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Dr. R Ghasemi
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Dr. R Ghasemi
Neurophysiology
SENSORY SYSTEM
By Dr. Rasoul Ghasemi
Outlines
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 Introduction
 Sensory System
 Principles
 Categorization
 Tactile Sensation
 Receptor types
 Dorsal Root Ganglion
 Nerve fibers
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Outlines
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 Proprioceptors
 Thermoreceptors
 Pathways to the CNS
 Somatosensensory cortex
 Pain
 Pain pathways
 Control of pain
 Hyperalgesia
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Somatosensory system
Four functions of Sensory systems
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 What do the senses do for us?
 Perception
 control
of movement
 regulation of internal organs
 maintenance of arousal
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Sensory system
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1- General somatic senses
 receptors
Touch
are widely spread
(Discriminative touch)
Movement,
Pain
(nociception)
Temperature
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pressure, vibration
Sensory system
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2- Special somatic senses
 Hearing
 Balance
 Vision
 Smell
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Sensory system
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3- Proprioceptive senses :
 Detect
stretch in tendons and muscle
 Body sense – position and movement of body in
space
4- Visceral sensory:
 General visceral senses – stretch, pain,
temperature, nausea, and hunger
Widely felt in digestive and urinary tracts,
and reproductive organs
5- Special visceral senses - taste
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The sensory systems encode four elementary
attributes of stimuli
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 Modality
 location
 intensity
 timing
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Sensory Receptors
Sensory Receptor Neurons
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 Structures vary
–
–
–
–
a. naked nerve endings surrounded by accessory tissues
 Pacinian corpuscle
b. cilia like structures - mechanosensory, olfactory, retina
c. those with true axons - mechanosensory, olfactory
 use electrical signaling
d. those without true axons - gustatory,
 primarily use chemical signaling (may generate an AP)
 Sensory cells respond to a specific stimulation
–
can respond to other stimulation if great enough (eyeball trick)
 Excite downstream neurons
–
if these are excited then will perceive the appropriate
stimulation (amputated limbs)
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Classes of receptors
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 Exteroceptors
 Detect stimuli near the outer surface of the body and
include those from the skin (thermal, touch, pressure
and vibration
 Interoceptors
 Detect stimuli from inside the body pH, oxygen
level/carbon dioxide in the blood, concentration,
osmolality of body fluids, distention and spasm (e.g.,
gut), and flow (e.g., urethra)
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Classes of receptors (continued)
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 Proprioceptors
 Monitor joint position & muscle contraction
 DO NOT ADAPT
 Structurally complex – are 2 types
 Tendon organs – monitor tendon strain
 Muscle spindles – monitor muscle length

Most information from these receptors is monitored
subconsciously

Are vital for normal skeletal motor function
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Types of Sensory Receptors
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 A ) Mechanoreceptors


(Skin tactile sensibilities)
Muscle endings
Muscle spindles
 Golgi tendon receptors


Hearing


Equilibrium


Sound receptors of cochlea
Vestibular receptors
Arterial pressure

Baroreceptors of carotid sinuses and
aorta
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Types of Sensory Receptors (continued)
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 B) Thermoreceptors
Cold receptors
 Warm receptors

 C) Nociceptors

Pain (Free nerve endings)
 D) Electromagnetic receptors
Rods
 Cones

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Types of Sensory Receptors (continued)
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 E) Chemoreceptors
 Taste
 Receptors

Smell


Receptors in or on surface of medulla and in aortic and carotid bodies
Blood glucose, amino acids, fatty acids


