Sensory Systems: Exteroception, Body Sense, Interoception

Sensory Systems Introduction

• Sensory systems bring in information for the brain to interpret and act upon.
• Sensory modalities are grouped into three functional categories: exteroception, body sense, and interoception.
• Exteroception determines what is out in the world, dealing with vision, audition, somatosensation, olfaction, and gustation; it is mostly conscious.
• Body sense determines where the body is, involving the vestibular sense and proprioception; it is mostly unconscious.
• Interoception determines the state of the body itself, including baroreceptors, chemosensors, surface temperature receptors, and nociceptors; it can be both conscious and unconscious.

Exteroception

• Determines what is out in the world and decides on the best action, such as approaching food, avoiding danger, navigating paths, or interacting with potential mates.
• Vision uses light, audition uses sound, somatosensation uses touch, olfaction uses smell, and gustation uses taste.
• Vision, audition, somatosensation, olfaction and gustation are mostly conscious senses.

Body Sense

• Determines how the body is positioned, where its parts are, and how they are moving.
• Body sense enables maintaining balance, coordinating actions, and adjusting for movement.
• Body sense determines verticality / horizontality, leg position during walking, and head movement.
• Vestibular sense perceives head movements and gravity.
• Proprioception perceives body position.
• Vestibular sense and proprioception are mostly unconscious.

Interoception

• Determines the state of the body and keeps it operating optimally through homeostasis and allostasis.
• Interoception is tied with motivations and emotions.
• Interoception determines respiration rate, heart rate, need to eat, heat loss, and fear level.
• Interoception uses baroreceptors to sense blood pressure.
• Interoception uses chemosensors to measure hunger, thirst, CO2, and pH.
• Interoception surface temperature receptors.
• Interoception uses nociceptors to measures pain.
• Interoception can be both conscious and unconscious.

Auditory System

• The auditory system transmits sound to the sensory organ.
• The auditory system transduces sound energy into a neural signal.
• The auditory system transmits the neural signal to the brain.
• The auditory system processes the neural signal to provide meaningful auditory information.

Path of Sound

• Sound first enters the outer ear: pinna then auditory canal.
• The outer ear then channels sound to the eardrum, or tympanic membrane.
• Sound moves into the middle ear where ossicles concentrate vibrations of the tympanic membrane onto the oval window.
• Finally transmitting into the inner ear where the stapes action at the oval window produces pressure changes that propagate through cochlear.
• Pressure then causes basilar membrane to vibrate in the inner ear.

Ear Anatomy

• Outer ear contains the auricle or pinna and the external auditory canal.
• Middle ear contains the ossicles.
• Inner ear contains the semicircular canals and cochlear.

Outer Ear

• The auricle (pinna) collects sound waves and channels them into the auditory canal.
• The auricle plays an important role in localising sounds.
• Folds in the ear selectively reflect sounds of various frequencies around the ear and into the auditory canal.
• Frequency profile changes as a sound source changes its location relative to the head.
• The frequency profile offers a cue to the location of the source.
• The auditory canal channels sound energy to the tympanic membrane.
• The tympanic membrane (ear drum) vibrates in response to air pressure changes of the sound waves.
• Middle ear ossicles are attached to the tympanic membrane.

Middle Ear

• The ossicles are used for impedance matching because sounds are in air, but sensory systems are in fluid.
• Middle ear concentrates the vibrations of the tympanic membrane on a very small area on the oval window.
• Sound energy would be lost if transmission was directly from air to fluid.
• Middle ear creates 17 fold increase in pressure by concentrating pressure on a small area.
• The lever action of the ossicles amplifies the vibrations by approximately 1.3 times.
• Middle ear accounts for a 22 fold increase in the strength of vibrations hitting the tympanic membrane.
• The middle ear houses the acoustic reflex for loud sounds and speech.

Inner Ear

• Action of the stapes at the oval window produces pressure changes that propagate through the cochlear.
• Pressure causes the basilar membrane to vibrate.

Transduction

• At the auditory threshold, the hair cell displacement is 100 picometers
• This is equivalent to 10mm at the top of the Eiffel Tower

Pitch Perception

• Auditory processing is tonotopic.
• The basilar membrane is a mechanical analyser of frequency.
• The basilar membrane structure changes continuously along the length.
• The basilar membrane is much wider at the apex than at the base.
• Point along membrane responds preferentially to a different frequency: high at the base, low at the apex.
• Basilar membrane processing is preserved throughout early processing and involves outer hair cells.

Auditory Pathways

• There is no major pathway like in vision.
• The auditory network is a complex network.
• The networks start with the First ipsilateral cochlear nuclei.
• The system ultimately passes through the medial geniculate nucleus of the thalamus (MGN) then the primary auditory cortex (A1).

