Neuroscience: Introduction Chemical Senses: Unit 2: lecture 1

The Chemical Senses:Chemoreceptors

sensory receptor selective for chemicals:

The Chemical Senses:Gustation (taste)

detects environmental chemicals

The Chemical Senses:Olfaction (smell)

detects environmental chemicals

Both gustation and olfaction…

• Strong and direct connections with our internal needs (thirst, hunger, emotion, and certain forms of memory)

• But their chemoreceptors differ structurally and mechanistically

Basic Tastes:

saltiness, sourness, sweetness, bitterness and umami

The traditional "tongue map" is a persistent scientific misconception

All basic tastes can be detected across all regions of the tongue where taste buds are present

Organs of taste and smell:

The anatomical structures we use to detect chemicals by smell and taste are physically connected

Pharynx

connects the mouth with the nasal passages allowing exchange of chemicals from food

Tongue

is a sensory organ for both chemo-sensation and somato-sensation

Filiform papillae do not contain taste buds

🡺 Food grip and texture

Foliate papillae

Fungiform papillae              taste buds

Circumvallate papillae

A taste bud:

cluster of taste cells (receptor cells) 

1 taste buds =50 - 200 taste receptor cells 

1 papillae = 1-100 taste buds

Taste receptor cells

Small opening

The taste cell is exposed to the contents of the mouth

Chemicals cause the taste cells to change their membrane potential and the sensory neuron to fire action potentials

• Most taste receptors respond to 1 taste

• NaCl: Salty

• Quinine: Bitter

• HCl: Sour

• Sucrose: Sweet

Mechanism of Taste Transduction

Transduction – process of environmental stimuli causing an electrical response in a sensory receptor

• Taste Transduction: Tastants may

• (1) Directly pass through ion channels (salt and sour)

• (2) Bind to and block ion channels (sour)

• (3) Bind to G-protein-coupled receptors in the membrane 🡪 activate secondary messengers 🡪 open ion channels (bitter, sweet, umami)

Taste Selectivity and Concentration: Threshold concentration

• The lowest concentration of a chemical (tastant) that can reliably evoke a taste response

Taste Selectivity and Concentration: Near threshold

Taste receptor cells and fibers may show relatively selective responses to particular tastes

Taste Selectivity and Concentration: Above threshold

• Increasing the concentration recruits more taste receptor cells and fibers, producing broader and more overlapping responses

• Overall: Taste coding is not strictly one receptor/one taste. Instead, it relies on overlapping population coding, with different taste receptor cells and neural fibers responding preferentially to different Tastants
  
  

• Important: A papilla is not dedicated to a single taste; each papilla contains multiple taste buds, and taste buds contain different types of taste receptor cells

Taste stimuli that directly pass through ion channels:

SALT AND SOUR

Transduction: Bitter, Sweet, Umami

• Two families of genes:

  • T1R – taste receptor protein 

  • T2R – taste receptor protein 

G-protein 

Taste stimuli via G-protein receptors:

Central Taste Pathway

TASTE BUDS 🡪 GUSTATORY AXONS 🡪 MEDULLA🡪 THALAMUS 🡪 CEREBRAL CORTEX

• Anterior tongue 🡪 cranial nerve VII (7) (facial nerve)

• Posterior tongue 🡪 IX (9) (glossopharyngeal)

• Throat (glottis, epiglottis, pharynx) 🡪 X (10) (vagus nerve)

3 cranial nerves carry gustatory axons to the MEDULLA (in the brain stem)

GUSTATORY  NUCLEUS (NTS)

🡪 diverges into parallel pathways that process:

• conscious perception (Thalamocortical Pathway)

• autonomic responses (Hypothalamic Pathway)

• emotional associations with food (Limbic / Amygdala Pathway)

Vagus Nerve (CN X) Innervation:

Taste buds are located on the posterior wall of the pharynx and epiglottis

Thalamocortical pathway

• conscious taste perception

  • NTS → VPM nucleus of thalamus → primary gustatory cortex (insula/frontal operculum)

  • Allows you to consciously perceive: “This is sweet,” “This is bitter,” etc

Hypothalamic pathway

• autonomic responses

  • NTS → hypothalamus and autonomic centers

  • Produces physiological responses associated with eating, such as salivation, digestive responses, and changes in appetite

Limbic/amygdala pathway

• emotional/reward associations

  • Taste information reaches limbic structures, including the amygdala

  • Contributes to emotional/behavioral responses to food, such as food preference, aversion, and reward

Neuronal Coding of Taste

Why don’t we have very specific taste receptors?

• (1) need a lot of receptors

• (2) couldn’t taste new things

How do we get around that?

• Population Coding – response of a large number of broadly tuned neurons, rather than a small number of precisely tuned neurons, are used to specify the properties of a particular stimuli

Olfaction

The sense of smell

• Why do we smell?

1. Helps us identify foods

   • Increasing or decreasing their enjoyment

2. Complements the information from the sense of taste

3. Warns us of potential dangers: smoke, spoiled food, etc

   • Out of several hundred of thousands of  molecules only 20% are pleasant

4. Mode of communication: reproductive behavior (pheromones), mark territory, identify individuals, indicate aggression, or submission

The Organs of Smell

Olfactory epithelium – small, thin sheet of cells lining part of the nasal passages that contains olfactory receptor neurons

3 main cell types:

1. OLFACTORY RECEPTOR – sight of transduction

   • Actual neuron

   • One of the few that are regularly replaced throughout life

2. Supporting cells – helps produce mucus

   • Similar to glia

   • Odorants (chemical stimuli in the air) – dissolve in the mucus before reach receptor

3. Basal Cells – source of new receptor cell growth

   • Every 4 to 8 weeks

Olfactory receptor during stimulation 

wo depolarizing events in the cilia: 

•  Receptor potential

• fCa²⁺-activated Cl⁻ current

Olfactory Response may terminate for several reasons:

1. Olfactory Adaptation (Sensory Fatigue)

2. Clearance of Odorant Molecules        

3. Pathological or Physiological Damage

OLFACTORY ADAPTATION

If you are exposed to a smell long enough, you stop smelling it!

Olfactory Receptor Proteins

• 1000s of them 

• 350 of them 

• They are G-protein-coupled receptors 

• Typically, each odorant receptor cell expresses only 1 receptor type

Population Coding

• Each receptor binds different odorants with less or more affinity, so each receptor is less or more sensitive to those odors:

  • Broadly tuned

• Population Coding:

• Central olfactory pathways decode the information from the whole neuronal population 

• Activity distributed over a large number of neurons

Glomeruli

cluster of neurons in the olfactory bulb that receive input from olfactory receptor neurons

First synapse occurs in the glomeruli of the olfactory bulb

Ratio: 25,000 axons synapse onto 100 2^nd order neurons 

CONVERGENCE

• Receptor axons of cells expressing the same odorant receptors project to the same glomeruli: 

• 🡺 spatial odorant maps

• "This particular combination of locations is associated with this odor."

Temporal Coding in the Olfactory System

• Temporal coding – depends on the timing of spikes

• For smell – encode quality of odors

Pathways

Olfactory summary

1. Each odor is represented by the activity of a large population of neurons

2. The neurons responsive to particular odors may be organized into spatial maps

3. Timing of action potentials may be an essential code for particular odors

Test yourself

• What are the 5 flavors? What purpose does each serve?

• What is the mechanism of sensory transduction for each flavor? What could block tasting each?

• Why have so many subtypes of bitter receptor?

• What is the anatomy of the olfaction circuit?

• What is adaptation?

• Compare & contrast the labeled-line hypothesis with population coding.

• What is spatial coding? Temporal coding?