• Audition
• Stimuli
• Transduction of sound by hair cells
• Detection of pitch
• Somatosenses
• Skin receptors
• Pain perception

Structures of the Ear

• Outer ear:
• Channel to tympanic membrane
• Middle ear:
• The ossicles
• Inner ear:
• Cochlea

The Cochlea

• Three chambers:
• S. vestibuli
• S. media
• S. typmani
• Organ of Corti:
• Basilar membrane: base
• Tectorial membrane: roof
• Hair cells in between

A Detailed View of Hair Cells

• Cilia project from top of hair cell and are attached to tectorial membrane
• Cilia are interconnected by tip links: move laterally as a unit
• Lateral movement opens ion channels, Ca⁺⁺ enters, produces receptor potential

• Afferent pathways:
• Through cochlear nuclei
• To superior olivary nuclei
• To inferior colliculus
• To medial geniculate
• To auditory cortex
• Efferent pathway:
• Olivocochlear bundle

Place Coding of Pitch

• Different frequencies produce maximal distortion of basilar membrane
• Vibration produces a travelling wave
• High frequency: near base of basilar membrane
• Moderate frequency: near apex of basilar membrane
• Different regions of the basilar membrane project to different areas of cortex: tonotopic representation

• Observations of travelling waves: von Bekesy
• Antibiotics:
• Induce hair cell loss first at base of basilar membrane
• Corresponding loss of high frequency sounds
• Cochlear implants

• A=Apex
• B=Base
• Multiple areas in auditory cortex

• Lesions placed at different levels of the auditory system:
• Bilateral auditory cortex: animal can detect pitch, intensity diff, but not "tunes"
• Brachium of inf. colliculus: animal cannot detect frequency or intensity differences
• Lateral lemniscus: animal is deaf

Pain as Information

• Pain serves a functional role for survival
• Denotes tissue destruction induced by
• Thermal stimuli
• Mechanical force
• Degree of localization?
• Motivational force
• Escape source of tissue destruction

• Receptors for pain: free nerve endings
• Pain receptors are found in:
• Skin
• Sheath around muscles, internal organs
• Cornea of the eye
• Pulp of the teeth
• Receptors are activated by mechanical, chemical stimulation

CNS Lesions and Pain

• VPM, VPL: No touch, temp, pinprick pain
• Parafascicular nuc: pain gone, touch remains
• Dorsomedial: no emotional response to pain
• Cortical lesions can reduce pain (temporal lobe, parts of the parietal lobe)

• Hypnosis
• Massage
• Acupuncture
• Opiates
• Placebo
• Attention shifts

• Exogenous opiates reduce pain reactivity
• Brain produces several endorphins
• Naloxone reverses opiate activity
• Opiates act on pain via mu receptor
• Focal brain stimulation can reduce pain
• PAG in particular is effective
• Appears to activate a descending pathway (Basbaum and Fields model)