05 The Somatosensory System
Psychology 355
Introduction
Somatic Sensation
A.
Touch, sharp pain, ache, chill/burn
B.
Proprioception: Body position, location of
limbs. Interacts with vestibular sense and
vision; interacts with touch for “haptics”.
Baroreceptors.
C.
Somatic sensory system: Different from other
systems: several sense systems
1.
Receptors: Distributed throughout body
2.
Responds to widest variety of stimulus types
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Touch
Types and layers of skin
A.
Hairy and glabrous (hairless)
B.
Epidermis (outer) and dermis (inner)
I.
Functions of skin
A.
Protective function
B.
Prevents evaporation of body fluids
C.
Provides direct contact with world
II. Mechanoreceptors
A.
Most somatosensory receptors are
mechanoreceptors
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Touch
Mechanoreceptors
A.
Pacinian corpuscles
B.
Ruffini's endings
C.
Meissner's corpuscles
D.
Merkel's disks
Free nerve endings
Krause end bulbs
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Touch
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Touch
Mechanoreceptors
Åke Vallbo and colleagues
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Touch
Mechanoreceptors
A.
Åke Vallbo and colleagues
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Touch
Mechanoreceptors
Two Point Threshold
Braille letters consist of
dots 2.5 mm apart and
can be read at ~600
characters / minute.
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Touch
Mechanoreceptors
I.
II.
III.
IV.
Punctate
Diffuse
RA
RA-Punctate
Meissner Corpuscles
RA-Diffuse
Pacinian Corpuscles
SA
SA-Punctate
Merkel Disks
SA-Diffuse
Ruffini Endings
Rapidly Adapting (RA) -respond to changes in
stimulation, but do not continue to respond to constant
stimulation
Slowly Adapting (SA) -respond to constant stimulation
Punctate - small receptive fields with distinct
boundaries
Diffuse - large receptive fields with non-distinct
boundaries
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Touch
Mechanoreceptors
I.
Meissner Corpuscles (RA-punctate) responds best to
active touch involved in object exploration; texture
II. Pacinian Corpuscles (RA-diffuse) extremely sensitive
over a large receptive field to high frequency changes
in pressure; its membrane is deformed – e.g., a
breeze on the palm of your hand; vibrations
III. Merkel Disks (SA-punctate) constant sources of
stimulation over a small area – e.g., carrying a pebble
IV. Ruffini Endings (SA-diffuse) constant stimulation over
a larger area – e.g., the feeling of clothing; skin
stretch
V. Free nerve endings - pain fibers & thermal
conductance fibers
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Touch
Mechanoreceptors
Receptive field size and adaptation rate
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Touch
Mechanoreceptors
adaptation rate: Pacinian Corpuscle
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Touch
Primary Afferent Axons
A.
Aa, Proprioception
B.
Ab, Touch
C.
Ad, Fast Pain,
temperature
D.
C, Slow Pain,
Temperature, Itch
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Touch
Primary Afferent Axons
Enter spinal cord
through the dorsal
root
Dorsal Root
Dorsal Root
Ganglion Cell
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Touch
I.
The Spinal cord
A.
Spinal segments (30)- spinal nerves within 4
divisions of spinal cord
B.
Dermatomes- 1-to-1 correspondence with
segments with overlap.
What happens when a dorsal root is cut?
Shingles
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Touch
The Spinal cord
Sensory Organization
I.
II.
III.
IV.
Cervical (C)
Thoracic (T)
Lumbar (L)
Sacral (S)
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Touch
The Spinal cord
Sensory Organization
I.
Cervical (C)
II.
Thoracic (T)
III.
Lumbar (L)
IV.
Sacral (S)
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Touch
The Spinal cord
Sensory Organization
I.
Cervical (C)
II.
Thoracic (T)
III.
Lumbar (L)
IV.
Sacral (S)
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Touch
Division of spinal gray matter:
1. Dorsal horn
Myelinated Ab axons (mechano)
2. Intermediate zone
3. Ventral horn
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Cerebral
Cortex
Touch
Dorsal Column–
Medial Lemniscal Pathway
Ipsilateral without
synapses along spinal cord
Touch information ascends
through dorsal column,
dorsal nuclei,
medial lemniscus, and
ventral posterior nucleus
to primary somatosensory cortex
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Touch
Dorsal Column–
Medial Lemniscal Pathway
dorsal column,
dorsal nuclei,
medial lemniscus, and
ventral posterior nucleus
to primary somatosensory cortex
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Touch
The Trigeminal Touch Pathway
A.
Trigeminal nerves: Face, mouth, tongue
(cranial V)
B.
Other cranial nerves (throat and abdominal
cavity: cranial IX, X)
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Touch
Somatosensory Cortex
A.
Primary: BA 3b
B.
Other areas
1.
Postcentral gyrus
BA 1, 2
2.
Posterior Parietal
Cortex
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Touch
Somatosensory Cortex
A.
Brodmann’s Area 3b (or S1): Primary
somatosensory cortex
1.
Receives dense input from VP nucleus of the
thalamus
2.
Neurons: Responsive to stimuli
3.
Lesions impair somatic sensations
4.
Electrical stimulation evokes sensory
experiences
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Touch
Somatosensory Cortex : 3b
Columnar Organization
Somatotopic Organization
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Touch
Somatosensory Cortex
Cortical Somatotopy
1.
Homunculus
2.
Importance of mouth
i.
Tactile sensations: Important for speech
ii.
Lips and tongue: Last line of defense
iii. Infants and non-primate mammals touch
primarily with their mouths
3.
