SENSORY SYSTEM
RECEPTORS & SENSORY
PATHWAYS
LECTURE 3
DR. ZAHOOR ALI SHAIKH
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Sensory system or Input system
 What type of sensation do you know?
 Touch
 Fine
 Crude
 Pressure
 Position
 Vibration
 Two point discrimination
 Pain
 Temperature
 Cold
 Warm
 Stereognosis
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How we feel the sensation?
 Sensory receptor
 Sensory pathway
 Sensory cortex
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Cont….
COMPONENTS
 Receptors
 Peripheral nerves
 Spinal cord
 Tracts
 Brain stem
 Thalamus
 Thalamocortical projection
 Somatosensory cortex
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Sensory Receptors
 These are transducers that convert various
forms of energy into action potentials .
 The particular form of energy to which a
receptor is most sensitive is called as its
adequate stimulus.
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Adequate stimulus
Each type of receptor is most sensitive to
a specific form of energy, called adequate
stimulus, the receptor is almost nonresponsive to the normal intensities of
other forms of energy.
e.g. rods & cones are stimulated by light
not heat.
Touch receptors are stimulated by touch.
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Sensory receptors
 Sensory receptors can be subdivided into:
1) Mechanoreceptors.
2) Thermoreceptors.
3) Nociceptors.
4) Proprieceptors.
5) Visceral receptors
6) Special senses.
Pressure
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Tactile
Receptors
in the Skin
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Skin Receptors
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Receptor Properties
1.Receptor potential or Generator Potential
 When we apply pressure, we generate
depolarization in the receptor it is called
Generator potential or Receptor potential.
 Non propagated depolarizing potential.
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What is the difference between
generator potential and action
potential?
Receptor potential
Action potential
In the Receptor
Graded
Doesn’t obey all or none
rule
Can be summated
Unpropagated
In the Sensory Nerve fiber
Not Graded
Obeys all or none rule
Not summated
Propagated
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RECEPTOR POTENTIAL & ACTION POTENTIAL
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Relation between receptor potential and action potential of sensory nerve
Note: if stimulus to the receptor is sub
threshold then no action potential is
generated in sensory nerve fiber
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Sensory Transduction
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2.Adaptation or
Desensitization
 When stimulus of constant strength is applied
to a receptor, some receptors can diminish
the extent of their depolarization inspite of
sustained stimulus. This is called adaptation.
Degree of adaptation varies in different
receptors.
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Classification of receptors
 Rapidly adapting or phasic receptors e.g.
pacinian corpuscles, Meissners corpusle.
Because of that we are not aware of sitting on
the chair, wearing watch, wearing clothes.
 Slowly or non adapting or tonic receptors
e.g. Nociceptors, muscle spindles.
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ADAPTATION OF RECEPTOR
Tonic
Phasic
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RESPONSE OF PHASIC & TONIC MECHANORECEPTOR
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3.Coding of sensory
information
Why we feel touch, pain, warm
sensation when all sensory
nerve carry the information to
brain as action potential?
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Coding of sensory
information
 Due to the Doctrine of specific nerve
energies.
We have;
 Specific receptors.
 Specific sensory pathways.
 Specific part of the brain they activate.
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Law of projection
 If we stimulate sensory pathway along its
course to the sensory cortex, the conscious
sensation produced is refered to the location
of receptors. This principle is called as Law of
projection.
 If we stimulate sensory cortex area which
receives impulses from left hand, patient
reports sensation in the left hand, not in the
head.
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Cont…..
Q. In patients whose limb was amputed (cut off)
they complained of pain in the absent limb
(Phantom limb), Why?
Due to law of projection, the ends of nerves cut
at the time of amputation when stimulated,
sensation evoked are projected to where the
receptor used to be present,i.e. in the limb.
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Receptor field
• Receptor field of a sensory unit is the area
from which a stimulus produces response
in that unit.
• Smaller the receptive field – More precise
the information e.g. Finger tips
• Larger the receptive field- less precise the
information e.g. arms, legs.
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Discriminative
ability of
regions with
small versus
large receptive
field
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Recruitment of sensory units
 Weak stimulus activates receptors with
lowest threshold.
