the physiology of sensory system: receptors, pathway, sensory cortex

2. 3
•Sensory receptors
•Specialized cells or multicellular structures that
collect information from the environment
•Stimulate neurons to send impulses along sensory
fibers to the brain
•Sensation
•A feeling that occurs when brain becomes aware of
sensory impulse
•Perception
•A person’s view of the stimulus; the way the brain
interprets the information

3. • The receptor area which when
stimulated results in a response of a
particular sensory neuron
• Receptive fields of adjacent neurons
overlap

5. Sensory unit:
Single sensory afferent with all its receptor
ending.
Receptive field of a neuron:
Region cotaining all receptors which its
stimulation elicits stimulation in that neuron.

6. Classification of receptors

8. 13
•Senses associated with skin, muscles, joints and viscera
• Exteroceptive senses (exteroceptors)
•Senses associated with body surface such as touch,
pressure, temperature, and pain
• Visceroceptive senses (interoceptors)
•Senses associated with changes in the viscera such as
blood pressure stretching blood vessels and ingestion of
a meal
• Proprioceptive senses
•Senses associated with changes in muscles and
tendons such as at joints

9. Mechanoreceptors
 Cutaneous (touch, pressure, vibration) eg. Pacinian, Meissner’s corpuscle,
free nerve endings
 Proprioceptors (joint position receptors) eg. Muscle stretch receptors,
tendon organs
 Baroreceptors
 Auditory/vestibular hair cells
•
• Chemoreceptors
 Taste buds and smell receptors
 Visceral chemoreceptors sensitive to Pco 2,Hp ,cte ytilalomso
• Thermoreceptors
 Cold and hotreceptors

10. • Mainly cutaneous
Touch
Pressure
Vibration
• Crude or Fine mechanosensations
• Others: auditory, vestibular, stretch,
proprioceptors

12. Properties of receptors

13. • Receptors are stimulated by a certain type of
energy (adequate stimulus).
• Stimulation of a receptor usually produces only
one sensation
modality specific
• But some receptors are stimulated by more
than one sensory modality (polymodal)
eg. free nerve endings

14. • Receptor cells are specific cells that are
sensitive to different forms of energy from
the environment
• These cells contain membrane
receptors coupled to ion channels
• They transform the stimulus into
electrical signals

15. Receptors act as :
detectors
transducers

16. • TRANSDUCTION
Stimulus energy is converted to action
potentials
Inside the nervous system signals are always
action potentials
Language of the nervous system contains only
1 word: action potentials

17. • Receptors transform
an external signal into
a membrane potential
• Receptor Potential
- separate receptor
• Generator Potential
-a specialized ending
of an afferent neuron

18. 7
• Stimulation of receptor causes local change in its receptor potential
•A graded electrical current is generated that reflects intensityof
stimulation
•If receptor is part of a neuron, the membrane potential maygenerate
an action potential
•If receptor is not part of a neuron, the receptor potential mustbe
transferred to a neuron to trigger an action potential
• Peripheral nerves transmit impulses to CNS where they are analyzed
and interpreted in the brain

22. Physical stimulus causing mechanical deformation on the capsule

23. Mechanical deformation is transmitted to the inside
Opens up mechanosensitive Na+ channel
Causes depolarisation and thus receptor potential

24. local current
Current flow through a local circuit

25. Action Potentials
are generated
Opening of voltage gated Na+ channels causes generation of
action potentials

26. • Non propagated local excitatory state.
• This is a graded potential as It does not follow
“all-or-none law”
• Its amplitude depends on the strength of the
stimulus.
• Summated (no refractory period)
• Long duration

27. Stimulus
Receptor
potentials
3-Relation between strength of stimulus and
magnitude of receptor potential.

28. Resting
Membrane
Potential
-70
Threshold - 55
+30
Stimulus
Receptor potential

29. 8
The ability to diminish the extent of their
depolarization despite sustained stimulus strength
Types of receptors according to their speed of
adaptation
• Tonic: Do not adapt or adapt slowly
e.g. pain receptors
muscle stretch receptors
and joint proprioceptors
• Phasic
(Rapidly adapting receptors e.g. Tactile receptors)

30. Time
Muscle
spindle
Temperature
Smell
tasteTouch and
pressure
Pacinian
corpuscles

32. • In the Pacinian corpuscle
mechanical deformation is transmitted
throughout the capsule and pressure is
redistributed.
Na+ channels inactivates after some time

