cap I_curs 2_3_4_General sensory systems 2014_2015_.pdf

Lect. univ. dr. Loredana - Cristina MEREULaboratory of Biophysics & Med. Physics, Faculty of Physics, 'Alexandru Ioan Cuza' University of Iasi

I. General principles of sensoryphysiology

II. The somatosensory SystemIII. Chemical Senses: Taste and

OlfactionIV. VisionV. Hearing and

Equilibrium

I. General Principles of Sensory Physiology

1. Biophysics of sensory perception2. Receptor physiology3. Sensory pathways4. Sensory coding5. Sensory Integration Dysfunction

Sensation is part of the complex processes ofperception, which involves the integration ofexperience and comparison with other sensations, inorder to evaluate/compare the quality, intensity andimportance of sensations.

In fact, one of the most important functions of thenervous system is to collect and interpret informationin a manner that results in appropriate mental andmotor responses. More than 99% of all sensoryinformation is discarded by the brain as irrelevant.

1. Biophysics of sensory perception

The Function of Each Sensory System Is to Provide the CNS (Central Nervous System) with a

Representation of the External World.

Biophysics of sensory perception

Detection of a stimulus and recognitionthat an event has occurred usually arecalled sensation.

Interpretation and appreciation of thatevent constitute perception.

Perception of a sensory experience canchange even though the input remains thesame.


Methods for studying sensory processing:

1. Psychophysics - Is the Quantitative Study ofSensory Performance - use behavioral testing toestablish the sensitivity of a sensory system and therules of its operation.

A psychophysical experiment determines thequantitative relationship between a stimulus and asensation in order to establish the limits of sensoryperformance (Stevens, 1957)

The basic law of psychophysics emerges: how anobserver perceives a stimulus is a linear function of theintensity of that stimulus.


Methods for studying sensory processing:

2. Electrophysiological recording from the singleneurons or small groups of neurons along thesensory pathway to find out how the neuralcircuitry gives rise to the perceptual abilities.

3. Imaging in humans that are doing perceptualtasks to identify the brain areas responsible.


Sensory systems appear to be very diverse.

Yet they all solve the same task:

they convert environmental signalsinto neural activity that can influencethe motor system of the animal.


Sensory perception reception and perception ofinformation from outer and inner medium.

From outer medium: Vision, hearing, smell, tasteand sense of touch

From inner medium: information on position, activeand passive movement (vestibular organ, nerve-endings in the musculoskeletal system). Also:changes in composition of inner medium andpain.

Complex feelings: hunger, thirst, fatigue etc.


Brain and spinal cord

Cranial nerves and spinal nerves

Somatic General - transmit impulses from skin, skeletal muscles, and jointsVisceral General - transmit impulses from visceral organs


The Autonomic Nervous System

Innervates non-skeletal (non-somatic) muscles

Gives sensory input to autonomic nervous system


The Autonomic Nervous System - is the subdivision ofthe peripheral nervous system that regulates bodyactivities that are generally not under conscious control.


Visceral sensory neurons

Monitor temperature, pain, irritation, chemicalchanges and stretch in the visceral organs

Brain interprets as hunger, fullness, pain, nausea,well-being

Receptors widely scattered localization poor (e.g.which part is giving you the gas pain?)

Visceral pain is induced by stretching, infection andcramping of internal organs but seldom by cutting (e.g.cutting off a colon polyp) or scraping them.


Visceral pain: not as well localized as pain originatingfrom the skin - pain impulses travel on secondary axonsdedicated to the somatic afferents - referred pain


Anterior skin areas to which pain is referredfrom certain visceral organs.


Pain in visceral organs is often perceived to besomatic in origin: referred to somatic regions of bodythat receive innervation from the same spinal cordsegments.

Classification of Sensory System by Structural Complexity

Somatic (= general) senses1. Touch2. Temperature3. Vibration4. Nociception5. Itch 6. Proprioception

Special senses1. Vision2. Hearing3. Taste4. Smell5. Equilibrium

The sensory system includes any organ or partinvolved in the perceiving and receiving of stimuli.


Commonly recognizedsensory systems arethose for:1. vision2. hearing3. somatic sensation

(touch)4. taste5. olfaction (smell)

Five Senses

In short, senses are transducers from the physicalworld to the realm of the mind.

Special SensesBiophysics of sensory perception

Localized confined to the head regionSpecial Senses


A sensory system is a part of the nervoussystem responsible for processing sensoryinformation.

A sensory system consists of: sensory receptor cells neural pathways parts of the brain

involved in sensoryperception.


a) Stimulus-energy source:InternalExternal

Properties of sensory systems Biophysics of sensory perception

b) Receptors: Sense organs - structures specialized to respond

to stimuli Transducers - stimulus energy converted into

action potentials

d) TranslationCNS integration and information processingSensation and perception your reality

Properties of sensory systems


c) Conduction Afferent

pathway

Nerveimpulsesto the CNS


Principles of sensory systems:

1. Not all animal biosensors are equal! What you detectdepends on your biology/ecology.For example, many insects can detect U.V. light, buthumans cannot. Is human vision inferior? Notnecessarily, rather there is just no survival advantagefor humans.

2. When important, sensory organs are so sensitive asto be at the limit of the physical phenomenon.For example, best photoreceptor systems can detect asingle photon. There is nothing less than a singlephoton that could be detected!

Biophysics of sensory perceptionPrinciples of sensory systems:

3. Sensory systems do not evolve to provideexact/correct information, rather they evolve toaccentuate important information and minimize un-important information.For example, retinal edge detectors and contrastenhancement. Note: the enhancement and minimizationof information often involves a process of filtering.

