Gustatory and Auditory Systems: Anatomy and Physiology

• Lecture notes on the Anatomy, Physiology, and Psychoacoustics of the Gustatory and Auditory Systems

Gustatory System Overview

• Taste signals are produced in the oral cavity and are essential for survival.
• Papillae are mounds or projections of tissue on the tongue and mouth roof, embedding taste buds within their grooves.
• Saliva carries food particles into taste pores, leading to taste buds.

Cellular Components of Taste

• Taste buds contain multiple taste receptor cells.
• Tastants are dissolved chemicals that stimulate taste receptor cells.
• Microvilli are invaginated receptor cell membranes on taste receptor cells specialized for different functions at the "top" (apical) and "bottom" (basal) parts.
• Receptor cells are specialized within each taste bud, ranging from 50-150 cells.
• Receptor cells transduce chemical signals, behave like neurons (though they aren't), have limited lifespans, and are constantly replaced.
• There are three types of taste receptor cells, each with distinct functions.
• Not all taste receptor cells synapse with taste nerve fibers.

Taste Receptor Cell Types

• Type I cells have glia-like functions ("housekeeping") and constitute 50% of cells in a taste bud.
• Type II cells respond to sweet and bitter tastes, making up about 1/3 of taste bud cells, and express G-protein coupled receptors (for sweet, bitter, umami) and their downstream effectors.
• Most type II cells have only one type of tastant receptor
• These cells don't make synapses with afferent fibers but release ATP to signal adjacent cells or nerve fibers.
• Type III cells respond to sour tastes.
• They make up ~2-20% of taste bud cells, don't express GPCRs, but have the machinery to detect sour tastants.
• These cells have chemical synapses with synaptic vesicles and synapse with afferent nerve fibers.

Mechanisms of Taste Transduction

• Taste transduction involves ion channels and G-protein coupled receptors.
• Ion channels mediate salty (Na+) and sour (H+) tastes.
• G-protein coupled receptors mediate bitter and sweet tastes.

Ion Channels and Taste

• For salty tastes (NaCl), Na+ ions enter the ion channel.
• For sour tastes (HCl), H+ ions enter an ion channel.
• Undissociated organic acids permeate the membrane, dissociate into acid and base, increase intracellular fluid acidity, and close a K+ channel, leading to depolarization.

G-Protein Coupled Receptors and Taste

• Type 1 taste receptors (T1Rs) function as heterodimers (require T1R3)
• Sweet T1R2-T1R3 receptors include ligands like sugars
• Type 2 taste receptors (T2Rs) are bitter-taste receptors that function as monomers or dimers with ligands like alkaloids.

Taste Transduction Process

• Tastant-transducing channels/GPCRs are activated, leading to depolarization directly (ion channels) or indirectly (GPCRs).
• Voltage-gated Na+ and K+ channels are activated (graded and action potentials), and voltage-gated Ca2+ channels open.
• In type III cells, Ca2+ causes neurotransmitter (serotonin) release into the synaptic cleft, with ATP and GABA possibly involved.

Neural Pathways for Taste

• Taste receptors are innervated by sensory (afferent) neurons.
• Afferent taste neurons are pseudounipolar with cell bodies in ganglia outside the brainstem.
• Taste information travels via the VIIth (facial), IXth (glossopharyngeal), and Xth (vagus) cranial nerves that emerge from the brain stem.

Tongue Map

• The traditional tongue map is a scientific urban legend.
• Receptors for all four basic tastes are distributed over the entire tongue.
• Regional coding suggests different tongue regions have slightly different thresholds for various tastes.
• Real-world taste intensities are produced by the summation across fibers with varying thresholds.

Neural Coding of Gustatory Signals

• Neural coding (gustation) represents the identity, concentration, and pleasurable/aversive values of a tastant.
• Taste stimuli are distinguished through labelled-line coding and cross-fiber coding.
• Labelled-line coding involves different receptors and their associated sensory fibers transmitting highly specific information.
• Cross-fiber coding involves different qualities of a sensory modality distinguished by the pattern of nerve discharges across a large population of fibers.

Combinatorial Model

• Some taste receptor cells are selectively sensitive to particular tastants while others are broadly tuned.
• Taste coding shows features of both labelled lines and cross-fiber (combinatorial) coding.
• Responses of a large number of broadly tuned neurons specify properties of a stimulus.

Gustatory Processing in the Brain

• Taste signals are transmitted via cranial nerves to the nucleus of the solitary tract (NST), which is the first relay site.
• NST axons project to a nucleus in the thalamus (VPMN), the secondary relay site.
• The primary gustatory cortex, located in the frontal lobe within the Sylvian fissure; insula and surrounding areas.
• Cells in the insular cortex are preferentially sensitive to different taste primaries, supporting labelled line coding.
• Taste projects ipsilaterally in the cortex.
• Different tastants activate different patterns, stereotyped across animals, with some overlap between modalities.

