Lecture 11 - Hearing, Vestibular, Taste, and Smell

Auditory cortex

• not involved in basic auditory discrimination - can be accomplished at lower levels of system

• rather for processing complex sounds, which most sounds are

Two main streams of auditory processing in cortex

• a dorsal stream

• a ventral stream

Dorsal stream

focuses on localization

Ventral stream

analyzes components of sound

Primary auditory cortex

considerable plasticity

• evidence that music training, especially at different times of life, can influence size and responsiveness of auditory cortex to music

• cells responsive to one frequency can shift their tuning curve depending on experiences

Three kinds of deafness

• conduction deafness

• sensorineural deafness

• central deafness

Conduction deafness

problem in the outer or middle ear prevent transmission of vibrations — not a nervous system problem

Sensorineural deafness

auditory nerve unable to become excited and conduct signals

• can be treated with cochlear implants — auditory nerve is still functional

How does deafness occur?

can happen for a variety of reasons — genetic mutation, toxic drug, loud sound

• frequently involves damage to hair cells

currently working on ways to regrow hair cells

Central deafness

something wrong in brain - e.g., a stroke, leading to damage of auditory cortex or MGN projections to cortex

Deafness

can be complete lack of conscious awareness of sounds or can be quite selective (word deafness)

Auditory brainstem implants (ABIs)

developed more recently

• bypass auditory nerve and directly stimulate brainstem nuclei

Vestibular perception

informs you about forces acting on the body (esp. the head) — gravity and acceleration

• particularly important for balance and body/head position awareness

Three components for awareness in space

• vestibular system

• visual system

• proprioceptive system

Three components for vestibular system

• utricle

• saccule

• semicircular canals

Utricle and saccule

important for detecting linear forces (horizontal and vertical)

• tell you about static position of head — contain otoliths that enhance sensitivity of receptors

Semicircular canals

important for detecting rotational forces

How do we detect these rotations?

• each canal

• ampulla

Each canal

located along a different axis

Ampulla

enlarged region of each semicircular canal — located at junction of canal and the utricle

• hair cells located in ampulla respond to precise kind of mechanical rotational force

• depend on specific orientation of hair cells

Utricle

saclike structure at ends of canals

Saccule

another saclike structure just below utricle

Evolution of vestibular system

probably evolved from lateral-line system found in some fishes and amphibians

Lateral-line system

array of receptors along side of body

• movements of water stimulate receptors — tells animals about current and nearby animals

• auditory system most likely developed from vestibular system

Vestibular system

nerve fibers go to vestibular nuclei in brainstem, though some bypass this and go directly to cerebellum

Vestibular nucleus

motor nuclei of eyes, thalamus, and cortex

Vestibulo-ocular reflex

enables you to precisely control the muscles of the eye (and hence your gaze) even as you move your head

• after spinning too much, conflict between vestibular system and visual system reflex

Taste and smell

• forms of chemodetection

• can only detect five tastes

Distinct flavors of foods/drinks

taste and smell (10,000 odors)

Taste

processed by the gustatory system

Papillae on tongue

each with one or more taste buds

• all three types of papillae can detect all five tastes

  • salty

  • sour

  • sweet

  • bitter

  • umami

Each bud

50-150 receptor cells

• taste cells constantly being replaced

Salty tastes

sodium transported across cell membrane via channels — leads to depolarization

Sour tastes

not completely understood — depends on acidity of substance — hydrogen ions

• e.g., hydrogen ions enter through channels and depolarize

Sweet tastes

detected by combination of two members of the T1R family - GPCRs

Bitter tastes

often evoked by toxic substances — want to be highly sensitive

• T2R receptors (about 30 of them) — GPCRs

• Bitter-taste cells express all of them

• As a result, poor discrimination of bitter tastes

• genetic differences in sensitivity to bitter

Umami tastes

meaty, savory taste

• two kinds of receptors — one responds to glutamate (amino acid)

• another responds to most amino acids

  • amino acids present in high levels in meat

Smell (olfactory system)

• considerable variability among species in sensitivity

• mice and humans — 1000 genes for odor receptors

• but. in humans, only 350 appear to be fully functional

• many mammals have 10 times the number of olfactory neurons that humans have

• presumably, sensitivity reflects evolutional needs

We have olfactory receptor cells….

whose cilia extend into the olfactory mucosa

• allows them to “sample” the chemicals breathed into the nose

• receptor cells then send axons to glomeruli in the olfactory bulb

• cells in olfactory bulb (mitral cells) send their axons back further into the brain

Olfactory information reaches…

cortex without going through thalamus first

Odor

collection of odorant molecules

How does olfactory transduction work?

• odorant binds to GCPR leading to G protein activation

• G protein - activates adenyl cyclase, which produces cAMP

• cAMP binds to channels, opening them, leading to depolarization

How do we perceive different odors?

• humans have 350 olfactory receptor genes but can discriminate 5000 odors

• odors must be perceived by the activation of a particular combination of receptors

• anatomy of odor processing

Topographic organization

odor maps

• glomeruli receive input from same kind of olfactory receptor neurons

• this separation is generally retained throughout nervous system

• some cells in cortex are responsive to specific combinations of odors

Vomeronasal system

• second chemical detection system in nose of some animals (some mammals, amphibians, and reptiles)

• cells in vomeronasal organ (next to olfactory epithelium)

• detect pheromones

• receptors important for organizing reproductive behavior

Humans do not appear to have a functional VNO — both receptors’ genes are nonfunctional

Receptors

• detect major histocompatibility complexes (MHCs) — important for detecting degree of relatedness to other animals

• send information to accessory olfactory bulb —> medial amygdala —> hypothalamus

Pheromones

behavioral evidence suggests humans are sensitive to pheromones

• so the pheromone and olfactory systems may not be so separate