Source: A Brain-Mind Odyssey, UC Berkeley
Tags: sensation, perception, chemotaxis, electromagnetic spectrum, visible light, olfaction, olfactory receptor, GPCR, anosmia, taste buds, umami, capsaicin, TRP receptors, flavor
Chapter 11 introduces the distinction between sensation (collecting environmental information) and perception (interpreting it), then surveys sensory capabilities across organisms, from bacterial chemotaxis to echolocation, electroreception, and magnetic field detection. Chapter 12 covers the olfactory system in detail: how airborne molecules bind to GPCR-type olfactory receptors, the neural pathway from nose to cortex and limbic system, and the role of pheromones. Chapter 13 does the same for taste, covering the five canonical tastes, ion channels and GPCRs in taste perception, TRP receptors for hot and cool sensations, and how "flavour" is really a multi-sensory combination.
Sensation
The collection of information from the environment via sensory organs and receptors.
Perception
The analysis and interpretation of sensory information by the nervous system, contributing to conscious perceptual awareness.
Chemotaxis
The movement of an organism toward or away from a chemical stimulus. In bacteria, amino acids interact with receptor proteins on the outer membrane, influencing the organism to swim toward nutrients.
E. coli motility: runs and tumbles
E. coli swims in a straight line (a "run") when flagella rotate in one direction, forming a propeller-like bundle. When flagellar rotation reverses, the bundle flies apart and the bacterium tumbles randomly. Runs continue in the direction of increasing nutrient concentration.
Phototaxis
An entire organism moves toward light.
Phototropism
An organism bends or grows toward light. Serves to increase exposure to light for photosynthesis or to grow toward open space for spore/seed dispersal.
Naive realism
The epistemological notion that what we perceive is identical to what actually exists in the world. Easily disproved by optical illusions.
Five canonical senses
Vision, hearing, taste, smell, and touch.
Vestibular sense
Detected by the semicircular canals of the inner ear. Receptor cells are hair cells; physical stimuli are gravity and acceleration. Perceptual experience: balance. Typically operates below conscious awareness.
Proprioception
Stretch receptors in muscles and joints detect information about muscle tension and joint position. Tunes body alignment and coordinated movement.
Visible light
A narrow band of the electromagnetic spectrum, roughly 400–700 nm. The only portion that engages in non-damaging interactions with molecular and cellular structures. The full electromagnetic spectrum spans from high-energy gamma rays to low-energy radio waves, a range of 18 orders of magnitude.
Karl von Frisch
Early 20th-century scientist who demonstrated that honeybees have colour vision and sensitivity to ultraviolet light and light polarisation. Nobel Prize in Physiology or Medicine, 1973.
Ultraviolet sensing (honeybees)
Honeybees can see UV patterns on flowers that are invisible to humans. UV is slightly higher in energy than visible light.
Infrared sensing (pit vipers)
Pit vipers have pit organs that detect infrared radiation (slightly lower in energy than visible light), allowing them to locate prey in complete darkness.
Polarisation of light
The alignment of the electromagnetic field's vibration in specific planes. Varies across the sky relative to the sun. Many insects, birds, and reptiles detect polarised light and use it for navigation (discovered by von Frisch).
Infrasound
Very low-frequency sound (below 20 Hz). Elephants can produce and detect infrasound.
Ultrasound
Very high-frequency sound (above 20,000 Hz). Used by bats, dolphins, and whales for echolocation.
Echolocation
Biological sonar. Animals emit high-frequency sounds and listen for reflections to navigate and hunt. Shorter wavelengths (higher frequencies) permit finer spatial detail.
Electroreception
The detection of electric fields generated by living organisms. Sharks use electroreceptive structures called ampullae of Lorenzini, dispersed over the head, to locate camouflaged prey. Platypuses use electroreceptors in the bill. Limited to aquatic environments where electric fields are stronger.
Magnetic field detection
Sensing the Earth's magnetic field for navigation. Many migratory animals (birds, fish, turtles) use this. Classic experiment: attaching a small magnet to a homing pigeon disrupts its navigation. Pigeons may use magnetic particles in the eye.
