Neuroscience Chapter 12 Notes
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Transcript of Neuroscience Chapter 12 Notes
![Page 1: Neuroscience Chapter 12 Notes](https://reader034.fdocuments.net/reader034/viewer/2022051215/563dbbae550346aa9aaf5193/html5/thumbnails/1.jpg)
Bear: Neuroscience: Exploringthe Brain 3e
Chapter 12: The Somatic SensorySystem
FROM and to THE BODY
(catchall for non hearing, seeing, tasting, smelling, equilibrium)
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DO WE REALLY ONLY HAVE FIVE SENSES?
We also sense:
PAIN
POSITION
TEMPERATURE
DISTENSION
DIRECTIONtransduced
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Somatosensory SystemHapsis – fine touch and pressureProprioception – location and movement
Nociception – pain and temperature
Some are rapidly adapting, some are slowly adapting
Many are mechanoreceptors - ion channels sensitiveto physical distortion such as bending or stretching.
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• Types and layers of skin– Hairy and glabrous (hairless)– Epidermis (outer) and dermis (inner)
• Functions of skin– Protective function– Prevents evaporation of body fluids– Provides direct contact with world
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• Mechanoreceptors (Cont’d)– Pacinian corpuscles– Ruffini's endings– Meissner's corpuscles– Merkel's disks– Free nerve endings, hair follicles
Krause end bulbsBalls of String lip edges
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MECHANORECEPTORSSensitivity depends on structure.
Pacinian corpuscles – pressure, stretchlargest, sensitive to flutter – vibrations (feel the music on your speakers 200-300 Hz.)(up to 2 mm x 1mm!) Poorly localized sensations.large football shaped capsule, with onion skin of 20 – 70 layersand fluid in between
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Pacinian corpusles-
When pressure is applied:Capsule compressesEnergy goes to axon terminalTerminal is deformedChannels openReceptor potential activates transmitter
When pressure is maintainedLayers slip past each otherEnergy movement stopsAxon terminal stops deformingReceptor potential stopsCell has adapted
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Ruffini’s endings (aka lamellar)
pressure– Vibration - a little smaller, better localized sensations
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Meissner’s corpusles – very smallreceptor fields, in glabrous skin only (example: raisedparts of your fingerprints) vibrations (50 Hz).
superficial touch , gentle fluttering, well-localized
Glabor = bald
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Merkel’s disksmall receptor fields consisting ofnerve terminal and non- neuralepithelial cells
steady skin indentations
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free nerve endings – most common
Temperature, pain, pressure
hair follicle receptor (around follicle)
Some follicles also have erectile muscles (goose pimples).
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Touch
• Mechanoreceptors
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Touch
• Mechanoreceptors (Cont’d)– Receptive field size and adaptation rate
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BUT WHERE IS THE CELLBODY??
Dorsal root ganglion
Cells have “neurites” that branch
One branch ends in the skinreceptors
One branch heads towards the CNS
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cell bodiesare outsidespinalcord
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based on size of receptor fields
what are receptor fields?
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TWO POINT DISCRIMINATION
Simple measure of detail discrimination.
Varies across the body.
Fingertips are most sensitive.
Basis for Braille fluency (600 letters/min.)
On fingers:High density of receptorsSmall receptor fields
In brain:More brain devoted to processingSpecial neural mechanisms for high-resolution discrimination
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• Primary AfferentAxons
– Aα, Αβ, Αδ, C
– C fibers mediate painand temperature
– Aβ mediates touchsensations
recallSPEED
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musclestretch
tendon stretch
joint movement
Proprioception
static andmovement
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Touch
• The Spinal cordSpinal segments(30)- spinal nerveswithin 4 divisions ofspinal cordDivisions
– Cervical (C)– Thoracic (T)– Lumbar (L)– Sacral (S)
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Dermatomes – skin innervationfrom single nerve1-to-1 correspondence withsegments
Shingles
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• Spinal cord Sensory Organization of thespinal cord
• Division of spinal gray matter:• Dorsal horn (sensory)• Intermediate zone (variable, Substance P)• Ventral horn (motor)
– Sensory Myelinated Aβ axons (touch-sensitive)
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• Dorsal Column–MedialLemniscal PathwayTouch– ascends through dorsal
column to dorsal nucleimedial lemniscus (wander– through brain stem)
– to ventral posteriornucleus
– to primary somatosensorycortex
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The Trigeminal Touch Pathway
Trigeminal nerves
CN V
also to VP
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Touch
• Somatosensory Cortex– Primary SI (3b)
– LAYER IV– Broadman’s 1,2,3a, 3b– Other areas
• Postcentral gyrus (1,2,3a)• Posterior Parietal (5,7)
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• Somatosensory Cortex(Cont’d)– Brodmann’s Area 3b (or
S1): Primary somatosensorycortex
• Receives dense input from VPnucleus of the thalamus
• Neurons: Responsive tostimuli
• Lesions impair somaticsensations
• Electrical stimulation evokessensory experiences
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• Somatosensory Cortex– Cortical Somatotopy
• Homunculus• Importance of mouth
– Tactile sensations: Important forspeech
– Lips and tongue: Last line of defense• IMPORTANT, so large• Or large, so important?
