2. Sensory functions
• Humans do not have receptor for every
possible stimuli
• There are different sensory modalities that
human brain can perceive
• Arrival of information is sensation
4. Modality specificity
• 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
5. Sensory pathway
• Once a receptor is stimulated
impulse travels through a particular pathway
known as sensory pathway or ascending pathway
up to the brain
7. Receptors
• 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
10. Cutaneous mechanoreceptors
• Pacinian corpuscle
• Meissner’s corpuscle
• Krause’s corpuscle
• Ruffini’s end organ
• Merkel’s disc
• Hair end organ
• Free nerve endings
11. Mechanoreceptors
• 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
16. 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
Merkel's disks
small arrays of small disks which may have
synapses 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
18. What happens inside a receptor?
• 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
• At the brain opposite happens in order to feel
the sensation
PERCEPTION
19. Receptor potentials
• When a stimulus activate a receptor initially a
“receptor potential” is generated
• This is also called “generator potential”
• This is a graded potential
• It does not follow “all-or-none law”
• Its amplitude depends on the strength of the stimulus
22. Coding of sensory stimuli
• Stimulus strength is coded as the frequency of
AP
• Higher the stimulus more frequent are the APs
• Amplitude of AP is constant
24. Sensory coding
• 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
25. Transduction in different receptors
• Different receptors have different ion channels
• Their opening causes receptor potential
34. Adaptation
• “getting used to”
• after a period of time sensory receptors adapt
partially or
completely
• different types
Rapidly adapting receptors
slowly adapting receptors
35. Adaptation
• after a period of time sensory receptors adapt
partially or completely
• different types
fast adapting receptors
slowly adapting receptors
37. Mechanism of adaptation
• In the Pacinian corpuscle
mechanical deformation is transmitted throughout
the capsule and pressure redistributes
Na+ channels inactivates after some time
41. Two ascending pathways
• Dorsal column - medial lemniscus pathway
fast pathway
• Spinothalamic pathway
slow pathway
These two pathways come together at the level of thalamus
44. Dorsal column pathway Spinothalamic pathway
• touch: fine degree
• highly localised touch
sensations
• vibratory sensations
• sensations signalling
movement
• position sense
• pressure: fine degree
• Pain
• Thermal sensations
• Crude touch & pressure
• crude localising sensations
• tickle & itch
• sexual sensations
45. Dorsal column nuclei
(cuneate & gracile nucleus)
Dorsal column
Medial lemniscus
thalamus
thalamocortical tracts
sensorycortex
internal capsule
1st
order
neuron
2nd
order
neuron
3rd
order
neuron
46. dorsal column - medial lemniscus pathway
• 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
47. dorsal column - medial lemniscus pathway
• 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
48. dorsal column - medial lemniscus pathway
• spatial orientation in the thalamus
medial: upper part of the body
lateral: lower part of the body
49. Dorsal column nuclei
(cuneate & gracile nucleus)
Dorsal column
Medial lemniscus
thalamus
thalamocortical tracts
sensorycortex
internal capsule
1st
order
neuron
2nd
order
neuron
3rd
order
neuron
50. spinothalamic pathway
• 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
54. Thalamocortical tracts
• from the thalamus 3rd order neuron ascends up
through the internal capsule
• up to the sensory cortex
• thalamocortical radiation
tracts diverge
61. Sensory cortex
• Different areas of the body are represented
in different cortical areas in the sensory
cortex
• Sensory homunculus
somatotopic representation
not proportionate
distorted map
upside down map
65. Sensory cortical areas
• Primary somatosensory cortex (SI)
postcentral gyrus
(Brodmann areas 3a, 3b, 1, 2)
• Secondary somatosensory cortex and
Somatosensory association cortex
Posterior parietal areas
(Brodmann areas 5, 7)
66. Somatosensory cortex
•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
67. Somatosensory cortex
•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)
68. Secondary somatosensory cortex and
Somatosensory association cortex
• 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
69. Secondary somatosensory cortex and
Somatosensory association cortex
• 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. Receptive fields
• The receptor area which when stimulated
results in a response of a particular sensory
neuron
• Receptive fields of adjacent neurons overlap
71.
72. Two-Point Discrimination
• Whether a stimulus feels like one sensation or two
distinct sensations depends on the size of the
receptive fields of the sensory receptors
• Different areas of the body have sensory receptors
with different sized receptive fields
• Smaller receptive fields result in greater sensitivity
• Fingers are more sensitive than backs
73.
74. Lateral Inhibition
• The capacity of an excited neuron to reduce the activity of its
neighbors
• When the skin is touched by an object
several sensory neurons in the skin next to one another are stimulated
neurons that are firing suppress the stimulation of neighbouring neurons
only the neurons that are most stimulated and least inhibited will fire
so the firing pattern tends to concentrate at stimulus peaks
• Lateral inhibition increases the contrast and sharpness
• Weaker signals get weaker, stronger signals get stronger
77. Sensory abnormalities
• 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
78. Types of sensory abnormalities
• Sensory loss
• Anaesthesia
absence of sensation
• Paraesthesia (numbness or pins-needles-
sensation)
altered sensation
• Neuropathic pain
• Hemianaesthesia
Loss of sensation of one half of the body
• Astereognosis
• Spatial neglect
79. Localisation of the abnormality
• Peripheral nerve
innervated area affected
• Roots
dermatomal pattern of sensory loss
• Spinal cord
a sensory level
• Internal capsule
one half of the body
• Cortical areas
Other features
80. Examples of sensory lesions or
sensory disorders
•Carpal tunnel syndrome
Median nerve lesion at the wrist
Numbness of thumb, index and middle
fingers
Pain in the hand
Pain could radiate
upwards
81. Examples of sensory lesions or
sensory disorders
•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
82. Examples of sensory lesions or
sensory disorders
• Cervical radiculopathy
Cervical root lesion
Compression of nerve root
as it comes out through
intervertebral foramina
Numbness and sensory loss
of relevant dermatomes
Commonly affected are C56
dermatomes
83. Examples of sensory lesions or
sensory disorders
• Spinal cord lesion (cervical myelopathy)
• Damage to the spinal cord
• Sensory loss or numbness below the
level of the spinal cord lesion
• eg. Sensory loss at T10
84. Examples of sensory lesions or
sensory disorders
• Sensory stroke
Internal capsule lesion
Numbness and sensory loss of one side of the body
85. Examples of sensory lesions or
sensory disorders
• Dorsal column disease (eg. Diabetes, tabes dorsalis)
• Dorsal column pathways are affected
• Vibration, proprioception affected early in disease process
86. Examples of sensory lesions or
sensory disorders
• Syringomyelia
Spinal cord central canal lesion
Dissociated sensory loss
Temperature and pain sensations affected in early in disease process
Touch and dorsal column functions not affected