33. Impulse
Redistribution of pressure inside the capsule
Stimulus
No
Impulse

34. •Rapidly adapting receptors
phasic or rate or movement receptors
detect changes in stimulus strength
eg. Pacinian corpuscle, hair end-organ
•Slowly adapting receptors
tonic receptors
detect continuous stimulus strength
eg. muscle spindles, Golgi tendon organ,
baroreceptors, Ruffini endings and Merkel’s discs, pain
receptors

35. • A receptor must convey the
type of information it is
sending  the kind of
receptor activated
determined the signal
recognition by the brain
• It must convey the intensity
of the stimulus  the
stronger the signals, the
more frequent will be the
APs
• It must send information
about the location and
receptive field, characteristic
of the receptor

36. Modality of sensation:
Adequate stimulus
Mullers law of specific nerve energy
Labeled line principle

37. Low of projection
phantom limb

38. Number of receptors
Frequency of impulses :
 weber feshner principle
R=log S * K
 Stevens power principle
R=ASK
Tactile sensations

39.  Special Sensations: Vision, Hearing, Taste, Smell
 General sensations:
 Somatic sensation
 Organic sensations: e.g. hunger
 Emotional sensation: e.g. fear, sadness

40. Mechanoreceptive sensations
Tactile sensations
Position sensations: static and kinetic
Thermoreceptive sensations: Cold, Warm
Pain sensation

41. Touch :
Crude touch
Fine touch
Tactile localization
Tactile discrimination
Texture
Stereognosis
Pressure: crude & fine
Vibration
Itching & tickling

42. _ A fibers:Myelinated
• Subtypes: some overlap in ranges
• Fastest conducting and largest diameter – m/sec,
– B fibers: Slower myelinated
– C fibers: Un myelinated
• Slower conduction and smallest diameter (0.5 m/sec,
0.5 )
Nerve Fiber Classification

43. – I, II, III fibers: Myelinated
• Subtypes: Ia, Ib
• Fastest conducting and largest diameter – Ia
– IV fibers: Unmyelinated
• Slower conducting than IIIs and smallest diameter

44. Modality of sensationType of
fiber
ProprioceptorsI
Aα
Fine touch, pressure,
stereognosis, vibration
II
Aβ
Fast pain, temperature,crude
touch
III
A
Slow pain, temp., tickle and
itching
IV
C

45. • Once a receptor is stimulated
impulse travels through a particular pathway
known as sensory pathway or ascending pathway
up to the brain

46. Receptor
Sensory
modality
Sensory nerve
Ascending
Sensory pathway
Central Connections
Sensory area
in the brain
Touch stimulus

47. • Dorsal column - medial lemniscus pathway
fast pathway
• Spinothalamic pathway
slow pathway
These two pathways come together at the level of thalamus

48. Posterior (dorsal)
Dorsal root ganglion
Dorsal root
Dorsal columns
Dorsal horn
Spinothalamic
tracts
Anterior (ventral)

49. Dorsal column pathway
Lateral
Spinothalamic
tract
Anterior
Spinothalamic
tract

50.  Fine touch
 fine pressure
 stereognosis
 vibratory sense
 movement sense
 position sense
Spinothalamic pathway
• Pain
• Thermal sensations
• Crude touch & pressure
• tickle & itch

51. Dorsal column nuclei
(cuneate & gracile nucleus)
Dorsal column
Medial lemniscus
thalamus
thalamocortical tracts
internal capsule
1st
order
neuron
2nd
order
neuron

52. • after entering the spinal cord
•lateral branch: participates in spinal cord reflexes
•medial branch: turns upwards
• forms the dorsal columns
• spatial orientation:
•medial: lower parts of the body
•lateral: upper part of the body

53. • synapse in the dorsal column nuclei
nucleus cuneatus & nucleus gracilus
• 2nd order neuron cross over to the opposite
side and ascends upwards as medial
lemniscus
• as this travels along the brain stem fibres
from head and neck are joined (trigeminal)
• ends in the thalamus (ventrobasal complex)
 ventral posterolateral nuclei

54. • after entering the spinal cord
synapse in the dorsal horn
• cross over to the opposite side
• divide in to two tracts
lateral spinothalamic tract:
pain and temperature
anterior spinothalamic tract
crude touch

56. from the thalamus 3rd order neuron
ascends up through the internal capsule
up to the sensory cortex
)thalamocortical radiation(