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•-Why? ––Sometimeswe need tokeepsensationsseparate
–(what iscoming fromwithin vs.environment).
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Touch• Somatosensory Cortex (Cont’d)
– S1: Rat• Vibrissae• “Barrel cortex”
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Touch
• Somatosensory Cortex (Cont’d)– Cortical Map Plasticity– Remove digits or overstimulate – examine
somatotopy before and after• Conclusions of experiments
– Reorganization of cortical maps» Dynamic» Adjust depending on the amount of sensory
experience
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PHANTOM LIMB –borders?Ramachandran box 12.3
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Touch• The Posterior Parietal Cortex
• Involved in somaticsensation, visual stimuli,and movement planningBINDING
• Agnosia (no sensing)• Astereoagnosia (touching
does not work)• Neglect syndrome (damage
to right - contralateral)
Hemispatial neglect
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Pain• Nociceptors free, branching• Pain and nociception
– Pain - feeling of sore, aching, throbbing– Nociception - sensory process, provides signals
that trigger pain
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Nociceptors: Transduction of PainBradykinin (peptide formed following damage)
Lactic acid (H+ - activate channels )??Mast cell activation: Release of histamine
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Red hot chili peppers
chemical (capsaicin) activates thermal receptors(not in birds)
also following tissue damage (why injured skin sensitive to cold)
– Types of Nociceptors:
•Polymodal nocireceptors• Mechanical nocireceptors• Thermal nocireceptors• Chemical (smallest itch histamine)
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Hyperalgesia
• Primary andsecondaryhyperalgesia
• Bradykinin –also LONGlasting effects
• Prostaglandins –make receptorsmore sensitive
•
ALSO– cells themselves up-regulate
- substance P– AXON BRANCHING!
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Pain
• Primary Afferents and Spinal mechanisms– First pain and second pain
– Referred pain: Angina– Mixed axons in spinal cord
– Converge on same spinothalamictrack cells
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2 PAIN PATHWAYS:
FAST PAIN PATHWAY (warning)•few synapses (direct - free of emotion?)•myelinated fibers (fast)does not last long
2) SLOW PAIN PATHWAY•more synapses (indirect)•unmyelinated fibers (slow)•interacts with emotion pathways (limbic) •involves prefrontal cortex•lasts a long time
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Pain
• Ascending Pain Pathways– Differences between touch and pain
pathway• Nerve endings in the skin• Diameter of axons• Connections in spinal cord
– Touch – Ascends Ipsilaterally– Pain – Ascends Contralaterally
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TWO PATHWAYS
injury MIGHTresults in different losses
Brown-Séquard Syndrome
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• Somatosensory Pathways
Two pathways to cortexARE A LITTLE BIT DIFFERENT
NOTE: VP sensoryVL motor
VP and VL –deep brain stimulation
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Perception of painaltered by attention and expectation(placebos work)
•anxiety (Lamaze works)•suggestibility
children and paininfantile amnesia?
cortex not “hooked up” (myelinated)?
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CANNABIS AND PAIN?
315 AD
CANNABINOID RECEPTORS – MOST PREVALENT G PROTEIN RECEPTOR IN CNS!
MEDULLA
SIDE EFFECTS INCLUDE MEMORY LOSS (surprise)
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• Ascending Pain Pathways (Cont’d)– Spinothalamic Pain Pathway
• Brown-Séquard Syndrome -
– The Trigeminal Pain Pathway (different)
– The Thalamus and the Cortex• Touch and pain systems remain segregated• Pain and temperature information sent to
various cortical areas
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Pain• Ascending Pain Pathways (Cont’d)
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Pain
• The Regulation of Pain
– Afferent Regulation (gate)– Descending Regulation (periaqueductal gray PAG)– The endogenous opiates
• Opioids and endomorphins
• PLACEBOS??? Acupuncture??• Naloxone (opioid antagonist) blocks placebo and acupuncture
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Pain• The Regulation of Pain (Cont’d)
– Descending regulation
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Gate theory of pain
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spinal cord interaction can be complexpain is especially puzzling
The gate theory for pain
all lessen pain:
morphineacupuncturerubbing
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Pain• Ascending Pain Pathways (Cont’d)
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Temperature• Thermoreceptors
– “Hot” and “cold” receptors (mint)– Varying sensitivities
IMPT – remember bio labs?
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Temperature
• Thermoreceptors– Hot and cold receptors
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Temperature
• The Temperature Pathway– Organization of temperature pathway
• Identical to pain pathway– Cold receptors coupled to Aδ and C– Hot receptors coupled to C
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……topy• Somatotopy
• Retinotopy
• Tonotopy
Stimulate PAG?
Kill off substance P neurons ?
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• Sensory systems exhibit similarorganization and function
• Sensory types are segregated within thespinal cord and cerebral cortex
• Repeated theme– Parallel processing of information
• Perception of object involves the• coordination of somatic sensory
information “binding”