Subcortical Sound Localisation

• Localisation of sound sources is primarily mediated subcortically at the superior olives (SO).
• The sound impinges on each ear slightly different depending on where the sound is coming from.
• The auditory system can determine sound origin because there are two detectable differences.
• Sound origin can be detected through interaural time difference and interaural intensity difference.
• Expert echolocators can discriminate target offsets of 1.2 degrees (similar to bats)
• Echolocator acuity is similar to visual acuity in far periphery.

Interaural Time Difference

• As a sound source moves left or right of centre, time to each ear differs.
• Medial SO generates a map of time differences using coincidence detectors.

Interaural Intensity Difference

• The head acts to block sound reaching one ear.
• Lateral SO is for intensity comparison.
• Lateral SO relies on cross inhibition.

Auditory Cortex

• The auditory cortex is organised tonotopically as a columnar organisation.
• The tonotopic representation is like in V1 to the visual cortex.
• The difference is that it's based on frequency rather than orientation.
• Both ears contribute to processing from early processing.

Auditory Dysfunction - Hearing Loss

• There are three broad classes of hearing loss: Conduction, Sensorineural, and Central.
• Conduction deafness involves damage to the tympanic membrane and ossicles and does not involve the nervous system. Conduction deafness is treated with hearing aids or bone conduction implants.
• Sensorineural deafness involves the auditory nerve fibres not being stimulated properly. Sensorineural deafness is typically permanent.
• Central deafness is caused by brain lesions in the temporal cortex or brainstem; this rarely results in hearing loss.

Conduction Deafness

• Conduction deafness may occur because ossicles become fused and no longer transmit sound vibrations from the outer ear to the cochlea.
• Hearing aids or bone conduction implants are used for treatment.

Sensorineural Deafness

• The auditory nerve fibres are not stimulated properly resulting in permanent loss.
• Infection, trauma, exposure to toxic substances, and loud sounds (e.g. noise pollution, personal headsets) can induce sensorineural deafness.
• Streptomycin (an antibiotic) has ototoxic properties.
• Tuberculosis patients that are treated with streptomycin had cochlear damage.
• In some cases, all the hair cells in the cochlea were destroyed, leading to total deafness.
• For sensorineural deafness Cochlear Implant may be used.
• Auditory nerve fibres are stimulated directly by bypassing hair cells.
• Sound is turned into digital code by an external processor.
• The internal electrode array in the cochlear stimulates the nerve accordingly.

Cochlear Implant

• An internal electrode array in the cochlear stimulate the nerve accordingly implementing tonotopic principles.
• It takes time and training to learn to interpret the signals.

Central Deafness

• Central deafness is caused by brain lesions in the temporal cortex or brain stem.
• Central deafness rarely results in hearing loss - people with cortical lesions in auditory areas often have normal audiograms.
• Individuals with central deafness typically have more complex issues.
• These issue are Aphasias, Agnosias, and Auditory hallucinations.

Vestibular System

• The vestibular system is especially important for movement and position of the head, whole body balance, and control of vision.
• The vestibular system has five receptor organs that sense accelerations of the head.
• Three semicircular canals sense head rotations.
• Two otolith organs (utricle and saccule) sense linear acceleration for horizontal movement and tilt.
• Vestibular system measures accelerations.
• Each receptor has cluster of hair cells that transduce head motion/position.

Labyrinth of the Inner Ear

• The labyrinth of the inner ear contains the semicircular canals, ampullae, utricle, saccule, vestibular part of the cranial nerve VII, facial nerve, auditory nerve, and cochlea.

Semicircular Canals

• Three perpendicular canals (horizontal, anterior vertical, posterior vertical) sense rotations around the three principle axes.
• Yaw is rotation around z-axis.
• Roll is rotation around x-axis.
• Pitch is rotation around y-axis.
• Ampulla contains diaphragm - cupula and hair bundles.
• Inertia of fluid exerts force on hair cells.
• Start rotation, fluid lags therefore cupula distorts.
• Stop rotation, fluid keeps going thus cupula distorts.
• No change occurs In between.
• Hair cells deform to either excitation or inhibition.
• Hair cells depolarises in exciation.
• Hair cells hyperpolarises in inhibition.

Otolith Organs

• Hair cells are in flat membrane covered in tiny 'stones'.
• Linear acceleration exerts force that moves the membrane, distorting the hair cells.
• Translational motion and gravity are detected.
• Otolith system cannot distinguish between tilt and linear acceleration.
• Tilt is used to simulate G-force in VR.
• Head tilts produce displacements of otolithic membrances similar to certain accelerations.

