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VESTIBULAR SYSTEM (Balance/Equilibrium) the Vestibular Stimulus Is Provided by Earth's Gravity, and Head/Body Movement. Locate

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VESTIBULAR SYSTEM (Balance/Equilibrium) The vestibular stimulus is provided by Earth’s gravity, and head/body movement. Located in the labyrinths of the inner ear, in two components: 1. Vestibular sacs - gravity & head direction 2. Semicircular canals - angular acceleration (changes in the rotation of the head, not steady rotation) 1. Vestibular sacs (Otolith organs) - made of: a) Utricle (“little pouch”) b) Saccule (“little sac”) Signaling mechanism of Vestibular sacs Receptive organ located on the “floor” of Utricle and on “wall” of Saccule when head is in upright position - crystals move within gelatinous mass upon head movement; - crystals slightly bend cilia of hair cells also located within gelatinous mass; - this increases or decreases rate of action potentials in bipolar vestibular sensory neurons. Otoconia: Calcium carbonate crystals Gelatinous mass Cilia Hair cells Vestibular nerve Vestibular ganglion 2. Semicircular canals: 3 ring structures; each filled with fluid, separated by a membrane. Signaling mechanism of Semicircular canals -head movement induces movement of endolymph, but inertial resistance of endolymph slightly bends cupula (endolymph movement is initially slower than head mvmt); - cupula bending slightly moves the cilia of hair cells; - this bending changes rate of action potentials in bipolar vestibular sensory neurons; - when head movement stops: endolymph movement continues for slightly longer, again bending the cupula but in reverse direction on hair cells which changes rate of APs; - detects “acceleration” in all 3 planes semicircular Ampulla Cross- canal section Cross- section Perilymph Membrane Cupula Hair Endolymph cells Vestibular nerve Vestibular Ganglion 3. Vestibular pathway to the nervous system: - vestibular bipolar sensory cell bodies located in vestibular ganglion, which looks like a nodule (enlargement) on the vestibular nerve - axons from vestibular neurons get together with axons of the spiral ganglion (auditory) and give rise to vestibulocochlear nerve = VIII cranial nerve - vestibular axons synapse within vestibular nuclei in medulla, and in cerebellum; - vestibular neurons send axons to cerebellum, spinal cord, medulla and pons - medullary responses to vestibular stimulation involved in motion sickness (nausea, emesis) - responsible for controlling neck muscles for keeping head upright - controls eye movement; compensates for sudden head movement = Vestibulo-Ocular Reflex (VOR) Chap. 8- 4 Nerves out of the “vestibule” . and entering the brain AUDITORY SYSTEM (Hearing) I. AUDITORY (ACOUSTIC) STIMULUS - waves of energy = sound waves - propagates through: gases (air), liquids (water), and solids (metals) 1 cycle Intensity (loudness): measured on a logarithmic scale (decibels, dB); very sensitive & wide range - range: 0 - 160+ dB (>140 dB = painful) Frequency: Normal range (humans): 20-20,000 cps - cps = cycles per seconds = Hertz (Hz) - frequency = pitch - ex., women’s voice higher pitch than men’s - range gets considerably narrower with age Complexity: additivity of simple waves gives rise to complex waves - most sounds are complex waves that can be analyzed with Fourier transform analysis into their component simple waves. II. EAR A. OUTER EAR a. Pinna (external ear) b. Ear canal Hammer Ossicles (middle Anvil Stirrup}ear bones) Oval window Auditory nerve Bone Cochlea Vestibule Ear canal Tympanic Round membrane Pinna window Eustachian tube (connects with throat) B. MIDDLE EAR a. Eardrum (tympanic membrane) b. Ossicles 1. Hammer 2. Anvil 3. Stirrup C. INNER EAR = COCHLEA C. COCHLEA a. Oval window b. Round window c. Endolymph d. Cochlear duct - contains Organ of Corti 1. Tectorial membrane (top) 2. Basilar membrane (bottom) 3. Hair cells Tectorial Cilia of Inner membrane hair cell hair cell Outer (rigid) hair cells Deiter’s cells Cochlear Axons of auditory nerve duct Basilar membrane - mobile Organ of Corti Cochlear nerve Spiral ganglion Bone Membrane surrounding cochlea Slice through cochlea Signaling mechanism for hearing: - sound waves produce movement of basilar membrane; - movement of basilar membrane induce movement of cilia of hair cells; - cilia movement increase or decrease polarization of hair cells, which increase or decrease neurotransmitter release onto axon terminals of bipolar auditory neurons; - this increases or decreases action potentials in bipolar auditory neurons. - it is the inner hair cells that provide the auditory signal to the brain; - the outer hair cells are believed to control the “tightness” of the basilar membrane, and therefore provide some modulation of hearing. D. CODING OF FREQUENCY: Pitch (frequency) perception: 1. Place coding a. Different spots on basilar membrane vibrate to different frequencies (Fig. A above) b. Works for moderate to high frequencies, 100-200 to 20,000 Hz; - near oval window (base): very high frequencies (20,000 Hz) - near apex: moderate frequencies (100-200 Hz) 2. Rate coding a. Frequency of sound waves over a large portion of basilar membrane = frequency of action potentials b. Works for low frequency sounds (below about 100-200 Hz) Chap. 8- 8 E. CODING OF INTENSITY (LOUDNESS): Determined by action potential frequency; ex. soft sound = fewer AP, loud sound = more AP F. CODING OF SOUND LOCALIZATION: Based on: 1. Arrival time: ex., click sound generated to the left arrives at left ear first 2. Phase difference: ex., continuous sound waves will reach each ear at slightly different phases of the oscillating sound waves - these mechanisms work best with sounds of moderate frequencies 3. Intensity difference: ex., sound generated to the left are sensed slightly louder on the left side - this mechanism works best with sounds of high frequencies Note. Low frequencies (< 100 Hz) are nearly impossible to localize (that’s why you need only one “sub-woofer” in a home-theater system) G. AUDITORY SYSTEM PATHWAY Spiral ganglion - contains bipolar neurons - receive information from hair cells - send their axons to brain via VIII nerve (auditory component of vestibulocochear nerve) Lateral fissure Auditory cortex Forebrain Thalamus: Medial geniculate body Midbrain Inferior colliculus Dorsal cochlear nucleus Lateral lemniscus Ventral cochlear nucleus Trapezoid Medulla body Auditory nerve Superior (VIII nerve) olivary complex Frequency organization kept throughout auditory system all the way to auditory cortex.
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