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‫گیرنده های حسی‬
‫پیکری‬
Sensory Transduction
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Sensory Receptors
• Sensory receptors are structures that are
specialized to respond to changes in their
environment
• Such environmental changes are called
stimuli
• Typically activation of a sensory receptor by
an adequate stimulus results in
depolarization or graded potentials that
trigger nerve impulses along the afferent
fibers coursing to the CNS
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Classification of Sensory System by Structural
Complexity
Somatic (= general)
senses
1.
2.
3.
4.
5.
6.
Touch
Temperature
Vibration
Nociception
Itch
Proprioception
Special senses
1.
2.
3.
4.
5.
Vision
Hearing
Taste
Smell
Equilibrium
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‫تقسیم بندی انواع گیرنده ها؛‬
‫بر اساس‪:‬‬
‫‪ -1‬مكان قرارگیری )‪(Location‬‬
‫‪ -2‬نوع تحریك )‪(Type of stimulus detected‬‬
‫‪ -3‬ساختار )‪(Structure‬‬
‫‪ -4‬تطابق پذیری )‪(Adaptation‬‬
‫•‬
‫•‬
‫•‬
‫•‬
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1- Classification by Location
• Proprioceptors : monitor degree of stretch
• Interoceptors (visceroceptors) : receive stimuli from
internal viscera
– Monitor a variety of stimuli
• Exteroceptors : sensitive to stimuli arising from
outside the body
– Located at or near body surfaces
– Include receptors for touch, pressure, pain, and
temperature (, …)
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Proprioceptors
• Muscle spindle
• Golgi tendon organ
• Joint receptor
Structure of Proprioceptors
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Proprioceptors provide information about the position of body parts
Muscle Spindles
• Encapsulated fibers
within the muscle belly
• Monitor changes in
muscle length
• Monitor the rate of
change in muscle length
• Respond by causing
muscle contraction
Structure of Muscle Spindle
Each spindle is 3 to 10 mm long.
 It is built around 3 to 12 tiny intrafusal muscle
fibers that are pointed at their ends and
attached to the glycocalyx of the surrounding
large extrafusal skeletal muscle fibers.
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Structure of Muscle Spindle
Each intrafusal muscle fiber is a tiny skeletal
muscle fiber.
central region of each of these fibers has few
or no actin and myosin filaments.
Therefore, this central portion does not
contract when the ends do. Instead, it
functions as a sensory receptor.
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Motor Innervation of Muscle Spindle
The end portions that do contract are excited
by small gamma motor nerve fibers (gamma
efferent fibers) that originate from small type
A gamma motor neurons in the anterior horns
of the spinal cord.
the large alpha efferent fibers (type A alpha
nerve fibers) innervate the extrafusal skeletal
muscle.
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Sensory Innervation of the Muscle Spindle
The receptor portion of the muscle spindle is
its central portion.
In this area, the intrafusal muscle fibers do not
have myosin and actin contractile elements.
 sensory fibers originate in this area.
They are stimulated by stretching of this
midportion of the spindle.
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Muscle spindle, showing its relation to the large extrafusal
skeletal muscle fibers. Note also both motor and sensory
innervation of the muscle spindle.
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Sensory Innervation of the Muscle Spindle
muscle spindle receptor can be excited in two
ways:
1) Lengthening the whole muscle stretches the
midportion of the spindle and, therefore,
excites the receptor.
2) contraction of the end portions of the
spindle's intrafusal fibers stretches the
midportion of the spindle and therefore
excites the receptor.
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Sensory Innervation of the Muscle Spindle
Two types of sensory endings are found in this
central receptor area of the muscle spindle :
1) primary ending
2) secondary ending
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Primary Ending
In the center of the receptor area, a large sensory
nerve fiber encircles the central portion of each
intrafusal fiber, forming the so-called primary
ending or annulospiral ending.
 This nerve fiber is a type Ia fiber averaging 17
micrometers in diameter
it transmits sensory signals to the spinal cord at a
velocity of 70 to 120 m/sec, as rapidly as any type
of nerve fiber in the entire body.
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Secondary Ending
Usually one but sometimes two smaller
sensory nerve fibers- type II fibers with an
average diameter of 8 micrometers-innervate
the receptor region on one or both sides of the
primary ending, This sensory ending is called
the secondary ending;
sometimes it encircles the intrafusal fibers in
the same way that the type Ia fiber does, but
often it spreads like branches on a bush.
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Division of the Intrafusal Fibers into
Nuclear Bag and Nuclear Chain FibersDynamic and Static Responses of the
Muscle Spindle
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Division of the Intrafusal Fibers
two types of muscle spindle intrafusal fibers:
1) nuclear bag muscle fibers (one to three in each
spindle), in which several muscle fiber nuclei are
congregated in expanded "bags" in the central
portion of the receptor area
2) nuclear chain fibers (three to nine), which are
about half as large in diameter and half as long as
the nuclear bag fibers and have nuclei aligned in a
chain throughout the receptor area.