Receptors of aortic and carotid bodies
Blood CO2


Receptors of olfactory epithelium
Arterial oxygen


of taste buds
Receptors in hypothalamus
Osmolality

Neurons in or near supraoptic nuclei
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Sensory Physiology: General Principles
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Sensory systems convert one form of energy into
an electrical signal
This conversion of one energy form (eg. light) into
an electrical signal (receptor potential) is known as
stimulus transduction
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General Principles
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The process of stimulus transduction involves the opening or
closing of ion channels on a specialized plasma membrane.
The size of the receptor potential, just like the synaptic
potential is graded and is determined by:
•
•
•
•
The magnitude of the stimulus
The rate of change of the stimulus strength
The temporal summation of other receptor potentials
The degree of ‘adaptation’
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Spatial and Temporal Summation
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General Principles
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Receptor potentials have the same properties as
synaptic potentials
A receptor may be either a specialized nerve
ending of an afferent neuron or a separate cell
that is intimately associated with the peripheral
endings of the neuron.
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Receptor Potentials
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Receptor Adaptation
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 Different rates of adaptation

Slowly adapting (SA)


steady pattern of firing
Rapidly adapting (RA)

fire only at onset of stimulus
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Sensory receptors that produce a steady rate of firing in
response to an unchanging stimulus are called “tonic
receptors”
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Sensory receptors that reduce their rate of firing in
response to an unchanging stimulus are called
phasic receptors
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Adaptation of sensory receptors can be produced by changes
the mechanical properties of the receptor or by the electrical
properties of the receptor or the synapse
The Pacinian corpuscle is a very rapidly adapting
receptor
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Dorsal Root Ganglion
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Dorsal Root Ganglion Neuron Is the Sensory Receptor in the Somatic Sensory
System
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Receptors
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 Bare nerve ending
(Protopathic sensations )
 Encapsulated nerve ending (Epicritic sensations )
 In
superficial layers
Meissner's
corpuscle/ Merkel disk receptor
 In deep subcutaneous tissue
 Pacinian
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corpuscle/ Ruffini ending
The somatic sensory receptors
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Tactile receptors
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 Merkel
receptor fires continuously while
stimulus is present.
 Responsible
for sensing fine details
 Meissner
corpuscle fires only when a stimulus is
first applied and when it is removed.
 Responsible
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for controlling hand-grip
Figure 14.1 A cross section of glabrous (without hairs or projections) skin, showing the layers of the
skin and the structure, firing properties and perceptions associated with the Merkel receptor and
Meissner
corpuscle - two mechanoreceptors that are near the surface of the skin.
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Tactile receptors- continued
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 Two types located deeper in the skin
 Ruffini
cylinder fires continuously to stimulation
Associated with perceiving stretching of the
skin
 Pacinian
corpuscle fires only when a stimulus is
first applied and when it is removed.
Associated with sensing rapid vibrations and
fine texture
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Figure 14.2 A cross section of glabrous skin, showing the structure, firing
and perceptions associated with the Ruffini cylinder and the
Dr.properties
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Pacinian corpuscle - two mechanoreceptors that are deeper in the skin.
Distribution
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Distribution
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Receptive Fields
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 Skin region influencing single neuron
 small, sharply defined
 fine spatial detail, tactual acuity
 large, less distinct regions
 fine acuity not as important ~
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Receptive Field Size
Adaptation
Small
Rapid
Slow
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Large
Meissner’s
Corpuscle
Merkel’s
Disks
Pacinian
Corpuscle
Ruffini
Ending
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Two-point discrimination
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Two-Point Discrimination
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Receptor Threshold
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The Spatial Characteristics of Objects Are
Signaled by Populations of Mechanoreceptors
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Lateral Inhibition Can Aid in Two-Point
Discrimination
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Thermal Receptors
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 Humans recognize four distinct types of thermal
sensation:
cold, and cool,
 warm, and hot

