CNS Receptors 2nd Year - Tanta University

Central Nervous Central Nervous systemsystem

A system that controls all of the activities of the body.

The nervous system is made of:

The brain The spinal cord

The nerves The senses

Functional unit of Functional unit of nerve Cellsnerve Cells

NeuronsNeurons

Sensory nervous Sensory nervous systemsystem

IntroductionIntroductionHuman life would be very Human life would be very

different without the ability different without the ability to sense and perceive to sense and perceive external stimuliexternal stimuli

Imagine your world without the Imagine your world without the ability to see, hear, smell, ability to see, hear, smell, touch, and feeltouch, and feel

Reflex actionReflex action

The nervous system also allows you to react to a stimulus.

A stimulus is a change in the environment.

Example: A hot stove

Or… tripping over a rock

ReceptorsReceptors

Range in complexity from the single-Range in complexity from the single-celled,celled,

Simple dendritic ending of a neuronSimple dendritic ending of a neuron

Complex sense organsComplex sense organs

receptorsreceptorsThey are specialized structure They are specialized structure

situated at the peripheral situated at the peripheral termination of afferent neurons termination of afferent neurons

They detect and respond to They detect and respond to external or internal change in the external or internal change in the environment i. e(environment i. e( stimuli )stimuli )

Inform the central nervous system Inform the central nervous system about these changesabout these changes

ReceptorsReceptors

• Sensory receptors also serve as afferent Sensory receptors also serve as afferent pathways for reflex action with or without pathways for reflex action with or without conscious sensation.conscious sensation.

ReceptorsReceptors Most of them are protein in Most of them are protein in

naturenature Located either in the:Located either in the:The plasma membrane (common)The plasma membrane (common)The cytoplasmThe cytoplasm One cell may contain many One cell may contain many

receptorsreceptors

ReceptorsReceptorsThey are stimulated by either one of these They are stimulated by either one of these stimuli.stimuli.

MechanicalMechanical

ThermalThermal

ChemicalChemical

ElectromagneticElectromagnetic

Functions of the receptorsFunctions of the receptors

1- Detector1- Detector

They detect any change in external or They detect any change in external or internal environment and inform the internal environment and inform the central nervous system about these central nervous system about these changes.changes.

Functions of the receptorsFunctions of the receptors

2- Transducer2- Transducer

The receptors act as transducer, they The receptors act as transducer, they transform different forms of stimuli transform different forms of stimuli (mechanical, thermal, chemical and (mechanical, thermal, chemical and electromagnetic) to electric changes in the electromagnetic) to electric changes in the form of receptor or generator potentialform of receptor or generator potential

ReceptorsReceptors

11.Mechanoreceptors.Mechanoreceptors respond to mechanical respond to mechanical stimuli stimuli

Skin tactile sensibilities –free nerve endings, Skin tactile sensibilities –free nerve endings,

Deep tissue sensibilities –Pacinian corpuscles, Deep tissue sensibilities –Pacinian corpuscles, muscle spindles, Golgi tendon organmuscle spindles, Golgi tendon organ

Hearing –sound receptors of cochlea•Hearing –sound receptors of cochlea•

Equilibrium –vestibular receptors•Equilibrium –vestibular receptors•

Arterial pressure baro receptors.Arterial pressure baro receptors.

ReceptorsReceptors

2.2.ThermoreceptorsThermoreceptors Respond to Respond to thermal Stimulie either Cold or Warm.thermal Stimulie either Cold or Warm.

ReceptorsReceptors

3.Nociceptors3.NociceptorsPain –free nerve endings.Pain –free nerve endings.

