lect.5, pain & thermal sensation for 2nd year mbbs students

7/29/2019 LECT.5, PAIN & THERMAL SENSATION for 2nd year mbbs students

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Somatic Sensations II

PAIN & THERMAL

SENSATION:


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DEFINITION: A protective / warningsensation, when a noxious stimulus is applied toa part of the body. Part becomes flexed &

withdrawn from source of noxious stimulus &this prevents tissue damage.

Pain in a viscera warning to seek thetreatment. (example: pain of appendicitis or MIis a warning).


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Types of pain:

2 types of pain:

FIRST / FAST PAIN

SECOND / SLOW PAIN


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FIRST / FAST PAIN

is pricking, sharp, short & is localized.

Example: pin prick or cut with a sharp weapon. Fast pain

impulses are carried by A delta fibers from

periphery to spinal cord(5-30 m/sec).


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SECOND / SLOW PAIN

is burning, dull, diffuse & of prolong duration. Itindicates tissue damage.

Example: if fingers touch something very hot.

Slow pain impulses are carried by C fibers fromperiphery to spinal cord (unmyelinated, velocity: 0.5 to2 m/sec).

Pain impulses are carried by lateral spinothalamic tract.

Noxious stimuli are of different types:Mechanical, thermal, chemical.


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Mechanical noxious stimuli:

Pin prick,

cut with a sharp edge,

heavy pressure. In viscera, distention or increase in pressure

(example: a segment of gut is distended).

Increased intracranial pressure

headache.


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Thermal noxious stimuli:

Extremes of temperature:

burning hot or freezing cold.


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Chemical noxious stimuli:

Histamine,

bradykinin,

serotonin, insect bites,

ischemic products (like substance P).


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PAIN SENSITIVITY IN BODY

TISSUES: All body tissues are not pain sensitive.

Compact bone is without pain receptors (receptors are inperiosteum).

If a patient gets osteomyelitis no pain till periosteal involvement.

In brain, no pain receptors. In meninges, pain receptors. In braindisorder, until meninges are involved, no pain.

In lung tissue, no pain receptors, but in pleural membrane.

In intestines, pain receptors are in peritoneum.

In liver & spleen tissue, no pain receptors, but in their capsule. Pain receptors are free nerve endings called nociceptors.


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ASPECTS OF PAIN:

Pain sensation has 2 aspects:

Pain perception (feeling of pain).

Reaction to pain (emotional aspect of pain).


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INDIVIDUAL PAIN

SENSITIVITY:

Reaction to pain varies in individuals. It includes:

vocalization (noise making),

clenching of hand,

facial muscle contraction,

narrowing of palpebral fissure,

fall or increase in B.P,

sweating & skin pallor.

In some individuals violent movements of body.


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SIGNIFICANCE OF PAIN

RECEPTORS:

Pain receptors are almost non-adapted & that isbeneficial.

As long as there in noxious stimulus, pain is felt,this has got a protective value.


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LOCALIZATION OF PAIN DEPENDS

UPON No. OF RECEPTORS:

Regarding (visceral pain), it is not so localized, becausethere are less number of pain receptors in viscera.Superficial pain is more localized.

Visceral pain is also referred.

It radiates to a distant structure.

Ischemic pain (behind upper sternum left shoulder left arm neck may radiate to right arm.Cause: Dermatomal rule.


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Dermatomal rule:

The viscera where pain is produced & wherepain is referred are supplied by same sensory /dermatome of spinal cord.


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Fast pain is more localized than slow. Touchreceptors also play a role in pain localization.


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Pain sensitivity can be tested in different ways:

Pricking the skin with a measured pressure.

Pinching the skin (rough method).Applying a hot stimulus (temp: above 45 degrees

centigrade).


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THERMAL SENSATION:

Human beings can perceive various grades of thermalsensation.

2 types of thermal receptors: Cold receptors Warmth receptors

At extremes of temperature, pain receptors are alsostimulated. When there is burning hot / freezing cold pain receptors are also stimulated along withthermal.


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THERMAL RECEPTORS:

Present just beneath the skin in the form ofaggregations / spots (cold spots & warm spots).

4-10 times more cold spots as compared to warm spots.

From warmth receptors impulses are carried by Cfibers.

From cold receptors impulses are carried mainly by Adelta fibers & some by C fibers.


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Mucus membrane of mouth is relatively insensitive to warm ascompared to cold temperature.

Exposed areas are relatively less sensitive to temperature changesas compared to covered areas of body like face, scalp & hand.

In exposed areas, temperature change can be detected: 0.5 to 1degree centigrade while covered areas can detect change of 0.2

degrees even.

*Neutral zone (adaptation of thermal receptors) is between 29 to36 degrees centigrade. We dont feel warmth / cold in this zone.

When temperature is below 29 or above 36, we feel warmth /

cold.

Here humidity factor is important. If humid, even 36 degrees isuncomfortable. In dry weather, even 45 degrees is tolerable.


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If we take 2 meterials: Wood

MetalWith same temperature, metal is felt cooler, because it conducts

more heat.

Thermal receptors respond to static temperature but much more tochanging temperature.

If temperature is falling, we feel more cold, as compared to fixed

cold temperature (during snow fall).

When there are thermal changes, these effect metabolic rate inreceptors, so accordingly receptors are stimulated.


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lect.5, pain & thermal sensation for 2nd year mbbs students

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