Psychology 355
05 The Somatosensory System
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Introduction
Somatic SensationA. Touch, sharp pain, ache, chill/burnB. 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 body2. Responds to widest variety of stimulus types
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Touch
Types and layers of skinA. Hairy and glabrous (hairless)B. Epidermis (outer) and dermis (inner)
I. Functions of skinA. Protective functionB. Prevents evaporation of body fluidsC. Provides direct contact with world
II. MechanoreceptorsA. Most somatosensory receptors are
mechanoreceptors
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Touch
Mechanoreceptors A. Pacinian corpusclesB. Ruffini's endingsC. Meissner's corpuscles D. Merkel's disks
Free nerve endingsKrause end bulbs
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Touch
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MechanoreceptorsÅke Vallbo and colleagues
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Touch
MechanoreceptorsA. Åke Vallbo and colleagues
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Touch
MechanoreceptorsTwo Point Threshold
Braille letters consist of dots 2.5 mm apart andcan be read at ~600characters / minute.
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Mechanoreceptors
I. Rapidly Adapting (RA) -respond to changes in stimulation, but do not continue to respond to constant stimulation
II. Slowly Adapting (SA) -respond to constant stimulationIII. Punctate - small receptive fields with distinct boundariesIV. Diffuse - large receptive fields with non-distinct boundaries
Punctate Diffuse
RA RA-PunctateMeissner Corpuscles
RA-DiffusePacinian Corpuscles
SA SA-PunctateMerkel Disks
SA-DiffuseRuffini Endings
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Touch
MechanoreceptorsI. Meissner Corpuscles (RA-punctate) responds best to
active touch involved in object exploration; textureII. 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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MechanoreceptorsReceptive field size and adaptation rate
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Mechanoreceptorsadaptation rate: Pacinian Corpuscle
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Touch
Primary Afferent AxonsA. AProprioception
B. ATouch
C. AFast Pain, temperature
D. C, Slow Pain, Temperature, Itch
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Touch
Primary Afferent AxonsEnter spinal cord through the dorsal root
Dorsal Root
Dorsal RootGanglion Cell
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I. The Spinal cordA. Spinal segments (30)- spinal nerves within 4
divisions of spinal cordB. Dermatomes- 1-to-1 correspondence with
segments with overlap.What happens when a dorsal root is cut?Shingles
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The Spinal cord Sensory Organization
I. Cervical (C)II. Thoracic (T)III. Lumbar (L)IV. Sacral (S)
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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 A axons (mechano)
2. Intermediate zone 3. Ventral horn
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Touch
Dorsal Column–Medial Lemniscal PathwayIpsilateral without synapses along spinal cord
Touch information ascends through dorsal column, dorsal nuclei, medial lemniscus, and ventral posterior nucleus
to primary somatosensory cortex
Cerebral 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 PathwayA. Trigeminal nerves: Face, mouth, tongue
(cranial V)B. Other cranial nerves (throat and abdominal
cavity: cranial IX, X)
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Touch
Somatosensory CortexA. Primary: BA 3bB. Other areas
1. Postcentral gyrusBA 1, 2
2. Posterior ParietalCortex
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Somatosensory Cortex A. Brodmann’s Area 3b (or S1): Primary
somatosensory cortex1. Receives dense input from VP nucleus of the
thalamus2. Neurons: Responsive to stimuli3. Lesions impair somatic sensations4. Electrical stimulation evokes sensory
experiences
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Somatosensory Cortex : 3bColumnar OrganizationSomatotopic Organization
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Somatosensory CortexCortical Somatotopy
1. Homunculus2. Importance of mouth
i. Tactile sensations: Important for speechii. Lips and tongue: Last line of defenseiii. Infants and non-primate mammals touch
primarily with their mouths3. Fingers and Thumb
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Somatosensory CortexCortical Somatotopy
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Somatosensory CortexCortical Magnification
The receptive fields and cortical representations give more acuity to fingers, mouth, nose and tongue
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Somatosensory CortexS1: Rat
1. Vibrissae2. “Barrel cortex”
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Somatosensory CortexS1: Rat
1. Vibrissae2. “Barrel cortex”
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Somatosensory Cortex
S1 – Owl monkey
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Somatosensory Cortex A. Cortical Map PlasticityB. Remove digits or overstimulate – examine
somatotopy before and after1. Conclusions of experiments
Reorganization of cortical maps
a. Dynamic
b. Adjust depending on the amount of sensory experience
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TouchSomatosensory Cortex
A. Cortical Map PlasticityB. Remove digits or
overstimulate – examine somatotopy before and after
1. Conclusions of experimentsReorganization of
cortical maps
a. Dynamic
b. Adjust depending on the amount of sensory experience
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Touch
Somatosensory CortexSquirrel Monkey
Cortical Map Plasticity
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Touch
The Posterior Parietal Cortex1. Involved in somatic sensation, visual stimuli,
and movement planning2. 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 pain1. Mechanical2. Thermal3. Chemical4. Polymodal
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PainTransduction of Pain
A. Mechanically-gated ion channelsB. 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
thermoreceptorsProstaglandins – 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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I. Primary Afferents and Spinal mechanismsA. First pain and second painB. Referred pain: Angina
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Ascending Pain PathwaysDifferences between touch and pain pathway
1. Nerve endings in the skin2. Diameter of axons3. Connections in spinal cord
i. Touch – Ascends Ipsilaterallyii. 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
Spinothalamic tract
Ascending Pain Pathways Spinothalamic Pain Pathway
Dorsal Root – immediactly decussates
Spinothalamic Tract
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Pain
ComparingPathways
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Pain
Reflex Arc
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Pain
I. The Regulation of PainA. Afferent RegulationB. Descending RegulationC. The endogenuos opiates
1. Opioids and endomorphins
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Pain
The Regulation of Pain
Descending regulation
Periaquaductal gray matter
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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 TheoryI. 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 neuronsV. 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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The cortical areas for the face annex the cortical areas for the arm and fingers.
Some of those neurons were previously specialized for pain.
Phantom Pain
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Temperature
ThermoreceptorsI. 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 temperatureIII. 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 PathwayA. Organization of temperature pathway
Identical to pain pathwayB. Cold receptors coupled to A and CC. 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 themesParallel processing of informationColumnar OrganizationCortical 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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