Secondary Gustatory Cortex

• The orbitofrontal cortex (OFC)/secondary taste cortex processes higher aspects of taste functions, such as motivational effects of hunger and satiety.
• Some neurons in the OFC are multimodal (integration area).

Miracle Fruit

• Miracle fruit is a berry derived from Synsepalum dulcificum that is tasteless to humans.
• It contains miraculin, a taste-altering glycoprotein that binds to sweet taste receptors and makes sour-tasting foods taste sweet.
• It naturally grows in tropical West Africa, where natives chew it to make acidic foods more pleasant.

Sweet Receptor

• The sweet receptor is a heterodimer (T1R2-T1R3), with sweet chemicals binding to the "Venus flytrap" portion.
• Small molecules bind to T1R2, while large molecules bind to T1R3.
• Ligand binding activates the receptor, which activates a G-protein signaling pathway, ultimately increasing Ca2+ in the intracellular fluid.

Basic Tastes

• Basic taste is any of the four taste qualities that generally agreed to describe human taste experience, including sweet, salty, sour, and bitter.
• The complexity of sensations evoked by foods is attributed to the olfactory system.
• People are born either liking or disliking basic tastes (hardwired affect).

Taste Sensations and Physiological Relevance

• Salt: Na+ is important to maintain nerve and muscle function (consider the action potential!)
• Bitter: avoidance as these often signal poison/toxicity.
• Sour: acids will damage external and internal body tissues at high concentrations.
• Sweet: sugars are the principal energy source for humans.
• Each of the four basic tastes is responsible for a certain nutrient or antinutrient.

Eating

• Taste provides information about what humans should ingest (or not).
• Foods can be pleasing or displeasing, with affective experience and reflexive responses being innate.

Beyond Four Basic Tastes

• Umami is purported to evoke the sensation of savouriness.
• Fats consist of fatty acids attached to a support structure.

Determining Basic Taste

• Key criteria include the presence of receptors for chemicals in the mouth, activation of distinct sensations, and hardwired affect.
• Some would argue that umami and fatty should be added to the list of basic tastes.
• Receptors in the mouth regulate the palatability of protein and fat, like the umami receptor that consists of TAS1R and TAS1R3, support this claim.
• Proteins and fats are big molecules mostly broken down by the process of digestion
• Conscious sensations about fats are evoked by the somatosensory system (e.g., oily, creamy).

How Miraculin Works

• Miraculin can bind to the heterodimer at a neutral pH, but no signalling pathway is activated.
• Acidity changes the protein, allowing miraculin to activate the taste receptor and signalling pathway.
• This leads to less activation and fewer available receptors for natural sweeteners to bind when miraculin is bound.

Supertasters

• Supertasters perceive taste sensations with the most intensity.
• An individual's perception depends on various factors, including the density of fungiform papillae and genetics.
• Supertasters experience the most intense sensations of oral burn and are more finicky eaters because bitter sensations are more intense.

Factors Influencing Taste

• Food preferences are learned, with innate preferences (e.g., salt, sugar) combining with learned olfactory affect to determine food likes and dislikes.
• This explains gustatory affect, but it does not explain all eating behaviour and food choices due to social factors.

Measuring Taste Perception

• Psychophysical techniques are used to measure the two perceptual aspects of taste.
• Electrogustometry involves delivering a small electric current to a specific point on the tongue, which is useful for measuring taste detection thresholds and locating lesions.
• Chemogustometry can be regional, where tastants are localized, or involve the whole-mouth.

Taste Abnormalities

• Ageusia is the total loss of taste, often due to injury to gustatory nerves, medications.
• Hypogeusia is a reduction in taste sensitivity, caused by dry mouth, smoking, or illness.
• Dysgeusia is when taste perceptions are distorted, and is, metallic taste for cancer patients.
• Taste detection thresholds indicate the intensity at which tastes are identified.
• Bitter < sour < salty < sweet threshold

Factors Affecting Taste Thresholds

• Other tastants in the mixture, temperature, location on the tongue, age, stimulation area, and genetics

Adaptation

• Adaptation: prior or ongoing stimulation reduces the perceived intensity or sensitivity to a tastant
• Cross-adaptation: perceived intensity of a compound decreases because of adaptation to a different compound of the same taste quality

Taste Attributes

• The four classical taste qualities don't cross-adapt and are consistent with labeled-line coding.
• Mixture suppression (masking) occurs when one taste quality suppresses another
• E.g., the sugar in tonic water makes it taste less bitter
• Humans express 25 bitter receptor proteins.
• Some respond to specific compounds, while others are generalists.
• Tasters can taste organic powders and are born with dominant versions of the TAS2R38 gene.
• Non-tasters are unable and are born with two recessive versions.

