Transcript Size of the receptive field

Mapping the human somatosensory cortex – the sensory
homunculus – perception of touch, temperature, pain,
proprioception, kinesthetics, haptics, sexual sensation,
tickle, & itch
The model represents
what the body would look
like (in this case a male
body) if it grew in
proportion to the area of
the cortex that was
dedicated to its sensory
perception
Big Picture of the Somatosensory
Systems: types of “sensing”
•
•
•
•
Cutaneous – related to skin
Tactile – related to touch
Haptic – related to search
Sensations:
– Discriminative touch (haptic search)
– Touch related to the skin
– Pressure and vibration
Big Picture of the Somatosensory
Systems: types of “sensing”
• Kinesthesis:
– Sensation “below the surface”
– The sense of position or location in space of the limbs
and body parts
– The sense of movement of the limbs and body parts
– Muscle stretch, joint movement and tendon tension
• Proprioception:
– All of the above
– But, integrated with information from the vestibular
system (Balance)
Big Picture of the Somatosensory
Systems: types of “sensing”
• Thermoreception & pain:
– Temperature
– Pain
– Tickle and itch
Cutaneous sensation – stimulation of receptors
in the skin:
The largest and the heaviest organ of the body
Protection from bacteria & disease
Maintains the integrity of our body (contains our
fluids)
Informs us about our immediate surroundings
(touch)
Cross-section of skin
“Mechanoreceptors” process different types of tactile,
proprioceptive and kinethetic stimulation
* Located in the Epidermis & Dermis
* Mechanical (physical) change of the outer cells
activate internal nerve endings (transduction
from mechanical to neural impulse)
Cross-section of skin
Slow Adapting (SA)
Rapid Adapting (RA)
Firing
Stimulus
on
off
on
off
Meissner corpuscle
Merkel receptor
Ruffini cylinder
Pacinian corpuscle
•Mechanoreceptors – Merkel receptor: located between
Epidermis & Dermis
•Sensitive to ongoing pressure on the skin, particularly where
stimulation is “tonic” (ongoing, maintained) such as in the
fingertips; used in processing pressure.
•Action potential caused by deformation of myelinated nerve
ending
•Slow Adapting (SA1) with small receptive field; fires in
“continuous stimulation”
Copyright © 2002 Wadsworth Group. Wadsworth is an imprint of the
Wadsworth Group, a division of Thomson Learning
Fine detail of touch is processed by Merkel
receptors (slow adapting)
•Mechanoreceptors – Meissner corpuscle: tactile corpuscle
located in Dermis just below the Epidermis
•Sensitive to light touch, “flutter” (vibration) or tapping on the
skin: fingertips, palms, soles, lips, tongue, face, nipples,
genitalia
•Action potential caused by deformation and reformation of
unmyelinated nerve ending
•Rapid Adapting (RA1) with small receptive field; fires in “onoff” bursts
•Mechanoreceptors – Ruffini Cylinder: located in Dermis
•Sensitive to stretching of the skin or movements of the joints
•Action potential caused by deformation of the nerve ending
•Slow Adapting (SA2) with large receptive field; fires in
continuous stimulation
• Mechanoreceptors – Pacinian corpuscle: located
deeper in the dermis, near joints, on internal organs
• Sensitive to deep & diffuse pressure & vibrations
• Action potential caused by deformation and
reformation of unmyelinated nerve ending that are in
the middle of the corpuscle (outer layered
mechanism)
Pacinian corpuscle
• Rapid Adapting (RA2) with large receptive field;
fires in “on-off” bursts
• “Graded response” – small or greater deformations
associated with smaller/greater response
• The larger the deformation/reformation the higher
the firing frequency of the neuron
Surface receptor – small receptive field
RA
Meissner: fine vibration –
recognize texture
Hair: responds to a
localized “flutter”
SA
Merkel: sensing fine
details of a surface or
edge
Deep receptor – large receptive field
Pacinian: a diffuse
vibration (holding a
power saw) or deep
pressure (when firmly
grabbed and
released)
Ruffini: a skin
stretch (sensing
the position of
your fingers)
Thermoreceptors & Temperature
• Receptors
– Warm fiber
– Cold fiber
• Rate of neural firing determines the
degree of increased or decreased
temperature
• Firing rate continues as long as the
stimulus is present
Cold fibers fire “best” to 30-deg (range
of 20-to-45-deg C)
Impulses per second
15
10
Warm fibers fire “best” to 44-deg (range
of 30-to-48-deg C)
(remember: body temperature is 37deg C)
Warm fibers
Cold fibers
5
20
25
30
35
40
Temperature (deg-C)
45
50
Tactile Acuity: Merkel receptors account for much of the
information that is disproportionately represented in the
cortex
Two-point threshold
test: what is the
smallest distance
between two
different points on
the body that can be
discriminated as
different?
