Sensory Systems: Taste, Smell, and Perception in Psychology

Sensation

The bottom-up process by which sensory organs detect and send information from the environment to the brain.

Perception

The top-down process where the brain interprets and organizes sensory information into meaningful experiences.

Difference between sensation and perception

Sensation collects raw input; perception gives it meaning.

Examples of chemical senses

Gustation (taste) and olfaction (smell).

Chemoreceptors

Specialized receptor cells that detect changes in chemical composition inside and outside the body.

Functions of chemical senses

Detect danger, aid in feeding and reproduction, and link strongly to emotion and memory.

Gustation definition

The sense of taste, detecting chemicals dissolved in saliva.

The six basic tastes

Bitter, Sour, Sweet, Salty, Umami, Fatty.

Function of bitter taste

Detects poison alkaloids; triggers avoidance.

Function of sour taste

Detects acidity (low pH), often signaling spoiled or unripe foods.

Function of sweet taste

Detects sugars; signals energy-rich foods.

Function of salty taste

Detects sodium chloride; helps maintain electrolyte balance.

Function of umami taste

Detects glutamate; signals protein-rich foods.

Function of fatty taste

Detects fatty acids; signals high-energy food.

Definition of flavor

Combination of gustatory, olfactory, and somatosensory information (texture, temperature).

Evolutionary purpose of taste

Survival mechanism for identifying safe and nutritious foods.

Papillae

Visible bumps on the tongue that house taste buds.

Taste buds

Clusters of 20-50 taste receptor cells located on papillae.

Taste receptor cells (TRCs)

Cells within taste buds that detect specific taste molecules and send signals to the brain.

Life span of taste receptor cells

Approximately 2 weeks; they regenerate constantly.

Why taste receptor cells regenerate

Because they are directly exposed to environmental damage through food and chemicals.

Function of gustatory afferent axons

Carry taste information from TRCs to the brain.

Transduction definition

The process of converting an environmental stimulus into an electrical signal within a sensory receptor cell.

Ionotropic transduction

Direct action of ions on receptors (used by salty and sour tastes).

Metabotropic transduction

Involves G-protein coupled receptors and second messengers (used by sweet, bitter, and umami tastes).

Salty Taste Transduction Steps

1. Na⁺ enters TRC → depolarization → opens VG Na⁺/Ca²⁺ channels → serotonin release → neuron fires.

Sour Taste Transduction Steps

1. H⁺ blocks K⁺ channels → depolarization → VG Na⁺/Ca²⁺ channels open → serotonin release → neuron fires.

Sweet, Bitter, Umami Transduction Steps

1. Food binds GPCR → G-protein activates cascade → Ca²⁺ release → depolarization → ATP release → neuron fires.

Key difference between ionotropic and metabotropic taste transduction

Ionotropic uses direct ion channels; metabotropic uses G-protein cascades.

Neurotransmitter released by salty/sour cells

Serotonin.

Neurotransmitter released by sweet/bitter/umami cells

ATP.

Cranial nerves carrying taste info

CN VII (Facial), CN IX (Glossopharyngeal), CN X (Vagus).

First brain relay for taste

Gustatory nucleus (nucleus of the solitary tract) in the medulla.

Thalamic relay nucleus for taste

Ventral posterior medial (VPM) nucleus of the thalamus.

Final cortical area for taste

Gustatory cortex (insula and frontal operculum).

Summary of taste pathway

Taste buds → CN VII/IX/X → Medulla → Thalamus (VPM) → Gustatory cortex.

Labeled line model

Each taste receptor or neuron is specialized for one taste quality.

Population coding model

The brain decodes taste by comparing patterns of activity across many neurons.

Advantage of population coding

Allows perception of countless complex flavors beyond the five/six basic tastes.

Olfaction definition

The sense of smell, detecting airborne chemicals called odorants.

Functions of olfaction

Identifying foods, detecting danger, social communication, emotional memory.

Adaptive importance of smell

Survival, reproduction, and pleasure from eating.

Example of trained olfaction

Perfumers and sommeliers improve detection and discrimination of odors with practice.

Olfactory epithelium

Sheet of cells in the upper nasal cavity where smell transduction begins.

Three main cell types of olfactory epithelium

Olfactory receptor neurons, supporting cells, and basal cells.

Olfactory receptor neuron structure

Dendrites with cilia that bind odorants; axons form CN I (olfactory nerve).

Path of odorants

Odorant → dissolves in mucus → binds to receptor on cilia → triggers electrical signal.

Cribriform plate

Tiny holes in the skull that olfactory axons pass through to reach the olfactory bulb.

Olfactory bulb

First brain structure to process olfactory information.

Mitral cells

Neurons in the olfactory bulb that receive input from ORNs and send output to the brain.

Olfactory glomeruli

Structures in the olfactory bulb where ORNs synapse onto mitral cells.

Steps of olfactory transduction

1. Odorant binds GPCR → activates G-protein → opens Ca²⁺/Na⁺ channels → depolarization → AP → mitral cell.

Type of receptor used in olfaction

G-protein coupled receptor (GPCR).

Number of olfactory receptor subtypes in humans

About 339.

How we smell thousands of odors

Combinatorial coding: each odor activates a unique combination of receptors.

Olfactory adaptation

Decreased sensitivity to a continuous odor over time.

Primary olfactory pathway

ORN → olfactory bulb → mitral cells → olfactory cortex and temporal lobe structures.

Unique feature of olfaction

It bypasses the thalamus initially.

Secondary olfactory pathway

Olfactory tubercle → medial dorsal thalamus → orbitofrontal cortex.

Role of amygdala in olfaction

Connects smell with emotional responses.

Role of hippocampus in olfaction

Links smell with memory.

Role of orbitofrontal cortex in olfaction

Conscious identification and perception of odors.

Why flavor is complex

Combination of gustatory, olfactory, and somatosensory inputs (texture, temperature).

Why food tastes bland when you have a cold

Blocked nasal passages reduce olfactory input, limiting flavor perception.

Three reasons we perceive countless flavors

Population coding of taste, combinatorial coding of smell, and cortical integration.

Six Tastes Mnemonic

SSSBUF (Sweet, Sour, Salty, Bitter, Umami, Fatty).

Taste Pathway Mnemonic

Tongue → Nerves (VII, IX, X) → Medulla → Thalamus → Cortex.

Smell Pathway Mnemonic

Odor → ORN → Bulb → Mitral → Cortex (No Thalamus first!).

Ion vs GPCR Mnemonic

Simple tastes use ions; complex tastes use cascades.