Chapter 7

1. Chapter 7 Audition, the Body Senses, and the Chemical Senses

2. Audition • The stimulus • Sounds vary in their: • Pitch – a perceptual dimension of sound; corresponds to their fundamental frequency • Loudness – corresponds to intensity • Timbre – corresponds to complexity • Anatomy of the ear • Sound is funneled via the pinna (external ear) through the ear canal to the tympanic membrane (eardrum), which vibrates with the sound • The middle ear is located behind the tympanic membrane and includes the middle ear bones, the ossicles (malleus, incus and stapes) • The malleus connects with the tympanic membrane and transmits vibrations via the incus and stapes to the cochlea, the sructure that contains the receptors

4. Anatomy of the ear • The cochlea is part of the inner ear; it is filled with fluid, therefore sounds transferred through the air must be transferred into a liquid medium; the ossicles aid in this transmission • The cochlea is divided into 3 sections: the scala vestibuli, scala media, and scala tympani • The receptive organ, the organ of Corti, consists of the basilar membrane, the hair cells, and the tectorial membrane • The auditory receptor cells are called hair cells, and they are anchored, via Deiter’s cells, to the basilar membrane • Sound waves cause the basilar membrane to move relative to the tectorial membrane, which bends the cilia of the hair cells; this bending produces receptor potentials

6. Audition • Hair cells • Hair cells contain cilia (hair-like appendages involved in movement or in transducing sensory info) • The hair cells form synapses with dendrites of bipolar neurons whose axons bring auditory info to the brain • The Auditory pathway • The organ of Corti sends auditory info to the brain by means of the cochlear nerve, a branch of the vestibulocochlear nerve (8th cranial nerve) • The pathway goes through the midbrain to the auditory cortex located in the temporal lobe • Auditory info is represented tonotopically, i.e. topographically organized mapping of different frquencies of sound that are represented in a particular region of the brain

7. Perception of pitch • Place coding • Detecting moderate to high frequencies • The system by which info about different frequencies is coded (i.e. neural representation of info) by different locations on the basilar membrane • Good evidence is seen for place coding with cochlear implants (an electronic device surgically implanted in the inner ear that can enable a deaf person to hear) because most speech sounds are of higher frequencies, and cannot be represented by rate coding • Rate coding • Detecting low frequencies • The system by which info about different frequencies is coded by the rate of firing of neurons in the auditory system

8. Auditory perception • Perception of loudness • The axons of the cochlear nerve inform the brain of the loudness of a stimulus by altering their rate of firing (Louder the sound, higher rate of firing) • Perception of timbre • When the basilar membrane is stimulated by a complex sound (such as a musical instrument), different portions respond to each of the overtones (the frequency of complex tones that occurs at multiples of the fundamental frequency)

9. Perception of spatial location • Neurons in our auditory system respond selectively to different arrival times of the sound waves at the left and right ears in order to perceive the spatial location of a sound (phase difference) • By analyzing the timbre of a sound, we can perceive if a sound is in front or behind us

10. Vestibular system • Consists of the vestibular sacs (respond to the force of gravity and inform the brain about the head’s orientation) and the semicircular canals (respond to angular acceleration, i.e. changes in head rotation, but not to steady acceleration) • The functions include balance, maintenance of the head in an upright position, and adjustment of eye movement to compensate for head movements • Anatomy • Vestibular sacs: • Utricle & saccule • In the canals, there is an enlargement called the ampulla, which is where the sensory receptors reside • The sensory receptors are hair-like and their cilia are embedded in the cupula

12. Vestibular system • Pathway • Vestibular nerve is part of the 8th cranial nerve • Project to cerebellum, spinal cord, medulla, and pons • Also connects to 3rd, 4th and 6th CN (control eye muscles) in order to adjust eyes during any head movements