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Division of the Intrafusal Fibers
The primary sensory nerve ending (17-micrometer) is
excited by both the nuclear bag intrafusal fibers and
the nuclear chain fibers.
Conversely, the secondary ending ( 8 micrometer) is
usually excited only by nuclear chain fibers.
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Details of nerve connections from the nuclear bag
and nuclear chain muscle spindle fibers.
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Dorsal
+
M
S
-
+
+
+
Ventral
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Golgi Tendon Organ Helps Control Muscle
Tension
is an encapsulated sensory receptor
About 10 to 15 muscle fibers are usually
connected to each Golgi tendon organ
is stimulated when this small bundle of muscle
fibers is "tensed" by contracting or stretching
the muscle.
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Thus, the major difference in excitation of the
Golgi tendon organ versus the muscle spindle is
that
the spindle detects muscle length and
changes in muscle length,
whereas the tendon organ detects muscle
tension as reflected by the tension in itself.
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Golgi Tendon Organs (GTO)
• Encapsulated
receptors
• Located at the
musculotendinous
junction
• Monitor tension
within the tendon
• Respond by causing
the muscle to relax
Golgi Tendon Organ Helps Control Muscle
Tension
 The tendon organ, like the primary receptor of the muscle
spindle, has both a dynamic response and a static response :
 dynamic response : reacting intensely when the muscle tension
suddenly increases
 static response : settling down within a fraction of a second to a
lower level of steady-state firing that is almost directly
proportional to the muscle tension.
 Thus, Golgi tendon organs provide the nervous system with
instantaneous information on the degree of tension in each
small segment of each muscle.
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Transmission of Impulses from the Tendon Organ
into the Central Nervous System
are transmitted through large, rapidly conducting
type Ib nerve fibers that average 16 micrometers
in diameter.
These fibers, like those from the primary spindle
endings, transmit signals both into local areas of
the cord and, after synapsing in a dorsal horn of
the cord, through long fiber pathways such as the
spinocerebellar tracts into the cerebellum and
through still other tracts to the cerebral cortex
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The local cord signal excites a single inhibitory
interneuron that inhibits the anterior motor
neuron (by releasing Glycine).
This local circuit directly inhibits the individual
muscle without affecting adjacent muscles.
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Inhibitory Nature of the Tendon Reflex and Its
Importance
When the Golgi tendon organs of a muscle
tendon are stimulated by increased tension in the
connecting muscle, signals are transmitted to the
spinal cord to cause reflex effects in the respective
muscle.
 This reflex is entirely inhibitory.
Thus, this reflex provides a negative feedback
mechanism that prevents the development of too
much tension on the muscle and a protective
mechanism to prevent tearing of the muscle or avulsion of
the tendon from its attachments to the bone.
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Dorsal
GTO
Inhibits alpha
motor neuron
-
+
Ventral
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Classification by Location
• Interoceptors
• Detect stimuli from inside the body and
include receptors that respond to pH, oxygen
level in arterial blood, carbon dioxide
concentration, osmolality of body fluids,
distention and spasm (e.g., gut), and flow
(e.g., urethra)
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Classification by Location
• Exteroceptors
– Sensitive to stimuli arising from outside of the
body
– Typically located near the surface of the body
– Include receptors for
•
•
•
•
•
•
•
Touch
Pressure
vibration
Pain
Temperature
Special sense receptors
…..
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2- Classification by Stimulus
Detected
2- Classification by Stimulus Detected
• Mechanoreceptors – respond to mechanical
forces
• Thermoreceptors – respond to temperature
changes
• Chemoreceptors – respond to chemicals in
solution
• Photoreceptors – respond to light – located in
the eye
• Nociceptors – respond to harmful stimuli that
result in pain
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3- Classification by Structure
• General sensory receptors
– Widely distributed
– Nerve endings of sensory neurons monitor:
• Touch, pressure, vibration, stretch
• Pain, temperature, proprioception
• Divided into two groups
• Free nerve endings
• Encapsulated nerve endings
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Skin
Receptors
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Skin Sensors
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Krause Receptor
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4- Classification by Adapting
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Receptor
(Generator)
Potential
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Tonic & Phasic Receptors
• Transduction: translation of a stimulus into
an action potential.
• Transferred to an afferent fiber which then
travels to the CNS.
• Tonic Receptors-always sending signals.
• Phasic Receptors: only when the
conditions they monitor change.
• Fast-adapting receptors are phasic and slowadapting are tonic.
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Receptor Adaptation
• Different rates of adaptation
–  response to continued stimulation
• Slowly adapting (SA)
– steady pattern of firing
• Rapidly adapting (RA)
– fire only at onset of stimulus
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Adaptation –
static & dynamic responses
(dynamic)
(static)
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Receptive field
sizes vary by type:
Meissner’s corpuscles small
Pacinian corpuscles large
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Receptive Field Size
Adaptation
Small
Rapid
Slow
Meissner’s
Corpuscle
Merkel’s
Disks
Large
Pacinian
Corpuscle
Ruffini
Ending
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