SENSORY SYSTEM

LECTURE 1

RECEPTORS

DR. ZAHOOR ALI SHAIKH

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Sensory system or Input system • What type of sensation do you know?• Touch

– Fine

– Crude

• Pressure• Position• Vibration• Two point discrimination• Pain• Temperature

– Cold

– Warm

• Stereognosis

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How we feel the sensation?

• Sensory receptor

• Sensory pathway

• Sensory cortex

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Cont….COMPONENTS

• Receptors

• Peripheral nerves

• Spinal cord

• Tracts

• Thalamus

• Thalamocortical projection

• Somatosensory cortex

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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

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Sensory Receptors

• These are transducers that convert various forms of energy into action potentials in neurons.

• The particular form of energy to which a receptor is most sensitive is called as its adequate stimulus.

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Adequate stimulusAdequate stimulus

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

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Types of receptors

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10

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Free nerve ending Epithelium of Skin Non adapting Pain

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Schematic representation Schematic representation of sensory nerve endings of sensory nerve endings in the skin:in the skin:

A.A. Free nerve endings in the Free nerve endings in the dermis and epidermis (Pain)dermis and epidermis (Pain)

B.B. Tactile corpuscle (Meissner) Tactile corpuscle (Meissner) in a dermal papilla (Touch)in a dermal papilla (Touch)

C.C. Bulbous corpuscle (Krause) Bulbous corpuscle (Krause) in the dermis (Cold)in the dermis (Cold)

D.D. End organ (Ruffini) in the End organ (Ruffini) in the dermis (Heat)dermis (Heat)

E.E. Nerve terminals around hair Nerve terminals around hair follicles (Touch)follicles (Touch)

F.F. Free nerve terminals in a Free nerve terminals in a hair papilla and deep layers hair papilla and deep layers of dermis (Pain)of dermis (Pain)

G.G. Lamellated corpuscle Lamellated corpuscle (Vater-Pacini) in superficial (Vater-Pacini) in superficial fascia (Pressure)fascia (Pressure)

Sensory Modality Receptors

1 Touch

•Meissner’s corpuscles,•ruffini’s endings, •krause’s corpuscles, •hair end organs •pacinian corpuscles

2 Pressure, Vibration Pacinian corpuscles

3 Warmth Free nerve endings

4 Cold Free nerve endings

5 Pain or nociception Free nerve endings

6 Joint position & movement Receptors in & around joints

7 Muscle length Muscle spindle

8 Muscle tension Golgi tendon organ

Receptor Type Sensation Signals Adaptation

Meissner corpuscle 

Encapsulated & layered 

Touch: Flutter & Movement

Frequency/Velocity & Direction 

Rapid

Pacinian corpuscle 

Encapsulated & layered 

Touch: Vibration  Frequency: 30-800 Hz  Rapid

Ruffini corpuscle 

Encapsulated collagen 

Touch: Skin Stretch  Direction & Force Slow

Hair follicle Unencapsulated  Touch: Movement Direction &

VelocityRapid

Merkel complex 

Specialized epithelial cell 

Touch, Pressure, Form  Location & Magnitude Slow

Free Nerve Ending 

Unencapsulated Pain, Touch, or Temperature 

Tissue damage, Contact, or

Temperature change

Depends on information

carried

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Receptor Properties

Receptor potential or Generator Potential Transduction (excitability)

• When we apply pressure, we generate depolarization in the receptor it is called Generator potential or Receptor potential.

• Non propagated depolarizing potential.

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.