Sensation is the raw data collection by sensory organs
Perception is the brain's interpretation of that data into conscious experience
Naive realism (the assumption that perception equals reality) is wrong, as optical illusions easily demonstrate
Bacterial chemotaxis: E. coli uses runs and tumbles, modulated by nutrient gradients
Phototaxis (whole-organism movement toward light) and phototropism (bending/growing toward light)
Honeybees see UV and detect light polarisation
Pit vipers detect infrared with pit organs
Bats, dolphins, and whales use echolocation (ultrasound)
Sharks detect electric fields (ampullae of Lorenzini)
Many animals navigate using the Earth's magnetic field
Spans gamma rays to radio waves, 18 orders of magnitude
Visible light is a tiny band (400–700 nm), the only energies that interact non-destructively with biological tissue
Higher-energy radiation (UV, X-rays, gamma) is damaging; lower-energy radiation (radio waves) is too weak to excite photoreceptors
20 to 20,000 Hz
Below 20 Hz: infrasound (elephants); above 20,000 Hz: ultrasound (bats, dolphins)
Olfaction
The sense of smell. Begins when airborne volatile molecules (odorants) enter the nasal passages during inhalation and exhalation.
Olfactory epithelium
Moist, mucous-lined tissue in the interior of the nasal passages. Volatile molecules become trapped here and contact olfactory receptor cells.
Olfactory receptor cells
Cells embedded in the olfactory epithelium. Have cilia that increase surface area and contain olfactory receptor proteins (GPCRs).
Olfactory stem cells
Cells within the olfactory epithelium that can differentiate into various types of olfactory receptor cells. Receptor cells are replaced every 1–2 months.
Olfactory receptor proteins
G-protein coupled receptors (GPCRs) located on the cilia of olfactory receptor cells. Each GPCR responds to molecules with specific molecular shapes.
Number of olfactory receptor proteins by species
Fish: roughly 100
Mammals (general): roughly 1,000
Mouse: roughly 1,300
Human: roughly 350 functional receptors (plus 650 pseudogenes)
The 350 human GPCRs can discriminate roughly 100,000 different odours through combinatorial patterns of activation
Pseudogenes
Nonfunctional genes that appear to code for olfactory GPCRs but are altered so they do not produce a working receptor protein. Humans have roughly 650 olfactory pseudogenes.
Essential oil
The oily concentrate of volatile, aromatic molecules from a plant. Hydrophobic. Often prepared by distillation (a technique originally invented for perfume-making).
Thiols
Molecules containing the –SH (sulfur-hydrogen) group. Often smell stinky. Examples: skunk spray, asparagus-urine odour.
Specific anosmia
Loss of sensitivity to a specific type of smell. Most likely caused by a genetic variation in one of the 350 olfactory GPCRs. Example: not everyone can smell the characteristic odour in urine after eating asparagus.
General anosmia
Loss of sensitivity to a wide range of aromas, potentially a complete lack of olfactory sensitivity. Causes include head trauma, nasal congestion, developmental factors, and degenerative brain disease.
Olfactory bulb
Located immediately above the nasal cavity. Olfactory receptor cells send their axons here. The nerve fibres between the nose and the olfactory bulb form cranial nerve I.
Mitral cells
Neurons in the olfactory bulb whose dendrites form synapses with incoming olfactory receptor cell axons. Mitral cells send axons to the pyriform cortex and the amygdala.
Olfactory neural pathway
Olfactory receptor cells → olfactory bulb (synapse with mitral cells) → two main routes:
Pyriform cortex → thalamus → orbitofrontal cortex (conscious odour identification)
Amygdala, temporal cortex, hypothalamus (emotional/limbic connection)
Pheromones
Chemicals that carry signal information for social communication between members of the same species. Extensively studied in insects (used for identity, social status, mate attraction, territorial marking, danger signalling).
Vomeronasal organ
A distinct olfactory sensory structure with its own neural pathway, found in many vertebrates (especially rodents). Responds somewhat selectively to pheromone molecules.
