8 Chemical senses

a = papillae

b = vallate papilla, showing locations of the taste bud

c =

• taste bud = cluster of taste cells

• taste pore = exposure to mouth

• microvilli extend to pore

vallate papillae

basal cells (in olfactory system the same)

keeps food from entering in the windpipe/lungs

2 weeks

—> tongue falls back, obstruction because of Epiglottis

VII = facial nerve

• sensory anterior part of the tongue

IX = glossopharyngeal nerve

• sensory posterior part of the tongue

X = vagus nerve

• glottis, epiglottis, pharynx —> taste axons —> vagus nerve

afferents from tongue and epiglottis (cranial nerves)

—> left gustatory nucleus (Medulla)

—> VPM=left ventral posterior medial nucleus (Thalamus)

—> primary gustatory cortex

(mostly ipsilateral)

anterior Insula

microvilli —> interaction with a tastant —> change in membrane potential

—> voltage-gated calcium channels open

—> Ca2+ influx

—> Neurotransmitter release

• ATP, 5-HT, GABA

Salty, Sourness, Bitterness, sweetness, umami

Salty: mainly due to Na+ influx

Sourness: mainly due to H+ influx

Bitterness, sweetness, umami: Gq protein coupled

salty taste receptors = Na+ channel = ENaC = epithelial

sodium channel

• Na+ influx

• depolarization

• concentration gradient towards basal part

• voltage-gated Na+ and Ca2+ channels open at the basal part

• depolarization

Sour taste receptor: H+ channel, Otopetrin-1 = protonselective channel

• opening H+ channels, inhibition of K+ channels (efflux)

• H+ influx, no K+ efflux

• depolarization

• concentration gradient towards basal part

• voltage-gated Na+ and Ca2+ channels open at the basal part

• depolarization

Gq protein coupled receptors, PLC (IP3+ Ca2+)

2 types

T2R dimer

• Bitter receptors

T1R

• Sweet receptors T1R2+T1R3

• Umami receptors T1R1+T1R3

population coding

= a taste is encoded by shared activity pattern of many broadly tuned neurons

expressed in different taste cells

→ connected to different gustatory axons

(same for umami)

• olfactory sensory neurons

• glomerulus = synapsing of olfactory sensory neurons with mitral cells

• periglomerular cell = inhibitory interneuron

• mitral cell = main output neuron

• Direct Pathway (Primitive Cortex) through olfactory tract

  = emotional and motivational processing

• Conscious Perception Pathway (Neocortex)

  = conscious discrimination

  —> olfactory tract —> olfactory tubercule

  —> medial dorsal nucleus (thalamus)

  —> orbitofrontal cortex

—> direct pathway to the CNS!

odorants dissolve in olfactory epithelium (mucus layer)

+bind to specific odorant receptor proteins on the cilia of the receptor neurons

—> G-protein called Golf​

—> adenylyl cyclase

—> cyclic adenosine monophosphate (cAMP)

—> binds directly to a cyclic nucleotide-gated cation channel

—> Na+ u. Ca2+ influx

—> Ca2+-activated chloride (Cl−) channels get open

—> Cl- outlflow (Cl- high intracellular)

—> further depolarisation

Olfactory Population Coding

• Each odor produces a unique activity pattern (population code)

• encoded by combined activity of receptor neurons

Olfactory Maps

• All olfactory cells with the same receptor type send their axons to the same glomerulus —> “map”

Temporal Coding

• encoded by the timing of action potentials

different colours = different olfactory receptor types

hunger —> endocannabinoids produced and released —> CB1 receptors activated —> sense of smell enhanced

Kallmann-Syndrome

X-linked recessive genetic disorder

during development

• axons of the olfactory receptor neurons do not connect to the olfactory bulb

• GnRH-1 cells do not migrate from the nasal region to the forebrain

infertility

—> lack of GnRH-1 cells in Hypothalamus

—> FSH, LH are not released

Olfactory receptor can instruct new pathway finding