Tactile Perception and Pain: Mechanoreceptors, Pathways, and Brain Processing

What causes tactile perception

Tactile perception arises from mechanical deformation of the skin.

• Includes indentation, vibration, bending, and stretching.

This system allows us to perceive object properties such as:

• Shape

• Texture

• Hardness

Mechanoreceptors

are specialized sensory receptors located in the skin and surrounding tissues.

They respond to physical forces like touch, pressure, vibration, and stretch.

Different types of mechanoreceptors

Different mechanoreceptors are tuned to specific types of stimulation:

• Light touch • Pressure • Vibration • Skin stretch

• These receptors are located at different depths of the skin, contributing to how we interpret tactile information.

Sensitivity

Sensitivity refers to the ability to distinguish between different types of touch stimuli.

This ability depends on:

• The interaction of multiple mechanoreceptors

• Each receptor's specialization for specific mechanical inputs

(• Tactile sensitivity varies widely across the body.)

Sensitivity is often measured using what types of threshold

Sensitivity is often measured using two-point threshold

Key pattern:

• Low sensitivity (large thresholds): areas like

the calf, thigh, and back

•High sensitivity (small thresholds): areas like

the fingers and lips

• This variation reflects differences in receptor density and cortical representation.

homunculus

• The homunculus is a distorted map of the human body based on sensory receptor density.

• Body parts with more receptors take up larger cortical space: Hands • Face

• This results in:

• Greater tactile acuity

• More precise discrimination

Highly sensitive areas include of Mechanoreceptor

Mechanoreceptor density differs across body regions, shaping how we experience touch.

• Highly sensitive areas include: • Hands

• Feet • Face • Genitals

• In real-world contexts (e.g., tattoos):

• Areas with higher sensitivity produce stronger sensations. •This can lead to increased pain perception.

Types of depth for Mechanoreceptors

Mechanoreceptors are located at different depths of the skin.

• Shallow receptors (closer to the surface)

• Deep receptors (located further beneath the skin):

Mechanoreceptors: Shallow receptors

• Shallow receptors (closer to the surface):

• Merkel's disks → detect sustained pressure and fine detail

• Meissner's corpuscles → detect light touch and low-frequency

vibration

Mechanoreceptors: Deep receptors

• Deep receptors (located further beneath the skin):

• Ruffini endings → detect skin stretch and sustained pressure

• Pacinian corpuscles → detect deep pressure and high-frequency vibration

• Depth influences: • Type of stimulus detected • Size of receptive field

what is receptive field (RF) and what are the two types

• A receptive field (RF) is the area of skin that activates a single neuron.

• Two key types:

• Small receptive fields (typically shallow receptors)

• Large receptive fields (typically deeper receptors)

Small Receptive fields (RFs)

• Small Receptive fields (RFs):

• Respond to very specific, localized stimulation.

• Provide high spatial resolution.

Common in:

• Fingertips

• Lips

Large Receptive Fields (RFs)

Large Receptive Fields (RFs): • Respond to stimulation over a broader area. • Provide less precise localization.

• Trade-off: • Better for detecting general pressure or movement • Worse for fine detail

Both Small Receptive fields (RFs) and Large Receptive Fields (RFs)

• Small receptive fields → High sensitivity and acuity

• Example: fingertips

• Large receptive fields → Lower sensitivity but greater coverage

• Example: back

Key takeaway:

• The tactile system balances precision vs. coverage depending on body region.

does mechanoreceptors differ in how they respond? whats the distinction

• Mechanoreceptors differ in how they respond over time.

Key distinction:

• Fast-adapting (FA) → respond to changes in stimulation

• Slow-adapting (SA) → respond continuously while stimulus is present

This allows us to perceive:

• Texture

• Motion

• Vibration

• Sustained pressure

Fast-adapting mechanoreceptors respond primarily to...

• Onset and offset of stimulation

• Changes across time

Best for detecting: • Movement across the skin • Vibrations • Dynamic touch

Everyday example: • Feeling a cat's purring vibrations • Detecting motion when something brushes across your skin

Slow-adapting mechanoreceptors

• Slow-adapting mechanoreceptors continue firing as long as the stimulus remains.