4. What is signal to one organism is noise to another.For example, barn owls adapted to hear rustlingnoises this is music to the barn owl! A major force inevolution of sensory systems is to improve thesignal/noise ratio.


Principles of sensory systems:

5. Sensory systems are often more sensitive than anyone receptor. Sensory systems use signal averaging.

6. Sensory processing is dynamic there is not asingle, stable sensitivity or filtering process. Ratherthere is real-time adjustment of parameters.

Behaviour involves adaptive responses (i.e.changes) to stimuli (i.e. changes in the environment). Anorganism is subjected to many different types of stimuli.

Classes of Environmental Stimuli

Stimuli are detected by sensoryreceptors.

2. Receptor physiologySensory receptors Definition and Properties

All receptors work in basically the same way: TheyConvert Different Forms of Energy into ElectricalSignals.

Thus they serve as biological transducers,changing the particular form of energy ( e.g.mechanical, chemical, thermal, or electromagnetic) of astimulus into the electrical signal (action potentials).

Receptor physiology. Sensory receptors Definition and Properties

Sensory Receptors are special nerve endingsor specialized neurons or epithelial cells, distributedthroughout the body (in the skin, muscles, vessels,bones and joints, in lungs, heart, and another organs)that exist singly or in groups with other cell types insensory organs, such as eyes or ears.


Our body contains 20 types of receptors thatcan detect heat, pressure, stretch, acceleration, sound,light, smells, taste, partial pressure, concentration ofsalts, hormons...and other forms of stimuli (Onlyreceptors for ionizing radiation are missing!!!)


Each type of receptor is specialized to respondto only one type of stimulus there is no such thingas a generalized receptor.

Since every receptor generates action potentialson stimulation, if each receptor responded to severaldifferent types of stimuli, they would all generate actionpotentials so there would be no way of discriminatingbetween the different stimuli.

The ability to discriminate between differentstimuli is because there are different receptors foreach type of stimulus.


Different receptors are connected to differentsensory neurones, which pass to different parts ofthe brain, which interpret the incoming signals indifferent ways to produce the different sensations.


The visual areainterprets incomingactions potentials as lightto enable us to see,whereas the auditory areainterprets then as soundto enable us to hear.

Perhaps the first to evolvewere sensory neurons.

One cytoplasmic extensionof these bipolar cells facing theexternal environment becamespecialized to detect stimuli muchweaker than those activatingindependent effectors, whereasthe other pole became specializedto transmit information aboutthese stimuli to a group ofindependent effectors.


[Fundamental Neuroscince Third edition, Larry Squire, Darwin Berg, Floyd Bloom,Sascha du Lac, Anirvan Ghosh, Nicholas Spitzer, 2008, Elsevier Inc]

Experimental evidence indicates that sensoryneurons provide four major selective advantages inevolution:

Increased stimulus sensitivity

Faster effector cell responses

Stronger behavioral responses because multipleeffector cells are infl uenced

Sensory neurons responding to different stimulusmodalities can be distributed strategically in differentbody regions


[Fundamental Neuroscince Third edition, Larry Squire, Darwin Berg, Floyd Bloom, Sascha du Lac, Anirvan Ghosh, Nicholas Spitzer, 2008, Elsevier Inc]

The bipolar shape of sensory neurons is fundamentallyimportant. Information normally flows in one direction throughmost neurons, and thus through most neural circuitsfromdendrites and cell body, the input or receptive parts of theneuron, to a single axon, the output or effector part.

In other words, most neurons have two classes ofprocesses: one or more dendrites detecting inputs, and a singleaxon conducting an output that can influence multiple cellsthrough branching or collateralization.

At least in early developmental stages, all sensoryneurons have this fundamental bipolar shape, and over thecourse of evolution they have become specialized to detect aremarkablevariety of stimuli from light, temperature, and a widerange of chemicals and ions, to vibration and other mechanicaldeformations.


[Fundamental Neuroscince Third edition, Larry Squire, Darwin Berg, Floyd Bloom, Sascha du Lac, Anirvan Ghosh, Nicholas Spitzer, 2008, Elsevier Inc]

The receptive field (RF) is the specific part of theworld to which a receptor organ and receptor cellsrespond. For instance, the part of the world an eye cansee, is its receptive field; the light that each rod or conecan see, is its receptive field. Receptive fields have beenidentified for the visual system, auditory system andsomatosensory system, so far.

Receptor physiology. Receptive field

A sensory unit is asingle afferent neuronand all receptorsassociated with it. Allthe receptors of anafferent neuron canrespond to an adequatestimulus in an areacalled a receptive field.

The RF is defined as that region of sensory spacewhose stimulation results in a change in discharge(usually firing rate) of the neuron.

Each receptor responds only to stimulation withinits receptive field. A stimulus that affects an area largerthan the receptive field of one receptor will activateadjacent receptors.

The size of a stimulus therefore influences thetotal number of receptors that are stimulated.

A large object, such as a basketball, heldbetween both hands will contact and activate moretouch receptors than a pencil grasped between thethumb and index finger.


The density of sensoryreceptors in the retinaand the size of thereceptive field for eachreceptor determine theresolution of a visualimage.

The density of receptors in a given part of the bodydetermines how well the sensory system can resolve thedetail of stimuli in that area.


A dense population of receptors leads to finer resolutionof spatial detail because the receptors have smallerreceptive fields.

These differences in receptor density are reflected inthe central nervous system in the maps of the bodycreated by the topographic arrangement of afferentinputs.


A common principle is used for estimating where astimulus is located- topographic mapping. Thismeans that points close together on the sensorysurface are represented close together in the brain. Inthe somatosensory surface this is calledsomatotopy, in the visual system retinotopy andin the auditory system tonotopy.