Vestibular System Integration

• Most movements illicit complex patterns of vestibular stimulation.
• Individual organ signals may be ambiguous due to combined movement, tilts and gravity.
• The vestibular system integrates information to interpret and head and body in space.
• Combines information from 3 canals, 2 otoliths, visual and somatosensory systems.

Vestibulo-Ocular Reflex

• Vestibulo-Ocular reflex (VOR) head movements illicit compensatory eye movements necessary to maintain fixation and minimise motion on the retina
• Loss of VOR causes oscillopsia which is "bouncing vision".
• Signals from the vestibular organs unavailable
• Compensatory eye movements are not made

Somatosensory - Exteroception

• Five senses involve touch.
• Mechanoreceptors detect stroke, pressure, vibration, stretch, light stroke, and affective touch.
• Temperature receptors detect cool, warm, cold, and hot sensations.
• Nociceptors detect sharp, burn, freeze, and slow burn sensations.
• Fibre diameters and myelination varies.
• Conduction speed can vary greatly.
• Touch and proprioception are fast, while thermal and nociception are slower.

Somatosensory - Proprioception

• Sensory and motor systems represent info about state of muscles and limbs.
• There is information about Muscle length and speed, muscle stretch, muscle contraction, joint angle, excess stretch or force.
• A variety of receptors are embedded in muscles, tendons, and joint capsules.
• Receptors are involved in conscious sensation of muscle activity as well as unconscious monitoring of body for posture in reflexes.
• Patellar reflex stretches muscle spindle in quadricep resulting in a spinal reflex to contract quad and relax hamstring.

Somatosensory - Interoception

• Visceral sensations represent status of internal body organs.
• Drive behaviour for survival such as respiration, hunger, thirst, nausea (food aversion), and arousal.
• The sensation of loss of air is a feeling of suffocation which becomes all consuming and results in breath holding.
• O2 and CO2 sensors in carotid bodies and in respiratory centres of medulla and hypothalamus.

Somatosensory Pathways

• Somatosensory information ascends from each side of the body to the cortex via two major pathways.
• The Dorsal Column-Medial Lemniscus carries information about touch and proprioception.
• The Anterolateral System carries information about pain and temperature.
• Both are fed by dorsal roots of spinal nerves or trigeminal sensory nerves in the head.

Dermatomes

• Afferent nerve fibres over a specific area of the body converge on specific dorsal roots in the spinal cord.
• There is considerable overlap so damage or loss causes negligible deficit in sensation.

Dorsal Column-Medial Lemniscus Pathway

• Touch and proprioception pathway.
• Ascends ipsilaterally in dorsal columns and synapse on dorsal column nuclei in medulla.
• Decussates then ascends via medial lemniscus to contralateral VPN.
• Project primarily to S1.

Dorsal Column-Medial Lemniscus Pathway

• Ascends ipsilaterally in dorsal columns and synapse on dorsal column nuclei in medulla.
• Decussates then ascends via medial lemniscus to contralateral VPN.
• Project primarily to S1.

The Anterolateral System Pathway

• Pain and temperature pathway. slow - small myelinated and unmyelinated.
• Synapses on cord entry.
• Decussates and ascends contralaterally up the anterior lateral spinal cord
• Multiple tracts.
• Spinothalamic to thalamus in several nuclei.
• Conveys noxious.
• Thermal and Visceral signals.
• Projects from thalamus TO to S1, cingulate cortex, insula cortex.
• Has Spinothalamic trad, Spinotectal tract, and Spinoreticular tract.

Cortex

• Primary (S1) and secondary (S2) somatosensory cortex sit on in anterior parietal.
• Primary (S1) and secondary somatosensory are also homunculus structures as well as tonotopic.
• Are controlled contralaterally in S1 but bilaterally in S2.
• S1 and S2 output to association cortex - posterior parietal
• Damage to S1 does not result in marked major deficits in sensation as several parallel pathways are operating.

Interoception

• Insular cortex (insula) has primary cortical substrate for interoceptive representation
• Topographic and modality-specific signals are relayed by the post insula.
• Integrated in the ant insula.
• Ant IC interacts with FT networks, enabling conscious access to interoceptive signals and their integration with more complex emotional and social cognitive processes.
• Post insula correlated with objective painful heat intensity, ant insula correlated with subjective pain intensity ratings.
• Individuals with high score on interoceptive sensitivity and are better able to utilize bodily signals to guide higher processes. These include decision making, subliminal fearning, and implicit memory

Dysfunctional Interoception

• Kids hypersensitive to sensory input may overreact to interoceptive sensations.
• They may eat more to avoid feeling hunger pangs and may go to the bathroom more because they don't like the way a full bladder feels.
• Kids who are under-responsive to sensory input may not feel or respond to sensations when they should.
• They may learn toilet or have more accidents and may not eat as often because they may not feel hunger or thirst.