Best for detecting:

• Sustained pressure

• Shape and form of objects

• Grip force

Everyday example:

• Holding an object and sensing its shape and firmness

• Maintaining a stable grip over time

Fast Adapting receptors vs Slow Adapting receptors

• Fast Adapting receptors → change detection • Motion, vibration, dynamic touch

• Slow Adapting receptors → continuous information • Shape, pressure, object properties

• Combined, they allow us to: • Interact with objects • Adjust grip • Detect both what is touching us and how it is changing

Tactile Perception: key takeaway

Big Picture Takeaway

• The tactile system is not just about "touch"

• It is a multi-receptor system that integrates:

• Depth (shallow vs deep)

• Receptive field size (small vs large)

• Time dynamics (FA vs SA)

This integration enables:

• Precision (fingertips)

• Detection (whole body)

• Interaction (grasping, exploration)

All together mechanoreceptors

• Small receptive fields (Type I, shallow):

• Meissner's corpuscles (FA1) → light touch, motion

• Merkel's disks (SA1) → sustained pressure, fine detail

• Large receptive fields (Type II, deep):

• Pacinian corpuscles (FA2) → vibration, rapid changes

• Ruffini endings (SA2) → skin stretch

Meissner's corpuscles detect, type, why it matters?

What they detect

Light touch — gentle contact on the skin

Fine texture changes — subtle differences in surfaces

Motion across the skin — small movements like swiping or brushing

Type

Fast‑adapting (FA1) — they respond quickly when something changes, then quiet down

Why they matter

Object manipulation — adjusting grip so things don't slip

Surface exploration — feeling details to identify materials

Everyday examples:

Using a touchscreen (taps, swipes)

Feeling differences in fabric textures

Controlling finger pressure while playing instruments

Merkel's discs detect, type, why it matters?

What they detect

Edges — sharp boundaries and outlines

Fine spatial detail — tiny differences in shape or pattern

Sustained pressure — continuous, steady indentation of the skin

Type

Slow‑adapting (SA1) — they keep firing as long as the pressure remains

Why they matter

Provide high tactile acuity, allowing precise shape and detail recognition

Everyday examples

Reading Braille

Using tools that require precise pressure control

Sensing fine detail in art or sculpting

Pacinian corpuscles detect, type, why it matters?

What they detect:

Rapid vibrations — high‑frequency changes in pressure

Deep pressure — strong indentation deeper in the skin

Type: Fast‑adapting (FA2) — respond quickly to changes, especially vibration, then stop firing

Why they matter:

Enable detection of vibratory signals through objects, helping you sense motion or impact indirectly

Everyday examples:

Feeling vibrations from tools or machinery

Detecting impact quality in sports (e.g., how a bat contacts a ball)

Noticing abnormal vibrations such as road hazards while driving

Ruffini endings detect, type, why it matters?

What they detect:

Skin stretch — tension across the skin surface

Sustained pressure — continuous force over time

Joint and limb position — slow changes in finger, hand, and limb angles

Type: Slow‑adapting (SA2) — continue firing as long as stretch or pressure is present

Why they matter:

Contribute to grip control and proprioception, helping you sense body position and adjust movement smoothly

Everyday examples:

Adjusting pressure during handwriting

Sensing stretch during yoga or movement

Maintaining balance and posture through subtle stretch feedback

mechanoreceptor transduction are...

• The specific mechanisms of mechanoreceptor transduction are poorly understood.

• Current understanding:

• Mechanical force may open ion channels in the membrane

• Some receptors may use mechanisms similar to "tip-link" systems (like in hearing)

• Key takeaway: • Physical deformation → electrical signal → perception

What are the two major pathways for tactile information?

Pathways

Two major pathways carry signals to the brain:

• Lemniscal (dorsal column) pathway: • Touch • Vibration • Proprioception

Spinothalamic pathway • Pain

• Temperature

• Key idea: touch vs. pain pathways

What does S1 do and how is it organized? Location

• Located in the parietal lobe (postcentral gyrus)

• Responsible for: • Processing touch and body sensation

• Organized as a somatotopic map (homunculus) • Body areas with greater sensitivity take up more cortical space

• Key idea: more sensitivity = more brain space

Somatotopic Maps, key features, key idea.