In each map the most denselyinnervated regions of the bodyoccupy the largest areas whilesparsely innervated regions occupysmaller areas because of thesmaller number of inputs.

Bear et al.

This weird picture ofa man is a visualrepresentation of therelative proportions of theneurons in the sensorycortex of the right side ofthe cerebral hemispheres.

Skin areas on thetrunk and limbs are poorlysupplied with sensoryneurones while the lips,hands and feet areextremely sensitive.


Sensory homunculus:This model shows what aman's body would look like ifeach part grew in proportion tothe area of the cortex of thebrain concerned with itssensory perception.


Somatic receptors

Chemoreceptors (taste, smell)

Thermoreceptors(temperature)

Photoreceptors (vision) Baroreceptors (sound,

balance) Proprioreceptors (muscle

stretch)

Visceral receptors

Chemoreceptors(chemicals in blood, osmoreceptors)

Baroreceptors (blood pressure)

Receptor physiology. Classification of sensory receptors

Classification by Structure (complexity)Free nerve-endings (pain)Encapsulated nerve endings = sensory bodies

(sensitive nerve fibre + fibrous envelope - cutaneoussensation)

Sensory cells (parts of sensory organs) - specificityNon-specific: receptors of pain - react on various

stimuli.Receptors :

simple (skin) complex (eye, ear)

Receptor physiology. Sensory receptors Classification

Classification by StructureReceptor physiology. Sensory receptors Classification

Classification by LocationAccording to the place of origin and way of theirreception:

1. Interoceptors (visceroceptors) receive stimuli from internal viscera receptors within

the organs (heart, lungs, kidney)

monitor a variety of stimuli detect stimuli from inside the body and include

receptors that respond to pH, oxygen level in arterialblood, carbon dioxide concentration, osmolality ofbody fluids, distention and spasm (e.g., gut), and flow(e.g., urethra)


Classification by LocationAccording to the place of origin and way of theirreception:2. Exteroceptors Include receptors for touch, pressure, pain, vibration

and temperatureSensitive to stimuli arising from outside of the bodyTypically located near the surface of the body3. Proprioceptorsmonitor degree of stretch informing about the length of

muscles and ligaments are placed in muscles, in bones and joints inform about body position and movement


Classification by Location


Classification by Stimulus Detected (or by function)According to the acting energy:1. Mechanoceptors sense physical deformation cause by forms of

mechanical energy such as pressure, touch, vibration,stretch, motion, sound and itch etc.

they transform mechanic energy into electric signal(e.g. exteroreceptor, baroreceptors, pulmonarystretch receptors).


Baroreceptors detect changesin fluid pressure

Lighttouch

HairStrongpressure

Hypodermis

Nerve Connectivetissue

Dermis

Epidermis

Touch receptors(light and deeptouch) areembedded inconnective tissue.

Mechanoceptors Receptor physiology. Sensory receptors Classification

Classification by Stimulus Detected

According to the acting energy:2. Thermoceptors - respond to temperature changes-heat or cold and help regulate body temperature by

signaling surface and body core temperature.- located in skin and anterior hypothalamus



2. Thermoceptors

Mammals havemany

thermoreceptors eachfor a specifictemperature range.


Cold HairHeat


Thermoreceptors

Capsaicin triggers thesame thermoreceptorsas high temperature

Menthol triggers thesame receptors as cold(

Classification by Stimulus DetectedAccording to the acting energy:3. Chemoceptors - respond to chemical stimuli in

solution(e.g., smell, taste, changes in blood chemistry)taste receptors in the tongue, smell receptors within anose, osmoreceptors in hypothalamus.Osmoreceptors detect changes in concentration ofsolutes, osmotic activity

General receptors: respond to totalsolute concentrationsSpecific receptors: respond toconcentrations of specific molecules



According to the acting energy:4. Photoceptors or Electromagnetic receptors

- respond to various forms of electromagnetic energysuch as visible light, electricity, and magnetism.- receptors containing photopigments (rods and conesat retina


This rattlesnake and other pit vipers have a pair of infrared receptors, one between each eye and nostril. The organs are sensitive enough to detect the infrared radiation emitted by a warm mouse a meter away.

Eye

Infraredreceptor

Some migrating animals, such as these beluga whales, apparently sense Earths magnetic field and use the information, along with other cues, for orientation.

Snakes can have verysensitive infraredreceptors detect bodyheat of prey

Animals can use earthsmagnetic field lines toorient themselvesduring migration(magnetite in body) orientation mechanism

Electromagneticreceptors



Pain Hair


Defensive function- stimulatedby things that are harmful hightemperature, high pressure,noxious chemicals, inflammations

5. Pain receptors, or nociceptors (non-specific)

are a class of naked dendritesin the epidermis (in skin, inorgans); respond to harmfulstimuli that result in pain


Statolith

Sensorynerve fibers

Ciliatedreceptor cells

Cilia

Hair of different stiffness and length vibrate at differentfrequencies and pick up sound waves and vibrationsStatocyts: organ with ciliated receptor cells surrounding achamber containing statoliths in invertebrates sense gravity

Sensing gravity and sound in invertebrates:

1 mm

Tympanicmembrane

Tympanic membrane stretched over their ear help sense vibrations

Receptor physiology

Sensing gravity and sound in humans:

Human ear: sensory organ for hearing andequilibrium

Our organ for hearing hair cells aremechanoreceptors because they respond tovibrations

Moving air pressure is converted to fluidpressure

Receptor physiology

Sensory receptors = Protein Receptors in Sensory Cell Membrane

Receptor physiology

In biophysics, the receptors are energy transducers above all.

