Lecture 4 + Assignment 4 Flashcards
Spinal segment order
Cervical
Thoracic
Lumbar
Sacral
What do the spinal segments control
SLTC on the diagram
Trigeminal nerve
= front of head
Cervical
= behind ear/headband area to fingertips
Thoracic
= chest and underside of arms
Lumbar
= above hips down
Sacral
= butt and bits, the back of legs
look at pic of person touching the floor without bending their knees
Spinal cord slice components + how is thoracic different
Grey and white matter
White matter = myelinated axons
Gray matter = cell bodies and dendrites
Gray matter makes a butterfly with dorsal horns
Thoracic = smaller butterfly so less motor control
From fingertip to CNS pathway
DCML and STT
- Receptor endings
2.a) Abeta mechanosensory afferent fiber
b) Adelta or C axons pain and temperature afferent fibers
- Dorsal root ganglion (neuron cluster)
4.a) Abeta to dorsal column
b) Adelta/C CROSSES OVER to ventral spinothalamic tract
=anterolateral
Three types of somatosensory axons - facts + examples
- A Beta
Mechanoreceptive
touch
merkel, meissner, pacinian, and ruffini cells - A Delta
Thermo/nociceptive
pain, temp.
free nerve endings - C
Thermo/nociceptive
pain, temp., ITCH
unmyelinated free nerve endings
Three types of somatosensory axons - speeds + diameters
A beta
- 35-75 m/s
- 6-12 nm
A delta
- 5-30 m/s
- 1-5 nm
C
- 0.5-2 m/s
- 0.2-1.5 nm
First and second pain
- touch sensation through Abeta axons
- first pain through Adelta axons
- second pain through c axons
- fastest to slowest axon conductance speeds
- due to myelination and diameter
- we become aware of sensations when APs reach the cortex
Why does second pain last much longer
- there is a range of velocities
- all different “racers”
- large diameter racers in a “team” (axon type) move faster
- keep feeling the sensation until all team members have passed the finish line
- difference between fastest and slowest Adelta speeds still much smaller than for the C axons
- also gap until first of second pain racers gets there
Somatosensory tracts
DCML
- A beta
- decussates in the caudal medulla (in brain)
- after synapse 1/3 at the dorsal column nuclei
STT
- A delta and C axons
- decussates immediately
Somatosensory tracts - topography
DCML
- more caudal entry = more medial
(longest in the middle)
STT
- more caudal entry = more lateral
(longest on the side)
SWITCH TO OPPOSITE SIDE
Somatosensory tracts - diagrams
Dorsal
(A betas)
- stays on the same side
CTLS SLTC
TLS SLT
LS SL
S S
Ventral
(A deltas and Cs)
- swaps sides immediately
SLTC CTLS
SLT TLS
SL LS
S S
Picking lesion locations
- Touch or pain/temperature
touch = DCML (dorsal)
p/t = STT (ventral) - Right or left
DCML: injury right = feel on right
ipsilateral
STT: injury right = feel on left
contralateral - Dermatome location
CTLSSLTC dorsal
SLTCCTLS ventral
Central cord syndrome / syringomyelia
no pain/temp in bolero region
C4-T1
single lesion in center of spinal cord
- prevents decussating of second order neurons
- affects both sides
Brown-sequard syndrome
Lower thoracic injury
Ipsilateral loss of touch sensation
Contralateral loss of pain and temperature sensation
Sensory cell pathway - touch
4 things
All A beta
- Meissner corpuscle
- Merkel-cell neurite complex
- Ruffini ending
- Pacinian corpuscle
Different sensory cells - mechanoreceptors
receptive field size + respond best to
Meissner
- small
- low frequency vibrations (2-50)
Merkel
- small
- static indentation
Ruffini
- large
- skin stretch (dynamic)
Pacinian
- large
- high frequency vibrations (50+ Hz)
Benefit of vibration sensation
- lets us feel textures, slippage, and light vibration of tools
- low frequency = feeling texture
Stimulus-response graphs
- adaption types
Slowly adapting
- merkel and ruffini
- first fast dynamic phase, then static phase
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Rapidly adapting
- meissner and pacinian
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- only respond initially
Ellen Lumpkin - merkel mechanotransduction
what do piezo let in
Merkel cells
- mechanically-gated piezo cation channels sensitive to pressure
- cation nonspecific, allow any in once they feel pressure
- merkel cells release neurotransmitters to Abeta axon
- similar to neurons bc have channels (piezo and Ca) + release neurotransmitters
Ardem Patapoutain
- Nobel prize in physiology or medicine in 2011
- piezo mechanically gated ion channels
Merkel cell mechanotransduction overview
- Piezos in Abeta open and let in ions
- Causes dynamic phase action potentials
- Piezos in Merkel open and let in ions
- Depolarizes Merkel = opens voltage-gated Ca channels
- Neurotransmitter (norepinephrine) released onto Abeta terminal
- Causes static phase action potentials
Two-point discrimination
If points have a faster spike rate in the single middle circle (b) than in the two side circles (a/c):
AP will be felt as one point
Two-point discrimination - best and worst spots
Best (most accurate)
- shoulder + arm
- torso + leg
Worst (least accurate)
- toes + fingers
- cheek + nose + lip + forehead
Goldreich two-point discrimination experiment
2POD: two-point orientation discrimination
- instead of 2v1 points = tactile spatial acuity..