Somatotopic Maps • The cortex contains a map of the body (homunculus)

• Key features: • Distorted proportions reflect sensitivity • Hands, lips, and face are overrepresented

• Key idea: brain representation ≠ actual body size

Cortical Plasticity

Cortical Plasticity

• The brain can reorganize itself based on experience

• Occurs even in adulthood

• Examples: Learning motor skills (dance, instruments) Improving sensory discrimination

What is pain perception. involves what types of components

Pain is a complex, multidimensional experience essential for survival. It involves both:

-Sensory components (what it feels like)

-Emotional components (how unpleasant it is)

Pain typically signals:

-Actual or potential tissue damage

How does pain influence attention and behavior?

Pain captures and prioritizes attention. It plays a critical role in protecting the body by

- guiding avoidance and learning.

Pain receptors (nociceptors) respond to:

-Intense mechanical force

-Extreme heat

-Chemical signals (e.g., capsaicin)

What are the two key dimensions of pain?

1. Affective (emotional) dimension

- Drives avoidance and protective behavior

- "This feels bad — stop it"

2. Sensory (discriminative) dimension

- Identifies location and quality

- Examples: sharp, dull, burning, throbbing

The course focuses more on the sensory/discriminative dimension.

What are the three major categories of pain?

-Nociceptive pain (most common)

-Inflammatory pain

-Neuropathic pain

What is nociceptive pain and what causes it? 2 subtypes of nocieptive pain

Nociceptive pain arises from tissue damage or potential damage.

Common causes:

- Cuts, burns, bruises

Fractures, sprains, arthritis

Two subtypes:

- Somatic pain->skin, muscles, joints (well localized)

- Visceral pain-> internal organs (diffuse, harder to localize)

What are nociceptors and what do they respond to?

Nociceptors are pain receptors found in free nerve endings. They respond to potentially harmful stimuli.

Important note:

-Not all free nerve endings signal pain

-Some contribute to pleasant touch (e.g., gentle cat kneading)

How do nociceptor thresholds change after injury?

• Nociceptors typically have high activation thresholds. • They respond only to strong or harmful stimuli.

• After injury: Thresholds decrease temporarily.

• This process is called sensitization. • Makes the area more responsive to stimulation • Helps promote protection and healing

• Key connection: • Sensitization is a core feature of inflammatory pain

What is inflammatory pain and why does it occur?

Inflammatory pain is part of the body's immune response to:

• Tissue damage

• Infection

• Illness

• Unlike nociceptive pain, which typically fades as healing occurs, inflammatory pain persists while inflammation remains active.

This type of pain serves an important function:

• Encourages rest and protection of the injured area

• Allows time for tissue repair and recovery

What are common real‑world examples of inflammatory pain?

• Common real-world examples include:

• Dermatitis (skin inflammation)

• Tendonitis (inflamed tendons from overuse)

• Rheumatoid arthritis (chronic joint inflammation)

What is neuropathic pain and what causes it?

• Neuropathic pain results from damage or dysfunction in the nervous system.

Common causes:

• Nerve compression (e.g., herniated disc)

• Injury or disease affecting nerves

Characteristics:

• Often chronic (long-lasting or persistent)

• Frequently difficult to treat

How is neuropathic pain assessed clinically?

In clinical settings (e.g., physiotherapy):

• Practitioners assess peripheral nerve function

Symptoms such as:

• Muscle weakness

• Sensory changes (numbness, tingling) → may indicate nerve damage

• Because nerves map onto specific spinal levels:

• Clinicians can localize the source of dysfunction

What are dermatomes and why are they important?

• Peripheral nerves correspond to specific spinal segments (dermatomes)

This organization allows clinicians to:

• Identify which spinal level is affected

• Trace symptoms back to specific nerve pathways

Key takeaway:

• Pain location can provide clues about underlying neural

damage

What types of fibers carry pain signals?

• Pain perception depends on how signals are transmitted to the brain.

• When tissue damage occurs, nociceptors send signals through two types of fibers:

• A-delta fibers • C fibers

What are A‑delta fibers and what type of pain do they carry?

A-delta fibers are myelinated.

