CHAPTER 6. INTEGUMENTARY SYSTEM SKIN Largest organ by weight ACCESSORY STRUCTURES.
The Skin: Largest Organ in the Body
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The
Ski
n: L
arge
st O
rgan
in th
e B
ody
Distortion due to variation in the size and density of sensory
neuron ‘receptive fields’
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The Stimuli of Somatosensation
SKIN (body surface)– Mechanical pressure: this is ‘touch’– Vibration (Hz): this is generally ‘texture’– Damage/Temp (pain/hot/warm/cold)– Chemical (example is menthol)
MUSCLES &TENDONS (body position)– Stretch, Tension– Kinesthesis, Proprioception
BODY SURFACE + BODY POSITION = “HAPTIC PERCEPTION”
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Rec
epto
rs c
an b
e ch
arac
teri
zed
in te
rms
of:
1.S
tim
ulat
ion
Type
(pre
ssur
e, v
ibra
tion
, tem
p, d
amag
e)
2.S
ize
of R
ecep
tive
Fie
ld (a
mou
nt o
f br
anch
ing)
3.R
ate
of A
dapt
atio
n (s
low
, med
ium
, fas
t)
Basket Cell
Pacinian corpuscle
Free Nerve Endings
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Pacinian CorpusclesDetect Vibration (texture)
PressureStimulus
PacinianActivity
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Mus
cle
Spi
ndle
– S
tret
ch R
ecep
tor
Gol
gi T
endo
n O
rgan
– T
ensi
on R
ecep
tor
Rec
epto
rs i
n M
usc
les
and
Ten
do
ns
En
cod
e B
od
y P
osi
tio
n
Mus
cle
fibe
r
Red
is m
otor
Blu
e is
sen
sory
Sens
ory
fibe
rat
tach
ed t
o te
ndon
Bone
Muscle
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This is a Muscle spindle, but other Mechanoreceptors (i.e., Basket Cells and Pacinian Corpuscles) also work this way
The cytoskeletal strands are like the ‘tip links’ of inner hair cells.
Ion channels ‘pulled open’ by
mechanical force.
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Skin Senses: 2 Pathways to Cortex
Lemniscal Pathway (mechanorecepetors)
– Tactile, pressure, Basket Cell (detect)– Tactile, vibration/texture, Pacinian Corpuscle (ID)
Spinothalamic Pathway (free nerve endings) – Tissue Damage, pain, Nociceptor (detect)– Temperature, hot/cold, Thermal Receptor (ID)– This pathway is ‘gated’ in the spinal cord
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D o r s a l – S e n s o r y
V e n t r a l – M o t o rHypothalamus
Thalamus
Spinal Cord
Thalamus
Cortexparietal
Hindbrain
Dorsal
Ventral
Thinking in 3D• Dorsal/Ventral Organization• Left/Right Crossing
Cortexfrontal
DORSAL
SENSORY NEURONSMUSCLES
MUSCLES
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Som
atot
opic
Org
aniz
atio
n
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2D ReceptorArray
Thalamus
Primary Cortex
However…
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Dense 2D Receptor Array
Thalamus
Cortex Is‘Modular’
Means the size and density of cortical columns is fixed
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Lemniscal System(mechanoreceptors)
Spinal Cord
Thalamus
Cortexparietal
Hindbrain
Cortexfrontal
PACINIANCORPUSCLE
Fuzzy
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Receptive fieldsperiphery vs. cortex
Stimulation anywhere within this large receptive field goes to one cortical column Stimulation
within this tiny receptive field goes to one cortical column
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Two-Point DiscriminationConverging neurons = Less discrimination,Lower threshold
Less convergence= More discrimination,Higher threshold
Detect Identify
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Sensory(parietal)
Motor(frontal)
The brain has no ‘sense’ of itself
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Two-
Poi
nt T
hres
hold
sWhere best for Braille?
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Pressure(Detect)
Two-Point(ID)
Thresholds for Detection and Identification
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Receptive fieldsperiphery vs. cortex
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MonkeyCortex
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Experience Changes Cortical Maps
what happens if you lose a finger? cortical maps will readjust experience alone can readjust - the
example of violin training - young vs. old
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What is the result of all this ‘experience’?