determine orientation
Why receptive fields smaller on finger than forearm
1 A-beta axon:3-5 merkel cells
Merkel cells more densely packed on fingers
Smaller receptive field contains the same # of cells
Thalamus
contains what
VPL
- ventral posterior lateral nucleus receives signals from the body axons
- from the dorsal column nuclei
LGN
- lateral geniculate nucleus info from the retina
- different parts of the nucleus receive info from different parts
Primary somatosensory cortex (S1)
+ subdivisions
More caudal than central sulcus
In order:
3a - proprioceptive (movement, action, location)
3b - light touch, small receptive fields
1 - light touch, large multi-digit rfs
2 - proprioceptive and light touch
shape: ^
3a/3b by central sulcus
1 at top
2 by posterior parietal complex
Homunculus
- coronal slice
most lateral
- tongue
- face
- arm
- trunk
- leg
- foot
- toes
- genitalia
Ratunculus
- vibrissae / whiskers
- barrels in cortex level IV respond to specific whiskers
Owl monkey amputation
- somatic sensory cortex
- third finger amputated
- second and fourth finger representations take up the brain space after 2 months
= somatosensory plasticity from deafferentiation
Phantom limb
- nerve endings still send pain signals although limb is gone
- feel like the limb is still there
- especially first few years after amputation
- phantom limb shrinks over time
Sensory cell pathway - temp and pain
- Free nerve endings
- Axons
STT pathway
- Nociceptors and thermoreceptors
- Adelta and C axons
- Spinothalamic tract
- Thalamus / VPL nucleus
- Thalamocortical axons
- Parietal cortex (S1)
Hot vs. cold receptors
- some Abeta heat receptors, some cold
- both slowly adapt
- fire more when first made hot (heat receptors)
- fire more when first made cold (cold receptors)
- both fire less over time
= adaptation in thermoreceptors
How heat receptors work
- ion channels open in response to heat
“transient receptor potential channels”
- open in response to heat = heat-gated
- TRPV1 permeable to Na and Ca cations in
- causes depolarizations and APs
- allows us to know were getting warm
Capsaicin - david julius
TRPV1 also responds to capsaicin in spicy peppers
- lipophilic
- goes straight through bilayer
- binds to INTRAcellular TRPV1 channel
- opens channel and makes us feel heat
Menthol - Julius and Payapoutain
- menthol/mint opens cold channels
TRPM8
- also permeable to Na and Ca
Referred pain
- you feel pain from an internal organ somewhere else in your skin
- skin and organ afferent nerves converge onto a single dorsal horn neuron
- brain infers pain from the place pain is more often felt = skin
Gate hypothesis - rub stubbed toe to get rid of pain
- Abeta mechanoreceptor and C fiber nociceptor neurons converge onto dorsal horn projection neuron (second order)
- between Abeta and dorsal horn is an inhibitory interneuron/local circuit neuron
- releases GABA or glycine = open Cl channels
- when mechano activated, inhibits neuron that nociceptor also touches
- makes slightly less pain bc reduces # of APs
Extracellular recording
oscilloscope measures voltage changes
audio amplifiers hear APs
- electrode oustide neuron bc brain inflates and deflates with heartbeats
= hard to get inside brain that’s bouncing
extracellular graph kinda like upside down graph of AP
- smaller amplitude bc more ions not in a confined space
- Na enters cell so leaves extracellular space and makes it more negative
Microelectrode mapping - vernon mountcastle
- mapped different hand segments to parts of the brain
cortical columns
- same receptive field
- vertical
different receptive fields horizontally
Cortical columns - order
2 mm thick in all mammals
- pyramidal cell
- local axon collateral
- stellate cell
- dendrites
- descending axon -> output
Pyramidal vs. stellate cells
prevalence, function
Pyramidal cells
- projection neuron axons travel far
- excitatory (glutamate)
- 70% of cortical neurons
Stellate cells
- non-pyramidal
- interneuron
- aka local circuit neuron
- excitatory (glutamate) or inhibitory (GABA)
- 30% of cortical neurons
Cortical column inputs and outputs
1 no cell bodies, yes axons and dendrites
2/3 sends axons to and receives axons from other cortical areas (including the opposite hemisphere)
4 receives axons from thalamus
5 sends axons to brainstem and spinal cord
6 sends axons to thalamus
Enkephalinergic inhibitory local circuit interneuron
Enkephalin = peptide neurotransmitter
- synapses axon terminal to axon terminal (instead of to dendrites) of C-fiber
= axo-axonic synapse
- blocks voltage-gated Ca channels and opens K channels
- reduces the release of neurotransmitters from the C-fiber onto the dorsal horn projection neuron
Pain is personal
two ppl might feel the same amounts of pain differently
ex. placebo effect
pain -> amygdala -> raphe nuclei -> brain stem enkephalinergic interneuron APs
- bc linked to amygdala activity activating enkephalin
- goes to raphe nucleus
no enkephalin = no placebo
