1. Evolution of the Vestibular and Auditory End Organs
The vestibular system evolved from the lateral-line system found in fishes and some amphibians.
The system has receptors along the side of the body.
Tiny hairs that emerge from sensory cells in the skin are embedded in cupulae, like those in the mammalian ampulla

2. External and Internal Structures of the Human Ear
Sound, a mechanical force, is transduced into neural activity.
The external ear, the pinna, and the ear canal collect sound waves.
The shape of the external ear transforms sound energy.
The middle ear concentrates sound energies.
Three ossicles—malleus, incus, and stapes—connect the tympanic membrane (eardrum) to the oval window.
Two muscles in the middle ear vary the linkage of the ossicles:
Tensor tympani—attached to the malleus and tympanic membrane
Stapedius—attached to the stapes
When activated, the muscles stiffen and reduce sound’s effect.
Inner ear structures convert sound into neural activity.
Mammals have a fluid-filled cochlea, a spiral structure with a base and an apex.
The base is nearest the oval window membrane.
The cochlea has three parallel canals:
Scala vestibuli—vestibular canal
Scala media—middle canal
Scala tympani—tympanic canal
The round window is a membrane that separates the scala tympani from the middle ear.
The organ of Corti has three main structures:
Sensory cells, or hair cells
Framework of supporting cells
Basilar membrane, which vibrates in response to sound

3. External and Internal Structures of the Human Ear (Part 1)
Sound, a mechanical force, is transduced into neural activity.
The external ear–the pinna and the ear canal–collects sound waves.
The shape of the external ear transforms sound energy.

4. External and Internal Structures of the Human Ear (Part 2)
The middle ear concentrates sound energies.
Three ossicles–malleus, incus, and stapes–connect the tympanic membrane (eardrum) to the oval window.
Two muscles in the middle ear vary the linkage of the ossicles:
Tensor tympani–attached to the malleus and tympanic membrane
Stapedius–attached to the stapes
When activated, the muscles stiffen and reduce sound’s effect.

5. External and Internal Structures of the Human Ear (Part 3)
Inner ear structures convert sound into neural activity.
Mammals have a fluid-filled cochlea, a spiral structure with a base and an apex.
The base is nearest the oval-window membrane.
The cochlea has three parallel canals:
Scala vestibuli–vestibular canal
Scala media–middle canal
Scala tympani–tympanic canal
The round window is a membrane that separates the scala tympani from the middle ear.

6. External and Internal Structures of the Human Ear (Part 4)
Inner ear structures convert sound into neural activity.
Mammals have a fluid-filled cochlea, a spiral structure with a base and an apex.
The base is nearest the oval-window membrane.
The organ of Corti has three main structures:
Sensory cells, or hair cells
Framework of supporting cells
Basilar membrane, which vibrates in response to sound

7. External and Internal Structures of the Human Ear (Part 5)

8. Auditory Nerve Fibers and Synapses in the Organ of Corti
OHCs extend into the tectorial membrane on top of the organ of Corti.
Afferent and efferent nerve fibers carry messages between the hair cells and the brain.
Different neurotransmitters are active at each type of synapse
Dancing Outer Hair Cell - Video

9. How Stereocilia Sense Auditory Stimulation
The organ of Corti has two sets of sensory cells:
Inner hair cells (IHCs)
Outer hair cells (OHCs)
Stereocilia, or hairs, protrude from each hair cell.
Thin fibers called tip links run across each hair cell’s stereocilia.
Vibration makes stereocilia sway, causing ion channels to open.
The hair cell depolarizes, and calcium influx at the base of the cell causes neurotransmitter release.

10. Pitch Information Is Encoded in Two Complementary Ways
The ability to detect a change in frequency is measured as the minimal discriminable frequency difference between two tones.
The detectable difference is about 2 Hz for sounds up to 2000 Hz; above these frequencies, larger differences are required.
Two theories of pitch discrimination:
Place coding theory—pitch is encoded in receptor location on the basilar membrane.
Temporal coding theory—firing rate of auditory neurons encodes the frequency of the auditory stimulus.

