Receptors andReceptors andSense OrgansSense Organs

Dr Fawzia ALRoug, MBBS, Master, Ph.D Dr Fawzia ALRoug, MBBS, Master, Ph.D Assistant Professor, Department of Assistant Professor, Department of Physiology, College of Medicine, King Physiology, College of Medicine, King Khalid University Hospital, Riyadh, Khalid University Hospital, Riyadh, Saudi ArabiaSaudi Arabia

A sensory System

It is a system which gives information about sensory stimuli through skin and the body adjusts its responses through the motor system.

Components of Sensory System

• Receptors

• Peripheral nerves

• Spinal cord

• Tracts

• Thalamus

• Thalamocortical projection

• Somatosensory cortex

ReceptorReceptor

Peripheral Nerve

Peripheral Nerve

Dorsal HornOf

Spinal cord

Dorsal HornOf

Spinal cord

TractsMedullary

Nuclei

TractsMedullary

Nuclei

ThalamusThalamusSensory Cortex

SI & SIISensory Cortex

SI & SII

(Localization & Perception of sensation)

StimulusStimulus

3rd order 2nd order

1st order

How information about internal & externalHow information about internal & externalenvironment reaches the CNS?environment reaches the CNS?

Via sensors i.e., Receptors that are connected to Via sensors i.e., Receptors that are connected to the CNS by different cables [i.e., ascending tracts].the CNS by different cables [i.e., ascending tracts].

What are the receptors?What are the receptors?

They are transducers that convert various forms of They are transducers that convert various forms of energy in the environment [internal or external] into energy in the environment [internal or external] into action potentials in the afferent neurons.action potentials in the afferent neurons.

What is the difference between a sense What is the difference between a sense organ and a receptor?organ and a receptor?

A sensory receptor is a specialized structure or a A sensory receptor is a specialized structure or a cell that generates Ap in neurons.cell that generates Ap in neurons.

A sense organ forms when the receptor is A sense organ forms when the receptor is associated with non-neural cells surrounding it.associated with non-neural cells surrounding it.

What are the forms of energy converted by What are the forms of energy converted by receptors?receptors?

MechanicalMechanical

ThermalThermal

Electromagnetic [light]Electromagnetic [light]

Chemical energy [odor, Chemical energy [odor, taste, O2 content of blood]taste, O2 content of blood]

How receptors respond to stimuli [different How receptors respond to stimuli [different forms of energies]?forms of energies]?

By converting them into electrical response, By converting them into electrical response, generator potential which when reach a threshold generator potential which when reach a threshold value will cause as AP. This is what is called value will cause as AP. This is what is called receptor transduction.receptor transduction.

Do all receptors respond to the same Do all receptors respond to the same stimulus?stimulus?

NO.NO.

Because each receptor is most sensitive to a Because each receptor is most sensitive to a particular form of energy called its adequate particular form of energy called its adequate stimulus.stimulus.

The receptor respond to one particular form of The receptor respond to one particular form of energy at a much lower threshold than other energy at a much lower threshold than other receptors respond to this form of energy.receptors respond to this form of energy.

Do receptors respond to forms of energy Do receptors respond to forms of energy other than their adequate stimulus?other than their adequate stimulus?

YES.YES.

But the threshold for these non-specific responses But the threshold for these non-specific responses is much higher, e.g. pressure on the eyeball will is much higher, e.g. pressure on the eyeball will stimulate rods & cones, but the threshold of these stimulate rods & cones, but the threshold of these receptors to pressure is higher than threshold of receptors to pressure is higher than threshold of pressure receptors in the skin.pressure receptors in the skin.

Differential Sensitivity of Receptors:Differential Sensitivity of Receptors:

How do two types of sensory receptors How do two types of sensory receptors detect different types of sensory stimuli?detect different types of sensory stimuli?

This depends on the differential sensitivity of This depends on the differential sensitivity of receptors, i.e., each type of receptor is highly receptors, i.e., each type of receptor is highly sensitive to one type of stimulus for which it’s sensitive to one type of stimulus for which it’s designed & is not responsive to normal intensities designed & is not responsive to normal intensities of other types of stimuli. Normally pain receptors of other types of stimuli. Normally pain receptors are not stimulated by touch or pressure in normal are not stimulated by touch or pressure in normal skin, under normal conditionskin, under normal condition

Sensory Modalities. What is this?Sensory Modalities. What is this?

A sensory modality is the type(s) of sensation(s) we A sensory modality is the type(s) of sensation(s) we experience when a receptor is adequately experience when a receptor is adequately stimulated, e.g., pain, touch, sound.stimulated, e.g., pain, touch, sound.

