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‫االسراء اية ‪58‬‬
Sensory System
By
Dr. Abdel Aziz M. Hussein
Lecturer of Physiology
Member of American Society of Physiology
• Sensory receptors are classified
functionally according to the following
criteria, :1. their location in the body
2. the nature of tissues in which they are found
3. the nature of stimuli acting on them
4. the morphological structure of the receptor
• A specific stimulus produces a receptor
potential by:a. inhibiting Na + influx into receptor
b. inhibiting K + efflux from receptor
c. enhancing Na + influx into receptor
d. enhancing K + efflux from receptor
• Rapidly adapting receptors are involved in :• a. initiation of rapid reflex responses
• b. detection of joint movements
• c. regulation of heart rate
• d. only ‘a’ and ‘b’ are correct
• Receptors detect stimulus intensity by :a.
b.
c.
d.
lowering the threshold for stimulating receptors
raising the threshold for stimulating receptors
decreasing the magnitude of receptor potential
raising the magnitude of receptor potential
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
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
Def :
•It is feeling produced by the application of heavy
mechanical stimuli to the skin (which can cause
deformation of the different skin layers).
Receptors:
a. Rapidly adapting receptors (Pacinian corpuscles).
b. Slowly adapting receptors (Ruffini multi-branched
nerve endings)
•
They continuously inform the C.N.S about the
pressure stimuli
•
Are present in the deeper layers of the skin
Significance:
•Plays a role in the
orientation of body position
•Are important for
discrimination of weight
Pathway:
• Gracile and Cuneate
tracts (but the fibers end
on different neurons in
area 3, 1 & 2).
Rapidly adapting
mechanoceptive
Slowly adapting
mechanoceptive
Proprioceptive
sensations
Deep pressure
Def :
•It is a feeling of rhythmic pressure changes produced by
the rapid repetitive stimulation of certain
mechanoreceptors.
•Receptors:
a. Pacinian corpuscles responds to vibrations up to 700
Hz
b. Meissner's corpuscles respond to vibration up to 200
Hz
Pathway:
•
Gracile and Cuneate tracts
•
Afferents are A beta fibers
Test:
•
Placing the base of vibrating tuning fork on the skin
over bony prominence
Def :
• 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
• Or feeling caused by stimulation of
proprioceptors
• 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
1. Sense of position (static sense):
• It is conscious orientation of the relative
position of the different parts of the body to
each other.
2. Sense of movement (dynamic sense):
• It means conscious orientation of the changes
in the relative position of the different parts of
the body to each other as regard, onset,
termination, direction and the 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;
1) Existing length of the muscles
2) Changes in the muscle length & the rate of this
charge “velocity
3) Degree of tension developed in the muscle “force
generated in the muscles during contraction”.
Receptors:
•B) Joint receptors : are
a. Ruffini endings are slowly adapting receptors
•
continuously discharge impulses to inform the C.N.S
about the position of the joints.
b. Pacinian corpuscles are rapidly adapting receptors
•
Inform the C.N.S about, the onset, the termination
and the velocity of the movement
A) At conscious
level
Pathway From body:
dorsal column –
medial
leminiscal
system “Gracile
& Cureate tracts”
From face:
trigeminal
pathway
B) At subconscious level
From body spinocerebellar
tracts
a) Dorsal spinocerebellar
tract.
b) Ventral spinocerebellar
tract.
From face: Fibers from the
main sensory nucleus in
pons, relay through the
inferior cerebellar peduncle
to the cerebellum
PVNT
Sensory Radiations
Gracile and Cuneate Nuclei
A beta
Receptors
Medial Leminiscus
Gracile and
Cuneate tracts
cerebellar cortex
Vermis of cerebellum
Clark’s nucleus
A) At conscious level B) At subconscious
level
Site of
perception
1ry sensory area in Cerebellar cortex
CC
in
the
Significance Give rise to the Help
senses of position & regulation of body
movement.
equilibrium
and
feedback regulation
of
voluntary
movements.
THANKS