57. • parietal cortex
• a distinct spatial orientation exists

61. • Different areas of the body are represented
in different cortical areas in the sensory
cortex
• Sensory area
somatotopic representation
not proportionate
distorted map
upside down map

62. Representation
•upside down
•distorted
Map

64. • Primary somatosensory cortex
(SSI)
postcentral gyrus
(Brodmann areas 3, 1, 2)
• Secondary somatosensory cortex
and Somatosensory association
cortex (SSII)
Posterior parietal areas
(Brodmann areas 5, 7)

65. • Functions
To localise somatic sensations
To judge critical degree of pressure
To identify objects by their weight, shape,
form - stereognosis
To judge texture of materials
To localise pain & temperature

66. • Damage to the sensory cortex results in
decreased sensory thresholds
inability to discriminate the properties of
tactile stimuli
Inability to identify objects by touch
(astereognosis)

67.  Occupies area 40
 Receives input from SSI
 Spatial orientation :face ant. , arms
centrally, legs post.

68. • Located directly posterior to the
sensory cortex in the superior
parietal lobes
• Consists of areas 5 and 7
• Receives synthesized
connections from the primary
and secondary sensory cortices
• Neurons respond to several
types of inputs and are
involved in the understanding
of sensation

69. • Damage can cause
Tactile agnosia
inability to recognize objects even though the objects can
be felt
Spatial neglect
This typically happens with non-dominant hemisphere
lesions
Neglect can be so severe that the individual even denies
that their left side belongs to them

70. Mechanoreceptive sensations
How to test
Receptor
Afferent
pathway

71. Modality of sensationType of
fiber
ProprioceptorsI
Aα
Fine touch, pressure,
stereognosis, vibration
II
Aβ
Fast pain, temperature,crude
touch
III
A delta
Slow pain, temp., tickle and
itching
IV
C

72.  Fine touch
 fine pressure
 stereognosis
 vibratory sense
 movement sense
 position sense
Spinothalamic pathway
• Pain
• Thermal sensations
• Crude touch & pressure
• tickle & itch

73. pathwayafferentreceptorType of sensation
Dorsal columnAβMeissner &merkelTouch fine touch
Ant . spinothalamicA deltaFree nerve endingCrude touch
Dorsal columnAβ
A delta
Pacinian corpuscles&
spray type endings
pressure fine
crude
Dorsal columnAβmixturestereognosis
Dorsal columnAβPacinian 500hz
Meissners 80 hz
vibration
Dorsal column
AαMuscle spindles golgi
tendon organs
Ruffini endings
Prprioceptive
sensations
Ant . spinothalamicCFree nerve endingsTickling and itching
Lat. spinothalamicC
A delta& C
VR1 VRL1
CMR
Temp . Warm
cold
Lat. spinothalamicA delta
C
Free nerve endingPain fast
slow

74. • The receptor area which when stimulated
results in a response of a particular sensory
neuron
• Receptive fields of adjacent neurons overlap

76. Whether a stimulus feels like one sensation or two
distinct sensations depends on:
• Number of receptors
• the size of the receptive fields of the sensory
receptors:Smaller receptive fields result in greater sensitivity
 Fingers are more sensitive than backs
• Degree of convergence within the pathway
• Area of representation

81. Thermal receptors
a) Warm : free nerve ending ( C fibers )
b) Cold : free nerve endings ( C fibers and A
delta fibers )
c) Others :cold pain and warm pain

82. Tested by using three test tubes ( 5 °C ,
25 °C & 40 °C )
Touch any part of body by the test tube (
for 2 minutes )
Ask him to feel hot or cold

83. They respond to the temperature of
subcutaneous tissue.
Widely separated(wide area exposed for
better differentiation)
Moderately adapting receptors
distribution: lips > fingers > trunk

84. Cold sensation is more important than
warm sensation
Cold receptors:
- More numerous ( 4-10 times)
- More superficial (under skin )
- Adapt more slowly
- Carried on type C & A delta

85. Chemically by change in concentration of
metabolites
Respond markedly to changing
temperature

86.  cold pain ( 5-15 °C) maximum 5 °C
 cold receptors ( 10-43°C) maximum 25°C
 warm receptors ( 30-50°C) maximum 45 °C
 warm pain above( 45°C )
 Paradoxical pain sensation
 At 0 °C : anaesthesia

87. Cold
pain
Warm
pain

89.  Pain is a warning that something is wrong.
 It occurs whenever there is actual or potential tissue
damage.
Pain system:
• Pain receptors (nociceptors).
• Types of pain.
• Pain pathways.
• Pain control.