Taste and Health

• Some bitter foods are good as they are adaptive and may reduce instances of cancer later on in life.
• Women are more sensitive to bitterness during pregnancy, which is adaptive.
• Artificial sweeteners have been linked to increases in appetite as the body compensates.
• Taste receptor cells are lost with age, leading to increased thresholds and possibly increased salting of food.
• People can change their salt preference over time as taste buds can adapt to a low salt diet.

Auditory System Uses

• Auditory System is used for finding mates, avoiding predators, communication, finding food, and for please.
• It it the distant and fastest sense to convert physical signals into biological signals.

Sound Physics and Psychology

• Physics views sound as a vibrational disturbance of a medium with describable physical qualities
• Psychology views sound as a physical event converted into a biological signal, producing a perceptual experience of hearing.

Vibrational Properties of Sound

• Objects must have inertia and elasticity to vibrate.
• Inertia opposes changes in motion, while elasticity opposes inertia.
• A tuning fork exemplifies simple harmonic motion, displacing air molecules to create compression and rarefaction.
• Sound is a travelling wave of pressure disturbance.

Basic Characteristics of Sound

• Pure sounds are characterized as single sinusoidal function and can be detected using the auditory system.
• Amplitude: pressure change from peak to peak of a sound wave, is perceived as loudness.
• Frequency: the number of cycles per second, perceived as pitch.
• Tapping a tuning fork harder increases amplitude, while using a stiffer tuning fork increases frequency.
• Humans hear a limited range of frequencies present in environmental sounds, typically between 20-20,000 Hz for young, healthy individuals.

Measuring Sound Intensity

• Humans hear a wide range of intensities, which is measured in decibels (dB) on a log scale.
• SPL indicates that minimal audible sound used as reference, on a log (not linear) scale.
• Periodic sounds repeat at regular intervals and are not sinusoidal.

Complex Sounds

• Complex periodic sounds include human speech and musical notes, with many sounds having a harmonic series caused by a vibrating source.
• Aperiodic sounds include noise and static.
• Fourier analysis decomposes any complex waveform into sine-wave patterns, graphing a frequency spectra.

Sound Intensity

• Sound intensity diminishes with distance due to the spreading of sound energy over a larger surface area (inverse square law).
• Waves are either reflected (echoes or reverberation) or absorbed by the medium.

Auditory System

• Diffraction, where waves bend around an object, reforms, and continues, is easier for low frequency sounds
• The auditory system detects long-range signals, encoding both amplitude and frequency through a series of structures.
• Outer ear, middle ear, and inner ear.

Components of the Outer Ear

• Pinna: sound funnel, cartilage, bumps and grooves and helps us increase intensity of sound
• External auditory canal (EAC): channels sound to eardrum, lined by wax-secreting glands
• Tympanic membrane (aka eardrum): elastic membrane that seals off the EAC and vibrates in response to sound.

Middle Ear

• Auditory ossicles conduct vibration of the eardrum to the inner ear (malleus, incus, and stapes)
• Eustachian tube equalizes pressure across the eardrum (this is why yawning or chewing helps when flying).

The Inner Ear

• Part of the vestibular system (balance) and the auditory system (cochlea)
• Fluid-filled bony structure embedded in temporal bone
• Filled with watery fluids in 3 parallel canals separated by 2 membranes
• Basilar membrane- sounds are transduced
• Vibration in the cochlea mimics the stimulus but is characterized by liquid rather than air.

Organ of Corti

• Conversion of vibrational to neural signals occurs in the organ of Corti
• Composed of a structure that extends along basilar membrane and is attached to the soft gelatinous structure.
• Contains stereocilia, outer hair cells (3 rows), inner hair cells (single row), and auditory nerve fibers.

Vibration and Auditory Transduction

• A travelling wave causes the basilar membrane to move up and down.
• Hair cells move in opposite directions, shearing, and inner hair cells transform vibrational energy into an electrical signal (sensory receptors).
• Displacement of hair bundle toward tallest stereocilium stretches tip links, which open and depolarize the hair cell. It causes the release of neurotransmitters and send info to higher auditory centeres

Amplification

• Neurons transduce hair cells to generate a sinusoidal receptor potential in response to sinusoidal stimulus, thus preserving temporal information.
• Mechanoelectrical transduction is fast and sensitive.
• The tensor tympani and stapedius dampen loud sounds, producing auditory reflexes that protect the inner ear structure.