There is what we call submodality, e.g.:There is what we call submodality, e.g.:4 different submodalities of taste: sweet, salt, sour, 4 different submodalities of taste: sweet, salt, sour, bitter. Each of them is subserved by distinct type bitter. Each of them is subserved by distinct type of receptors.of receptors.

Classification of sensory receptors:Classification of sensory receptors:

1.1. Teleceptors: (distance receivers), detect events at Teleceptors: (distance receivers), detect events at distance e.g. rods & cones distance e.g. rods & cones light; sound: hair cells light; sound: hair cells

2.2. Exteroceptors: concerned with information of the ext. Exteroceptors: concerned with information of the ext. environment e.g. skin: pain, touch, temperatureenvironment e.g. skin: pain, touch, temperature

3.3. Interoreceptors: concerned with internal environment, Interoreceptors: concerned with internal environment, e.g. chemoreceptors, baroreceptors, osmoreceptorse.g. chemoreceptors, baroreceptors, osmoreceptors

4.4. Proprioceptors: provide information about body Proprioceptors: provide information about body position in space, e.g. receptors in muscle, tendons & position in space, e.g. receptors in muscle, tendons & joints: muscle spindles, Goli tendon organsjoints: muscle spindles, Goli tendon organs

5.5. Nociceptors: these are pain receptorsNociceptors: these are pain receptors

Free nerve ending Epithelium of Skin Non adapting Pain

Another ClassificationAnother Classification

Cutaneous receptorsMechanoreceptors: terminal of AB myelinated fibersThermoreceptorsnociceptors

Musculoskeletal receptorsproprioceptors

Visceral receptorse.g., chemoreceptors, baroceptors, mechanoreceptors

Transduction of sensory stimuli into nerve Transduction of sensory stimuli into nerve impulses:impulses:

The generator potentials [=receptor potential]: it’s The generator potentials [=receptor potential]: it’s the change in the membrane potential of the the change in the membrane potential of the receptor when excited by a stimulus.receptor when excited by a stimulus.

It’s a non-propagated depolarizing potential It’s a non-propagated depolarizing potential resembling EPSP.resembling EPSP.

When the magnitude of the generated potential is When the magnitude of the generated potential is 10mV 10mV AP is generated in the sensory nerve. AP is generated in the sensory nerve.

Transduction (receptor potential)

It is a change in membrane potential of a receptor. Due to:

1) Mechanical deformation of receptor membrane.

2) Application of chemical which binds to the receptor.

3) Change in temperature.

4) Application of electromagnetic energy. These lead to the opening of selective channels and

depolarization of receptor membrane.

Generator Potential(Receptor Potential)

• Non propagated depolarizing potential.

• It is produced in un-myelinated nerve terminal.

Depolarization of receptor membrane leads to local circuit which initiates action potential at the first node of Ranvier on the nerve attached to the receptor.

RECEPTOR POTENTIAL & ACTION POTENTIAL

Sensory Transduction

Source of generator potential:Source of generator potential:

The generator potential is produced in the The generator potential is produced in the unmyelinated nerve terminal of the receptorunmyelinated nerve terminal of the receptor

When pressure stimulus is applied When pressure stimulus is applied the terminal the terminal fibers of the corpuscle will be deformed fibers of the corpuscle will be deformed opening opening of Na+ channels of Na+ channels Na+ influx (to the interior of the Na+ influx (to the interior of the fiber) fiber) creates high positivity inside the fiber creates high positivity inside the fiber receptor potential receptor potential then the generator potential then the generator potential depolarizes the sensory nerve at the 1depolarizes the sensory nerve at the 1stst node of node of Ranvier. Once the firing level is reached, action Ranvier. Once the firing level is reached, action potential is produced.potential is produced.

Once the firing level is reached, action Once the firing level is reached, action potential is produced:potential is produced:

The node converts the graded response of the The node converts the graded response of the receptor into action potentialsreceptor into action potentials

The frequency of AP is proportionate to the The frequency of AP is proportionate to the intensity (magnitude) of the applied stimuliintensity (magnitude) of the applied stimuli

In other way: the more the receptor potential In other way: the more the receptor potential rises above threshold level, the greater becomes rises above threshold level, the greater becomes the action potential frequencythe action potential frequency

Ionic basis of receptor potentials:Ionic basis of receptor potentials:

Opening of channels of receptor membranes, e.g. Opening of channels of receptor membranes, e.g. Na+ channels as mentionedNa+ channels as mentionedG proteins: as in rods & conesG proteins: as in rods & cones

Properties of receptors:Properties of receptors:

1- Adaptation:= desensitization1- Adaptation:= desensitization

It’s the decrease in the frequency of APs in a It’s the decrease in the frequency of APs in a sensory nerve overtime when a maintained stimulus sensory nerve overtime when a maintained stimulus of constant strength is applied to its receptor.of constant strength is applied to its receptor.