90.  Free nerve endings attached to A delta & C, slowly
(non) adapting to prolonged stimulation
 4 types:
Mechanical pain receptors.
Thermal pain receptors.
Chemical pain receptors.
Polymodal pain receptors

91. Widely
distributed
Sup. Layers of skin
Pleura
Periosteum
periosteum
Arterial walls
Joint surface
duraof cranial cavity
Less
distributed
Deep tissues
& Viscera
Absent
Liver
Parenchyma
Lung alveoli
Brain tissue

92. Damaged tissues----chemical mediators
e.g. histamine, serotonin, substance P,
bradykinins, K+ -------sensitizes
painreceptors -------hyperalgesia
1ry hyperalgesia
2ry hyperalgesia

93. Two separate pathways:
I. Neospinothalamic pathway:
Conducting quick, localized pain
(Fast sharp pain)
II. Paleospinothalamic pathway:
For slow pain
Lateral
spino-
thalamic
tract

94. Ad fiber
glutamate
C Fibers
Subst. P
Substantia
Gelatinosa of
Rolandi (SGR)
glutamate

96. In the brain stem, 90% of fibers synpse
on:
 Reticular formation
 Intralaminar (non specific ) nuclei of thalamus
to activate the whole cortex

97. Motor reflexes (AHCs)
Arousal reaction (RAS)
Autonomic reactions(hypothalamus)
Emotional reactions(limbic system)

98. Perception of pain signals
Fast pain in thalamus & cortex
Slow pain in thalamus
Roles of the cortex in pain perception are
1.Localization of pain.
2.Discrimination of pain.
3.Modulation of pain.

99. According to oigin
Cutaneous
Deep
Visceral
Neuropathic

100. According to Quality :
1. Fast pain (sharp, acute, pricking, immediate)
2. Slow pain (burning, chronic, dull aching,
throbbing

101. Slow painFast pain
Felt within 1 sec or
more
Felt within 0.1 sec
May be prolongedShort duration
Poorly localizedWell localized
All types of receptorsMechanical or thermal
Skin, deep tissues &
viscera
Usually in skin, rare in
deep tissues

102. slow painFast pain
Carried by C, blocked
by local anaesthetics
Carried by Ad, blocked
by pressure & O2 lack
C release Substance PAd release Glutamate
Transmitted by
Paleospinothalamic T
Transmitted by Neo-
spinothalamic T
End in RF Non-
specific thalamic nuclei
whole cortex
Its fibers end in
sensory cortex

103. Felt in muscles , tendons , joints and
bones
Causes:
 Trauma
 Inflammation
 Muscle spasm
 Ischemic pain

104. Definitions : type of deep pain felt in
muscles when their blood supply is
reduced
Example : angina pectoris & intermittent
claudicating
Causes
Increase metabolites and proteolytic
enzymes.

105. Pain from viscera ( peritoneum , pleura
and pericardium )
C – fiber pain (poorly localized)
A delta fibers stimulated if from parital
layers of viscera
Most viscera contain only few pain
receptors
Sharp cut doesn't cause pain ( injury of few
nerve endings )
Diffuse stimulation cause severe pain

106. Causes
a) Over distension
b) Spasm
c) Ischemia
d) Inflammation and irritation
e) Infiltration by tumors

107. Characters
a) Dull aching
b) Poorly localized
c) Autonomic responses : decrease heart
rate and ABP
d) Guarding (Rigidity)
e) Referred

108. Pain due to nerve fiber damage
Examples :
a) Trigeminal neuralgia
b) Sciatica
c) Herpes zester
d) Diabetic neuropathy

109. Definition: pain not felt in diseased
viscus and felt in corresponding
dermatome
Examples:
Cardiac : left shoulder
Gall bladder : Right shoulder
Appendix : at umbilicus
Gastric : above umbilicus
Kidney & pancreas : back

110. Convergence projection theory
 Afferent pain fiber from the skin and viscus converge on
the same cell of the ( SGR ) that will finally activate the
same cortical neuron
 Whatever may be the source of pain , the cortex will
project to a skin area as the skin commonest source of
pain due to
- Skin is richer in pain receptors
- Skin is more exposed to stimulation
- Skin is topographically represented in the cortex while
viscera are not