Mechanical and chemical receptors sense the bodyscondition.

3. Sensory pathways

Sensory perception is an ability to distinguish, detect,

utilize some feelings, and answer to many information

that come to the brain through the reflex arc.

Sensory pathways

The reflex arc consists of:

receptor afferent nerve pathway the central nervous system (brain and spine) efferent pathway effector (muscle).

Sensory pathways

The sensory pathwaysconvey the type andlocation of the sensorystimulus.The type: because of thetype of receptor activated.The location: because thebrain has a map of thelocation of each receptor.

Sensory pathways

Stimulus transduction to electrical energy receptor potential action potential

Stimulus as physical energy sensory receptor acts as a transducer

Sensory pathways

Integration in CNS cerebral cortex or acted onsubconsciously

Stimulus > threshold action potential to CNS

Sensory receptors transduce stimulus energy andtransmit signals to CNS (central nervous system)

1. Reception: detection via extero- or intero-receptors2. Transduction: stimulus energy converted intochanges in membrane potential of sensory receptor(receptor potential)

AmplificationAdaptation

3. Transmission: action potentials transmitted to theCNS4. Perception: constructions formed in brain (color,smell, sounds)

Sensory pathways

1. Reception Excitation is manifested at the receptor cells.

These are either special cells (i.e. cells with hairs in thecochlea of the inner ear hair cells) or non-myelinatedendings of the afferent primary neurons (otherwiseknown as sensory or receptor neurons).

Intensity is a common dimension to all stimuli. It isa measure of energy (or the concentration of chemicalexcitation) which interacts with sensory receptors.

Stimulus is based on the sensory modality, suchas the traditional five senses: taste, smell, touch, sightand hearing. More complex stimuli (e.g. humidity) are acombination of primary ones (pressure andtemperature).

Sensory pathways

Biophysics-Lectures-2012-3 (Physiology)

Photoreceptors detect light, and they are thefunction of the senses of vision;

Chemoreceptors are in function of sensations oftaste and smell, and control of arterial blood;

Mechanoreceptors sense physical deformity andparticipate in the sense of touch and hearing, as well asin the detection of stress in the muscles and tendons;

Thermal receptors detect thermal changes;Proprioceptors are sensors that provide

information about joint angle, muscle length, andtension position of the limb in space;

Nociceptors detect harmful substances, etc.

Sensory pathways. Reception

1. Sensory reception

2. Transduction - Conversion of physical, chemical and other stimuli to change in membrane potential;

Information from an environment come to our body andare processed by our senses (Touch, Taste, Smell,Vision, Hearing). Reaching the body the informationare coded in 2 forms: as a local electric response(local potential), and the general action potential (AP)

Typically activation of a sensory receptor by anadequate stimulus results in depolarization or gradedpotentials that trigger nerve impulses along theafferent fibers coursing to the CNS

Sensory pathways.


.

The conversion of specific energy excitation in thereceptors potential (i.e. mechanical, heat, light orchemical energy into electrical energy) is called signaltransduction. It is based on the change ofmembrane permeability, either by mechanicaldeformation, binding of ligands, changing temperaturesor absorption of the electromagnetic radiation

Each excitation causes the receptor (generator)potential: a change in the resting potential of theneuron. This change is usually depolarization, but canbe hyper-polarization, or even both (for example,alternating in the case of hair cells in hearing).

Sensory pathways. Transduction


.


A chemical binds to the receptorson an afferent process, causesdepolarization and discharge.A hair cell is activated byvibration, becomes depolarizedand releases transmitter whichexcites an afferent fiber; itdischarges and this informationreaches the CNS.A photoreceptor is activated bylight; its release of transmitter isreduced. After a series of complexinteractions in the retina, ganglioncells discharge and send thisinformation to the brain.

(a) Depolarization of a free nerve ending leads to areceptor potential that spreads by local current flow tothe axon. (b) Action potentials are produced in the axonin response to a sufficiently large receptor potential


Events in sensory transduction.

Sensory Transduction Among the Vertebrates


Generating potential of most of the sensory receptorsexposed to the constant stimulus gradually declinesover time leading to a reduction of the frequency ofaction potentials. Thus, the receptor is adjusting tocontinuous stimulus by reducing the sensitivity -this iscalled adaptation. The adaptation occurs at the level oftransduction.

It is thought that adaptation occurs in order to:

1) reduce stimulus overload

2) ignore continuous, but less important stimulation,and to have the ability of perception of fast changes(and not just the magnitude) of stimulation.

Receptor adaptationSensory pathways. Transduction

It is usually caused by inactivation of sodiumchannels after extended period of depolarization, andcan weaken fundamental capacity of receptors toproduce the receptor potential (for example, lightinduced breakdown of photo-sensitive molecules in rodsand cones of the eye), and it may influence the actionsof supporting structures by the mechanism of negativefeedback (for example, too much light closes the eyepupil).

The adaptation degree is different for variousreceptors. Adaptation can be fast or slow.

Receptor adaptationSensory pathways. Transduction

Tonic receptors - slow acting; adapt just a little or not significantly at all. - continue to form impulses as long as the stimulus is

there (ex: proprioreceptors)Phasic receptors- quick acting, adapt: stop firing when stimuli are

constant (ex: smell)


The receptors with Low Adaptation are involved in acontrol of blood pressure, in control of breathing, inresponses of body to the pain sensation - protectionmechanism, etc.


The Law of Adaptation Adaptation - is an internal electric property(caused by membraneproperties of the receptor) to respond when the long-term stimulusof a constant intensity is applied. Actualy, it is a drop of thereceptor excitability to give rise the GP and then the Aps.