ReceptorsReceptors

4.Electromagnetic receptors 4.Electromagnetic receptors Respond toRespond to

Vision –rods and conesVision –rods and cones

ReceptorsReceptors

5.Chemoreceptors Respond to5.Chemoreceptors Respond toTaste –taste budsTaste –taste buds

Smell –olfactory epitheliumSmell –olfactory epithelium

Arterial oxygen –aortic and carotid bodies Arterial oxygen –aortic and carotid bodies

Osmolality–anterior hypothalamusOsmolality–anterior hypothalamus

Blood carbon dioxide –CNS (medulla)Blood carbon dioxide –CNS (medulla)

Receptor or generator Receptor or generator potentialpotential

It is studied in the pacinian corpuscles It is studied in the pacinian corpuscles (one of the mechanoreceptor) (one of the mechanoreceptor) becausebecause It is large in size and can be It is large in size and can be dissected.dissected. It form unmyelinated straight It form unmyelinated straight terminal part of afferent nerve fibres.terminal part of afferent nerve fibres. It is surrounded by a capsule It is surrounded by a capsule formed of onion like layer of connective formed of onion like layer of connective tissue.tissue. In this receptor, the first Node of In this receptor, the first Node of RanvierRanvier is present inside the is present inside the capsulecapsule

Receptor potentialReceptor potentialModerate stimulationModerate stimulation

of the receptor causes stretching of the receptor causes stretching or distortion of the nerve ending or distortion of the nerve ending and/or release of chemical and/or release of chemical transmitter, transmitter,

Receptor potentialReceptor potentialStimulus release eitherStimulus release either

Chemical messengerChemical messenger HormonesHormones NeurotransmittersNeurotransmitters Local autacoidsLocal autacoids Change of the temperatureChange of the temperature Electromagnetic radiationElectromagnetic radiation which combine with the receptorwhich combine with the receptor

Receptor potentialReceptor potential this will increase the this will increase the

permeability of the permeability of the membrane of the receptor to membrane of the receptor to sodium which passes from sodium which passes from outside to the inside of the outside to the inside of the cellcell

Receptor potentialReceptor potentialmore sodium influx than potassium efflux more sodium influx than potassium efflux (sodium move according to concentration (sodium move according to concentration and electric gradient. It is more and electric gradient. It is more concentrated outside the membrane of the concentrated outside the membrane of the receptor cells receptor cells

Also the inside of the receptor is more Also the inside of the receptor is more negative than its outsidenegative than its outside

Receptor potentialReceptor potentialThe movement of sodium causes The movement of sodium causes only partial depolarization of the only partial depolarization of the receptor in the nerve endings. The receptor in the nerve endings. The membrane of this nerve endings membrane of this nerve endings have a have a high thresholdhigh threshold, therefore the , therefore the movement of sodium cannot movement of sodium cannot completely depolarized this region.completely depolarized this region.

Receptor potentialReceptor potentialIf the amount of partial If the amount of partial depolarization is large and reach depolarization is large and reach a threshold value 10-15 ma threshold value 10-15 m (Due (Due to strong stimulation). to strong stimulation).

Receptor potentialReceptor potentialThe depolarization increase and The depolarization increase and passes for a long distance to passes for a long distance to reach the first Node of Ranvier reach the first Node of Ranvier (where the threshold of the (where the threshold of the membrane to stimulation is very membrane to stimulation is very low)low)

Receptor potentialReceptor potentialit causes opening of voltage gated it causes opening of voltage gated sodium channels, and when it lead to sodium channels, and when it lead to complete depolarization of the first complete depolarization of the first Node of Ranvier and an action Node of Ranvier and an action potential is then propagated along the potential is then propagated along the sensory nerve fiber associated or sensory nerve fiber associated or connected with the stimulated connected with the stimulated receptor to the central nervous receptor to the central nervous systemsystem

An impulse is self-propagating. Once started it continues, and moves only in one direction

Receptor or generator potentialReceptor or generator potential

The receptor potentialThe receptor potential partial depolarizationpartial depolarization

Is conducted by local flow for a short Is conducted by local flow for a short distance from the nerve endingdistance from the nerve ending

The magnitude of receptor potential The magnitude of receptor potential decreases with distance from its site decreases with distance from its site of origin till ended.of origin till ended.

The Receptor PotentialThe Receptor Potential..