• Myelin = fatty insulation that speeds up signal transmission

Characteristics of pain:

• Fast, sharp, and well-localized

• Occurs immediately after injury

Function:

• Acts as an early warning system

• Prompts rapid withdrawal from harm

What are C fibers and what type of pain do they carry?

C fibers are unmyelinated

• Signals travel more slowly

• Characteristics of pain:

• Dull, aching, burning, and diffuse

• Long-lasting and harder to localize

Functional role:

• Respond to mechanical, thermal, and chemical stimuli • Strongly associated with inflammatory pain

How does the body create a layered response to harmful stimuli?

• The body uses two pathways to create a layered response to harmful stimuli:

Immediate response (A-delta fibers):

• Rapidly transmit signals to the brain, producing quick awareness of danger and enabling fast protective actions.

Sustained response (C fibers):

• Send slower, longer-lasting signals that support recovery by promoting protective and healing behaviors.

How do pain signals travel to the brain?

• Pain signals travel to the brain through the spinothalamic tract.

• These signals reach the thalamus, which acts as the brain's central relay station for sensory information.

Which brain regions process pain signals?

-Somatosensory cortex: physical sensation & localization

-Insula: emotional awareness of pain

-Anterior cingulate cortex (ACC): emotional & motivational aspects

-Prefrontal cortex (PFC): context, decision‑making, response planning

What is emotional pain and how does the brain process it?

• Emotional pain (psychological distress) activates brain systems involved in emotion processing and regulation.

• It can be just as impactful as physical pain, engaging complex and overlapping neural networks.

Which brain regions are involved in emotional pain?

• Amygdala: Processes emotional significance and contributes to the intensity and memory of painful experiences.

• Anterior cingulate cortex (ACC): Evaluates emotional meaning, supports impulse control, and is involved in empathy.

• Prefrontal cortex (PFC): Regulates emotional responses and exerts cognitive control during stressful situations.

How does psychopathy affect emotional pain processing?

• Research on psychopathy shows key differences in brain structure and function compared to typical populations.

Individuals with psychopathy often show:

• Reduced amygdala volume and activity

• Abnormal functioning in the prefrontal cortex (PFC)

• Altered activity in the anterior cingulate cortex (ACC)

Why is pain perception essential?

• Pain perception is essential to sensation and perception, influencing biological, psychological, and social functioning.

It:

Signals danger

Promotes healing

Shapes emotional experiences

Supports social processes such as empathy and understanding others

What are the two major components of proprioception?

Vestibular sense

• Maintains balance and spatial orientation

Key idea: keeps you upright and stable

Joint-position sense

• Tracks the relative position and movement of body parts

• Key idea: tells you where your body is without looking

What does the vestibular system detect and why is it important?

• The vestibular system detects head position and movement.

• Guides reflexive eye movements

• Helps maintain balance and posture

• Located in the inner ear, next to the cochlea

• Key idea: movement detection system in the ear

What are the two main components of the vestibular system and what do they detect?

• Two main components:

Otolith organs (utricle & saccule)

Detect gravity and linear movement (up/down, forward/back)

Semicircular canals (3)

• Detect rotational movement of the head

• Key idea: different structures detect different types of motion.

How do the otolith organs detect movement?

Otolith Organs (utricle & saccule) • Contain hair cells + otoliths (tiny crystals)

• Respond to gravity and linear acceleration

When head is upright:

• Utricle → receptors on the "floor"

• Saccule → receptors on the "wall"

• Key idea: detects tilt and straight-line movement

What do the semicircular canals do? function throughout the brain

Semicircular Canals

• Three canals: superior, posterior, horizontal

• Positioned at right angles to each other

Function:

• Detect head rotation across planes

• Key idea: spinning and turning detection

• Filled with endolymph fluid

• Each canal has an ampulla

• Contains sensory receptors (hair cells)

• Key idea: fluid movement drives the signal

• Contains the cupula (gel-like structure)

• Hair cells are embedded within it

• Key idea: this is where motion gets translated into neural signals

• Head movement → fluid shifts

• Fluid movement → cupula bends

• Bending hair cells → neural signals sent to brain

• Key idea: movement → bending → signaling

How does vestibular fibers reach the brain?