Receptors with Rapid Adaptation of their Burst Activity - theirfire just for a short time, during the constant (maintained)stimulation (as typically seen in touch, pressure, taste and smellreceptors.)

Receptors with Low Adaptation of their Burst Activity -theyfire for a long time with only a low drop of their firing activity (asseen in the pain, cold, heat receptors, baroreceptors, inpulmonary stretch recepors, the chemoreceptors, carotidbaroreceptors or in the pulmonary stretch receptors).


1. Sensory reception2. Transduction

3. TransmissionSensory information is transmitted through the

nervous system as nerve impulses or action potential tothe Central Nervous System (CNS).

Some axons can extend directly into the CNS andsome form synapses with dendrites of other neurons.

Primary response of sensory cell to the stimulus:receptor potential and receptor current are proportionalto the intensity of stimulus. The receptor potentialtriggers the action potential.

Sensory pathways

Receptor Potential to Action Potential

Sensory receptors initiate coded messages:

total number of fibers transmitting

specific fibers carrying signal

total number of action potentials

frequency of action potentials

Sensory pathways. Transmission

How can we distinguish between color, taste andhearing if all are converted into the same type of actionpotential?

Local electric response- takes local place, it does notspread to the vicinity, when its magnitude reachesmore than 10 mV then, in turn AP is produced. Thistype of coding is so called AMPLITUDE. (i.e. thestronger is stimulus, the higher is amplitude ofresponse).

Action potential is a generally spreading electricity,being under the Law All or None. This type of codingis named FREQUENCY i.e. the stronger is stimulusthe higher is a rate of APs from the receptors. Thebrain knows that a higher frequency of actionpotentials means a stronger stimulus (and viceversa).


Receptorpotential

Generatorpotential

Actionpotential

Transformation of amplitude modulated receptorpotential into the frequency-modulated actionpotential.

Conversion function of receptors

Magnitude of receptorpotential controls the rate atwhich action potentials aregenerated (larger receptorpotential results in more frequentaction potentials)


Sensory neurons spontaneously generate actionpotential without stimulus at a low rate.

Conversion function of receptors

Increased intensity ofstimulus, i.e. increasedamplitude of receptorpotential evokes anincrease in actionpotential frequency.


Stretch receptors (mechanoreceptors) are dendritesthat spiral around small skeletal muscle fibers.

MuscleWeakmuscle stretchReceptor potential

Action potentials

Mem

bran

epo

tent

ial (

mV)

Time (sec)0 1 2 3 4 5 6 7

70

70

50

0

Stretchreceptor

Dendrites

Axon

Strongmuscle stretch

Time (sec)0 1 2 3 4 5 6 7

70

70

50

0

Crayfish stretch receptors have dendrites embedded in abdominalmuscles. When the abdomen bends, muscles and dendritesstretch, producing a receptor potential in the stretch receptor. Thereceptor potential triggers action potentials in the axon of thestretch receptor. A stronger stretch produces a larger receptorpotential and higher frequency of action potentials.


1. Sensory reception2. Transduction3. Transmission

4.Perception:

Action potential reach the brain via sensory neurons,generating perception of a stimulus

All action potentials have the same property, whatmakes the perceptions different are the part of thebrain they link to.

Sensory receptors are connected to different parts ofthe brain which interprets signal and generatessensation.

Sensory pathways

In every case,soon after peripheralinput arrives in thebrain, decussationsresult in one hemifieldbeing representedprimarily by the brainon the opposite side.

Sensory pathways. Perception


Within each ofthese areas, theorganized mappingthat is established byreceptors in theperiphery is preserved.

Each pathway has a unique nucleus in thethalamus and several unique fields in the cerebralcortex.

Laws of Sensory PerceptionBiophysical relation between the stimulus and sensation

Weber-Fechners Law: is a basic psychophysical Law.

The bigger is the intensity of stimulation, the higher isthe magnitude of sensation.

The intensity of sensation (IR ) increases with stimulusintensity (IS) non-linearly.

It was presumed earlier the sensation intensity isproportional to the logarithm of stimulus intensity:

IR = k1 . log(IS)


Today is the relation expressed exponentiallycalled Stevensons Law a modified form of Weber-Fechner law

IR = k2 . ISa

k1, k2 are the proportionality constants, a is an exponentspecific for a sense modality (a = 1 is valid formechanoreceptors, a < 1, for fotoreceptors, a > 1, forpain receptors).

The Stevens law expresses better the relationbetween the stimulus and sensation at very low or highstimulus intensities.





The lowest stimulus strength a subject can detectis termed the sensory threshold. Thresholds arenormally determined statistically by presenting a subjectwith a series of stimuli of random amplitude. Thepercentage of times the subject reports detecting thestimulus is plotted as a function of stimulus amplitude,forming a relation called the psychometric function.

Threshold is defined asthe stimulus intensitydetected on 50% of thetrials.

Laws of Sensory PerceptionBiophysical relation between the stimulus and sensationSensory pathways. Perception

The absolute sensorythreshold (curve b) is an idealizedrelationship between stimulusintensity and the probability ofstimulus detection. If the sensorysystem's ability to detect the stimulusis increased or the subject'sresponse criterion is decreased,curve a would be observed; curve cillustrates the converse.

Sensory Thresholds Are Modified by Psychological and Pharmacological Factors

Marijuana also increases pain thresholds, but does so byincreasing the response criterion rather than decreasing stimulusdetectabilitythe stimulus is just as painful but the subject ismore tolerant.

Laws of Sensory PerceptionBiophysical relation between the stimulus and sensationSensory pathways. Perception



The measurement of sensory thresholds is auseful diagnostic technique for determining sensoryfunction in individual modalities.