Does not propagateDoes not propagate

Graded the amplitude Graded the amplitude varies with the intensity of varies with the intensity of stimulationstimulation..

Differences Differences Between Between

Receptor potential Receptor potential

&&

action potentialaction potential

Items of differencesItems of differences Generator Generator potentialpotential

Action potentialAction potential

DepolarizationDepolarization PartialPartial completecomplete

DurationDuration 20 millisecond20 millisecond 2 milliseconds2 milliseconds

Response(AmResponse(Amplitude)plitude)

Graded Graded response response Amplitude Amplitude increases with increases with the increase the increase strength of the strength of the stimulusstimulus

No graded No graded responseresponse

Amplitude is Amplitude is Independent with Independent with the strength of the strength of the stimulusthe stimulus

ThresholdThreshold No thresholdNo threshold threshold 10 mthreshold 10 m

Items of differencesItems of differences Generator potentialGenerator potential Action potentialAction potential

All or non law All or non law Not obey this lawNot obey this law Obey this lawObey this law

SummationSummation summatedsummated Cannot be Cannot be summatedsummated

Refractory Refractory periodperiod

Has no refractory Has no refractory periodperiod

Has refractory Has refractory period period

ConductionConduction Conduction Conduction decrease with decrease with distancedistance

Conduction with Conduction with constant constant amplitudeamplitude

DurationDuration decrease decrease amplitudeVaries amplitudeVaries with the with the stimulusstimulus

constantconstant

Skin receptorsSkin receptors

Properties of the receptorsProperties of the receptors

1- Excitability1- Excitability

Is the ability of the receptor to respond to Is the ability of the receptor to respond to adequate stimulus to the form of receptor adequate stimulus to the form of receptor or generator potentialor generator potential

Properties of the receptorsProperties of the receptors2- Adequate stimulus or Muller's law of 2- Adequate stimulus or Muller's law of specific nerve energyspecific nerve energy

Each type of receptors is sensitive to only Each type of receptors is sensitive to only one particular kind of stimulus called one particular kind of stimulus called adequate stimulus and give rise to only adequate stimulus and give rise to only one type of sensation regardless to the one type of sensation regardless to the method of stimulation i.e. each receptor method of stimulation i.e. each receptor has a lower threshold to its adequate has a lower threshold to its adequate stimulus e.g. auditory receptors are stimulus e.g. auditory receptors are sensitive only to sound vibration sensitive only to sound vibration


3- Rate of afferent discharge 3- Rate of afferent discharge

Depend on the intensity of the stimulus, it Depend on the intensity of the stimulus, it is directly proportional to the intensities of is directly proportional to the intensities of the stimulus. Sensory receptor detect the the stimulus. Sensory receptor detect the different intensity of the stimulie by two different intensity of the stimulie by two means.means.


Increase rate of afferent discharge from a Increase rate of afferent discharge from a single receptor (frequency modulated single receptor (frequency modulated response).response).


Weber Feshner’s lawWeber Feshner’s law. State that the . State that the amplitude of generator potential and the amplitude of generator potential and the frequency of afferent discharge are directly frequency of afferent discharge are directly proportional to log 10 intensity of the proportional to log 10 intensity of the stimulusstimulus


Increase intensity of the stimulus Increase intensity of the stimulus 100 folds, the frequency of 100 folds, the frequency of afferent discharge (afferent discharge (No.No. of of impulses/unit time) increases 2 impulses/unit time) increases 2 folds, while increase the intensity folds, while increase the intensity to 1000 increase rate of afferent to 1000 increase rate of afferent discharge by 3 foldsdischarge by 3 folds

This explain the compression This explain the compression function of the receptorfunction of the receptor

. The receptor can detect marked variable . The receptor can detect marked variable changes in the intensities of the stimuli changes in the intensities of the stimuli and causes a smaller range in the and causes a smaller range in the frequency of afferent discharge which can frequency of afferent discharge which can be detected by cortical sensory area (the be detected by cortical sensory area (the receptor detect wide range of intensities, receptor detect wide range of intensities, but the cortical sensory area can detect but the cortical sensory area can detect only smaller range of frequency of afferent only smaller range of frequency of afferent discharge).discharge).


