Chapter 9: Sensory Systems - Taste & Smell

Taste

The ability to sense these tastes is determined genetically, but how we perceive these taste is culturally/environmentally determined

Tongue

The organ that houses papilla

Papilla

“Bumps” on our tongue that act to increase the total surface area of the tongue

Taste Buds

Housed by papilla, a cluster of 50-150 taste receptor cells

Taste Pore

On the surface of a taste bud, an opening where taste cells extend the process of cilia that detect tastants

Sensing Taste Selectivity

Each taste cell is sensitive to a single taste ➡ Each selectively express certain receptors for distinct chemicals

Distinct Processes in the Taste Cell

• Bitter, Sweet, and Umami all act through metabotropic receptors

• Salty and Sour act through ionotropic receptors

Salty

Sensed primarily by detecting sodium (Na+) ions through sodium channels;

• Can be both appetitive and aversive

Sour

Driven by acid sensing ➡ Acids all release protons (H+);

• Sour-sensing taste cells express an extremely selective ion channel, causing a rush of protons into the cell when detected

• Necessary for detecting carbonation

• Necessary for regulating thirst-driven water consumption

Bitter

Thought to have evolved to sense toxic substances;

• The T2R family of receptors are necessary for bitter taste

• Every bitter taste cell seems to express most or all bitter receptors, suggesting broad tuning

Sweet

Sensed by heteromers of metabotropic receptors in the T1R family;

• Combination of T1R2 and T1R3 make sweet taste possible

• We sense different sweets by their different interactions with these receptor complexes

Umami

Sensed by heteromers of metabotropic receptors in the T1R family;

• The combination of T1R1 and T1R3 make savory possible

• Umami is possibly also sensed by a metabotropic receptor for glutamate

Gustatory System

1. Taste cells transmit information through 3 cranial nerves which travel directly to the brainstem

2. The brainstem then sends information to the gustatory thalamus, to the gustatory cortex which lives in the insula

Optogenetics

A technique that allows researchers to use light to directly control the activity of different neurons in the brain

Channelrhodopsin

A family of light-gated cation channels found in green algae

Olfaction

The ability to sense smell is absolutely essential for survival

Olfactory Epithelium

A sheet of sensory receptor cells that lines the most dorsal aspects of the nasal cavity;

• Houses three cells that it make up:

  • Basal Cells

  • Supporting Cells

  • ~6 million Olfactory Receptor Neurons

Olfactory Receptor Cells

1. Each olfactory receptor extends a long process into the epithelium that terminates in a cilia that sits in the mucosal surface

2. On the other end, the cell sends a long axon that joins up with other olfactory axons to make the olfactory nerve (cranial nerve 1)

Olfactory Signal Transduction

1. Odorants enter the nasal cavity during inhalation

2. The volatile compounds in the air are dissolved into the fluids of the mucosal layer to interact with the receptors on the olfactory receptor neurons

3. Odorant receptors are a huge family of metabotropic GPCRs (G-coupled protein receptors) with a specific classification of G_olf

4. Humans have genes that code for over 1000 olfactory receptors, but only 400 are functional

Functional Olfactory Receptors

Even with only 400 functional receptors, we can perceive over 10,000 different odors ➡ We do so by population coding: summing the activity of many olfactory receptors

Rodent Olfactory System

Rodents have 1000 olfactory receptors divided into 4 different families ➡ These four families exhibit spatial segregation in the olfactory epithelium and in the olfactory bulb

Pheromones

A volatile chemical or odor signal emitted by an animal

Vomeronasal Organ (VNO)

Special olfactory organs that sense pheromones;

• Most mammals, amphibians, and reptiles have a VNO

• Phermones are sensed by V1R and V2R receptors

VNO Projection

The VNO projects to the accessory olfactory bulb, which projects directly to the medial amygdala

Human Olfactory System

1. Following processing in the olfactory bulb, smell information is sent directly to the entorhinal cortex amygdala, and pyriform cortex (only sense that doesn’t first go to the thalamus)

2. This rapid connection from smell to amygdala and entorhinal cortex is maybe why smell is so strongly connected to memory