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‫االسراء اية ‪58‬‬
Sensory System
By
Dr. Abdel Aziz M. Hussein
Lecturer of Physiology
Member of American Society of Physiology
Type of
sensation
Site of
sensation
Intensity of
sensation
Stimuli
4
Def
• It is the ability of the CNS to discriminate
(recognize) or identify the;
1. Type of stimulus (modality discrimination)
2. Site of stimulus (locality discrimination)
3. Strength of stimulus (intensity discrimination)
• Although, all stimuli reach the brain in the
same manner i.e. action potential
5
• Mechanism: explained by 2 mechanisms;
1. At the level of receptor: explained by Muller
law of specific nerve energy
2. At the central level: explained by Labelled line
principle
• Each sensation has specific receptor, pathway
and center in the cerebral cortex
• Stimulation of receptor, pathway, and center
gives specific sensation whatever may be the
method of stimulation
6
Cold sensation
At central
level
Cold stimulus
At receptor
level
7
Pain sensation
Painful
stimulus
8
Def.
• Is the ability of brain to accurately determine
the site of stimulus
Mechanism: law of projection
•
depends upon the topographic area of
representation
•
Each part in the body surface is represented by a
certain area in the sensory cortex
•
stimulation of certain areas in the cortex leads to
a sensation felt in the site or in the body surface
9
Phantom Limb
Mechanism:
• Depends upon the frequency of discharge
from the receptors along the afferent nerves
• ↑ stimulus strength →↑ frequency of AP due
to;
a) ↑ Frequency of AP from the receptor (Weber
Fechner law)
b) Number of receptors stimulated (recruitment of
receptors)
12
Stimulus Strength
Recruitment of Receptors
Recruitment of Receptors
1) Def.,
• Conscious awareness of a particular feeling caused by
stimulation of certain type of receptor by its adequate
stimulus
2) Classifications:
Sensations
General
Special
Emotional
1) Emotional sensations:
• E.g. anxiety, fear, sadness
2) Special sensations:
• E.g. anxiety, fear, sadness
3) General sensations:
• Arise from receptors distributed allover the body
• Are classified into;
a) Somatic sensations: from somatic structures e.g. skin
b) Visceral sensations: from viscera
c) Organic senses: e.g. thirst, hunger and sexual desire
Def,
• Sensations arise from somatic structures of the body
i.e. skin and deep tissues e.g. sk ms
Types:
1. Mechanoceptive sensations: include
a. Tactile: e.g. touch, pressure and vibration
b. Proprioceptive e.g. sense of position and movement
2. Pain (nociceptive) sensation.
3. Thermal sensation; cold and warm.
Def.,
• Feeling produced by application of light
mechanical pressure to the skin
Types:
• They include 2 types :
1. Crude touch
2. Fine touch
Def.,
• Poorly localized touch sensation produced by touching
the skin with diffuse ill defined object e.g. a piece of
cotton or the touch of clothes.
Receptors:
1. Free nerve endings
2. Hair end organs
Afferents:
a. A- nerve fibers
(5-30 m./s)
b. C-non myelinated
nerve fibers (0.5-2 m/s)
Pathway: Ventral Spinothalamic tract (part of anterolateral
system)
• Characterized by
a. Slow rate of transmission
b. Poor spatial arrangement of its fibers
c. Inaccurate information about site and size of crude touch
PVNT
Sensory Radiations
Spinal Leminiscus
Lamina IV, III
A delta and C
Receptors
Free nerve endings
Hair end organs
Ventral spinothalamic tract
Def.,
• Highly localized touch sensation produced by
application of a well localized object to the skin e.g. a
tip of a pencil or a head of a pin or teeth of a comb.
Receptors:
1) Meissner's corpuscles
(rapidly adapting).
2) Merkel's discs (slowly
adapting)
• These receptors are
numerous in the distal
than the proximal parts of
the body (more in tips of
fingers – tip of the nose –
lobule of ear – and nipples
of the breast).
Afferents:
•
A-beta rapidly conducting nerve fibers (30-70
meters/second
Pathway :
• Dorsal column – medial lemniscal system (Gracile &
Cuneate tracts)
• Characterized by
a. Fast rate of transmission
b. Good spatial arrangement of its fibers
c. Accurate information about site and size of fine touch
PVNT
Sensory Radiations
Gracile and Cuneate Nuclei
A beta
Receptors
Medial Leminiscus
Gracile and
Cuneate tracts
Types:
• It includes :
1. Tactile localization:
• It is the ability to localize exactly the point of touch
with eyes closed
2. Tactile (2 point) discrimination :
• It is the ability to feel 2 points of touch with eyes
closed as 2 separate points of touch provided that
the distance between the 2 points is greater than
minimal or threshold stimulus
• It is tested by the compass
Types:
Minimal distance:
• It is the distance between 2 points of fine touch
below it the 2 points are felt as a single point
whereas at or above it the 2 points are felt as
separate points.
• It is very short at tip of tongue (1 mm), finger
tips (3 mm) , lips (2 mm) and nipples of the
breast.
• It is long in the back of trunk (70 mm)
Types:
• Tactile localization and discrimination are more
accurate on the extremities than on the
proximal parts due to:
1. Great number of receptors
2. Great number of afferents
3. Little convergence of afferents centrally
4. Wide area of cortical representation →good
analysis and interpretation.
Types:
3. Stereognosis:
•
It is the ability to recognize a familiar object with eyes
closed e.g. recognition of a key, a pen or a coin.
4.Texture of materials:
• Ability of the person with his eyes closed to recognize
the nature of an object e.g. glasses and wood.
N.B.
Gracile and cuneate tracts carry the following
sensations:
1. Fine touch
2. Vibration
3. Pressure
4. Proprioceptive sensations
THANKS