13. Somatosenses • Provide info about what is happening on the surface of our body and inside it • Cutaneous sense – sensitivity to stimuli that involve the skin; touch • Kinesthesia – perception of the body’s own movement • Organic sense – a sense modality that arises from receptors located within the inner organs of the body • Cutaneous senses respond to pressure, vibration, heating, cooling, and events caused by tissue damage • Kinesthesia is provided by stretch receptors in skeletal muscles and tendons that report changes in muscle length to the CNS

14. Somatosenses • Anatomy of the skin and its receptive organs • Humans have both hairy and glabrous (hairless) skin • Hairy skin: • Free nerve endings – detect painful stimuli and changes in temp • Ruffini corpuscles – respond to indentation of skin • Pacinian corpuscles – respond to rapid vibrations • Glabrous skin • Free nerve endings, Ruffini and Pacinian corpuscles • Meissner’s corpuscles – touch-sensitive end organs • Merkel’s disk – the touch-sensitive end organs found adjacent to sweat ducts

16. Somatosenses • Touch perception • When the Pacinian corpuscle is bent relative to the axon, the membrane becomes depolarized • Most info about tactile stimulation is precisely localized • Adaptation • A moderate, constant stimulus applied to the skin fails to produce any sensation after it has been present for a while • Due to the physical • Temperature • 2 types of thermal receptors: one responds to warmth, the other to coolness • Pain • Accomplished through free nerve endings on skin • At least 3 types of nociceptors

18. Gustation • Stimuli • 5 qualities: bitter, sour, sweet, salty, umami • Flavor is the combination of gustation (taste) and olfaction (smell) • Anatomy of taste bud and gustatory cells • Tongue, palate, pharynx and larynx contain ~10,000 taste buds • Most of these receptive organs are arranged around papillae: • Fungiform papillae (anterior 2/3 of tongue) • Foliate papillae (edges of back of tongue) • Circumvallate papillae (posterior third of tongue) • Taste buds consist of groups of ~20-50 receptor cells, with cilia located at the end of each cell that project through the opening of the taste bud (pore) into the saliva • Taste receptor cells form synapses with bipolar neurons whose axons convey gustatory info through the 7th, 9th and 10th CN

20. Gustation • Perception of gustatory info • The tasted molecule binds with the receptor and produces changes in the membrane potential • Different substances bind with different types of receptors producing different taste sensations • Salty • Simple sodium channel, blocked by the drug amiloride • Sour • Respond to hydrogen ions in acidic solutions • Bitter • Typical stimulus is plant alkaloid such as quinine • Perhaps family of bitter receptors • GPCR called gusducin • Sweet • Respond to sugar molecules (e.g. glucose, fructose) • Also coupled to gusducin • Umami • Taste of MSG • Specialized metabotropic glutamate receptor may be responsible

22. Gustation • Gustatory pathway • Info from anterior tongue travels through chorda tympani, a branch of the facial (7th) nerve; info from posterior tongue travels through lingual branch of 9th CN; 10th CN carries info from palate and epiglottis • First “relay station” is the nucleus of the solitary tract (NTS), in the medulla, which then projects to the thalamus, then to the primary gustatory cortex, located at base of frontal cortex and in the insular cortex

23. Olfaction • Stimulus • Odorants • Anatomy • 6 million olfactory receptors located on olfactory epithelium, located at the top of the nasal cavity • Receptor cells are bipolar neurons whose cell bodies lie in the cribiform plate • Olfactory bulbs lie at the base of the brain on the ends of the olfactory tracts • Each olfactory cell sends an axon onto the olfactory bulb, where it synapses with dendrites of mitral cells (in the olfactory glomeruli), and the projects thorough the olfactory tracts to the amygdala, pyriform cortex, and entorhinal cortex

25. Olfaction • Transduction • A G protein called Golf activates an enzyme that opens sodium channels of the olfactory cell • In humans there are ~500-1000 different olfactory receptors • Perception of specific odors • How can a (relatively) small amount of receptors lead to such a vast array of smells? • A particular odorant binds to more than one receptor, thus different odorants produce different patterns of activity in different glomeruli • The spatial pattern of olfactory info is maintained in the olfactory cortex