Elevation of threshold may signal an abnormalityin sensory receptors (such as loss of hair cells in theinner ear caused by aging or exposure to very loudnoise), deficits in nerve conduction properties (as inmultiple sclerosis), or a lesion in sensory processingareas of the brain.

Sensory thresholds may also be altered as aresult of emotional or psychological factors related tothe conditions in which stimulus detection is measured.

4. Sensory codingA receptor must convey the type of information it

is sending the kind of receptor activated determinedthe signal recognition by the brain.

It must convey the intensity of the stimulus the stronger the signals, the more frequent will be theAps.

It must send information about the location andreceptive field, characteristic of the receptor.

4. Sensory coding

Stimulus type is coded by the receptor type andthe pathway activated when the stimulus is applied.The perception of a stimulation often results from thesimultaneous activation of more than one sensorypathway.

The finalperception resultsfrom integrationin the brain ofinformation fromvarious sensorysystems.(Pinocchioillusion)

What do Sensory Systems Sense?There are two main functions of any sensory system:1. The detection of a signal. Weak signals can be

detected without the animal being able to finelydiscriminate any of its features.

2. Discrimination of some aspects of a sensory input.This is often referred to as estimation.

A common strategy of sensory systems is to haveseparate neural pathways specialized for estimatingdifferent types of stimulus features. For example, thevisual system analyzes colour, shape and movement indifferent brain regions.

4. Sensory coding

What must be estimated from the input?

1. Qualitative features such as colour or odorant; thisis often referred to as modality- what is it?

2. Quantitative features such as magnitude- oftenreferred to as the intensity of a stimulus;

3. Temporal features such as duration or frequencyof a signal.

4. Spatial location of a stimulus- where is it?

Typically, all these aspects are estimated at once.

4. Sensory coding

How are these attributes represented in the brain?

1. Modality: the most basic mechanism for identifying

the nature of a sensory input is via labeled lines.

What this means is that input from the optic nerve is

always interpreted by the brain as visual input etc.

This extends to much finer discriminations: the

connections of pain and touch fibers in the

somatosensory system are entirely different and

electrical stimulation of either leads to the appropriate

sensation.

4. Sensory coding

How are these attributes represented in the brain?2. Intensity: the estimated intensity of a stimulus isnot a linear function of the actual intensity.The relation can be described as logarithmic or powerlaw. The reason is intuitively easy to understand.Increases of a weak signal generate a larger perceivedincrease than increases of a strong signal- the perceptsaturates.

Intensity is coded by the frequency of actionpotentials (frequency coding) and the number ofreceptors activated (population coding). Strongerstimuli produce a higher frequency of actionspotentials.

4. Sensory coding

How are these attributes represented in the brain?2. IntensityIntensity is interpreted by population coding becausethe more intense the stimulus, the greater the numberof receptors stimulated. This results from either asingle sensory unit (sensory afferent neuron) sendingmore action potentials to the CNS or more sensoryunits sending action potentials to the CNS.

4. Sensory coding

3. Temporal features.

Typically there are neurons in sensory systems that only

respond to the onset of a stimulus- these are generally

referred to as phasic responders and they are good for

estimating the time of occurrence of a signal. There

are other neurons that respond throughout the stimulus

presentation- tonic responders- these signal stimulus

duration. The frequency of a signal may be very

important in some senses (audition).

4. Sensory coding


4. Location. The receptive fields of specific afferentneurons code for stimulus location. Tactile receptors inthe skin illustrate this. The precision of stimulus locationis acuity.

Acuity depends on:

1. Size and number of receptive fields. Thesmaller the receptive field the greater the acuity. Forexample, tactile acuity can be measured by two-pointdiscrimination or the ability to perceive two fine pointspressed against the skin as two points and not as one.

4. Sensory codingHow are these attributes represented in the brain?

4. Location.

4. Sensory codingHow are these attributes represented in the brain?

Areas with smallerreceptive fields (lips,fingertips) havebetter two-pointdiscrimination thanthose with largerfields (back,shoulder).

The smaller the receptive fields the smaller the distancebetween two points of stimulation can be and still bediscriminated.

4. Location.

Acuity depends on:

2. Lateral inhibition - occurs when a strongstimulus applied to the receptive field of one neuroncauses that neuron to inhibit transmission of signals byneurons with neighboring receptive fields. Lateralinhibition increases acuity because it increases thecontrast of signals in the nervous system.

4. Sensory coding


4. Location. Acuity depends on: 2. Lateral inhibition

4. Sensory coding


In other words, thedifference betweenthe strength of thesignals coming fromthe central neuron inthe affected fieldand the neurons onthe periphery isincreased as theinformation isprocessed in thecentral nervoussystem.

Getting Sense Input into the CNS

Sensory input comes in many flavours. Information in

the CNS all comes in the same currency- action

potentials or spikes.

The reason that the CNS uses only one way to transmit

information is simple: it allows integration of different

types sensory input and the connection of sensory

input to motor output- all the neurons dealing with

these different systems use the same language of

spikes.

4. Sensory coding

Getting Sense Input into the CNS

The problem becomes: how to translate the differentkinds of sensory input into spikes. In all cases this isdone by specialized receptor cells in a processcalled sensory transduction which depends on thenature of the signal (Chemoreception,Mechanoreception, Vibration reception, Light).

The initial transduction process causes the receptor cellto depolarize and this leads to spike initiation insensory afferent fibers that then convey thisinformation to the brain.

The message sent by a receptor to the brain is in theform of a sequence of spikes- a spike train. What isthis message?

4. Sensory coding

In addition to place codes, neurons can signalinformation in the rate at which they respond and in thetemporal pattern of their response.

