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‫﴿و ما أوتيتم من العلم إال قليال﴾‬
‫صدق هللا العظيم‬
‫االسراء اية ‪58‬‬
Sensory System
By
Dr. Abdel Aziz M. Hussein
Assist Prof. of Medical Physiology
Def :
• Feeling caused by stimulation of proprioceptors
• It is the orientation of the position of the different
parts of the body in relation to each other as well
as movements of joints
• It arises from deep structures e.g. skeletal ms,
ligaments and joints
Types:
1. Sense of position
2. Sense of movement
• Both senses are called kinesthetic senses
A) Sense of position (static sense):
• Conscious orientation of the relative position of
the different parts of the body to each other.
B) Sense of movement (dynamic sense):
• Conscious orientation of the changes in the
relative position of the different parts of the
body to each other as regard,
 Onset
 Termination
 Direction
 Rate or velocity of this change.
Receptors:
•A) Muscle receptors : are
a. Muscle spindle in fleshy part of ms
b. Golgi tendon organs in ms tendons
•Both inform CNS about;
a) Existing ms length
b) Changes in ms length & the rate of change (velocity)
c) Degree of tension developed in ms “force generated
in the ms during contraction”.
Receptors:
•B) Joint receptors : are
a. Ruffini endings :
•
Slowly adapting receptors
•
Continuously inform the C.N.S about the position of
the joints.
b. Pacinian corpuscles
•
Rapidly adapting receptors
•
Inform the C.N.S about, the onset, the termination
and the velocity of the movement
Pathway:
1. From body: dorsal column – medial leminiscal
system “Gracile & Cureate tracts”
2. From face: trigeminal pathway
Site of perception:
• 1ry somatic sensory area in CC
Significance:
• Give rise to the senses of position & movement
Pathway:
1. From body spinocerebellar tracts:
 Dorsal spinocerebellar tract.
Ventral spinocerebellar tract.
2. From face: Fibers from the main sensory nucleus
in pons, relay through the inferior cerebellar
peduncle to the cerebellum
Site of perception:
•Cerebellar cortex
Significance:
•
Help in the regulation of body equilibrium and
feedback regulation of voluntary movements.
Dorsal Spinocerebellar Tract
Ventral Spinocerebellar Tract
Def.,:
• Feelings produced by stimulation of the cutaneous
thermal receptors
Thermal receptors:
1) Types:
a) Cold receptors :
• Mostly encapsulated nerve endings transmitting their
signals by type A- nerve fibers.
b) Warm receptors :
• Mostly free nerve endings transmitting their signals
mainly by type C-nerve fibers.
Thermal receptors:
2) Distribution:
• Distributed in a punctuate fashion where,
certain areas of skin contain warm receptors
only and others contain cold receptors only with
thermally insensitive areas in between.
• Cold receptors are greater than warm receptors
by about 3-10 times.
Cold receptors
Warm receptors
Paradoxical cold
Cold receptors
Pain receptors
Warm receptors
Pain receptors
Receptors:
3) Discharge:
a) Cold receptors :
• Discharge ( ) 10- 40 C with maximum rate at 24 C
b) Warm receptors :
• Discharge ( ) 25- 45C with maximum rate at 37 C
• Heat pain receptors discharge more than 45 C
• Cold pain receptors discharge ( ) 10 – 0 C with
maximum rate at 5 C .
• At 0 C pain receptors stop (local anesthesia)
• Paradoxical cold sensation occurs at 45 -50 C
Receptors:
4) Adaptation :
• Are slowly adapting Rs that show phasic response
• At the onset of stimulation show rapid increase in
discharge, then markedly the rate of discharge
decreases within short duration but not reach the zero
level i.e. remain steady at a low rate
• The brain discriminate the grade of temp by
the relative ratio of discharge of cold and warm
receptors.
Receptors:
5) Mechanism of stimulation
• No direct effect for the temp change on the thermal
receptors.
• Temp changes, lead to changes in the metabolic
activity of the receptors with changes in the
metabolites concentrations
• E.g. ↑ing skin temp  ↑es the metabolic activity of
the skin cells including receptors  ↑ed conc. of
metabolites → stimulates the warmth receptors i.e. the
thermal receptors stimulated by a chemical
mechanism.
Pathway: anterolateral system (lateral spinothalamic tract)
A) 1st order neuron :
• A delta and C afferent fibers
• Cold receptors connected with A delta and warm
receptors are connected with C fibers
B) 2nd order neuron :
• Lateral spinothalamic tract and in brain stem as spinal
leminiscus
• End in posteroventral nucleus of thalamus (PVNT)
Pathway:
C) 3rd order neuron :
• Axons of neurons of PVNT ascend in sensory
radiations
• End in primary somatic sensory area (area 3,1,2)
PVNT
Sensory Radiations
Spinal Leminiscus
Lamina SGR
A delta or C
Receptors
Cold and warmth
Lateral spinothalamic tract
Def :
•Pain is an unpleasant sensory and emotional experience
associated with actual or potential tissue damage
Significance:
1. Pain is a warning signal for tissue damage. It is the
prominent symptom of tissue damage
2. Pain has a protective function. It initiates protective
reflexes that;
•Get rid of the painful stimulus.
•Minimize tissue injury or damage.
•Pain Rs are morphologically similar but functionally they are
specific
1) Morphology: are specific free nerve endings
2) Highly specific i.e. respond to tissue damage only
• Classified according to their adequate stimulus into:a) Mechanical Pain Rs:
• Respond to strong mechanical trauma e.g. cutting
b) Thermosensitive pain Rs:
• Respond to excessive changes in temp (above 45°C and
below 10°C).
c) Chemical Pain Rs: respond to noxious chemical stimuli.
d) Polymodal Pain Rs: respond to a combination of
mechanical, thermal, and chemical noxious stimuli
3) Distribution:
a) Abundant in the skin and some internal tissue such as
the periosteum, arterial wall, joint surfaces, and the dura
of the tentorium cerebelli.
b) Few in deep tissues and all viscera. So, for pain to occur,
painful stimulus must by intense and widespread. The
deep & visceral pain is poorly localized.
c) Brain itself and the parenchymal tissues of the liver,
kidneys, and lungs have no pain receptors “pain
insensitive structures”
4) Threshold :
•It is the lowest intensity of injurious agent needed to
stimulate the pain receptors and produced pain
sensation
•Pain receptors are of high threshold: the pain receptors
needs sufficient degree of tissue damage to be
stimulated.
•Measured by;
1. By pricking the skin with a pin at measured levels.
2. By compressing the skin against hard objects.
3. Thermal method (more accurate) (45 C)
5) Adaptation:
•Slowly adapting receptors even non adapting receptors
•This is very important because it directs the subject to
get rid of the injurious agent
6) Mechanism of stimulation:
Chemical stimuli
Mechanical stimuli
Thermal stimuli
Strong acids or Alkalies
Cutting or pricking
temp. > 45 C and < 10 C
Tissue damage
1st class
K ions, Histamine,
Serotonin, and Bradykinin
Directly stimulate
Pain Receptors
Release of Pain
Producing
Compounds (PPS)
2nd class
PGE2, leukotriens and
Substance P
Sensitize the pain Rs
by lowering its
threshold to stimuli
Tissue
Damage
Direct
stimulators
Sensitizers
THANKS