This phenomenon depends on the type of the This phenomenon depends on the type of the receptor or sense organ.receptor or sense organ.

According to this we have:-According to this we have:-

1- Phase receptors:-1- Phase receptors:-These are rapidly adapting receptors.These are rapidly adapting receptors.e.g. touch receptorse.g. touch receptors

2- Tonic receptors:-2- Tonic receptors:-These are slowly adapting receptors: its generator These are slowly adapting receptors: its generator potential is prolonged and decays very slowly.potential is prolonged and decays very slowly.e.g. muscle spindles, cold & pain receptors, lung e.g. muscle spindles, cold & pain receptors, lung inflation receptorsinflation receptors

Mechanism of adaptation:Mechanism of adaptation:Accommodation of the sensory nerve fiber to the Accommodation of the sensory nerve fiber to the generator potential.generator potential.

RESPONSE OF PHASIC & TONIC MECHANORECEPTOR

ADAPTATION OF RECEPTOR

2- Excitability:2- Excitability:

Response of a receptor to an adequate stimulus by Response of a receptor to an adequate stimulus by producing a non-propagated potential = receptor producing a non-propagated potential = receptor potential.potential.

If magnitude is sufficient If magnitude is sufficient AP will propagate along AP will propagate along the sensory nerve.the sensory nerve.

How a receptor be stimulated?How a receptor be stimulated?

1.1. Mechanical deformation Mechanical deformation open ion channels open ion channels2.2. Application of a chemical to the receptor Application of a chemical to the receptor

membrane membrane open ion channels open ion channels3.3. Change of temperature Change of temperature change permeability change permeability4.4. Effect of electromagnetic radiation Effect of electromagnetic radiation (+) G (+) G

proteinprotein

Give examples for receptors!Give examples for receptors!

3.Adequate stimulus3.Adequate stimulus

Each type of receptor is most sensitive to a specific Each type of receptor is most sensitive to a specific form of energy, called adequate stimulus, the receptor is form of energy, called adequate stimulus, the receptor is almost non-responsive to the normal intensities of other almost non-responsive to the normal intensities of other forms of energy.forms of energy.e.g. rods & cones are stimulated by light not heate.g. rods & cones are stimulated by light not heat

What is the difference between generator potential and What is the difference between generator potential and action potential?action potential?

Generator potential

GradedDoesn’t obey all or none ruleCan be summatedUnpropagated

Action potential

NotObeys all or non ruleNot summatedPropagated

Sensory receptors in the skin: Sensory receptors in the skin: [Mechanoreceptors][Mechanoreceptors]

1- Expanded ends of sensory nerve fibers:1- Expanded ends of sensory nerve fibers: - Ruffini endings- Ruffini endings - Merkel’s discs- Merkel’s discs

2- Encapsulated endings:2- Encapsulated endings: - Meissner’s corpuscles- Meissner’s corpuscles - Pacinian corpuscles- Pacinian corpuscles - Krause’s end-bulbs- Krause’s end-bulbs

3- Naked nerve endings between cells in tissues.3- Naked nerve endings between cells in tissues.

Coding of sensory informationCoding of sensory information

If we know that:If we know that:APs in the nerve from a touch receptor are APs in the nerve from a touch receptor are essentially identical to those in the nerve from essentially identical to those in the nerve from warmth receptor.warmth receptor.

So, whySo, whyStimulation of a touch receptor causes a sensation Stimulation of a touch receptor causes a sensation of touch and not warmth? And how possible to tell of touch and not warmth? And how possible to tell that touch is light or heavy?that touch is light or heavy?

1- Doctrine of specific nerve energies: [Modality 1- Doctrine of specific nerve energies: [Modality discrimination]discrimination]The sensation evoked (=produced) by impulses generated The sensation evoked (=produced) by impulses generated in a receptor depends on the specific part of the brain in a receptor depends on the specific part of the brain they they ultimately activateultimately activate..

Because specific sensory pathway are discrete, from Because specific sensory pathway are discrete, from sense organ to cortex, when nerve pathways from certain sense organ to cortex, when nerve pathways from certain sense organ are stimulated sense organ are stimulated the sensation produced is the sensation produced is that for which the receptor is specialized no matter how or that for which the receptor is specialized no matter how or where along the pathway the activity is initiated. This is where along the pathway the activity is initiated. This is the doctrine of specific nerve energies. So, if a sensory the doctrine of specific nerve energies. So, if a sensory nerve from Pacinian corpuscle in the hand is stimulated at nerve from Pacinian corpuscle in the hand is stimulated at elbow, sensation produced is touch. If stimulated at dorsal elbow, sensation produced is touch. If stimulated at dorsal column of sp. cord column of sp. cord touch. touch.