 Unlike mechanoreceptors, cold receptors and
warmth receptors fire action potentials
continuously at low rates (2-5 spikes per second)
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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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Internal mechanoreceptors
 Stretch receptors that
monitor changes in
organ pressure in
distensible organs
 Rapidly adapting
 They monitor BP,
respiration, digestion,
and urinary control
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Chemoreceptors…
Only respond to dissolved chemicals
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 Rapidly adapting:



found in olfaction, taste & the CNS at:
Medulla – receptors are sensitive to pH/CO2 changes in CSF–
trigger respiratory adjustments
Aortic/Carotid bodies – sensitive to changes in pH/CO2/O2
blood levels – trigger adjustments in respiration and
cardiovascular activity
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Proprioceptors….
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 Monitor joint position & muscle contraction
 DO NOT ADAPT
 Structurally complex – are 2 types
 Tendon organs – monitor tendon strain
 Muscle spindles – monitor muscle length
 Most information from these receptors is
monitored subconsciously
 Are vital for normal skeletal motor function
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Muscle spindles respond to
muscle length or velocity
Golgi tendon organs
respond to muscle
force or contraction
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Nerve Fibers That Transmit Different Types of Signals,
and Their Physiologic Classification
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Afferent Fibers
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Somatosensory Pathways
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 The Anterolateral Pathway (also called Spinothalamic)
 The Dorsal Column Pathway
 The Spinocerebellar Pathway (important for
proprioception)
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 Pathways are composed of a first, second and third order neuron
 First Order neuron
 Second Order neuron
 Third Order neuron
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Sensory Pathways for Transmitting Somatic
Signals into the CNS
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 A) the dorsal column–medial lemniscal system






Touch sensations requiring a high degree of localization of the
stimulus
Touch sensations requiring transmission of fine gradations of
intensity
Phasic sensations, such as vibratory sensations
Sensations that signal movement against the skin
Position sensations from the joints
Pressure sensations having to do with fine degrees of judgment of
pressure intensity
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Dorsal column pathway
 Carries fine touch, vibration and
conscious proprioception signals
 1st neuron enters spinal cord through
dorsal root; ascends to medulla (brain
stem)
 2nd neuron crosses over in medulla;
ascends to thalamus
 3rd neuron projects to somatosensory
cortex
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Dorsal Columns/Medial Lemniscal System
Fasiculus cuneatus
(upper body)
Neuron #1
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Fasiculus gracilis
(lower body)
Origin:
Course:
Termination:
Dorsal root ganglion (cervical or lumbar)
Fasiculus gracilis/cuneatus
Nucleus cuneatus (upper body)
Nucleus gracilis (lower body)
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Dorsal Columns/ Medial Lemniscal System
Nucleus
gracilis
Nucleus
cuneatus
Internal
Arcuate
Fibers
Medial
Lemniscus
Neuron #2
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Origin:
Course:
Termination:
Nucleus gracilis/cuneatus
Medial Lemniscus
VPL (ventral posterolateral
nuclues) of Thalamus
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Dorsal Columns/ Medial Lemniscal System
To primary
somatosensory
cortex
VPL
Nucleus of
Thalamus
Neuron #2
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Origin:
Course:
Termination:
Laterality:
Nucleus gracilis/cuneatus
Medial Lemniscus
VPL of Thalamus
CONTRA
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dorsal
cloumn
pathway
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Dorsal column pathway
Primary somatosensory
cortex (S1) in parietal
lobe
Dorsal column
nuclei
Thalamus
Medulla
Dorsal column
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Medial
lemniscus
Spinal cord
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Spinocerebellar pathway
 Carries unconscious
proprioception signals
 Receptors in muscles &
joints
 1st neuron: enters spinal
cord through dorsal root
 2nd neuron: ascends to
cerebellum
 No 3rd neuron to cortex,
hence unconscious
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Spinocerebellar tract damage
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 Cerebellar ataxia
 Clumsy movements
 Incoordination of the limbs (intention
tremor)
 Wide-based, reeling gait (ataxia)
 Alcoholic intoxication produces similar
effects!
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Sensory Pathways for Transmitting Somatic Signals
into the CNS
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 B) The
anterolateral system
Pain
 Thermal sensations
 Crude touch and pressure sensations
 Tickle and itch sensations
 Sexual sensations