Odorants (airborne volatile molecules) enter the nasal passages and become trapped in the moist olfactory epithelium
They bind to GPCR olfactory receptor proteins on the cilia of olfactory receptor cells
Humans have about 350 functional olfactory GPCRs; each responds to specific molecular shapes
Different patterns of activation across the 350 receptor types allow discrimination of roughly 100,000 odours
Olfactory receptor cells are replaced every 1–2 months from stem cells in the epithelium
This is adult neurogenesis in the peripheral nervous system
Plant aromas consist of dozens of different molecules activating various receptor combinations
Even tiny molecular changes (e.g. geranial vs. geraniol, differing by 2 hydrogen atoms) can produce distinct smells (lemon vs. rose)
Olfactory receptor cells → olfactory bulb (cranial nerve I) → mitral cells → two branches:
To pyriform cortex → thalamus → orbitofrontal cortex (odour identification)
To amygdala and hypothalamus (emotional regulation, limbic system)
The direct connection to the limbic system explains the strong link between smell and emotion/memory
Pheromones carry social information within a species
Many vertebrates have a vomeronasal organ, a separate olfactory structure specialised for pheromone detection
Best studied in insects and rodents
Taste buds
Clusters of approximately 100 taste receptor cells. About 10,000 taste buds in the human mouth (primarily on the tongue, also on the upper palate and pharynx).
Microvilli
Filamentous structures at the ends of taste receptor cells. Increase the surface area exposed to tasteable substances (analogous to cilia in olfactory cells).
Gustatory cell replacement
Taste receptor cells are replaced approximately every 2 weeks by taste-receptor stem cells. Necessary because these cells are continually exposed to damaging substances from the environment.
Five canonical tastes
Salt, sour, bitter, sweet, umami.
Salt taste
Detected via ion channels that allow sodium ions (Na⁺) to flow across the membrane. The stimulus is NaCl.
Sour taste
Detected via ion channels sensitive to hydrogen ions (H⁺). The stimulus is acids.
Bitter taste
Detected via GPCRs. Humans have over 30 different bitter-receptor GPCRs, allowing detection of diverse molecular shapes associated with bitterness. Examples of bitter substances: plant alkaloids such as caffeine, cocaine, morphine, quinine.
Sweet taste
Detected via GPCRs (only 2 in humans). Sugar molecules bind as ligands to sweet-receptor GPCRs, causing a shape shift and initiating an intracellular signal. Fewer receptor types means less diversity in sweet taste compared to bitter.
Synthetic sweeteners
Non-nutritive molecules that taste sweet despite not being sugars. Saccharin (first discovered, 300x sweeter than sucrose) and aspartame (200x sweeter, a dipeptide) are classic examples.
Umami
The fifth taste, described as savoury, meaty, or mushroom-like. Identified by Japanese chemist Kikunae Ikeda, who proposed concentrating it as MSG. The taste receptor is a metabotropic GPCR glutamate receptor, activated by glutamate and some other amino acids.
Gustatory neural pathways
Cranial nerve fibres carrying taste information enter the brain via the lower brainstem (nucleus solitarius) and split into two pathways:
Thalamus → insula and somatosensory cortex (parietal lobe): the primary gustatory cortex, responsible for conscious taste interpretation
Hypothalamus and amygdala: limbic system, emotional responses to taste
Capsaicin
The single molecular constituent responsible for the "hotness" of chilli peppers (Capsicum annuum). Binds to TRPV1 receptors, opening ion channels that allow Ca²⁺ to flow in, depolarising the cell. These same receptors also respond to thermal heat, which is why chilli feels "hot." Signals enter the brain via the 5th cranial nerve and are processed in pain-associated regions.
TRP receptors (transient receptor potential)
A family of ionotropic channel receptors found throughout the body (mouth, skin, nervous system). Respond to temperature, capsaicin, menthol, and other stimuli. An example of biological reuse: a useful protein structure tweaked for different purposes in different locations.
Menthol
Found in mint plants. Binds to TRPM8, a TRP-family ionotropic receptor, opening a calcium channel and producing the perception of coolness.
Isothiocyanates
Molecules containing a sulfur-carbon-nitrogen group. Responsible for the pungent "hotness" of mustard, horseradish, and wasabi. Activate TRPA1 receptors.
Flavour
A combination of taste, aroma, pungency (hot/cool sensations), and texture. Aromatic properties are particularly important. Pungency (capsaicin, menthol) is a separate sensory system from taste, wired through different pathways.