For a given receptor, the firing rate or frequency ofaction potentials signals the strength of the sensoryinput.

The perceived intensity arises from an interactionbetween this firing rate and the number of neuronsactivated by a stimulus.

Together the number of neurons active with anysensory stimulus and the level of their activity gives riseto an intensity code.

4. Sensory coding

The Overall Plan of Sensory Systems

When a sensory neuron fires, it communicates to thebrain that a certain form of energy has been receivedat a specific location in the sense organ.

The details of the action potential code tell the brain: how much energy was received at that place when it began when it stopped how quickly the energy changed in intensity.

All sensory systems also have similar centralprocessing mechanisms.

4. Sensory coding

The Overall Plan of Sensory SystemsFor all sensory systems there is a common plan:Peripheral receptors respond to a specific stimulus

and convert it (directly or indirectly) into a spike train.The afferent fibers end in lower brain regions where

they are processed. Quite often there are manyparallel pathways present.When rapid responses are required, the processed

information might go directly to a motor system.However, for more detailed analysis, the information

proceeds to higher brain levels for further processing.In mammals and birds, the sensory input reaches the

forebrain where it somehow results in the perceptionof complex patterns.

4. Sensory coding

Parallel Pathways for General and Communication Signals

A very general principle of sensory coding isthat communication signals have their own separatechannels. An animal might encounter very differentenvironments depending on where it is born; forexample, a city rat and a country rat will likely encountervery different odors. So sensory systems need to adaptthemselves to the experiences of different animals.

In contrast, communication signals evolve overevolutionary time and are highly conserved for eachspecies. For example, many animals (including rats)use pheromones to communicate gender etc. These arefixed and so the olfactory system does not have tolearn about different pheromones.

4. Sensory coding

Sensory Integration is the neurological processof organizing information we get from our bodies and theworld around us for use in daily life. It takes place in thecentral nervous system, which consists of countlessneurons, a spinal cord, and - at the "head" - a brain.

5. Sensory Integration Dysfunction

The main task of our centralnervous system is to integratethe senses. Over 80 per centof the nervous system isinvolved in processing ororganizing sensory input, andthus the brain is primarily asensory processing machine.

Self-regulation is the ability to control one'sactivity level and state of alertness, as well as one'semotional, mental or physical responses to sensations.It is self-organization.

The vestibular, tactile and proprioceptivesenses are fundamental. They lay the groundwork forhealthy development. Sensory IntegrationDysfunction is the inability to process informationreceived through the senses. Also called SensoryIntegration Disorder, Sensory Integrative Disorder orSI for short.


Sensory integration dysfunction is the result ofinefficient neurological processing. Dysfunctionhappens in the central nervous system, at the "head" ofwhich is the brain. When a glitch occurs, the braincannot analyze, organize and connect - or integrate sensory messages.


The result is the person cannotrespond to sensory information tobehave in a meaningful,consistent way. He/she may alsohave difficulty using sensoryinformation to plan and organizewhat he needs to do. Thus, hemay not learn easily.

Because a person with Sensory Integration Dysfunctionhas a disorganized brain, many aspects of his behaviourare disorganized. His overall development is disorderly.

Behaviour problems are almost always present with aperson with Sensory Integration Dysfunction.

Self-regulation problems occur: the person is unableto "rev up" or calm down once aroused. He/she mayalso perform unevenly.


The brain-behaviour connection is very strong.

1. Unusually high activity level. The person may bealways on the go, move with short and nervousgestures, play or work aimlessly, be quick-tempered andeasily excited, and find it impossible to stay seated.2. Unusually low activity level. The person may moveslowly and in a daze, fatigue easily, lack initiative and"stick-to-it-iveness" and show little interest in the world.3. Impulsivity. The person may lack self-control and beunable to stop after starting an activity. For example,she may pour juice until it spills, run into trees andpeople, and talk out of turn.

BEHAVIOUR PROBLEMS ASSOCIATED WITH SENSORY INTEGRATION DYSFUNCTION


4. Distractibility. The person may have a shortattention span, even for activities he enjoys. The personmay pay attention to everything except the task at hand.The person may be disorganized and forgetful.5. Problems with muscle tone and motorcoordination. The person's body may be either tenseor "loose and floppy". The person may be awkward,clumsy, apparently careless, and accident-prone.



6. Problems motor planning.Motor planning is the ability to conceive of, organize,sequence and carry out complex movements in ameaningful way.The person may have trouble climbing stairs,negotiating obstacle courses and equipment, ridingbikes, dressing, getting in and out of the car, and usingeating and writing utensils.His ability to learn new motor skills, such as clappingout rhythms and skipping, may develop noticeably laterthan other children's.



7. Lack of a definite hand preference by the age offour or five. The person may not use one handconsistently when handling tools such as pens andforks. She may use either hand to reach for an object.She may switch the object from right to left whenhandling it, eat with one hand but draw with the other, oruse both hands to manipulate scissors.8. Poor eye-hand coordination. The person may havetrouble using pens, creating art projects, doing puzzles,eating neatly, or tying shoes. The person's handwritingmay be sloppy and uneven.



9. Resistance to novel situations. The person mayobject to leaving the house, meeting new people, tryingnew jobs, or tasting different foods. The person may bepanicky for no obvious reason.10. Difficulty making transitions from one situationto another. The person may seem stubborn anduncooperative when it is time to come for dinner, getinto (or out of) bathtubs, or change from one activity toanother. Minor changes in routine will upset this personwho does not "go with the flow".