How the Cochlea Codes

• Bulge in vestibular canal becomes bigger, causing deflections of basilar membrane, which is more intense.
• Frequency is topographically mapped onto the basilar membrane in a tonotopic fashion.
• The Basilar functions as a Fourier analyzer encodes the signals by analyzing frequencies to complex tones.

Nerve Fibres

• Fibres innervate signals to auditor nerve fibres
• Cell bodies of afferent nerves just outside the body of the cochlea
• Transmission occurs through the cochlear branch via the vestibulocochlear nerve (cranial nerve VIII)

Tuning Curve of the Auditory Nerves

• Each fibre has a tuning curve, the lowest point of which is the characteristic frequency that it is most sensitive to.
• Hair cells are innervated by auditory nerve (AN) fibres in a systematic and topographic manner.
• Tones increase firing rate if the tone is near the tuning curve
• Fibres on outer hair cells are from efferent axons, with input causing the outer hair cells to become physically longer, influencing movement of the basilar membrane.
• Multiple neurons encode higher frequencies as a group via fibers firing every few cycles, which is called the "volley principle".

Encoding Sound Intensity

• Rate saturation: As intensity increase, the more likely that a signal is processed.
• At intensities above threshold, AN fibres become less selective, and the brain cannot rely on a signal to determine

Different Afferent Neurons

• High (Low) Ap freq (Activatation Threshold)
• Saturates at low (high) intensities
• These can be sponataneous, which are innervated at different points along the synapse.

Sound Transference

• Auditory system determines the frequency and pattern of incoming sound waves.
• System allows spanning of differing sounds due to the way nerve process information.
• Response modification (influences in the cochlear nucleus) and tonotopic (aligns those which are sensitive)

Laterality of Auditory system

• Signals are processed differentially based on how different each the input to each ear
• Contra (excitatory) and ipsilateral (inhibitory)

Auditory Pathways

• Auditory signals are transferred via a series of 5 steps.
• Travels from the eardrum to the auditory coretx
• Relays for modification to relay and precess each signal.

Auditory Cortex Organization

• Located on the superior temporal gyrus (mound) in the temporal lobe, containing tonotopic maps with isofrequency streets.
• Secondary auditory cortex (A2/ belt area) surrounds and receives input from A1, processing more complex auditory signals
• Variabilities are dependent

Auditory Dysfuntion

• Variability in hearing loss ranges from impairment to deafness.
• Damage is typically described by location.
• Obstructed ear canal: sound waves cannot exert pressure on eardrum
• Conductive (mechanical) loss: middle ear bones lose ability
• Sensorineural loss - damage to the nerves or ear.
• Congenital vs Aquired.

Middle Ear Diseases

• Otitis Media is an ear inflammation
• Changes ear drum to the point, children at greater risk
• Treated with medical insertion or surgery.
• Otosclerosis can also contribute.

Sensoineural Hearing Loss

• Exposure, Trauma or Tumours contribute
• Can be acquired by genetic.

Presbycusis

• Typically effects how quickly the waves are sent, occurs more rapidly with age.
• Diagniosed via varying the conditions, or looking using an otoscope

Measuring Sensations

• Done with Audiologists to test what is possible and which is not.

Signal Output

• Typically higher if not occluded by the other means
• Psychoacoustics is defined in the environment
• Pitch, Loudness and tone are what is tested.

Thresholds

• Thresholds are at a given freq, this is the range in which audibility functions within.

Frequency

• Tested by having a listener equate sounds from differing areas
• Experimental tests

Complex Sounds

• Harmonic Sounds are assessed by each is Integer multiple
• Timbre/Attack are defined and assessed using visual output.

Acoustic

• If a signal contains information it can be detected
• Compared 2 signals by ear or location.

Azitmuth and Elevtion

• Compared intensity allows perception and comparison.
• It allows processing to make sounds easier to discern.

Cones of Confusion

• Sound intensity is defined with ear to pin point where it is
• Binoaural are the most common to test where difference is coded in superior olive.

Sound Cues

• Interual and the ear are used to get accurate spacial
• This allows the body to pick it up in relation to where

Auditory Distance

• The direction the signal is received effects the overall experience.
• Some sounds that that far are reverberrated
• Clicks can then be related to make sense of area.

Stimulae

• Scene contains the entirety, those who are sound waves

Ability of Brain

• Separates sound like someone at party, known as cocktail
• Can separate sounds or movements to simplify the process.
• This allows the listeners to cont to percieve even after damage/noise.

Properties of The System

• Gestalt grouping is used to detect threat faster.
• Acoustic startle is why loud sounds quickly trigger to defend self.
• Most ignore what is unnecessary and focus on current environment.