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Somatosensory Pathways
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 Thalamus




ventral posterior nuclei (VP)
pain & touch still segregated
pain also to intralaminar nuclei
also input from cortex
 Input to S1
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Somatosensory cortex
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 S1 - Postcentral Gyrus
 Somatotopic Organization
 topographic representation of body
 Distorted Homunculus
 disproportionate amount of cortex for body parts
 high sensitivity: large cortical area
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Primary Somatosensory Cortex
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Lesions in Somatosensory Areas Produce
Specific Sensory Deficits
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 Tabes dorsalis
 late consequences of syphilitic infection
 Destroy large-diameter neurons in the DRG
 Disruption of SSI
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Spinothalamic
damage
spinothalamic pathway
Left
spinal cord injury
Brown-Séquard
Syndrome
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Pain
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Pain
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Pain is an unpleasant sensory and emotional
experience associated with actual or potential tissue
damage or described in terms of such damage”
International Association for the Study of Pain
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Why feel pain?
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 Gives conscious awareness of tissue damage
 Protection:
 Remove body from danger
 Promote healing by preventing further damage
 Avoid noxious stimuli
 Elicits behavioural and emotional responses
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Pain Is Mediated by Nociceptors
free nerve endings in skin respond to noxious stimuli
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Selectively respond to
stimuli that can damage
tissue
Histamine,
K+ released from injured cells,
Bradykinin
Substance P and other related
peptides
Acidity (decreases in the local pH
around the nerve terminals),
ATP, serotonin, and acetylcholine.
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Pain
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Pain Receptors and Their Stimulation
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 Pain Receptors Are Free Nerve Endings
 Three Types of Stimuli Excite Pain Receptors

Mechanical,

Thermal

Chemical

Polymodal
 Tissue Ischemia as a Cause of Pain
 Muscle Spasm as a Cause of Pain
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Pain….
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• Superficial Somatic Pain arises from skin areas
• Deep Somatic Pain arises from muscle, joints,
tendons & fascia
• Visceral Pain arises from receptors in visceral
organs
–
localized damage (cutting) intestines causes no pain
–
diffuse visceral stimulation can be severe
•
distension of a bile duct from a gallstone
•
distension of the ureter from a kidney stone
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Acute and chronic pain
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 Fast Pain (acute)
 occurs
rapidly after stimuli (.1 second)
 sharp pain like needle puncture or cut
 not felt in deeper tissues
 larger A nerve fibers
 Slow Pain(chronic)
 begins
more slowly & increases in intensity
 in both superficial and deeper tissues
 smaller C nerve fibers
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Two Types of Peripheral Pain Neurons
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A-delta fibers
 Thick, myelinated, fast conducting neurons
 Mediate the feeling of initial fast, sharp, highly localized
pain.
C fibers
 Very thin, unmyelinated, slow-conducting
 Mediate slow, dull, more diffuse, often burning pain.
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spinothalamic
pathway
to reticular
formation
Aδ nerve
C nerve
nociceptor
nociceptor
Impulses transmitted to spinal cord by
Myelinated Aδ nerves: fast pain (80 m/s)
 Unmyelinated C nerves: slow pain (0.4 m/s)

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Ascending Pathways:
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Impulses ascend to brain via:

Spinothalamic tract(fast pain)
Spinoreticular tract to reticular formation (slow pain)
 Spinomesencephalic tract
 Cervicothalamictract
 Spinohypothalamic tract