11. Basilar Membrane Movement for Sounds of Different Frequencies
Sound vibrations cause the basilar membrane to oscillate.
Different parts respond to different frequencies:
High frequency–displaces narrow base of basilar membrane
Low frequency–displaces wider apex
Auditory neurons have tonotopic organization.
Slow Motion GUITAR Strings Video

12. Examples of Tuning Curves of Auditory Neurons
Mechanical responses of OHCs serve as a cochlear amplifier.
The cochlea itself produces sounds called otoacoustic emissions.
Tuning curves–graphs of auditory nerve fiber responses–show that additional sharpening takes place.

13. Auditory Pathways of the Human Brain
The vestibulocochlear nerve, cranial nerve VIII, contains auditory fibers from the cochlea.
Each fiber divides into two branches, going to cells in the ventral and dorsal cochlear nuclei.
The cochlear nuclei have multiple targets:
Superior olivary nuclei–receive bilateral input
Inferior colliculi–in the midbrain, which then send output to the medial geniculate nuclei in the thalamus

14. Tonotopic Mapping in the Cat Inferior Colliculus
Auditory neurons have tonotopic organization.
They are arranged in a map according to the frequencies they respond to.
PET and fMRI show main activation is in the primary auditory cortex on the superior temporal lobe.
The ability to detect a change in frequency is measured as the minimal discriminable frequency difference between two tones.

15. Tonotopic Organization of Auditory Cortical Regions in Three Species of Mammals
biopsych4e-fig jpg Two theories of pitch discrimination:
Place theory–pitch is encoded in receptor location on the basilar membrane.
Volley theory–firing rate of auditory neurons encodes the frequency of the auditory stimulus.
The two theories can be incorporated.
The frequency properties of a sound are coded in two ways:
Place coding or tonotopic representation
Temporal pattern of firing of cells
Species may differ in their sensitivity to sounds.
Ultrasound–very high frequency, at about 20,000 Hz
Infrasound–very low frequency–20 Hz

16. Long-Term Retention of a Trained Shift in Tuning of an Auditory Receptive Field

17. Congenital amusia (tone deafness)
Impaired music perception affecting approximately 4% of individuals
A deficit in processing musical pitch but not musical time
Genetically transmitted
Compare families with amusic individuals to control families
About one third of first-degree relatives In amusic families share the impairment compared to only a few percent for the control families
Genes do not directly control cognitive functions such as music perception
Genes responsible for congenital amusia influence brain development.

18. Figure 9.18 Structures of the Vestibular System
Parts of the vestibular system:
Semicircular canals—three fluid-filled tubes in different planes
Utricle and saccule—fluid-filled sacs that respond to static head positions
Ampulla—enlarged region of hair cells in the canal
Stereocilia of the hair cells are embedded in a gelatinous mass, the cupula.
Otoliths—bony crystals on a membrane in the vestibular system, that increase receptor sensitivity to movement
Nerve fibers from vestibular receptors synapse in the vestibular nuclei in the brainstem; some fibers go directly to the cerebellum.
The vestibulo-ocular reflex (VOR) allows you to gaze at a fixed point while the head moves.

19. Nerve fibers from vestibular receptors synapse in
Nerve Fibers from the Vestibular Portion of the Vestibulocochlear Nerve (VIII) Synapse in the Brainstem
Nerve fibers from vestibular receptors synapse in
the vestibular nuclei in the brainstem
some fibers go directly to the cerebellum.
The vestibulo-ocular reflex (VOR) allows you to gaze at a fixed point while the head moves.

20. The Vestibulo-Ocular Reflex
Motion sickness—the feeling of nausea caused by passive movements
Sensory conflict theory—motion sickness is due to conflict in visual and vestibular information.