Special Senses - Hearing

Special Senses ● Sensory receptors are within large, complex sensory organs in the head. ● Smell in olfactory organs ● Taste in taste buds ● Hearing and equilibrium in ears ● Sight in eyes HEARING AND BALANCE Sense of Hearing ● ○ ○ ● ○ ○ ○ ● Ear Organ of hearing ● Three (3) sections: ○ External ear ○ Middle ear ○ Inner ear ○ External Ear ● Auricle ○ Collects sounds waves ● External Auditory Meatus ○ Lined with ceruminous glands ○ Carries sound to tympanic membrane ○ Terminates wit tympanic membrane ● Tympanic Membrane ○ Vibrates in response to sound waves Middle Ear ● Tympanic Cavity ○ Air-filled space in temporal bone ● Auditory Ossicles ○ Vibrate in response to tympanic membrane ○ Malleus, incus and stapes ○ Hammer, anvil and stirrup ● Oval Window ○ Opening in wall of tympanic cavity ○ Stapes vibrates against it to move fluids in inner ear Auditory Tube ● Also known as the Eustachian tube ● Connects middle ear to throat ● Helps maintain equal pressure on both sides of tympanic membrane ● Usually closed by valve-like flaps in throat Inner Ear Complex system of labyrinths Osseous Labyrinth ■ Bony canal in temporal bone ■ Filled with perilymph Membranous Labyrinth ■ Tube within osseous labyrinth ■ Filled with endolymph 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 bony labyrinth ● Scala Tympani ○ Lower compartment ○ Extends from apex of the cochlea to round window ○ Part of bony labyrinth ● Cochlear Duct ○ Portion of membranous labyrinth in cochlea ● Vestibular Membrane ○ Separates cochlear duct from scala vestibuli ● Basilar Membrane ○ Separates cochlear duct from scala tympani *Hair Cells (Sensory Receptors): Convert mechanical sound waves into electrical signals. Types of sound conduction to the cochlea A. • • • • B. Air Conduction (Blue Dotted Arrow) Sound travels through the external auditory canal. Vibrates the eardrum and ossicles in the middle ear. Transmits vibrations to the cochlea, where hair cells generate neural signals. This is the primary pathway for normal hearing. Bone Conduction (Green Dotted Arrow) • • • C. • • Vibrations are transmitted directly to the temporal bone. The bone conducts sound waves directly to the cochlea, bypassing the outer and middle ear. Common in hearing aids for conductive hearing loss. Cartilage Conduction (Red Solid Arrow) Vibrations travel through the aural cartilage, which then transmits sound to the cochlea. This pathway is an alternative for individuals with ear canal obstructions. Organ of Corti • Hair cells (sensory receptors) o Converts mechanical sound waves into electrical signal o Inner Hair Cells: Transmit auditory information to the brain. o Outer Hair Cells: Amplify sound and improve frequency discrimination. • Basilar Membrane: Supports the hair cells and vibrates in response to sound frequencies. • Tectorial Membrane: Contacts hair cells and facilitates signal transduction. • Supporting Cells: Provide structural stability and support for hair cells. • Spiral Ganglion Neurons: Carry auditory signals to the brain via the cochlear nerve. Auditory Nerve Pathways 1. Sound Collection & Transmission ↓ 5. ↓ Inferior Colliculus (Midbrain) ↓ Medial Geniculate Nucleus (Thalamus) Cortical Processing & Perception ↓ Primary Auditory Cortex (Temporal Lobe) ↓ Higher Auditory Areas (Wernicke’s, Association Cortex) Clinical Application Disorder Description Specific Cause Examples Presbycusis Age-related gradual hearing loss, especially high frequencies Degeneration of cochlear hair cells Tinnitus Perception of ringing, buzzing, or clicking sounds Noise exposure, ear infections, ototoxic drugs Meniere’s Disease Inner ear disorder causing, vertigo, tinnitus, and hearing loss Endolymphatic fluid imbalance Otosclerosis Abnormal bone growth in middle ear leading to hearing loss Genetic trauma, facial nerve dysfunction Hyperacusis Increased sensitivity to normal environmental sounds Loud music, industrial noise, explosions Noise-Induced Hearing Loss (NIHL) Damage to cochlear hair cells due to prolonged or intense noise exposure Brain injury, developmental disorders Auditory Processing Disorder (APD) Difficulty processing and interpreting sounds despite normal hearing ability Outer Ear→Tympanic Membrane ↓ Middle Ear Stapes) ↓ 2. (Ossicles: Malleu, Incubus, Oval Window →Cochlea (Inner Ear) Conversion to Neural Signals ↓ 2+ Hair Cells (Organ of Corti)→K+& Ca Influx→Glutamate Release 3. ↓ Cochlear Nerve (CN VIII) Brainstem Processing ↓ Cochlear Nucleus (Medulla) ↓ Superior Olivary Complex (pons) →Sound 4. Localization Midbrain & Thalamic Relay Sense of Equilibrium ● Static Equilibrium ○ Vestibule ○ Senses position of head when body is not moving ● Dynamic Equilibrium ○ Semicircular canals ○ Senses rotation and movement of head and body Vestibule ● Utricle ○ Communicate with saccule and membranous portion of semicircular canals ● Saccule ○ Communicate with cochlear duct ■ Macula ● Hair cells of utricle and saccule Macula ● Responds to changes in head position ● Bending of hairs results in generation of nerve impulse Semicircular Canals ● Three (3) canals at right angles ○ Ampulla ■ Swelling of membranous labyrinth that communicates with the vestibule ○ Crista Ampullaris ■ Sensory organ of ampulla ■ Hair cells and supporting cells ■ Rapid turns of head or body stimulate hair cells Crista Ampullaris Vestibular System in Equilibrium Mechanism of Balance Control 1. Semicircular Canals – detects rotational (angular) movements of the head 2. Otolith Organs (Utricle & Saccule) – detects linear acceleration and head position relative to gravity 3. Vestibular Nerve – carries signals from the hair cells to the brainstem and cerebellum for balance control 4. Vestibulospinal Tract – sends motor commands from the brainstem to the spinal cord to adjust postural muscles 5. Vestibulo-Ocular Reflex – coordinates eye movements with head movements to maintain stable vision Integration with proprioception • Proprioceptors in muscles and joints send position and movement signals to the cerebellum. • Vestibular, proprioceptive, and visual inputs combine to maintain balance, posture, and coordination. • Dysfunction in the vestibular system can lead to vertigo, dizziness, and balance disorders. Clinical Application Disorder Descripti on Cause Example Psychological Impact Benign Paroxysmal Positional Vertigo (BPPV) Brief Episodes of vertigo triggered by head movement s Displaced otolith crystals in semicircul ar canals Anxiety, fear of movement, panic attacks Meniere’s Disease Recurrent vertigo, tinnitus, hearing loss, ear fullness eExcess endolymp h fluid in the inner ear Depression, stress, social withdrawal Vestibular Neuritis Sudden, severe vertigo with imbalance Viral infection affecting the vestibular nerve Anxiety, difficulty concentrating, frustration Vestibular Migraine Episodic vertigo linked to migraines Neurologic al dysfunctio n, genetic factors Mood swings. Irritability, cognitive fatigue