Taste-WK2

where is taste on the body

• on the tongue mediated by chemoreceptors

where are taste buds located

on the side of the papillae (the tongue)

• consist of a cluster of spindle shaped cells –some of these are the taste receptors, others are supporting cells

what is the microvilli

• they project into the taste pore

• at base of the taste buds which communicate through afferent neurons

describe sensing in taste

1. to interact with chemoreceptors, tastants must be in a solution

2. the tastants dissolve into saliva & mucous in the mouth

3. Stimulation of the chemoreceptor results in the receptor potential –a depolarisation–which causes release of neurotransmitter from the taste cell

4. The neurotransmitter stimulates an action potential in the afferent gustatory sensory neurons

5. Neurons project to the brainstem and the information is relayed through the thalamus to the gustatory cortex and hypothalamus

what are the 5 primary sub-modalities of taste

1. salty

2. sour

3. sweet

4. bitter

5. umami (savoury taste)

describe sour

• sour is associated with acidic food or drink & also spoiled food

• it is detected as H+ ions by distinct ion channels

• OTOP1 is a proton-selective ion channel

• the H+ entry causes cell acidification which closes K+ channels generating depolarisation

what is cell acidification

it causes k+ channels to generate signals to depolarise

describe salt

salt is essential in maintaining electrolyte balance

• it is sense as Na+ by ion channels like ENaC (epithelial sodium channel)

• Na+ is sensed in the conc. between 10-500mM

• the opening of the ENaC depolarises the cells which causes neurotransmitter release onto the sensory afferent neurone

describe sweet

• sugars & artificial sweeteners (saccharin, aspartame) are detected as sweets

• they are detected by GPCRS

• also activate a complex of T1R2/T1R3

• the GPCR uses a special G protein gustducin to signal

describe bitter

• it is correlated with poison

• there are 50-100 different bitter receptors, which respond to different chemicals

  • it is sensed by the different GPCRs T2R

  • many alkaloids chemicals are poisonous - bitter taste is a defence against poison

what is umami

• it means delicious in Japanese

• it is triggered by amino acids monosodium glutamate (MSG) - a flavour enhancer

• activates the GPCR dimer T1R1/T1R3 to signal

• the GPCRs use G_agus to signal & activate PLC

what are the 4 taste cell types

1. type i cells - glial (astrocytes)

2. type ii - receptor cells respond to sweer, bitter , umami

3. type iii- respond to sour & salty

4. type IV - are immature

describe type ii receptor signal transduction

type ii receptors cells respond to sweet, bitter, umami & releases ATP as a neurotransmitter to communicate with sensory afferent neuron

what is type iii signal transduction

• has a conventional synapses with sensory afferent neuron

• neurotransmitter is 5-HT

• they respond to sour via the OTOP1 channels - release neurotransmitter to communicate

• involved in salty detection through ENaC

describe taste signalling

1. sour & salt taste are detected by ion channels

2. sweet, umami & bitter taste are detected by GPCRs that bind to Gα gustducin

3. it stimulates the PLC-beta to increase the IP₃ & release Ca²⁺ from the intracellular stores

4. Ca²⁺ activates TRPM5 ion channels causing sodium influx & membrane depolarisation

5. it releases ATP from type ii taste cell which activates the sensory nerve fibre via purinergic receptor stimulation

6. the information is then carried through afferent neurons to the gustatory area of the brain

what are other taste sensations

• fat is also said to be sensed in the mouth - the K+ channels are activated by fatty acids

• the transporter molecule CD36 & 2 fat sensitive GPCRs are potentially involved

what is the somatosensory pathway

• it is the nerve that senses spice like capsaicin from chilli pepper & menthol in mint via TRPV1 ion channels