2-2- Increasing the no. of stimulated Increasing the no. of stimulated receptor i.e. recruitment of the receptor. receptor i.e. recruitment of the receptor. The more increase the intensity of the The more increase the intensity of the stimulus the more increase in the no. of stimulus the more increase in the no. of receptor stimulated the more the afferent receptor stimulated the more the afferent discharge.discharge.


4- Adaptation4- Adaptation

Is a decrease frequency of afferent Is a decrease frequency of afferent discharge inspiet of constant stimulation to discharge inspiet of constant stimulation to the receptor.the receptor.

Differences betweenDifferences between adaptation &fatigue adaptation &fatigue

AdaptationAdaptation FatigueFatigue

Caused byCaused by Constant stimulationConstant stimulation Previous activityPrevious activity

RecoveryRecovery ImmediateImmediate SlowSlow

SitesSites All tissue including the All tissue including the receptorsreceptors

Some tissues but not the Some tissues but not the receptorsreceptors

O2 lackO2 lack Not affectedNot affected EnhancedEnhanced

MechanismsMechanisms Discussed belowDiscussed below Accumulation of Accumulation of metabolites as lactic acidmetabolites as lactic acid

Mechanism of adaptationMechanism of adaptation

1-1- Pressure applied to the receptor causes Pressure applied to the receptor causes compression of its lamellae leading to deformity compression of its lamellae leading to deformity of the nerve endings and opening of sodium of the nerve endings and opening of sodium channels producing generator and action channels producing generator and action potential with afferent discharge. After a time the potential with afferent discharge. After a time the fluid in the receptor is redistributed and the fluid in the receptor is redistributed and the pressure become equally distributed and the pressure become equally distributed and the receptor potential disappears even though the receptor potential disappears even though the stimulus continue to be effective. This is a rapid stimulus continue to be effective. This is a rapid mechanism.mechanism.


2-2- Another slower mechanism is Another slower mechanism is accommodation which occur in the first accommodation which occur in the first node of Ranvier resulting from inactivation node of Ranvier resulting from inactivation of sodium channel and decrease in the of sodium channel and decrease in the excitability of the first node so the excitability of the first node so the frequency of afferent discharge decreases.frequency of afferent discharge decreases.


3-3- Loss of energy to surrounding tissues Loss of energy to surrounding tissues of the receptors by continuous stimulation. of the receptors by continuous stimulation. The energy of the stimulus is lost in the The energy of the stimulus is lost in the tissue surrounding the receptors before tissue surrounding the receptors before reaching them.reaching them.The energy loss increases and the amount The energy loss increases and the amount of energy reaching the receptor of energy reaching the receptor decreases, and the rate of afferent decreases, and the rate of afferent discharge decreases.discharge decreases.

Classification of Classification of receptorsreceptors

Classification of receptor Classification of receptor according to their rate of according to their rate of

adaptation. adaptation.

1-1- Slowly adapting or tonic receptors Slowly adapting or tonic receptors e.g. muscle spindle, Baroreceptor, e.g. muscle spindle, Baroreceptor, chemoreceptor and pain receptor. They chemoreceptor and pain receptor. They are responsible for initiation of vital reflex are responsible for initiation of vital reflex action essential to life.action essential to life.


adaptation.adaptation.2-2- Rapidly adapting receptors or phasic Rapidly adapting receptors or phasic receptors. e.g. touch receptors. They receptors. e.g. touch receptors. They suppress unnecessary information that suppress unnecessary information that disturb the central nervous system.disturb the central nervous system.


adaptation.adaptation.3-3- Moderately adapting receptors. e.g. Moderately adapting receptors. e.g. cold, smell and taste receptors. their rate cold, smell and taste receptors. their rate of adaptation between tonic and phasic of adaptation between tonic and phasic receptor. receptor.