Pain Perception

• Pain is adaptive as it stops from doing more damage.
• Pain also Encourages to seek treatment or to treat pain.
• The cortical representation of pain is diffuse since single structure is responsible.
• The primary and secondary sensorimotor cotices are responsive but not needed as it doesnt reduce pain when lesioned.
• Full lesion has little to no effect on pain.

Chemical Senses

• Chemicals in the environment are cues.
• The General state of the local environment is also relevant such as toxins pH or iconic environment.
• Senses are also tuned to detect the presence of Food, predator/prey mate scent, kin identification.
• Taste is non volatile and odours is volatile.
• Single cells react and respond to local chemical environment
• Chemical senses evolved very early and cause Many responses. Identification ; affective; initiate physiological changes.

Chemical Senses olfaction

• Receptors in upper nasal passage embedded in in olfactory mucosa are also Olfactory receptor neuron (ORN).
• Axons pass through cribriform plate into olfactory bulbs
• Synapse then project via olfactory tracts to brain.
• The receptors are not arranged in a sensible way.

Olfactory Receptors

• Olfactory receptors (OR) are G-protein coupled receptors located on cilia on the ORN dendrite.
• The ORN is composed of 400 receptors
• About 1000 genes total - most are broken receptors.
• Only 1 OR type per ORN.
• Has Component processing capabilities to identify odours with activity.
• Rapid turnover of ORNs occurs every 1-2 months through continual replacement
• Humans have 10 million ORNs against Dog = 200 million ORNs.

Olfaction Pathway

• The process proceeds through ORNs and undergoes many convergences before reaching the Olfactory Bulb.
• The signals then undergo projections to several structures in medial temporal lobe to trigger Amygdala Hypothalamus and limbic structures as well Piriform Cortex
• There are 2 main pathways from amygdala/piriform that are Limbic since responses to it are motivational, autonomic and emotional and Thelamic since consciousness occurs.
• The Thalamic-Orbitofrontal portion triggers many Perceptions of Odors and Memory.

Olfactory dysfunction

• Anosmia: the inability ability Common cause: blow to the head with brain shifts that shear off olfactory receptors at cribriform fracture.
• COVID-19 also has this symptoms Sudden smell loss in COVID-19 with prevalence of between 40-75 percent. Long-COVID Olfactory dysfunction with distortion. phantosmia Hallucination

Chemical Senses - Gustatory

• Five primary tastes: salty, sweet, sour, bitter, umami.
• Information provided by taste
• Umami protein
• Sweet carbs and high caloric density
• Salty ion/water balance
• Bitter and Sour warning system:
• Bitter Detected by humans and animals
• Can be detected by the organism 1,000x more precisely than salty.

Gustatory Pathway

• Taste is transduced at the Tangue and oral cavity inside of taste buds
• Located on Small protuberances.
• non neural therefore Synapse

Gustatory Transduction

• There are Few center tongue
• multiple 10-30 day turn over with Drops with age
• Located at non-taste secretory and Sensory touch
• Irritant is recognized through capsaicin.
• O2 Is determined through secretory means.
• high 33 receptor proteins are mapped are sour and salty
• there are 1 umami
• 2 are swee0t
• there are 30 bitter varieties.

Gustatory Transduction pathway

• Has Has 3 afferents, Solitary N then projects to:
• Ventral post nucleus
• which goes to superior areola also homunculus
• gustatory can cause Ageusia (rare) therefore many pathways active to trigger it

Super-tasters

• 25% or 25 super (versus and 25% nontasters)
• there are 120 to
• 670 per 2 therefore Much
• intensely can also trigger in the bitter
• has Much for high .More papialla .

Key Learnings Auditory

• Auditory: transmit sound, transduce, transmit neural, process
• Middle ear ossicles greatly amplify (impedance matching).
• Basilar membrane varies tonotopic Complex
• Complex brainstem network from cochlear nucleus to MGN then A1. Sub cortical localisation Interaural time and differences and SO

Vestibular

• Vesitublar measures accelerations of the head through 5 main organs.
• Semicicrular canals (rotational), otolith organs (linear including gravity).
• Has Vestibulo-occular reflex and oscollopsia. -Also Somatosensory interoDermatomr and pathways DCML ALS:
• Center-surround RFs are located at S1 and S2. Olfaction is very Direct and ipsilateral then conscious .
• The Gustory pathway is activated during tasting.