Sensory System 2

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ميحرلا نمحرلا الله مس ب

﴾لايلق لاإ ملعلا نم متيتوأ ام و﴿

ميظعلا الله قدص

58 ةيا ءارسلاا

Sensory System

By

Dr. Abdel Aziz M. Hussein

Lecturer of Physiology

Member of American Society of Physiology

Properties

Specificity

Excitabilit y

Adaptation

Rate of discharge

This is Muller’s Law of specific nervous Energy:

It states that;

Each type of receptor is most sensitive to a particular stimulus called the adequate stimulus

Each type of receptor when stimulated gives one type of sensation called modality (type) of sensation is specific to that receptor regardless the method of stimulation

Examples:

1. Rods and cones → more sensitive to light waves

2. Auditory receptors → more sensitive to sound

6

Cold

Sensations

Pressure

Touch

Pain

Cold

Receptors

Ice bag

Pain

Sensations

Pressure

Touch

Pain

Cold

Receptors

Pin

Low temp.

Cold stimulus

Cold sensation

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Painful stimulus

Pain sensation

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However, the receptors can respond to stimuli other than adequate ones, provided that these stimuli are very strong; but still the response is the same modality to which the receptor is specialized

E.g. Heavy blow to the eye

Def.

The ability of the receptors to respond to their adequate stimuli.

Mechanism:

Receptor potential may be;

1. Partial depolarization in most receptors

2. Hyperpolarization in photoreceptors (rods & cones)

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Def:

Receptor or generator potential is a potential changes that occur in the receptors on adequate stimulation

Usually in the form of partial depolarization

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Mechanism:

Differs according to type of receptor

In mechanoreceptors is caused by deformation of receptor → open ion channels

Better studied in Pacinian corpuscles due to; a. Easily stimulated by microglass rods b. Large in size and easily dissected.

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Mechanism:

Pacinian corpuscles

2) Capsule of C.T. consists of several concentric layers (onion like) consists of

1) Central nerve terminal

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Unmyelinated nerve fiber

(Receptor or Transducer region

Capsule of concentric layers

1

st

node of Ranvier

(spike generating region)

Stimulus

Na+

+ + + + + +

+ + + + + + +

+

+

+

Stimulus

Deformation

Electrotonic current

Action potential

+ + - - - + +

Receptor potential

- -

Properties of generator potential

1. Amplitude:

Maximal amplitude of receptor potential is around

100 mv

Occurs only at an extremely high intensity of sensory stimulus

Due to maximal opening of Na

+ receptor membrane channels in the

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Properties of generator potential

1. Amplitude:

Is graded amplitude i.e. not obey all or none rule i.e. directly proportional with stimulus intensity

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Low intensity

Moderate intensity

High intensity

Properties of generator potential

2. It is not followed by absolute refractory period

3. Can be summated by temporal and spatial summation

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Properties of generator potential

4. Its duration is more than 5 m.sec i.e. longer than duration of action potential

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Properties of generator potential

5. If reach firing level of the afferent nerve it leads to repetitive action potential

↑ strength of stimulus →↑ amplitude of receptor potential → ↑amplitude of electrotonic current →↑

No of action potential generated from receptor

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Properties of generator potential

6. Conduction: passively conducted to the 1 st

Node of Ranvier in the form of electrotonic current

7. Not blocked by local anesthetics

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Action potential

Stimulus

- -

- - - - - -

Receptor potential

Cocaine

Stimulus

- -

- - - - - -

NO Action potential

Receptor potential

Weber Fechner Law:

The rate of impulse discharged from a receptor is directly proportional with the log intensity of the stimulus

R = log S x K

Where,

R = rate of impulse discharge

S = strength of stimulus

K = constant

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Weber Fechner law:

So, 100 fold increase in intensity leads to only 2 fold increase in frequency of impulses

So, receptors compress marked changes in stimulus intensity into smaller changes in frequency of action potential (Compression function of receptor)

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3

2

2 (100 times)

3 (1000 times)

Log. Intensity of stimulus

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↑ the intensity of the stimulus → ↑ the rate of discharge by 2 means: a. By increasing rate of discharge from each receptor b. By increasing the number of excited receptors

(Recruitment of receptors)

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Stimulus

+ + + + + +

+ + + + + +

+

+

+

+

Stimulus

+ + - - - + +

+ + - - - + +

- -

- -

Electrotonic current

Action potential

Stimulus

+ - - - - - +

+ - - - - - +

- -

- -

Electrotonic current

Action potential

Amplitude of receptor potential

Recruitment of receptors

Def.,

• decline or decrease in the rate of discharge from the receptors inspite of prolonged constant stimulation

Types of receptors:

According to the speed (rate) of adaptation ;

A) Slowly adapting (tonic) receptors

B) Rapidly adapting (phasic) receptors

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Start

Rapidly adapting

(Phasic) Rs

Time

Slowly adapting

(tonic) receptors

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Rapidly adapting (Phasic) Receptors: start discharge at a high rate then discharge decreases rapidly and even stop within short period of time

Examples

1. Tactile receptors as hair follicle, Meissner’s and

Pacinian corpuscles in skin

2. Some mechanoreceptors in the joints

Importance :

They inform the CNS about ;

1. Onset

2. Termination of stimulation

3. Change in intensity of the stimulus

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Slowly adapting (tonic) Rs

Receptors continue discharge of impulses even though at a lower rate as long as the stimulus is applied

Examples:

1. Pain receptors

2. Mechanoreceptors e.g. ms spindle, Golgi tendon receptors

3. Arterial baroreceptors

4. Cold receptors

5. Chemoreceptors in aortic and carotid bodies

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Significance :

Maintain sensory information, so subserve vital functions e.g.:

1.Arterial baroreceptors : regulation of ABP&

Heart rate

2.MS spindles and joint receptors : maintenance of the body posture and equilibrium.

3.Pain receptors :initiate protective mechanisms.

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THANKS

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