LECT.4, PROPERTIES OF RECEPTORS by dr saidia zafar

7/30/2019 LECT.4, PROPERTIES OF RECEPTORS by dr saidia zafar

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PROPERTIES OF

RECEPTORS:


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1. SPECIFICITY:

A particular type of receptor responds to a specificstimulus.

Each type of receptor is sensitive to a particular typeof energy.

Stimulus to which a receptor is sensitive is calledADEQUATE STIMULUS (for thermal receptorstemp. stimulus).

Receptors can respond to stimuli other than the

adequate if stimulus is of high intensity (normallylight is stimulus for Rods & cones, but firm pressure oneye ball perceive light halos.


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SPECIFICITY(continued)

Each type of stimulus / sensation we perceiveMODALITY OF SENSATION (painsensation is one modality & thermal sensation is

another). For a specific type of sensation a specific

nervous pathway (for vision visual pathway)& stimulates a particular part of brain.

Specific nervous pathway for each sensationLABELED LINE PRINCIPLE.


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2. RECEPTOR / GENERATOR

POTENTIAL:

When we apply a stimulus to receptorlocalized change in membrane potentiallocalized hypo-polarization due to sodium influx

RECEPTOR POTENTIAL. It resembles EPP (End plate potential) or EPSP

(Excitatory post synaptic potential).

When receptor potential reaches threshold

action potential along sensory nerve fiber arisingfrom the receptor.


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Fig. 46.2, Guyton, 11th ed.


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RECEPTOR / GENERATOR

POTENTIAL: (continued) Amplitude of receptor potential is directly proportional to

strength of stimulus.

Frequency of discharge of impulses from receptor is directlyproportional to amplitude of receptor potential.

*Receptor potential is due to sodium influx, so when we applydifferent types of stimuli activation of sodium channelssodium influx (if we applymechanical stimulus receptormembrane is stretched sodium channels are activated, ifelectrical /chemical stimuli sodium influx).

*Frequencyof discharge of impulses from receptors one ofthe sensory input for brain to assess intensityof stimulus.


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Fig.46.4, Guyton, 11th ed.


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3.ADAPTATION OF SENSORY

RECEPTORS:

When a stimulus of CONSTANT INTENSITY

is applied CONTINUOUSLY to receptordischarge of impulses from receptor decreases

after sometime. (mushkilain itni pareen mujhpay k asaan ho gaeen!)

Depending upon degree & rate of adaptation, 2

types of receptors: PHASIC & TONIC.


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Rate & degree of adaptation:

PHASIC RECEPTORS:

Rapid adaptation.

Tactile / touch receptors.

Hair follicle receptors.

Meissners corpuscles.

Pacinian corpuscles.

Krauses corpuscles.

TONIC RECEPTORS:

Slow & incomplete

adaptation.

Pain receptors.

Cold receptors.

Muscle spindles.

Proprioceptors. Baroreceptors.


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Fig.5.3, Ganong 22nd ed.


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Physiological significance of

Rapid adaptation:

Touch receptors rapid adaptation perceive newevents /stimuli.

Rapid adaptation perceive change in stimulus

intensity (we take bath

wear clothes

rapidadaptation of touch receptorsdont feel theirpresence after sometime).

(insect crawls on skinperceive the new stimulus

already adapted to clothes

if does not bite

forgetits presence2nd insect crawlsperceive new eventdue to RAPID ADAPTATION).


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Rapid adaptation: (continued)

(if sitting on a chair & bag is resting on backof

chair skin is adapted to this intensity of

stimulus, if chair is pushed from behind can

perceive change in intensity of stimulus).


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Slowadaptation:

Pain is protective sensation (protective value persistsdue toincomplete & slow adaptation of painreceptors).

As long as cause of pain is there

we feel pain. Advantage: we seek the doctor in time.

Baroreceptors in carotid sinus & arch of aortaadaptin 24-48 hrs. If in sec could not regulate B.P (take

part in short term regulation of B.P only). Muscle spindles maintain muscle tone & posture

(slowly adapting). If rapidly adapting useless.


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1st Mechanism of Adaptation:

PACINIAN CORPUSCLEINDENTATION OF

MEMBRANE OF

PACINIAN CORPUSCLE

(UNIFORM

DISTRIBUTION

OF PRESSURE)

1. STRUCTURAL ADAPTATION

(INCREASED

LOCALIZED

PRESSURE)


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We apply a pressure stimulus there will be

indentation of the membrane of receptor.

Pressure will increase inside the corpuscle in that

areadischarge of impulses incr.

After some time increase in pressure inside the

corpuscle become evenly distributeddischarge of impulses decr.


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2nd Mechanism of Adaptation:

2.ACCOMMODATION:

Membrane of receptor / nerve endingaccommodated to stimulus due to

*inactivation of sodium channels.

Continuous stimulus to the receptor

continuously receptor becomesinactivated / accommodated (also the basis offatigue).


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4. MULLERS LAW OF SPECIFIC NERVE ENERGY &

LAW OF LOCALIZATION & PROJECTION:

From a particular type of receptorspecificnerve fibers carry impulses & stimulatespecific part of brain.

It means that specific nerve fibers carry impulsesfor a specific stimulus or sensation.

If a nerve fiber for a specific sensation are

stimulated at any point along the pathway

sensation perceived is that for which thepathway is specific for.


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MULLERS LAW OF SPECIFIC NERVE ENERGY & LAW

OF LOCALIZATION & PROJECTION: (continued)

If we stimulate a specific nerve fiber for a specificsensation at any point in pathway same sensation isperceived (along the pathway of touch, if we stimulateany point sensation perceived will be touch, no

matter what is the stimulus. Point to point connection between receptors in

different parts of body with the sensory cortex.

Specific nerve fibers carry impulses from receptors of a

particular part of body, e.g., from the hand. Specific nerve fibers carry impulses to sensory cortex &

with the help of these specific nerve fiberslocalization of stimulus.


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5. PHANTOMS LIMB:

A person having amputated leg sometimesc/o severe pain & proprioceptive sensations in

the amputated leg.

MECHANISM:

In stump of amputated leg, are cut ends of nerve

fibers specific for the amputated leg.

When stimulated (pressure / infection) c/opain in missing leg (Phantoms limb).


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6. DETERMINATION OF

INTENSITY OF STIMULUS:

Brain can determine intensity of stimulus by 2

types of sensory inputs:

FREQUENCYof discharge of impulses from

receptors. (directly proportional).

NUMBER OF RECEPTORS stimulated.

(directly proportional).

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LECT.4, PROPERTIES OF RECEPTORS by dr saidia zafar