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Relation of Receptor potential to Action potential:Relation of Receptor potential to Action potential:

When we increase the pressure, magnitude of receptor When we increase the pressure, magnitude of receptor potential rises, when it reaches to threshold level, action potential rises, when it reaches to threshold level, action potential is generated in the sensory fiber attached to the potential is generated in the sensory fiber attached to the receptorreceptorLet us take the example of Pacinian Corpuscle .Let us take the example of Pacinian Corpuscle .Corpuscle has central nerve extending through its Corpuscle has central nerve extending through its core.When pressure stimulus is applied core.When pressure stimulus is applied the covering of the covering of the corpuscle and central nerve fiber will be deformed the corpuscle and central nerve fiber will be deformed opening of Na+ channels opening of Na+ channels Na+ influx (to the interior of Na+ influx (to the interior of the nerve fiber) the nerve fiber) creates high positivity inside the fiber creates high positivity inside the fiber generator potential generator potential then the generator potential then the generator potential depolarizes the sensory nerve at the 1depolarizes the sensory nerve at the 1stst node of Ranvier. node of Ranvier. Once the firing level is reached, action potential is Once the firing level is reached, action potential is produced in the nerve fiber.produced in the nerve fiber.

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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

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

Receptor or Generator potential

In the ReceptorGradedDoesn’t obey all or none ruleCan be summatedUnpropagated

Action potential

In the Sensory Nerve fiberNot GradedObeys all or non ruleNot summatedPropagated

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RECEPTOR POTENTIAL & ACTION POTENTIAL

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Note: if stimulus to the receptor is sub threshold then no action potential is generated in sensory nerve fiber

Relation between receptor potential and action potential of sensory nerve

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Sensory Transduction

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Adaptation or Desensitization

• When stimulus of constant strength is applied to a receptor, the frequency of action potential in its sensory nerve decreases over the time. This is called adaptation. Degree of adaptation varies in different receptors.

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Classification of receptors

• Rapidly adapting or phasic receptors e.g. pacinian corpuscles.

• Slowly or non adapting or tonic receptors e.g. Nociceptors, muscle spindles.

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ADAPTATION OF RECEPTOR

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RESPONSE OF PHASIC & TONIC MECHANORECEPTOR

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Why we feel touch, pain, warm sensation when all sensory

nerve carry the information to brain as action potential?

Coding of sensory information

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Coding of sensory information

• Due to the Doctrine of specific nerve energies.

We have;

• Specific receptors.

• Specific sensory pathways.

• Specific part of the brain they activate.

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Law of projection

• If we stimulate sensory pathway along its course to the sensory cortex, the conscious sensation produced is refered to the location of receptors. This principle is called as Law of projection.

• If we stimulate sensory cortex area which receives impulses from left hand, patient reports sensation in the left hand, not in the head.

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Cont…..

Q. In patients whose limb was amputed (cut off) they complained of pain in the absent limb (Phantom limb), Why?

Due to law of projection, the ends of nerves cut at the time of amputation when stimulated, sensation evoked are projected to where the receptor used to be present,i.e. in the limb.

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Sensory Unit

• Single sensory axon with all its peripheral branches.

Receptor field

• Receptor field of a sensory unit is the area from which a stimulus produces response in that unit.

34RECEPTIVE FIELD

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Recruitment of sensory units

• Weak stimulus activates receptors with lowest threshold.

• Strong stimulus activates those receptors also which have high threshold .

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Intensity discrimination

Spatial summation

• Stimulus intensity which causes increase firing of many units (fibers ) due to overlap of one sensory unit to the other.

37SPATIAL SUMMATION

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Temporal summation

• If we give stimulus which causes increase frequency of nerve impulses in single sensory fiber. (same sensory unit)

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Physiological classification of nerve fibers that transmit different types of

sensations

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Physiological classification of nerve fibers Physiological classification of nerve fibers that transmit different types of sensationsthat transmit different types of sensations

IaIa

IbIb

IIII

IIIIII

IVIV

Muscle spindle, annulo-spiral Muscle spindle, annulo-spiral ending.ending.Golgi tendon organ.Golgi tendon organ.

Muscle spindle, flower-spray Muscle spindle, flower-spray ending, touch, pressureending, touch, pressure

Pain and cold receptors; some Pain and cold receptors; some touch receptorstouch receptors

Pain, temperature, and other Pain, temperature, and other receptorsreceptors

AA

AA

AA

AA

Dorsal root CDorsal root C

NumberNumber OriginOrigin Fiber typeFiber type