear (tympanic cavity) ]** - Air-filled chamber. - Communicates with nasopharynx through auditory tube. - Permits equalization of pressure on either side of tympanic membrane. - Contains 3 tiny ear bones called auditory ossicles. - **[Malleus (hammer)]** - **[Incus (anvil)]** - **[Stapes (stirrup) ]** When sound waves vibrate tympanic membrane, auditory ossicles conduct vibrations to internal ear. **Muscles of the Middle Ear** - Two small muscles protect the ear from very loud noises. - **[Tensor tympani ]** - Pulls on malleus and stiffens tympanic membrane. - **[Stapedius ]** - Reduces movement of stapes at oval window. **The Internal Ear** **[Labyrinth:]** winding passageway. **[Bony labyrinth]** surrounds and protects **[membranous labyrinth]**. - Perilymph flows between the two labyrinths. - Endolymph is within membranous labyrinth. Bony labyrinth can be subdivided into... - Vestibule - Semicircular canals - Cochlea **[Vestibule:]** encloses saccule and utricle. - Receptors detect gravity and linear acceleration. **[Semicircular canals:]** contains three semicircular ducts. - Receptors stimulated by rotation of head. **[Cochlea:]** contains cochlear duct of membranous labyrinth. - Receptors provide sense of hearing. **[Round window:]** thin, membranous partition that separates perilymph from air spaces of middle ear. **[Oval window:]** connected to base of stapes by collagen fibers. **Equilibrium** **[Equilibrium:]** state of physical balance. **The Vestibular Complex and Physiology of Equilibrium** - Sensations provided by receptors of vestibular complex (vestibule and semicircular canals). - **[Hair cells:]** sensory receptors of internal ear. - Provide information about direction and strength of mechanical stimuli. **The Semicircular Ducts: Rotational Movements** **[Anterior, posterior, and lateral semicircular ducts:]** continuous with utricle. - Each duct contains an expanded region (ampulla). - With gelatinous **[ampullary cupula]**. - **[Ampullary crest]** contains hair cells. - Each hair cell in vestibular complex has... - 80-100 stereocilia (resembles long microvilli). - A single large kinocilium. **The Utricle and Saccule: Position and Acceleration** - Hair cells provide sensations of position and linear movement. - Connected with **[endolymphatic duct]**, which ends in **[endolymphatic sac]**. **[Maculae:]** oval structures where hair cells cluster. - **[Macula of utricle:]** sense horizontal movement. - **[Macula of saccule:]** senses vertical movement. **[Otoliths (ear stones):]** densely packed calcium carbonate crystals on surface of gelatinous mass. **Pathways for Equilibrium Sensations** Sensory neurons in **[vestibular ganglia]**. - Monitor hair cells of vestibular complex. - Fibers from ganglia form **[vestibular nerve]** of vestibulocochlear nerve (VIII). - Synapse within **[vestibular nuclei]** at boundary between pons and medulla oblongata. There are four functions of vestibular nuclei... - Integrate sensory information about balance and equilibrium from both sides of head. - Relay information from vestibular complex to cerebellum. - Relay information from vestibular complex to cerebral cortex. - Providing conscious sense of head position. - **Send commands to motor nuclei in brainstem and spinal cord.** **Reflexive Motor Commands from Vestibular Nuclei** - Distribute to motor nuclei for cranial nerves involved with eye, head, and neck movements. - Instructions descending in vestibulospinal tracts of spinal cord. - Adjust peripheral muscle tone. - Complement reflexive movements of head and neck. **Automatic Movements of Eyes** - Directed by superior colliculi of midbrain in response to sensations of motion. - Attempt to keep gaze focused on a specific point. - If spinning rapidly, eyes make jerky movements. - **[Nystagmus:]** trouble controlling eye movements when body is stationary. - Caused by damage to brainstem or internal ear **Hearing** - Sound waves are converted into mechanical movements by vibration of tympanic membrane. - Auditory ossicles conduct vibrations to internal ear. - Vibrations are converted to pressure waves in fluid. - Detected by hair cells in cochlear duct. - Information is sent to auditory cortex of brain. **An Introduction to Sound** **[Pressure wave:]** sine wave (S-shaped curve). - Consists of a region where air molecules are crowded together, - And adjacent zone where they are farther apart. **[Wavelength:]** distance between two adjacent wave crests. **[Frequency:]** number of waves (cycles) that pass a fixed reference point in a given time. - Measured in **[hertz (Hz):]** number of cycles per second. **[Pitch:]** our sensory response to frequency. **[Amplitude:]** height of a sound wave. **[Intensity:]** amount of energy in a sound wave, determines how loud it seems. - Measured in **[decibels]**. **Anatomy of the Cochlear Duct** **[Cochlear duct (scala media)]** lies between **[scala vestibuli (vestibular duct)]** and **[scala tympani (tympanic duct)]**. - Hair cells lie in **[spiral organ (organ of Corti).]** - Rests on **[basilar membrane]**. - Separates cochlear duct from scala tympani. - Hair cells lack kinocilia. - Stereocilia contact overlying **[tectorial membrane]**. **Auditory Discrimination** - Range from softest to loudest tolerable sound represents trillion-fold increase in power. - Young children have greatest hearing range. - With age, damage accumulates... - Tympanic membrane gets less flexible. - Articulations between ossicles stiffen. - Round window may begin to ossify. **The Physiology of Hearing** There are six basic steps in process of hearing... - Sound waves arrive at tympanic membrane. - Movement of tympanic membrane displaces auditory ossicles. - Movement of stapes at oval window produces pressure waves in perilymph in perilymph of scala vestibuli. - Pressure waves distort basilar membrane on their way to round window of scala tympani. - Vibration of basilar membrane causes hair cells to vibrate against tectorial membrane. - Information about stimulation is relayed to CNS over cochlear nerve. - Spiral ganglion contains cell bodies of bipolar sensory neurons that monitor cochlear hair cells. **Auditory Pathways** - Afferent fibers of sensory neurons in spiral ganglion form cochlear nerve. - Axons enter medulla oblongata and synapse at cochlear nucleus. - Information ascends to... - Superior olivary nuclei of pons. - Inferior colliculi of midbrain. - Midbrain coordinates unconscious motor responses. - Ascending auditory sensations synapse in medial geniculate body of thalamus. - Projection fibers deliver information to auditory cortex of temporal lobe.