2- Projection: [Locality discrimination]2- Projection: [Locality discrimination]This principle is that:This principle is that:No matter where a particular sensory pathway is No matter where a particular sensory pathway is stimulated along its course to the cortex, the conscious stimulated along its course to the cortex, the conscious sensation produced is referred to the location of the sensation produced is referred to the location of the receptor.receptor.

Examples:Examples:When the cortical receiving area for impulses from the When the cortical receiving area for impulses from the left hand is stimulated left hand is stimulated patient reports sensation in patient reports sensation in the hand not the head.the hand not the head.

Phantom limb: amputee complain of pain and Phantom limb: amputee complain of pain and proprioceptive sensation in the absent limb (phantom proprioceptive sensation in the absent limb (phantom limb) as if it were still in place.limb) as if it were still in place.

Mechanisms: The ends of nerves cut at time of Mechanisms: The ends of nerves cut at time of amputation often forms neuromas. They may amputation often forms neuromas. They may discharge spontaneously or when pressure put on discharge spontaneously or when pressure put on them, impulses are generated in nerve fibers that them, impulses are generated in nerve fibers that previously come from sense organs in the amputated previously come from sense organs in the amputated limb & sensation produced is projected to where limb & sensation produced is projected to where receptors used to be.receptors used to be.

3- Intensity discrimination3- Intensity discriminationIntensity (strength) discrimination depends on:Intensity (strength) discrimination depends on: 1- Number of receptors stimulated: stronger stimuli1- Number of receptors stimulated: stronger stimuli stimulate more receptorsstimulate more receptors 2- Frequency of action potentials (generated by2- Frequency of action potentials (generated by activity in a given receptor) reaching the cortex.activity in a given receptor) reaching the cortex.

R = KSR = KSAA

R = sensationR = sensationS = intensity of the stimulusS = intensity of the stimulusK & A = constantsK & A = constants

N.B.N.B.Tactile localization:Tactile localization:- one-point localization: ability to localize one-point of touch one-point localization: ability to localize one-point of touch [locality discrimination].[locality discrimination].- tactile discrimination: two-point discrimination, ability to feel tactile discrimination: two-point discrimination, ability to feel two points touched simultaneously as two separate points two points touched simultaneously as two separate points depends on:-depends on:- 1- Number of receptors stimulated1- Number of receptors stimulated 2- degree of overlap in receptive field2- degree of overlap in receptive field

Overlap Overlap discrimination discrimination

It’s accurate on tips of fingers, lips & tongue, WHY?It’s accurate on tips of fingers, lips & tongue, WHY?1- greater number of receptors1- greater number of receptors2- greater number of afferents2- greater number of afferents3- less convergence3- less convergence4- more area of cortical representation4- more area of cortical representation

Note:Note:

Labeled line principle:Labeled line principle:

The specificity of nerve fibers for transmitting only one The specificity of nerve fibers for transmitting only one modality of sensation is called “labeled line principle”modality of sensation is called “labeled line principle”

e.g. if a touch fiber is stimulated by exciting a touch e.g. if a touch fiber is stimulated by exciting a touch receptor electrically or in any other way, the person receptor electrically or in any other way, the person perceives touch because touch fibers leads to specific perceives touch because touch fibers leads to specific touch areas in the brain.touch areas in the brain.

So, the sensation felt depends on which area in the NS So, the sensation felt depends on which area in the NS the fibers leads [ultimately stimulated].the fibers leads [ultimately stimulated].

RECEPTIVE FIELD

Sensory Pathways

DORSAL COLUMN PATHWAY

• CARRIES FINE TOUCH, POSITION, PRESSURE, VIBRATION, TWO POINT DESRIMINATION stereognosis

• AFFERENT SENSORY FIBERS Aβ TYPE.

• VERY FAST VELOCITY 30 – 70 m/s

• 3 NEURON SYSTEM(SEE THE DIAGRAM)

ANTEROLATERAL PATHWAY• CARRIES PAIN &

TEMPRATURE (lat. Sp.Th)• CRUDE TOUCH &

PRESSURE ( VENT, Sp. Th)• AFFERENT SENSORY

FIBERS Aδ (MYELINATED) FAST PAIN

• C FIBERS( UNMYELINATED) SLOW PAIN

• RELATIVELY SLOW VELOCITY Aδ – 6 – 30 m/s. C – 0.5 – 2 m/s.

• 3 NEURON SYSTEM(SEE THE DIAGRAM)

Spinal Tracts