Neuro3: Taste, Smell And Hearing

Front 1

Labeled Line Theory, in smell, taste and touch

Back 1

Mechanoreceptors and Chemoreceptors project afferents to thalamus, synapse and then 2nd orders project to cortex for multi-modal processing

In some cases the mechanoreceptor itself is the afferent (free nerve endings)

Chemical stimulation at chemoreceptor = sense of taste or smell

Mechanical stimulation at skin = sense of touch

Stimulation of the 2ndary neuron along that path independently ALSO has same sensation

This comes into play with "crossed modalities" (other senses simulating a sensation) as well as lesions causing sensations

Taste and smell are particularly linked

Smell bypasses thalamus completely too

Front 2

Flavor

Back 2

Combination of the sensation of taste (CN VII or IX), smell (CN I), and free nerve endings (spiciness and temperature) (CN V) combined with emotional and cognitive valence (rewards from memory, emotion) to form the sensation of flavor

Front 3

Gustatory and Olfactory system receptor structures, cell turnover

Back 3

Both have numerous receptors in taste buds and nasal epithelia

Olfactory - G-Protein coupled receptors ONLY (about 1000 distinct smell combos)

Taste - G protein coupled receptors AND ion channels (such as transient receptor potential (TRP) channels). TRPs exist for spicy, heat, cold/menthol, etc.

Cell turnover - both undergo constant lifetime turnover

Olfactory - Granule neurons in olfactory bulb and primary epithelial cells (CN I)
Gustatory - modified epithelial cells (taste buds)

Front 4

Senses most tied to emotion and memory

Back 4

Olfactory and Gustatory (olfactory more)

Because tied to limbic system and papez's loop

Olfactory is most likely most conserved system

Olfactory system is one that BYPASSES THALAMUS

Front 5

Taste bud types and fxn

Back 5

Three main types of lingual buds located within papillae

Foliate

Fungiform

Circumvallate

2000-5000 buds with 50-150 taste cells based on which type of bud it is

From tip of tongue its mostly fungiform (anterior 2/3), then foliate (on edges middle 1/3), then circumvalate (posterior 1/3)

chemicals enter and act on plasma membranes of taste cells (G-protein coupled receptors, ion receptor channels, TRP channels). Depolarization and passive spread can produce APs (USUALLY DO NOT), leads to Ca2+ entry and glutamate release to transmit signal. ATP is also released and thought to be main NT for taste. Release is onto primary afferent neuron

Front 6

Primary NT for taste

Back 6

When sensation hits microvillae and taste cells, they either depolarizes (rarely have an AP) and Ca2+ in, glutamate and ATP out onto primary afferent neuron

ATP is primary one for the sensation

Front 7

Innervation of tongue parts

Back 7

Facial nerve (VII) innervates taste for anterior 2/3 of tongue via chorda tympani (fungiform and some foliate papillae)

Glossopharyngeal (IX) innervates posterior 1/3 for taste (vallate papillae and some foliate papillae)

Trigeminal (V) innervates GSA anterior 2/3 for touch, pain

Glossopharyngeal (IX) innervates GSA posterior 1/3 for touch, pain

Epiglottis is by the vagus

Soft palate is by facial nerve (VII)


Vagus and glossopharyngeal GSA is mostly for gag relfex

Front 8

Innervation course from tongue nerves

Back 8

SSA (special sensory afferents) travel via chorda tympani (VII) and glossopharyngeal (IX) to the rostral medulla at the SOLITARY NUCLEUS (also gets some Vagus input)

nucleus is the principal visceral sensory relay with caudal medulla receiving input from the "viscera"

Front 9

5 modalities of taste and channels involved, effectors

Back 9

Sweet - response to glucose (GPCR)
Salty - sodium channel
Sour - acid - proton channels
Bitter - G protein coupled
"Umami" - response to Glutamate

TRP channels see "PAIN, heat and spiciness"

Sodium (salty) and proton (sour) channels cause direct depolarization and calcium channel openings

All have some combination of direct depolarizatoin and Ca2+ influx to cell

Front 10

Anatomical path for taste sensation

Back 10

Solitary nucleus in rostral medula for reflexes (X and IX gag reflex)

Rostral pons - parabrachial nucleus (not impt for humans)

VPM - thalamus (head sensory) via cortical relay, BIL innervated via central tegmental tract from rostral pons

Ultimately reaches primary gustatory complex in the insular/operculum

Front 11

Cortical processing of taste

Back 11

Goes from opercular/insular cortex to orbitofrontal cortex (integration with information such as smell) then projects to amygdala, hypothalamus, striatum (memory, emotional valence)

Front 12

Broad tuning of taste path from solitary nucleus to orbital cortex

Back 12

Goes from many different traces that represent signals from multiple taste submodalities per taste cell projecting to the solitary nucleus to being isolated and processed into a single submodality by the time it reaches the orbital cortex cell

Each "taste" in solitarius cell has multiple submodalities condensed to one by the time it reaches the orbital cortex cell

Front 13

Labeled Line pathway vs across fiber pathway

Back 13

Labeled line pathway would say hitting one taste submodality (ex. salty) would only excite that submodality on it's afferent path to the solitary nucleus

HOWEVER, the Across-Fiber pattern would suggest that stimulating one modality would also enchance others slightly less that are linked to it (ie. stimulating salty also gives some stimulation to sweet and sour) on the afferent paths.