Vestibular + cochlear nerves → cranial nerve VIII

Carries balance + sound information

Vestibular fibers synapse in:

-Vestibular nuclei

-Cerebellum

• These regions help: • Coordinate balance • Refine movement and posture

Key idea: one nerve carries both hearing and balance signals

The Vestibular Pathway ...

The Vestibular Pathway

Vestibular nuclei send projections to:

• Spinal cord: Adjust posture and maintain balance

• Cranial nerve nuclei (eye muscles)

• Controls eye movements

• Enables vestibulo-ocularreflex(VOR)

Cortex (via thalamus)

• Integrates vestibular, visual, and tactile info

• Supports spatial awareness

Key idea: widespread system for balance, vision, and perception

What is joint‑position sense?

Provides awareness of body position and movement

Allows:

• Accurate, coordinated movements

• Even without visual input

Information comes from:

• Skin stretch receptors (SA-D)

• Receptors in muscles and tendons

What do muscle spindles and Golgi tendon organs detect?

Muscle & Tendon Sensation

Muscle spindles (intrafusal fibers)

• Detect muscle length and stretch

Golgi tendon organs (GTOs)

• Detect tension and force in tendons

Key idea: length vs. tension are measured separately

What are the three muscle response patterns?

• Slow stretch → steady spindle firing

• Fast stretch → rapid, strong spindle response

• Added weight / tension → increased GTO activity

What are reflexes?

• Reflexes are:

• Involuntary

• Automatic’

• Consistent responses to stimuli

Example: • Patellar (knee-jerk) reflex

• Key idea: fast protective responses

What is the monosynaptic stretch reflex? what does it help with

• Muscle stretch detected by muscle spindles

• Signal sent to spinal cord → immediate response

Key feature:

• Direct synapse onto motor neuron (one synapse)

• Result: Muscle contracts to resist stretch'

Helps with:

• Adjusting to sudden weight changes

• Maintaining posture

• Example: Prevents you from collapsing when weight is added

What are polysynaptic reflexes and what do they do?

Polysynaptic Reflexes

• Involve multiple synapses and interneurons

• Example: Golgi tendon organ reflex regulates muscle force

• Function: Prevents excessive tension and injury

• Real-world note:

• Reduced GTO feedback (e.g., anesthesia) can increase strength but raises injury risk

What are the chemical senses and what do they detect?

• Chemical senses detect the structure of molecules that are airborne or dissolved in saliva.

• The two chemical senses, smell and taste, are closely interconnected.

Why is smell considered an ancient and important sense?

• Smell is a very old and very well-preserved sense, older than both vision and hearing.Olfaction remains critical for survival, playing key roles in:

• Assessing environmental hazards.

• Evaluating food sources.

• Recognizing kin, strangers, and potential mates.

What are odorants and what are their key characteristics?

• Odorants: chemical compounds that can be detected by the olfactory system, which allows organisms to perceive smells.

Key characteristics:

• Volatile: they evaporate and disperse through the air so they can enter the

nasal cavity and reach the olfactory receptors.

• Shape and functional groups: the specific molecular shape and functional groups of odorants determine their binding to different olfactory receptors.

This leads to a distinct smell in our perceptions

How does concentration affect odor detection?

Detection depends on concentration in the air

Strength + distance of source affect intensity

Perceived quality can change with concentration

What does the detection threshold slide show

Sensitivity varies widely across odorants

Some detectable at extremely low concentrations

How does the olfactory system identify odorants?

depends on their compatibility with particular receptors.

• Imagine the olfactory receptors in your nose as a series of locks. Each lock (receptor) has a unique shape.

• For a smell to be detected, the odorant (key) must match the shape of the receptor (lock).

• If the key fits the lock, it can unlock (activate) the receptor, leading to olfactory transduction, resulting in smell recognition (olfaction).

How do context and expectations influence odor identification?

• The ability to identify odors is also heavily influenced by context and expectations.

Expecting a smell (e.g., coffee) → easier identification

Brain is prepared for the expected "key"

Context primes recognition

• If you enter a room expecting to smell coffee and encounter that aroma, you'll likely recognize it immediately because your brain is prepared for that specific "key" to fit the anticipated "lock."

What happens when an odor appears out of context?