111. .1Convergence Projection theory

112. Afferent from pain fibers of diseased
viscera, give subliminal fring to a nearby
SGR (receiving afferents from an area of
skin), increasing its excitability.
The result is pain felt in this skin area and
lowering of its pain threshold

113. 2. Facilitation theory
Viscus

114. • Various types of sensory abnormalities can
occur when the sensory pathways are
damaged
• Sensory loss, altered sensations or pain
could occur as a result
• In addition, motor pathways could also be
affected resulting in motor weakness

115. • Hypothesia
• Anaesthesia
absence of sensation
• Paraesthesia (numbness or pins-needles-
sensation) altered sensation
• Hemianaesthesia: Loss of sensation of one half of
the body
• Neuropathic pain
• Analgesia
• hyperalgesia
• Spatial neglect

116. Primary hyperalgesia
Secondary hyperalgesia

117. • Peripheral nerve
innervated area affected
• Roots
dermatomal pattern of sensory loss
• Spinal cord
a sensorylevel
• Internal capsule
one half of the body
• Cortical areas
Other features

118. •Polyneuropathy
All sensory nerves of both upper and
lower limbs are degenerated
Numbness of hands and feet
Glove and stocking type of sensory
loss
Diabetic or nutritional neuropathy

119. • Sensory stroke
 Internal capsule lesion
 Numbness and sensory loss of one side of the body

120. • Dorsal column sensations are affected
• Vibration, proprioception affected early in disease process
• Sensory ataxia
• Romberg sign

121. Spinal cord central canal lesion
Dissociated sensory loss
Temperature and pain sensations affected in
early in disease process and crude touch
affected later.
Touch and dorsal column functions not
affected

122. Posterior (dorsal)
Dorsal root ganglion
Dorsal root
Dorsal columns
Dorsal horn
Spinothalamic
tracts
Anterior (ventral)

123. Dorsal column pathway
Lateral
Spinothalamic
tract
Anterior
Spinothalamic
tract

124. At the level of lesion
loss of all sensations supplied by the affected
dorsal roots
Below level of lesion
On the same side :loss of dorsal column sensations
On the opposite side: loss of spinothalamic tract
sensations

125. Dorsal column pathway
Lateral
Spinothalamic
tract
Anterior
Spinothalamic
tract

126. What are the characters of thermal
receptors?
What is the mechanism of thermoceptive
stimulation?
What are the types of pain receptors?
Do you know the pain insensitive
structures?
What are the types of pain sensations?

127. What is the neuropathic pain?
Give example to neuropathic pain?
What are causes of visceral pain?
Mention the characters of visceral pain
Explain mechanisms of referred pain?
Enumerate some examples of referred
pain?

129. 17

130. • Pacinian corpuscle
• Meissner’s corpuscle
• Krause’s corpuscle
• Ruffini’s end organ
• Merkel’s disc
• Hair end organ
• Free nerve endings

131. • Pacinian corpuscle
deep, pressure sensitive, fast adapting, large receptive field
• Meissner’s corpuscle
superficial, sensitive to touch, small receptive field
• Ruffini’s end organ
deep, tension sensitive, slow adapting, large receptive field
• Merkel’s disc
superficial, touch, pressure and texture sensitive, slowly
adapting, small receptive field
• Krause’s endings
vibration sensitive

132. • Hair end organ
• Free nerve endings
Crude mechanosensations
(Pain, temperature)

136. Pacinian
corpuscles
looks like onion, large receptive field, rapidly
adapting
Hair follicle
receptor
nerve endings around root of hair in hairy skin,
small receptive field, either slowly or rapidly
adapting
Ruffini's
ending
looks like small Pacinian, large receptive fields,
slowly adapting
small arrays of small disks which may have
Merkel's diskssynapses to nerve endings, small receptive fields,
slowly adapting
Meissner's
corpuscles
hang under ridges of glabrous skin, small
receptive fields, rapidly adapting
Krause end
bulbs
look like knotted balls of string in skin in border
between dry skin and mucous membrane in
mouth, genitals, anus

137. Summation
• Spatial
• Temporal
Relaying of signals through neuronal pools
• Threshold---Subthreshold stimuli
• Excitation or Facilitation
Divergence and Convergence of signals passing through
neuronal pool
After Discharge
• Synaptic after discharge
• Reverberatory circuit
• Continuous signal output
• Rhythmical signal output