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Central Pain Pathways: Fast Pain
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Fast pain and A-delta fibres
 A-delta fibers synapse on cells in the spinal cord that
lead to an area of the thalamus called the ventrobasal
complex

ventrobasal complex also receives neurons that mediate
touch

sends its output to the somatosensory cortex

allows us to localize where pain originates
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Central Pain Pathways: Slow Pain
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Slow pain and C fibres
 C fibres synapse on cells in the spinal cord
 Relays to a midline nucleus in the thalamus and
 to the limbic system
 responsible for motivational and emotional aspects
of pain
 Those connections are important for the
interpretation of pain.
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Ascending Pathways:
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Visceral pain
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Notable features of visceral pain:
Often accompanied by strong autonomic and/or
somatic reflexes
Poorly localized;
may be “referred”
Mostly caused by distension of hollow organs or
ischemia (localized mechanical trauma may be
painless)
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Visceral Pain
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 Causes of True Visceral Pain
 Ischemia
 Chemical Stimuli
 Spasm of a Hollow Viscus
 Overdistention of a Hollow Viscus
 Insensitive Viscera
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Afferent innervation of the viscera.
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Many visceral afferents are specialized nociceptors, as
in other tissues small (Ad and C) fibers involved.
Large numbers of silent/sleeping nociceptors,
awakened by inflammation.
Nociceptor sensitization well developed in all visceral
nociceptors.
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Referred pain
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Pain originating from organs perceived as coming from skin
Site of pain may be distant from organ
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Referred pain
Convergence theory:
This type of referred pain occurs
because both visceral and somatic
afferents often converge on the
same interneurons in the pain
pathways.
Excitation of the somatic afferent
fibers is the more usual source of
afferent discharge,
so we “refer” the location of visceral
receptor activation to the somatic
source even though in the case of
visceral pain.
The perception is incorrect.
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“Pain Gate” Theory
Melzack & Wall (1965)
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A gate, where pain impulses can be “gated”
The synaptic junctions between the peripheral
nociceptor fiber and the dorsal horn cells in the spinal
cord are the sites of considerable plasticity.
A “gate” can stop pain signals arriving at the spinal cord
from being passed to the brain


Reduced pain sensation
Natural pain relief (analgesia)
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How does “pain gate” work?
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The gate = spinal cord interneurons that
release opioids.
The gate can be activated by:
Simultaneous activity in other sensory (touch)
neurons
 Descending nerve fibers from brain

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Applications of pain gate
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Stimulation of touch fibres for pain relief:
 TENS (transcutaneous electrical nerve
stimulation)
 Acupuncture
 Massage
Release of natural opioids
 Natural childbirth techniques
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Pain Relief
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 Aspirin and ibuprofen block formation of
prostaglandins that stimulate ociceptors
 Novocain blocks conduction of nerve
impulses along pain fibers
 Morphine lessen the perception of pain in
the brain.
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Peripheral
sensitization to pain:
Some definitions:
Hyperalgesia increased
sensitivity to an already painful
stimulus
Allodynia normally non
painful stimuli are felt as painful (i.e
.light touch of a sun-burned skin)
Analgesia
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Hyperalgesia
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 The skin, joints, or muscles that have already
been damaged are unusually sensitive. A light
touch to a damaged area may elicit excruciating
pain;
 Primary hyperalgesia occurs within the area of
damaged tissue;
 Secondary hyperalgesia occurs within the tissues
surrounding a damaged area.
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Agents that Activate or Sensitize Nociceptors:
Cell injury  arachidonic acid  prostaglandins   vasc. permeability
(cyclo-oxygenase)
 sensitizes nociceptor
Cell injury  arachidonic acid  leukotrienes   vasc. permeability
(lipoxygenase)
 sensitizes nociceptor
Cell injury   tissue acidity   kallikrein   bradykinin   vasc. permeability
 activates nociceptors
  synthesis & release of PGs
Substance P (released by free nerve endings)  sensitize nociceptors
  vasc. perm., plasma extravasation
(neurogenic inflammation)
 releases histamine (from mast cells)
Calcitonin gene related peptide (free nerve endings)  dilation of peripheral capillaries
Serotonin (released from platelets & damaged endothelial cells)  activates nociceptors
Cell injury  potassium  activates nociceptors
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