If you smell coffee in an unexpected place (e.g., bathroom), you may misidentify it

Shows how context shapes odor recognition

Humans are surprisingly bad at identifying odors without context

-This indicates how context sets the expectation for certain "keys," making it more attuned to detecting those and potentially confused or slower to identify when odors appear out of the expected context.

What did Cain (1979) discover about odor identification? what does it suggest?

- reveals the challenges of odor identification.

- Participants were given common odors with no context

- Participants struggled with no context

-Odors: coffee, peanut butter, chocolate

-Identification accuracy was low

-Consistency improved slightly across sessions

-Humans struggle to identify odors without context

-Olfactory system relies heavily on other sensory cues

What did Yeshurun & Sobel (2010) study? and results/suggest?

reveals that odor identification capabilities vary across the lifespan.

-Odor identification across lifespan

-Groups: children, young adults, middle‑aged adults, elderly

-No contextual cues

-Odors: banana, cherry, lemon, wintergreen, clove, licorice

-clear age related cues

-Odor identification peaks in young adulthood

-Declines gradually with age

What did Parr et al. (2002) investigate? Results?

reveals the effects of expertise on odor identification.

-Experts: chefs, perfumers

-Novices: no training

-No contextual cues

-Experts significantly better at recognizing odors

-BUT expertise did not improve detection sensitivity- meaning Detection sensitivity = the lowest concentration of an odor you can notice (Experts do not smell faint odors better than novices.)

What is odor adaptation?

• Odor adaptation: a normal reduction in the perception of an odor after prolonged exposure to that scent.

• This neural adaptation, resulting in a reduction in odor response, helps to refocus attention on new or changing stimuli, which is advantageous for survival.

• For example, you get used to a certain scent of perfume.

-Olfaction responds mainly to changes

-quickly adapt to environments

What is cross‑adaptation?

• Cross adaptation is a phenomenon in which exposure to one odorant can reduce the sensitivity to a second odorant with a similar chemical structure.

(Exposure to one odor reduces sensitivity to a second odor with similar chemical structure)

Cross‑adaptation study

• The initial training phase involved the rodents habituating (adapting) to the single odor.

-Rodents habituated to one odor

• Adapting to a first odorant generally reduced the sensitivity of olfactory receptor neurons to a second and proceeding odorant with a similar chemical structure.

Sensitivity decreased for chemically similar odors

What is the overall conclusion about cross‑adaptation?

Adapting to one odor reduces sensitivity to similar odors

Due to shared chemical structure

How does cross‑adaptation affect perfume workers? Wine tasting?

Smelling many perfumes reduces sensitivity

Coffee beans act as an "olfactory palate cleanser"

Wines with similar aromatic compounds cause cross‑adaptation

Water or crackers reset sensitivity

What is anosmia and how does it relate to aging? EX covid 19

complete loss of the sense of smell resulting from various causes.

Smell declines with age

Graph shows odor ID scores decreasing across age groups

• Many patients with COVID-19 also experience temporary anosmia, a loss of sense of smell with or without other respiratory symptoms.

What are the five steps of odor perception?

1. Inhalation

2. Contact with the Olfactory Epithelium

3. Binding of Odorants to Receptors

4. Transmission to Olfactory Bulbs

5. Olfactory Pathway

1. What happens during inhalation?

• Air intake: an odorant's journey begins when inhaled through the nose.

• Air containing odorant molecules enters the nasal cavity through both nostrils and the pharynx, where it is warmed, moistened, and filtered by the nasal mucosa.

2.Contact with olfactory epithelium

• High up in the nasal cavity lies the olfactory epithelium, which is critical for detecting odors.

• This area contains olfactory receptor neurons (ORNs) equipped with cilia, hair cells similar to those in the cochlea that process auditory information.

3.Binding of Odorants to Receptors

The cilia contain olfactory receptors that bind to odorant molecules.

- When an odorant molecule with the correct shape (key) binds to its corresponding olfactory receptor (lock), this triggers olfactory transduction.

-Each olfactory receptor receptor neuron (ORN) expresses only one type of olfactory receptor, and each receptor is tuned to detect a few specific types of odorant molecules based on their molecular structure.

- COVID-19 disrupts this process, resulting in temporary anosmia.