Classification of receptorsClassification of receptorsLocation:Location: Extero receptors Extero receptors Intero receptorsIntero receptors

Histological structureHistological structure Non capsulated receptorsNon capsulated receptors Capsulated receptorsCapsulated receptors

Stimulus the receptors recievesStimulus the receptors recieves MechanoreceptorsMechanoreceptors ThermoreceptorsThermoreceptors ChemoreceptorChemoreceptor Electromagnetic receptors Electromagnetic receptors

Sensory codingSensory coding

It is the ability of the nerve centers to get It is the ability of the nerve centers to get information about modality, intensity and information about modality, intensity and locality of the stimulus. locality of the stimulus.

As regard modalityAs regard modality

According to Muller’s law of specific nerve According to Muller’s law of specific nerve energies each spetial receptor is energies each spetial receptor is stimulated by its adequate stimulie with stimulated by its adequate stimulie with little amount of energy needed give one little amount of energy needed give one type of sensation.type of sensation.

Label line principleLabel line principle

There is specific pathway for each There is specific pathway for each sensation which transmit it to a specific sensation which transmit it to a specific area of the cerebral sensory cortex. area of the cerebral sensory cortex. Stimulation of this pathway at any point Stimulation of this pathway at any point evokes its specific sensation. evokes its specific sensation.

Free nerve endings (pain receptors) do not Free nerve endings (pain receptors) do not obey the Muller’s law. It can be stimulated obey the Muller’s law. It can be stimulated by chemical, thermal, mechanical and by chemical, thermal, mechanical and electric stimuli provided that the stimulus is electric stimuli provided that the stimulus is strong and causing pain or injury strong and causing pain or injury

As regard intensityAs regard intensity

1-1- Increase the number of active Increase the number of active receptors and increase afferent discharge receptors and increase afferent discharge from. These receptors reaching the nerve from. These receptors reaching the nerve center. This is called recruitment of the center. This is called recruitment of the receptors. receptors.

2-2- Increase the frequency of afferent Increase the frequency of afferent discharge reaching the nerve centers from one discharge reaching the nerve centers from one single receptor. single receptor.

Weber Feshner law state that the frequency of Weber Feshner law state that the frequency of afferent discharge is directly proportional to log afferent discharge is directly proportional to log 10 intensity of the stimulus.10 intensity of the stimulus.

Increase intensity of the stimulus increase the Increase intensity of the stimulus increase the frequency of afferent discharge reaching the frequency of afferent discharge reaching the nerve.center nerve.center

As regard localityAs regard locality

Law of projection, there is a separate area Law of projection, there is a separate area of representation for each part of the body of representation for each part of the body in the sensory area of the cerebral cortex, in the sensory area of the cerebral cortex, when impulse reaches the specific area in when impulse reaches the specific area in the cortex, it project this stimulus to its the cortex, it project this stimulus to its original site.original site.

somatic sensory afferentssomatic sensory afferents

Afferent nerve nerves are either peripheral Afferent nerve nerves are either peripheral branch of the dorsal root ganglia or branch of the dorsal root ganglia or sensory afferent fibers of cranial nervessensory afferent fibers of cranial nerves

velocity of conduction or unmyelinated velocity of conduction or unmyelinated They are either myelinated with high or They are either myelinated with high or with low velocity of conductionwith low velocity of conduction

.Also they are different in diameters the .Also they are different in diameters the high velocity with larger diameterhigh velocity with larger diameter

Classification of nerve fibersClassification of nerve fibers

Histologically they are either Histologically they are either

myelinated with high velocity of conductionmyelinated with high velocity of conduction

unmylinated with low velocity of conductionunmylinated with low velocity of conduction

Functionally they are eitherFunctionally they are either

Sensory Sensory

MotorMotor

Myelinatedneurons conduct action potentials more quickly by

1-Saltatory conduction

2-Axons of largediameter alsotransmit actionpotentials faster

CNS Receptors 2nd Year - Tanta University

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