Across-Fiber Path is what actually happens and serves to enhance signals via collateral excitement of neurons

Front 14

Anatomy of Olfactory Path

Back 14

Olfactory receptor neurons with many microvilli for SA piercing cribiform plate are real neurons of CN I (not the bulb or tract) which is just hte nerve

Proceeds to the primary cortex directly and BYPASSES THALAMUS

medial part goes to septal nuclei (pleasure)
Lateral part goes to amygdala, hippocampus (emotion and memory), and olfactory cortex

Front 15

Olfactory transduction

Back 15

CN I complex DO fire APs (unlike taste most of the time) to transmit signals to olfactory bulb on FINE UNMYELINATED AXONS using G-PROTEIN COUPLED RECEPTORS

G protein binds the receptor after activated and either directly or indirectly opens an ion channel which causes depolarization then AP threshold then AP.

Each GCPR for 1-2 chemicals

Each cell with SPECIFIC GCPRs converge to specific type of glomeruli leading to a single chemical response that is relayed to MITRAL cells (located in each glomeruli receiving input from each type of neuron with specific GCPR) that goes from the bulb down the tract

Synapses at neck so signals can be relayed to contralateral ofactory bulb

Directly projects to Periamygdaloid or Piriform Cortex (TEMPORAL LOBE) = PRIMARY CORTEX FOR SMELL and located in parahippocampal gyrus


BYPASSES THALAMUS but may have some indirect paths there

Front 16

Role of anterior olfactory nucleus

Back 16

inhibit contralateral olfactory bulb smell signals coming in

Front 17

Anosmia and clinical links

Back 17

Smell loss, can get if fracture cribiform plate

Linked to taste loss

Also linked to parkinson's which may start with anosmia since loss begins in olfactory system and medulla

Front 18

Uncinate seizures

Back 18

Begin in or near smell or taste context and can start with unpleasant sensations of both

Temporal lobe seizures

Front 19

Sound system overview path

Back 19

Cochlea senses mechanical transduction due to sound pressure waves on hair cells causing NT release

Innervation down cochlear nerve (VIII) with afferent for frequency/loudness and efferent for modulation (background noise) /sensitivity

Travels to superior olivary nucleus then to inferior colliculus via lateral lemniscus then to medial geniculate then to auditory cortex in Heshel's gyrus

Front 20

Role of Superior Olivary Nucleus
Inferior Colliculi

Back 20

SON - localize sound in space

IC - orient torwards a particular sound (less impt in humans)

Front 21

What is sound?, ear range

Back 21

Sinusoidal oscillation of compressions and rarefactions in an elastic medium (air, fluid, bone, etc)

Ear can hear from 20Hz to 20kHz

Front 22

Sound perception levels

Back 22

For every 10-fold increment in sound pressure level, humans perceive an equal increment in sound (logarithmic) so decibel system created this way

L = 20 x Log P/Pref = decibal level

Front 23

Roles of the ear parts

Back 23

The outer ear and canal are for sound collection

Middle ear is for transmission to the cochlea and vestibular apparatus

Cochlea in inner ear transduces mechanical stimulation to electrical transduction

Front 24

Cochlea structure

Back 24

Sound in thru oval window (Stapes) and out through round window

Fluid in utricle, saccule and semicircular canals (anterior vertica, posterior vertical, horizontal - spatial position), and down snail shell like cochlea

Spiral ganglia (snail shell) has three chambers:

Vestibule: connected to oval window, sound origination to

Scala Tympani - sound exits

Scalia media - solution different from CSF used for mechanical transduction of auditory and vestibular signals

Vestibular nerve attatched to utricle, saccule and cochlear nerve attached to cochlea snail shell

Front 25

Basilar membrane and role, high vs low freq, Oscillations

Back 25

Place where signal transduction takes place
Thin near oval window (sound enters) and thick at apex

High frequency sounds - interpreted where membrane is thin and taut

Low frequency sounds - where membrane is wider and more flexible

Oscillations - enter and cause deflections at a certain part of basilar membrane based on frequency. (max deflection for 200Hz cell is near hair cells). These combine to form tonotopic map

Front 26

Different solutions in chambers of cochlea

Back 26

For each unit of a scala tympani, scala media, scala vestibuli:

Endolypmh - in scala media, filtrate formed by the STRIA VASCULARIS. HIGH K+

Perilymph - found in scala vestibuli and scala tympani - similar to CSF, "normal K+"

Front 27

Organ of Corti and role

Back 27

Site of mechano-electric transduction using stereocilia coming in from hair cells

Inner hair cells perform auditory transduction

Outer hair cells (more numerous) perform the modulatory function

Goes Basilar membrane to organ of corti to hair cells projecting into scala media

Front 28

Hair cell structure

Back 28

One end is near perilymph "normal" K+ and the other is projecting into endolymph "high" K+ with stereocilia with one kinocillium at end

Front 29

Initial signal transduction in hair cells

Back 29

Cell is RMV of -45mV, endolymph is +80mV, perilymph is 0mV

Vibration of basilar membrane up and down, tectorial membrane (NOT CONNECTED TO CILIA) causes fluid to move back and forth over cilia, movement of stereocilia/kinocilia leading to K+ channel opening , depolarization and opening of Ca++ channels, NT release

K+ enters from endolyph side and exits to perilymph side via voltage dependent K+ channels or Ca2+ sensitive K+ channels

Ca2+ enters via a voltage channel and is pumped out by ion pumps to reequilibrate.

