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1. 15 The Special Senses: Part C

2. The Ear: Hearing and Balance The three parts of the ear are the inner, outer, and middle ear The outer and middle ear are involved with hearing The inner ear functions in both hearing and equilibrium Receptors for hearing and balance: Respond to separate stimuli Are activated independently

3. The Cochlea The cochlea is divided into three chambers: Scala vestibuli Scala media Scala tympani

4. The Cochlea The scala tympani terminates at the round window The scalas tympani and vestibuli: Are filled with perilymph Are continuous with each other via the helicotrema* The scala media is filled with endolymph The helicotrema is the part of the cochlear labyrinth where the scala typmani and the scala vestibuli meet.

5. http://www.ece.rice.edu/~dhj/cochlea.html http://www.egms.de/figures/journals/cto/2005-4/cto000007.f3.png

6. Properties of Sound Sound depends on elastic medium for its transition (can not be transmitted in vacuum). Sound is: A pressure disturbance (alternating areas of high and low pressure) originating from a vibrating object Composed of areas of rarefaction (less molecules) and compression (more /compressed molecules) Represented by a sine wave in wavelength, frequency, and amplitude

7. Properties of Sound Frequency – the number of waves that pass a given point in a given time Wavelength – the distance between 2 consecutive crests; it is constant for a particular tone Pitch – perception of different frequencies (we hear from 20–20,000 Hz) The higher the frequency – the higher the pitch Amplitude – Height of the wave - loudness Loudness – subjective interpretation of sound intensity

8. Transmission of Sound to the Inner Ear The route of sound to the inner ear follows this pathway: Outer ear – pinna, auditory canal, eardrum Middle ear – malleus, incus, and stapes to the oval window Inner ear – scalas vestibuli and tympani to the cochlear duct Stimulation of the organ of Corti Generation of impulses in the cochlear nerve

9. Transmission of Sound to the Inner Ear Figure 15.31 http://www.britannica.com/eb/art-536?articleTypeId=1

10. Resonance of the Basilar Membrane As the stapes rocks back and forth against the oval window, it moves the perilymph in the scala vestibuli into a similar back-and-forth motion A pressure wave travels through the perilymph from the basal end toward the helicotrema. Sounds of very low frequency (below 20 Hz) create pressure waves that take the complete route through the cochlea toward the round window through the scala tympani. Such sounds do not activate the spiral organ (are below the threshold of hearing). http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/sound/u11l2d.html

11. Resonance of the Basilar Membrane Sound waves of low frequency (inaudible): Travel around the helicotrema Do not excite hair cells Audible sound waves: Penetrate through the cochlear duct Vibrate the basilar membrane Excite specific hair cells according to frequency of the sound

12. The Organ of Corti Is composed of supporting cells and outer and inner hair cells The hair cells are arranged in one row of inner hair cells and three rows of outer hair cells - sandwiched between the tectorial and basilar membranes. Afferent fibers of the cochlear nerve are in contact with the bases of the hair cells. The hair cells have numerous stereocilia (actually long microvilli) and a single kinocilium (a true cilium) project from their apices. The “hairs” (stereocilia) of the hair cells are stiffened by actin filaments and linked together by fine fibers called tip-links They project into the K+-rich endolymph, and the longest of them are embedded in the overlying tectorial membrane

13. Excitation of Hair Cells in the Organ of Corti Transduction of sound stimuli occurs after the stereocilia of the hair cells are turn aside by movements of the basilar membrane. Bending the cilia toward the kinocilium puts tension on the tip-links, which in turn opens cation channels in the adjacent shorter stereocilia. This results in an inward K+ (and Ca2+) current and a graded depolarization Depolarization increases intracellular Ca2+ and so increases the hair cells’ release of neurotransmitter (glutamate), which causes the afferent cochlear fibers to transmit a faster stream of impulses to the brain for auditory interpretation. Bending the cilia away from the kinocilium relaxes the tip-links, closes the mechanically gated ion channels, and allows repolarization and even a graded hyperpolarization.

14. http://www.keele.ac.uk/depts/co/auditory/pages/projects.htm http://www.wadalab.mech.tohoku.ac.jp/corti-e.html

15. Excitation of Hair Cells in the Organ of Corti The outer hair cells send little information to the brain. Instead, they act on the basilar membrane itself. Most (90-95%) nerve fibers around the OHC are efferent (from the brain to the ear) In response to sound, the OHC send signals to the medulla and the pons sends immediately signals back In response, The OHC contract by about 15% of their height Because the OHC are attached to the basiliar membrane and the tectorial membrane, contraction decrease the ability of the basiliar membrane to vibrate. As a result, some areas of the duct send less signals to the brain which allow the brain to distinguish between more and less active hair cell. Give a more precise perception of different pitches

16. Mechanisms of Equilibrium and Orientation Vestibular apparatus – equilibrium receptors in the semicircular canals and vestibule Maintains our orientation and balance in space The position of the body with respect to gravity (static equilibrium) – the vestibule The motion of the body (dynamic equilibrium) – the semicircular canals

17. Static equilibrium • The receptors for static equilibrium are the maculae – one in the urticle and one in the saccule • The utricle is sensitive to a change in horizontal movement, • The saccule gives information about vertical movement http://www.tchain.com/otoneurology/disorders/unilat/utricular.html

18. Anatomy of Maculae Maculae are the sensory receptors for static equilibrium Contain supporting cells and hair cells Each hair cell has stereocilia and kinocilium embedded in the otolithic membrane Otolithic membrane – jellylike mass covered with tiny CaCO3 stones called otoliths Utricular hairs respond to horizontal movement Saccular hairs respond to vertical movement

19. Effect of Gravity on Receptor Cells When the head starts or stops moving in a linear direction, the otolithic membrane slides backward or forward like a plate over the hair cells, bending the hairs. The hair cells release neurotransmitter continuously but movement of their hairs modifies the amount they release. When the hairs are bent toward the kinocilium, the hair cells depolarize, increasing their pace of neurotransmitter release, and a faster stream of impulses travels up the vestibular nerve to the brain When the hairs are bent in the opposite direction, the receptors hyperpolarize, and neurotransmitter release and impulse generation decline. In either case, the brain is informed of the changing position of the head in space.

20. Crista Ampullaris and Dynamic Equilibrium The crista ampullaris (or crista): Is the receptor for dynamic equilibrium Is located in the ampulla of each semicircular canal Responds to angular movements Each crista has support cells and hair cells that extend into a gel-like mass called the cupula Dendrites of vestibular nerve fibers encircle the base of the hair cells

21. Activating Crista Ampullaris Receptors The cristae respond to changes in the velocity of rotation movements of the head. the endolymph in the semicircular ducts moves briefly in the direction opposite the body’s rotation, deforming the crista in the duct. As the hairs are bent, the hair cells depolarize and impulses reach the brain at a faster rate. Bending the cilia in the opposite direction causes hyperpolarization and reduces impulse generation. Because the axes of the hair cells in the complementary semicircular ducts are opposite, rotation in a given direction causes depolarization of the receptors in one ampulla of the pair, and hyperpolarization of the receptors in the other

22. Rotary Head Movement Figure 15.37d http://www.unmc.edu/Physiology/Mann/pix_9/left_mvt.gif

23. Equilibrium Pathway to the Brain • Pathways are complex and poorly traced • Impulses travel to the vestibular nuclei in the brain stem or the cerebellum, both of which receive other input • Three modes of input for balance and orientation • Vestibular receptors • Visual receptors • Somatic receptors