Compare your:
Finger tips, Lips &
Side of the hip
Size of the
“receptive field”
correlates with the
cortical
magnification factor
Size of the receptive field determines fineness of acuity
(i) Large fields more insensitive: near touch more likely to
activate same receptive field; (ii) Small fields more sensitive:
near touch more likely to activate different receptive fields
neural firing
from 1 receptive
field
no response
neural firing from
both receptive field
Notice that the size of the receptive field correlates with the
receptors depth in the skin
Fast Adapting
Slow Adapting
Haptic Perception: Three-dimensional
exploration of objects with the hands
• Haptic Perception: information-knowledge
acquired from sensations in skin, muscles, tendons
& joints involving “active exploration”
• Active v. passive touch: whose doing the touching?
You on the world or the world on you.
– Passive touch: “I feel a pricking sensation on my skin”
– Active search: “I feel a pointed object”
Human Haptics: perception of 3-D
objects with the hands
Haptic feedback is comprised of both tactile
(touch) and force (pressure) that is always
sensed when physically interacting with the
hands
Lateral motion
Pressure
Enclosure
Static contact
Unsupported Holding
Contour Following
Texture
Hardness
Global shape
Temperature
Weight
Precision shape
Factors contributing to specificity of
haptic information processing
• Type of stimulation and receptors’
specialization for processing that information
– “Object Affordance” – what an object “tells you”
• Size of the receptive field (small field more
sensitive)
Examples of Exploratory Procedures (EPs)
Exploratory
Procedures (EPs) of
active touch
(Lederman &
Klatzky, 1987) –
usually only use 1 or
2 methods to explore
an object.
Object recognition, mechanoreceptors and
cortical organization
How long would it take you to “recognize” these objects in
your hands?
Factors contributing to specificity of
haptic information processing
• Cortical magnification factor (the homunculus)
– Approximately 10 different “maps” as a function of
different types of information
– Form v. texture v. direction of movement, etc.
represented by different homunculi
– Cortical cells are similar in action to visual simple
and complex cells (showing sensitivity to edges,
ends of stimuli, etc.)