10,000 taste buds in the mouth, each with roughly 100 taste receptor cells
Taste cells are replaced every 2 weeks from stem cells (because they face constant environmental damage)
Microvilli increase the surface area of taste cells, just as cilia do for olfactory cells
Salt: ion channels allow Na⁺ to cross the membrane
Sour: ion channels sensitive to H⁺ (acids)
Bitter: over 30 GPCRs, enabling detection of a wide variety of potentially toxic molecules (e.g. plant alkaloids)
Sweet: only 2 GPCRs, so less diversity in sweet taste perception
Umami: metabotropic GPCR glutamate receptor, activated by glutamate and certain amino acids. Identified by Kikunae Ikeda; concentrated as MSG
Capsaicin (chilli) activates TRPV1, which also responds to thermal heat, same receptor, same "hot" perception
Menthol (mint) activates TRPM8, producing the sensation of coolness
Isothiocyanates (mustard, wasabi) activate TRPA1
TRP receptors are found not just in the mouth but in skin and throughout the nervous system
Pungency signals travel via the 5th cranial nerve to pain-processing regions, a completely different pathway from taste
Flavour = taste + aroma + pungency + texture
Aromatic (olfactory) properties contribute even more than taste itself
This is why food seems flavourless when you have a blocked nose
⚠️ Sensation vs. perception is a foundational distinction. Sensation = data collection by receptors; perception = interpretation by the nervous system.
⚠️ Know which senses use ion channels (salt, sour) vs. GPCRs (bitter, sweet, umami, smell). This is a common exam comparison.
⚠️ Humans have roughly 350 functional olfactory GPCRs but over 30 bitter-taste GPCRs and only 2 sweet-taste GPCRs. The numbers matter for explaining why bitter taste is more diverse than sweet.
⚠️ The olfactory pathway's direct connection to the amygdala and limbic system explains the powerful link between smell and emotion/memory. Expect a question on this.
⚠️ Capsaicin binds to TRPV1, the same receptor activated by actual heat. That is why chilli feels "hot." Do not confuse TRP receptors (pungency/temperature, ionotropic) with taste receptors (GPCRs or ion channels for the five canonical tastes).
⚠️ Flavour is NOT the same as taste. Flavour = taste + aroma + pungency + texture.
⚠️ Olfactory and gustatory receptor cells both regenerate from stem cells (olfactory every 1–2 months, gustatory every 2 weeks). Know both timescales.
⚠️ Know the specific TRP receptors: TRPV1 (capsaicin/heat), TRPM8 (menthol/cool), TRPA1 (isothiocyanates/pungency).
Q: What is the difference between sensation and perception?
A: Sensation is the collection of information from the environment via sensory organs and receptors. Perception is the nervous system's analysis and interpretation of that information, producing conscious awareness.
Q: How do the taste receptors for salt and sour differ mechanistically from those for bitter and sweet?
A: Salt and sour use ion channels (Na⁺ channels for salt, H⁺-sensitive channels for sour). Bitter and sweet use GPCRs. Bitter has over 30 GPCRs (detecting diverse toxins); sweet has only 2.
Q: Why is the olfactory system uniquely connected to emotion and memory?
A: The olfactory neural pathway sends signals directly to the amygdala and hypothalamus (limbic system), bypassing the thalamic relay that most other senses use. This direct limbic connection underlies the powerful emotional associations of smell.
Q: Why does capsaicin produce the sensation of heat?
A: Capsaicin binds to TRPV1 receptors, which are also activated by actual thermal heat. The same ion channel opens, the same Ca²⁺ influx occurs, and the brain interprets both stimuli through the same pain-associated pathway.
Q: How many functional olfactory receptor proteins do humans have, and how can they detect roughly 100,000 odours with that number?
A: About 350 functional olfactory GPCRs. Different odorants activate different combinations of these receptors, creating a vast number of distinguishable activation patterns through combinatorial coding.
Q: What is a pseudogene, and how many olfactory pseudogenes do humans have?
A: A pseudogene is a nonfunctional gene that appears to code for a protein but is altered so it does not produce a working product. Humans have roughly 650 olfactory pseudogenes alongside their 350 functional olfactory receptor genes.
Q: What is umami, and what type of receptor detects it?
A: Umami is the fifth canonical taste, described as savoury or meaty. It is detected by a metabotropic GPCR glutamate receptor, activated by glutamate and some other amino acids. Identified by Kikunae Ikeda.
Q: How is flavour different from taste?
A: Flavour is a multi-sensory combination of taste, aroma (olfaction), pungency (hot/cool, mediated by TRP receptors), and texture. Aroma is often the most important component.
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