Smart? Dumb? Dumber?
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Variation in Cortical MapsOverall brain weight differs by ~30%
Size of primary cortical areasdiffers by as much as 100%
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Touch Vision Audition
Blind
Normal C
ortex is allocated based on useT
he beauty of modular architecture
“Colum
ns is Colum
ns”
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Nociceptors respond to AND release chemical stimuli(the basis of inflammation)
Pain Is A Perception:The Stimulus is Tissue Damage
PainPainPainPain
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Convergent Excitation: lower thresholds (better detection) come at a cost of lousy ID.
Heart Attack? Lung Pain? Arm Pain?
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Spi
nal G
ate
The
ory:
Two
Way
s to
Inh
ibit
L-fibers are mechanoreceptors
S-fibers are free nerve endings
1.
2.
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Spinothalamic System(free nerve endings)
1. Nociceptor (excitatory)
3. Modulatory Brainstem neurons (excitatory)
Spinal Cord
Thalamus
Cortexparietal
Hindbrain
Cortexfrontal
Opiate Neuron (inhibitory)
2. Mechanoreceptors (excitatory)
2.
3.
1.
OUCH!
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Spinothalamic System(free nerve endings)
1. Nociceptor (excitatory)
3. Modulatory Brainstem neurons (excitatory)
Spinal Cord
Thalamus
Cortexparietal
Hindbrain
Cortexfrontal
Opiate Neuron (inhibitory)
2. Mechanoreceptors (excitatory)
2.
3.
1.
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Spinothalamic System(free nerve endings)
1. Nociceptor (excitatory)
3. Modulatory Brainstem neurons (excitatory)
Spinal Cord
Thalamus
Cortexparietal
Hindbrain
Cortexfrontal
Opiate Neuron (inhibitory)
2. Mechanoreceptors (excitatory)
2.
3.
1.
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Hindbrain(5-HT, NE)
Midbrain(DA)
Dopamine (DA)Serotonin (5-HT)Norepinephrine (NE)
“one-to-many”
Architecture of‘Modulatory’ Systems
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Endogenous Opiates
Common practice to name neurotransmitters after the plant-derived chemical that mimics their action in the brain
Morphine-like neurotransmitters Endorphins, Enkephalins Role in suppressing pain (i.e., inhibiting input
from nociceptors) Opiate neurons found throughout brain and
spinal cord – not all are involved in pain
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EPSP: glutamate IPSP: gaba or endogenous opiate
Synaptic potentials are brief (a few milliseconds) positive or negative changes in voltage.Occur at dendrites – positive and negative summate.
Caused by neurotransmitter released from a presynaptic neuron.EPSPs increase likelihood of action potentials, IPSPs decrease that likelihood.
+
-YES! NO!
Drug effect: excitatory agonist
Drug effect: inhibitory agonist
Agonist drugs can also produce positive or negative changes in voltage.They do so by mimicking the chemical structure of naturally-occurring neurotransmitters.
However, the DURATION of their effects can be minutes or hours.Neurons respond to this stimulation by making themselves more or less excitable.
Leads to dose tolerance and with persistent use, chemical dependence.
YEEEEEEEEEEEEEEEEEEEEEEEEEEEEEESSSSSSSSSSSSSSSSSSSSSSSSSS!
NOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO!
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Sho
rt te
rm e
ffec
t of
mor
phin
e is
incr
ease
in K
+ c
urre
ntL
ong
term
eff
ect o
f m
orph
ine
is in
crea
se in
Na+
cur
rent
Why
dru
gs p
rodu
ce c
hem
ical
dep
ende
nce.
. .
. . .
even
‘go
od’ o
nes
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The ‘Spinal Gate’ in actionCell bodies inbrainstem
dorsal
ventral
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Activation of a Nociceptor
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Activation of a Nociceptor: Inhibition by Enkephalin / Morphine
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Cone Snail Venom
Venom consists of a‘cocktail’ of proteins
One of these venomproteins blocks ‘N’type calcium channels
Nociceptors have ‘N’type calcium channelson their axon terminals
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Neurotransmitters
Ca++ K+ Na+
Where a venom (or drug) could work. . .
Receptor Agonists / AntagonistsReuptake Inhibitors