Fingers and Thumb
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Touch
Somatosensory Cortex
Cortical Somatotopy
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Touch
Somatosensory Cortex
Cortical Magnification
The receptive
fields and cortical
representations
give more acuity
to fingers, mouth,
nose and tongue
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Touch
Somatosensory Cortex
S1: Rat
1.
Vibrissae
2.
“Barrel cortex”
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Touch
Somatosensory Cortex
S1: Rat
1.
Vibrissae
2.
“Barrel cortex”
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Touch
Somatosensory
Cortex
S1 –
Owl monkey
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Touch
Somatosensory Cortex
A.
Cortical Map Plasticity
B.
Remove digits or overstimulate – examine
somatotopy before and after
1.
Conclusions of experiments
Reorganization of cortical maps
a.
b.
Dynamic
Adjust depending on the amount of
sensory experience
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Touch
Somatosensory Cortex
A.
Cortical Map Plasticity
B.
Remove digits or
overstimulate –
examine somatotopy
before and after
1.
Conclusions of experiments
Reorganization of
cortical maps
a.
Dynamic
b.
Adjust depending
on the amount
of sensory
experience
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Touch
Somatosensory Cortex
Squirrel Monkey
Cortical Map
Plasticity
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Touch
The Posterior Parietal Cortex
1.
Involved in somatic sensation, visual stimuli,
and movement planning
2.
Astereoagnosia
3.
Neglect syndrome
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Pain
Nociception: No pain, no gain? Hansen’s Disease.
A.
Pain - feeling of sore, aching, throbbing
B.
Nociception - sensory process, provides signals
that trigger pain
1.
Mechanical
2.
Thermal
3.
Chemical
4.
Polymodal
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Pain
Transduction of Pain
A.
Mechanically-gated ion channels
B.
Damage to cells release proteases (an enzyme which
digests proteins) which cause kaninogen to break
down to form bradykinin which binds to certain
nociceptors:
Hyperaglesia:
Bradykinin – increases the sensitivity of nociceptors and
thermoreceptors
Prostaglandins – caused by enzymatic breakdown of
membrane lipids – cause increased sensitivity of
nociceptors
Substance P – Released by nociceptors - Causes swelling &
Mast cell activation: release of histamine
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Pain
I.
Primary Afferents and Spinal mechanisms
A.
First pain and second pain
B.
Referred pain: Angina
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Pain
Ascending Pain Pathways
Differences between touch and pain pathway
1.
Nerve endings in the skin
2.
Diameter of axons
3.
Connections in spinal cord
i.
Touch – Ascends Ipsilaterally
ii.
Pain – Ascends Contralaterally
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Pain
Ascending Pain Pathways
Spinothalamic Pain Pathway
Dorsal Root
– immediactly decussates
Spinothalamic Tract
Spinothalamic tract
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Pain
Ascending Pain Pathways
Spinothalamic Pain Pathway
Dorsal Root
– immediactly decussates
Spinothalamic Tract
Spinothalamic tract
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Pain
Comparing
Pathways
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Pain
Reflex Arc
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Pain
I.
The Regulation of Pain
A.
Afferent Regulation
B.
Descending Regulation
C.
The endogenuos opiates
1.
Opioids and endomorphins
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Pain
Periaquaductal gray matter
The Regulation
of Pain
Descending
regulation
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Phantom Pain
I.
After surgical removal of a limb,
sensations resume in the limb
II. In 90% of patients, the sensations
are very painful
III. In 60% the pain is excruciating:
described sometimes as an arm on
fire, being torn or punctured, great
pressure
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Phantom Pain: Strange Facts
I. Stimulating certain areas of skin (e.g., face)
may aggrevate phantom pain.
II.Severing the nerve doesn’t help. Blocking the
nerve doesn’t help. Removing the portion of the
thalamus that relays the information to the brain
doesn’t help
III.Stimulating the nerve does help. Electric or
manual stimulation of the stump helps
tremendously electric more so).
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Phantom Pain: A Theory
I. Recall that the cortex is plastic and may
reorganize.
II. Normally this involves annexing juvenile or
unused neurons (indicated by low activity
level)
III.In amputation the entire area of say an arm
is no longer active in the brain
IV. Other areas attempt to annex these neurons
V. Because the neurons already had a
specialization (e.g., sharp pain) and are no
being stimulated by adjacent areas of
cortex, the subject feels pain.
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Phantom Pain
The cortical areas for the face annex
the cortical areas for the arm and
fingers.
Some of those neurons were
previously specialized for pain.
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Temperature
Thermoreceptors
I.
A uniquely tactile object property
II.
The rate at which heat is gained or lost between the skin
and an object - we do not detect absolute temperature
III. Metal objects, fluids etc. create a more extreme
sensation of temperature than do other objects (despite
no differences in absolute temperature) because heat
energy is transferred more easily to and from them
IV. If a metal and a wooden block are both 150°, the metal
block will feel hotter than the wooden block.Likewise
for the same blocks at 0° the metal block will feel colder
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Temperature
The Temperature Pathway
A.
Organization of temperature pathway
Identical to pain pathway
B.
Cold receptors coupled to Ad and C
C.
Hot receptors coupled to C
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Concluding Remarks
I.
Sensory systems exhibit similar organization and
function
II. Sensory types are segregated within the spinal
cord and cerebral cortex
III. Repeated themes
Parallel processing of information
Columnar Organization
Cortical Magnification
IV. Perception of objects (haptics) involves the
seamless coordination of somatic sensory
information
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End of Presentation
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