 Strong stimulus activates those receptors
also which have high threshold .
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SPATIAL SUMMATION
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Physiological classification of
nerve fibers that transmit
different types of sensations
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Physiological classification of nerve fibers
that transmit different types of sensations
Number
Ia
Origin
Fiber type
Muscle spindle, annulo-spiral
ending.
Golgi tendon organ.
A
II
Muscle spindle, flower-spray
ending, touch, pressure
A
III
Pain and cold receptors; some
touch receptors
A
IV
Pain, temperature, and other
receptors
Dorsal root C
Ib
A
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Sensory Pathways
Somatosensory pathways are
THREE Neuron system
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How Sensation are carried?
 From receptors impulses are carried by sensory
nerves, the sensory nerve have cell bodies in
dorsal root ganglia.
 Primary afferent fibers which carry cutaneous
sensations are
 Large myelinated fiber Aα and Aβ – propriception,
touch, pressure.
 Small myelinated fiber Aδ – fast pain
 Small unmyelinated C fibers – slow pain
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 Sensory fiber go to dorsal horn in the spinal cord.
 Dorsal horn represent a gate in which impulses in
the sensory nerve fiber are translated into
impulses in ascending tracts.
 Dorsal horns are divided into laminae I – VII. (I –
superficial, VII – deepest).
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Sensory pathways or tracts
areThree Neuron system
1. Dorsal column or posterior column or Gracilis
and cuneatus or leminscal system.

It carries sensations of fine touch, position,
viberation, two point discrimination &
stereognosis.
2. Anterolateral system

Ventral spinothalamic tract carries crude touch
and pressure,
 Lateral spinothalamic tract carries pain and
temperature.
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Spinal Tracts
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Dorsal
Column
System
Medulla
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Anterolateral
System
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Anterolateral
System
Dorsal
Column
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DORSAL COLUMN PATHWAY
 CARRIES FINE TOUCH,
POSITION, PRESSURE,
VIBRATION, TWO POINT
DESRIMINATION
stereognosis
 AFFERENT SENSORY
FIBERS Aβ TYPE.
 VERY FAST VELOCITY 30 –
70 m/s
 3 NEURON SYSTEM
(SEE THE DIAGRAM)
ANTEROLATERAL PATHWAY
 CARRIES PAIN & TEMPRATURE
(lat. Sp.Th)
 CRUDE TOUCH & PRESSURE
(VENT, Sp. Th)
 AFFERENT SENSORY FIBERS
Aδ – 6 – 30 m/s (MYELINATED)
FAST PAIN
 C FIBERS – 0.5 – 2 m/s
(UNMYELINATED) SLOW PAIN
 3 NEURON SYSTEM
(SEE THE DIAGRAM)
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Different Sensations
TEMPERATURE
 Skin has cold sensitive area & heat sensitive area.
Receptors are free nerve endings.
 Cold receptors are 4 – 10 times more than warm
sensitive spots.
 Cold receptor responds from 10 – 38 oC.
 Warm receptor responds from 30– 45 oC.
 Afferent from cold receptors – Aδ & C fibers.
 Afferent from warm receptors – C fibers.
 Temperature sensation is carried via lateral
spinothalamic tract.
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THERMORECPTOR
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Different Sensations
PAIN
 Pain receptors or nociceptors – free nerve endings.
 Pain sensation are carried by two types of nerve fibers.
 Aδ (myelinated) -2 – 5 μm.in diameter, conduction
velocity – 12 – 30 m/sec.
 For fast pain (sharp localized)
 C fibers (unmyelinated) 0.4 – 1.2μm. in diameter,
conduction velocity – 0.5 – 2 m/sec.
 For slow pain (dull, diffuse)
 Both Aδ & C fibers terminate in dorsal horn.
Pain is carried via lateral spinothalamic tract.
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PERCEPTION
 Perception is conscious interpretation of
external world by Brain due to sensory
impulses delivered to Brain from sensory
receptors.
We perceive sounds , colors, smell.
 Why we do not perceive X-Ray, light
waves,MRI.
Because we do not have receptors to
respond.
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Thank you
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