Front 30

Stereocilia opening system

Back 30

One of stereocilia contains a fine filament (tip link) which when basilar membrane movement induces fluid movement and cilia movement, the tip link puls open a K+ channel allowing K+ to enter cilia then rest of cell

Front 31

Encoding Frequency

Back 31

Each cell is "tuned" to a particular frequency corresponding to its place along the basilar membrane

High frequency most near round window and lowest is near end of basilar membrane

There is processing to inhibit surrounding signals on each side (both ears) to fine tune frequency to what is desired to be heard

Front 32

What causes most hearing loss

Back 32

Damage to hair cells

Front 33

How do cochlear implants work

Back 33

Bypass inner hair cells by using a microphone instead of the ear to pick up vibrations

Microphone connects to a frequency analyzer which picks out frequencies and loudness of the sound then plays them back onto the basilar membrane at the places they would be heard if hair cells were intact

Works because cochlear n. is stimulated right under basilar membrane at right spot, Brain interprets the frequency and only about 8-16 distinct ones needed to hear spoken language

Front 34

How is loudness encoded

Back 34

Sound pressure to middle ear, more pressure on oval window if louder, more pressure = more movement of hair cells

Primary IHC afferents have more NT release linearly related to sound pressure level (firing rate is parallel to pressure level)

CAN be saturated by very loud sounds near 500 spikes/second

Front 35

Innervation in the hair cells

Back 35

Afferents - Type I afferent to inner hair cell, Type II afferent to outer hair cell.

Medial olivocochlear efferent from superior olive - to outer hair cells

Lateral olivocochlear efferent - to inner hair cells

Front 36

Innervation ratios inner vs outer hair cells and roles

Back 36

IHCs - HEAVILY innervated, 1:10 cells to afferents. Mutliple projections, Major auditory output for loudness, frequency, location, etc with lots of parallel processing

OHCs - light innervation (10:1) cells to afferents, NO primary auditory signaling role, ENHANCE hearing sensitivity

But there are about 3,500 IHC vs 14,000 OHC

Front 37

Cochlear amplification steps

Back 37

Outer hair cells

Sound waves (basilar membrane motion) are dampened by cochlear fluids so need to apply mechanical energy to augment the basilar membrane

OHCs enhance sensitivity due to EXTENSIVE EFFERENT innervation

OHCs have very little afferent signals and mainly just boost sensitivity for afferent IHCs

Front 38

Innervation path past ear

Back 38

From cochlea goes to cochlear nucleus in medulla (spikes I and II)

then to superior olivary complex in pons (spike III)

Then to inferior colliculi in midbrain (V) via the lateral lemniscus (IV)

Then to medial geniculate in thalamus (VI) then to auditory radiations in thalamocortical region (VII)

Decussates at trapezoid body by the Superior Olivary Nucleus so sounds go to both cortices

Front 39

Damage where along auditory path causes UNIL hearing loss

Back 39

Just cochlear nucleus

Anytime after will still maintain BIL sound

Front 40

Sound localization overview

Back 40

Occurs in superior olive. If sound comes in midline it will hit both ears (and olives) at same time.

If off one one side will reach there first

Medial superior olive deals with timing difference

Lateral superior olive encodes sound loudness (intensity)

Combined this tells direction and distance to localize a sound

Inter-aural shape (ear shape allows one to tell if sound is coming from in front or behind

Front 41

Inferior Colliculi location, role

Back 41

In caudal midbrain (wine glass)

Role - receives input from lateral lemniscus and superior olivary nucleus

Cells sense differences in intensity and inter-aural timing

Output is concerned with directing attention to sound location

Also involved in integration and orientation

Front 42

Medial geniculate location, role

Back 42

In the rostral midbrain at the beginning of thalamus (attached to pulvinar).

PROCESSING prior to the cortex. Heavy pathways both directions from cortex and thalamus

Multimodal cells - cells that not only respond to auditory information but also information from lateral geniculate and cortex to combine somatosensory information and visual information

Medial geniculate has some

Front 43

Auditory cortex, tonatopy

Back 43

Heschl's gyrus or transverse temporal gyri deep within sylvian fissure

Higher freq. sounds = medial, lower freq. sounds = lateral

Front 44

Tuning/processing of auditory cortical neurons

Back 44

Integrate/encode frequency, loudness, and location all within the auditory cortex

NOT interpretable at primary auditory cortex buts must be further processed at Wernicke's area or other association cortices

This is how sound ends up being interpreted by differing cortical areas attuned to specific dB and degrees off midline, etc.