Located high in nasal cavity

Contains ORNs with cilia

Cilia detect odorants

'Odorant shape must match receptor shape

Binding triggers transduction

ORNs express only one receptor type

4. Transmission to the Olfactory Bulb

-The transduced electrochemical signals travel along the axons of olfactory sensory neurons, which converge through the cribriform plate to reach the olfactory bulb.

-In the olfactory bulb, the axons from various olfactory receptors connect in small structures known as glomeruli.

-Each glomerulus corresponds to neurons expressing the same type of olfactory receptor.

5. Olfactory Pathway

• The signals are then relayed via the olfactory tract to various brain regions, including the olfactory cortex, the orbitofrontal cortex, and parts of the limbic system

How many ORNs do humans, cats, and dogs have? How many receptor types do humans, cats, and dogs have?

Humans: 3-7 million ORN

Cats: 45-80 million ORN

Dogs: 200-500 million ORN

Humans: ~300 R

Cats: ~200 R

Dogs: ~1000 R

What kinds of molecules do human olfactory receptors detect?

While having 300 different types of receptors might seem a lot, our ability to detect different kinds of odor molecules is still limited.

Human olfactory receptors are primarily designed to respond to molecules made up of carbon, hydrogen, oxygen, nitrogen, and sulfur - essentially, basic organic compounds that are typical byproducts of living systems.

How are ORNs organized in the olfactory bulb?

-The axons of olfactory receptor neurons (ORNs) converge onto the same glomerulus in the olfactory bulb.

-Each glomerulus receives input from one type of ORNs, thereby leading to 300 different types of glomeruli in the olfactory bulb.

-This organization leads to 300 different receptor types in humans.

ORNs with same receptor type → same glomerulus

Humans have ~300 glomerulus types

How can humans detect many odors with only 300 receptors?

-Although we have about 300 receptors, we can discriminate many different odorants.

-This is achieved through a mechanism known as population coding, where specific odorants are identified by the pattern of activity across multiple receptors.

How many molecular shapes could humans theoretically discriminate?

-Theoretically, if each receptor type is tuned to recognize a unique molecular shape (key), then with approximately 300 different receptor types, we could potentially discriminate between 2350 different molecular shapes.

This amounts to an astronomically high number!

• Around 200 billion trillion trillion trillion trillion trillion trillion trillion trillion distinct shapes!

What is unique about olfactory processing?

Olfactory processing is ipsilateral - processed on the same side of the brain.

What is the primary olfactory cortex?

-The piriform cortex is considered the primary olfactory cortex.

-It receives input directly from the olfactory bulb and is primarily dedicated to olfactory information.

What limbic structures receive olfactory input?

• Olfaction stands out from other senses, such as vision, hearing, touch, and taste, because it does not route sensory information through the thalamus.

• Instead, the olfactory system has a direct pathway to the brain's limbic system, triggering amygdala, hypothalamus, and hippocampus.

What roles do the amygdala and hypothalamus play in olfaction?

Amygdala: immediate emotional reactions to smells.

• The smell of smoke might trigger an instant sense of danger.

• Hypothalamus: crucial for regulating the hormonal system and functions of 4Fs. • Feeding, fighting, fleeing, and fornication.

Why are smells strongly linked to memories?

• The entorhinal cortex is closely linked to the hippocampus, a crucial memory area.

• This direct connection from the olfactory bulbs to the hippocampus might explain why scents often provide powerful memory triggers.

What does the orbitofrontal cortex do in olfaction?

-The piriform cortex, amygdala, and entorhinal cortex all send signals to the orbitofrontal cortex.

-Activity in this region is linked to evaluating scents, such as determining whether they are unpleasant or threatening.

What is gustation?

• Gustation: a physiological process that allows us to detect and respond to dissolved molecules and ions, known as tastants, through receptors located on the tongue.

What are the five basic taste qualities and what triggers them?

• Taste is the actual sensation resulting from gustation. It is typically categorized into five basic qualities detected by specific tastants:

• Sweet: triggered by sugars and similar substances.

• Sour: triggered by acidic substances.

• Salty: caused by the presence of salts.

• Bitter: typically associated with alkaloid-rich foods and toxins.

• Umami: caused by amino acids and peptides, signaling protein-rich foods.