11. High level of frustration. Struggling to accomplishtasks that peers do easily, the person may give upquickly. He may be a perfectionist and become upsetwhen art or work projects don't meet his expectations.Insisting on being the winner, the best, orthe first, hemay be a poor game-player.12. Self-regulation problems. The person may beunable to "rev up" or calm down once aroused. Theperson may perform unevenly: "with it" one day, "out ofit" the next.



13. Academic problems. The person may havedifficulty learning new skills and concepts, and may beperceived as an underachiever.

14. Social problems. The person may be hard to getalong with and have difficulty making friends andcommunicating. He may need to control his surroundingterritory and have trouble sharing.



15. Emotional problems.He may be overly sensitive to change, stress, and hurtfeelings and be disorganized, inflexible, and irrational.He may be demanding and needy, seeking attention innegative ways.He may be unhappy, believing and saying that lie iscrazy, no good, a dummy, a loser, and a failure.Low self-esteem is one of the most telling symptoms ofpoor sensory control.



NEUROSCIENCE: Third Edition, Dale Purves et al., 2004 SinauerAssociates, Inc.

Fundamental neuroscience /by Larry Squire et al.3rd ed. 2008, Elsevier Inc. Coding of Sensory Information, Esther P. Gardner John H. Martin;

http://homepage.psy.utexas.edu/homepage/class/psy394U/hayhoe/IntroSensoryMotorSystems/week3/Kandel%20Ch%2021,%2022,%2023.pdf

http://www.cogsci.ucsd.edu/~ajyu/Teaching/Cogs160_sp12/Lectures/lect1.pdf http://freedownloadb.net/ppt/sensory-and-motor-mechanisms-6026576.html www.austincc.edu/rfofi/BIO2304/2304LecPPT/2304Sensory.ppt www.mohsenparviz.ir/lesson/L5-%20Sensory%20Receptors.ppt www.med.uottawa.ca/Courses/NSC5104/.../NeuralSystemsSensory1.ppt www.med.muni.cz/biofyz/files/en/HEARING-finx.ppt www.jfmed.uniba.sk/.../Biofysics_of_sensory_p._receptors__vision.ppt faculty.weber.edu/nokazaki/.../PPT%20notes/Sensory%20System.ppt http://humanservices.alberta.ca/documents/pdd/pdd-central-sensory-

integration-dysfunction.pdf

References

Slide Number 1Slide Number 2I. General Principles of Sensory Physiology1. Biophysics of sensory perceptionBiophysics of sensory perceptionBiophysics of sensory perceptionSlide Number 7Slide Number 8Biophysics of sensory perceptionBiophysics of sensory perceptionBiophysics of sensory perceptionBiophysics of sensory perceptionBiophysics of sensory perceptionBiophysics of sensory perceptionSlide Number 15Visceral sensory neuronsSlide Number 17Slide Number 18Slide Number 19Classification of Sensory System by Structural ComplexitySlide Number 21Biophysics of sensory perceptionSlide Number 23Properties of sensory systems Properties of sensory systems Slide Number 26Slide Number 27Slide Number 282. Receptor physiologySensory receptors Definition and Properties Receptor physiology. Sensory receptors Definition and Properties Receptor physiology. Sensory receptors Definition and Properties Receptor physiology. Sensory receptors Definition and Properties Receptor physiology. Sensory receptors Definition and Properties Receptor physiology. Sensory receptors Definition and Properties Receptor physiology. Sensory receptors Definition and Properties Receptor physiology. Sensory receptors Definition and Properties Receptor physiology. Sensory receptors Definition and Properties Receptor physiology. Sensory receptors Definition and Properties Slide Number 39Slide Number 40Slide Number 41Slide Number 42Slide Number 43Slide Number 44Slide Number 45Receptor physiology. Classification of sensory receptors Classification by Structure (complexity)Classification by StructureClassification by LocationClassification by LocationSlide Number 51Classification by Stimulus Detected (or by function)Slide Number 53Classification by Stimulus DetectedClassification by Stimulus DetectedSlide Number 56Classification by Stimulus DetectedClassification by Stimulus DetectedSlide Number 59Classification by Stimulus DetectedSlide Number 61Slide Number 62Slide Number 63Slide Number 643. Sensory pathwaysSensory pathwaysSensory pathwaysSensory pathwaysSensory pathwaysSensory pathwaysSensory pathwaysSensory pathways. ReceptionSlide Number 73Sensory pathways. TransductionSensory pathways. TransductionSensory pathways. TransductionSensory pathways. TransductionSensory pathways. TransductionSensory pathways. TransductionSensory pathways. TransductionSensory pathways. TransductionSensory pathways. Transduction The Law of Adaptation Slide Number 84Slide Number 85Slide Number 86Slide Number 87Slide Number 88Slide Number 89Slide Number 90Sensory pathways. PerceptionSensory pathways. PerceptionLaws of Sensory PerceptionBiophysical relation between the stimulus and sensationSlide Number 94Sensory pathways. PerceptionSensory pathways. PerceptionSensory pathways. PerceptionSensory pathways. Perception4. Sensory codingSlide Number 100What do Sensory Systems Sense?Slide Number 102How are these attributes represented in the brain?How are these attributes represented in the brain?How are these attributes represented in the brain?Slide Number 106Slide Number 107Slide Number 108Slide Number 109Slide Number 110Getting Sense Input into the CNSGetting Sense Input into the CNSSlide Number 113The Overall Plan of Sensory SystemsThe Overall Plan of Sensory SystemsParallel Pathways for General and Communication SignalsSlide Number 117Slide Number 118Slide Number 119Slide Number 120Slide Number 121Slide Number 122Slide Number 123Slide Number 124Slide Number 125Slide Number 126Slide Number 127Slide Number 128Slide Number 129

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