Factors contributing to specificity of
haptic information processing
• Neurons organized with a Center-Surround
onset-offset organization (also occurs in many
areas not involved with haptics)
– Throughout the thalamus and somatosensory cortex
Object recognition, mechanoreceptors and
cortical organization
• Many sources of information are coming from
various receptors in time-space synchrony
• Texture, hardness, orientation signaled by
different receptors & receptive fields,
differing CNS pathways, differing mid-brain &
cortical receptive areas, etc. in simultaneous
orchestration
• Typically, most people can recognize most
objects in a matter of one or two seconds
Tracts from receptors through the Central
Nervous System (CNS)
• Lemniscal System (dorsal-column mediallemniscal pathway)
– Phasic sensations (recurring cycle; vibrations)
– Sensations of movement against the skin
– Fine positional and pressure sensations
• Proprioception
– Touch sensations
• High degree of localization and specificity of stimuli
• Fine graduations in intensity of stimuli
lemniscal system pathway
•
Begins with somatosensory axons
entering the spinal cord via the dorsal
root
•
Ascends in the dorsal columns
ipsilaterally
•
First synapse point for this pathway is
in the dorsal column nuclei located
in the medulla
•
Axons of neurons originating in the
dorsal column nuclei decussate
(cross over) to contralateral side
•
Axon then ascends via the Medial
Lemniscus to the contralateral
ventral posterior thalamic nucleus
(VPN)
•
The majority of VPN neurons project
to the primary somatosensory
cortex (S1)
•
Remaining neurons project to the
secondary somatosensory cortex
(S2) of the posterior parietal lobe
Summary: lemniscal system pathway
(in your book: dorsal-column-medial-lemniscal (DCML) pathway)
• Secondary somatosensory
cortex (S2)
• Primary somatosensory
cortex (S1)
• Ventral posterior thalamic
nucleus (VPN)
• Dorsal column nuclei in
medulla
• Dorsal Root ganglion of
spinal cord
The somatosensory map on the cortex – the “homunculus”
The somatosensory map on the cortex
– the sensory “homunculus”
Tracts from receptors through the Central
Nervous System (CNS)
• Spinothalamic System
– Thermal sensations: cold and warm
– Pain sensations
– Crude (diffuse) pressure and touch
sensations
– Tickle and itch sensations
– Sexual sensations
Spinothalamic System Pathway
• Somatosensory axons
entering the spinal cord via the
dorsal root and synapsing
upon entry
• The majority of these 2ndorder axons decussate, and
ascend to the brain via the
anterolateral portion of the
spinal cord white matter
• This ascending system is
composed of three separate
tracts:
– spinothalamic tract
– spinoreticular tract
– spinotectal tract
Spinothalamic System Pathway
•
The spinothalamic tract projects to the
ventral posterior nucleus of the thalamus
– This tract is involved in the perception of
touch, temperature, and sharp pain
•
The spinoreticular tract projects to the
brain stem reticular formation on its way
to the parafasicular nucleus and
intralaminar nucleus of the thalamus
– This pathway seems to be selectively
involved in the perception of deep,
chronic pain
•
The spinotectal tract projects to the
tectum of midbrain
– This tract is likely involved in some aspect
of pain perception (not yet understood)
•
The tracts of the Spinothalamic System
project to both the primary and
secondary somatosensory cortex, and to
more posterior locations within the
parietal lobe
Object recognition, mechanoreceptors and
cortical organization
• Many sources of information are coming from
various receptors in time-space synchrony
• Texture, hardness, orientation signaled by
different receptors & receptive fields,
differing CNS pathways, differing mid-brain &
cortical receptive areas, etc. in simultaneous
orchestration
• Typically, most people can recognize most
objects in a matter of one or two seconds
Perception of pain
• Pain defined (International Association for
the Study of Pain):
– “Pain is an unpleasant sensory and emotional
experience associated with actual or potential
tissue damage, or described in terms of such
damage (Merskey, 1991)”
• Cannot be fully explained by physical
dimensions alone (sensory & emotional)
– Sensory experience: throbbing, hot, sharp, dull
– Emotional experience: annoying, torturing,
excruciating
Perception of pain
• Proof of the multimodal nature of pain? Clinicians’
treatment of pain symptoms
– Cutting nerve fibers in pain pathways from receptors to
higher cortical levels do NOT eliminate pain perception
– Best evidence: phantom limb findings
• Cognitive factors in pain perception
– Expectations – preparation helps
– Shifting attention – distraction from pain
– Content of emotional distraction – thinking “peaceful
thoughts”
– Individual differences in pain perception – differences in
tolerance and experience
Perception of pain: Melzack & Wall’s “Gate Control
Theory”
• Pain perception can be affected by psychological
dimensions (i.e., expectations) and competing stimulation of
pain pathways (i.e., simultaneous stimulation of cutaneous
receptors)
• Two types of fibers in the Dorsal Root Ganglion of the spinal
cord
– Short fibers carry pain information
– Long fibers carry tactile/non-pain information
– Activation of long fibers competes with activity of short fibers (hence,
diminishing perception of pain)
• Pain perception pathways far more complex than first
described by Melzack & Wall, but central thesis remains
basic: competing gating mechanisms