Download igher) order: thalamus

SOMATIC SENSORY SYSTEM
Modalities
What makes one different from another?
Receptors
Differ in their adequate stimulus (i.e., type of stimulus energy to
which receptor is specialized to respond)
Central pathways
Place coding: different modalities are processed in separate
places by brain.
Which of these is primary basis of different sensory experiences?
Müller: connections (not receptor sensitivity) is primary
Demonstration: if press eyeball, see flash
Implication: LABELED LINES (a specific pathway carries
information about a single modality; activity in the pathway
interpreted by brain as arising from that modality)
Relation to somatic sensory
Submodalities exist (separate, non-confused sensory qualities)
Tactile (touch)
Position (kinesthesis; proprioception)
Thermal (hot/cold)
Pain (fast, slow)
Question: do separate labeled lines underlie these submodalities?
Answer: apparently so. There is evidence for this even at periphery.
Peripheral elements
Fiber classes (table on blackboard)
CLASS
I
CLASS
A
DIAMETER
Large
MYELIN
+++
II
A
Medium
++
III
A
Small
+
IV
C
Very small
-
Tiny
-
Ventral Root
MODALITY
Position (?)
RECEPTOR
Spindle primary,
Golgi tendon organs
Fine tactile, Cutaneous mechanoposition (?)
receptors, spindle
secondaries
Fast pain,
Free nerve endings
temperature, (noci-, thermo-, and
crude tactile mechanoreceptors)
Slow pain,
Free nerve endings
temperature, (noci-, thermo-, and
crude tactile mechanoreceptors)
Visceral?
?
Functional associations
Origin of evidence: selective inactivation of specific fiber classes
Anoxia and pressure block thick fibers first
Local anesthetics block thin fibers first
Correlate with perceptual losses
Itemize functions of each fiber class
Receptor types
Many varieties
Morphology
Endings that are encapsulated or linked to specialized
structure
Free nerve endings (all over body, including viscera)
Physiology (adequate stimulus)
How well-correlated with fiber or function?
Muscle-related receptors: pretty good correlation
Spindles
Primaries  Ia
Secondaries  II
Functional distinction: encode dynamic vs. static
stretch
Golgi tendon organs  Ib
Also: joint receptors
Function: proprioception
Spindles are primary source of position information
(note that this requires that brain also know gamma
output; "corollary discharge"- brain keeps a "copy" of
gamma efferent signal)
But joint, cutaneous also contribute to position sense
Cutaneous receptors
Varieties
Morphology
Encapsulated
Pacinian corpuscle
Meissner's
Ruffini
Free nerve endings
Hair base
No specialization
Link of morphology to function or fiber type?
Crudely yes
Class II always associated with
encapsulated mechanoreceptors
Class III and IV always terminate as free
endings
But breaks down in detail
Encapsulated endings exhibit varied
morphology but all are mechanoreceptors
(though may be distinguishable from
one another on basis of speed of
adaptation)
Free endings all look the same, but may
differ radically in functional properties
(mechano-, noci-, or thermo-receptors).
Central Paths
If there is some evidence for labeled lines in the periphery, can these be
traced separately into the CNS?
Yes, though fortunately not one path per submodality
DORSAL COLUMN vs. ANTEROLATERAL (or spinothalamic) SYSTEMS
Get name from spinal location of ascending tract
Simplistic summary of distinction between two systems:
Fine cutaneous and proprio (EPICRITIC) via dorsal column
Pain and temperature (PROTOPATHIC) via anterolateral
DORSAL COLUMN SYSTEM
Review
Neurons:
1) Dorsal root ganglion
2) Medulla
3) Thalamus
Overview of pathway
CROSSED path to cortex
Implication: contralateral representation (as for corticospinal)
Decussation: importance of location, e.g. in localizing lesions
Primary fibers
Fiber classes: I and II
Division of roots: medial branch
Local connections: e.g., Ia's and monosynaptic reflex
Locating pathway in cross-sections
Dorsal column (show)
Dorsal column nuclei (show)
Medial Lemniscus
Formation at decussation of output fibers of dorsal column
nuclei (lower medulla)
Upper medulla; Medial Lemniscus abuts midline
At pons, it is horizontal, lies above pontine gray
At midbrain, lies ventrolateral
VPL of thalamus
Topographic order
Accretion of fibers onto lateral side of dorsal column as ascend
cord (hence lower body represented medially, upper body
laterally within dorsal funiculus)
Cuneate and gracile fasciculus and nuclei
Topography maintained in thalamus and cortex (homunculus)
TRIGEMINAL component supplies face component of same system
Cell bodies - trigeminal ganglion (Dorsal-root-ganglion
equivalent)
PRINCIPAL SENSORY NUCLEUS of V (Dorsal-column-nucleus
equivalent)
Decussation
VP-M of thalamus
Projects to face area of primary somatic sensory cortex
SUMMARY: Fine touch and position means Dorsal Column plus
Principal Trigeminal Nuclei
ANTEROLATERAL SYSTEM
Information
Pain / Temperature (PROTOPATHIC)
But also crude touch and position
Same as SPINOTHALAMIC
We will ignore anterior vs. lateral spinothalamic distinction
Summary:
CROSSED path to cortex (and brainstem)
Neurons
First order: Dorsal root ganglia
Second order: dorsal horn
Third (or higher) order: thalamus
Details
Primary fibers = Class III and IV
Lateral division of roots (fine fibers)
Lissauer's (topographic "smearing" because primary sensory
axons may course up or down a few segments before
synapsing in cord)
Synapse (sub. gelatinosa, marginal nucleus; also Rexed IV)
May be multiple synaptic links in dorsal horn before ascent
DECUSSATION of ascending fibers (second or higher-order
neuron)
As in Dorsal Column System, happens at same level of
neuraxis as the first synapse
But occurs in cord (ventral white commissure)
Anterolateral funiculus
Brainstem location of spinothalamic tract
Thalamus
Input not always directly from spinal cord
Target includes VPL (but others too)
Cortex
TRIGEMINAL COMPONENT
Spinal V (sub. gelatinosa equivalent)
Spinal tract of V (contains primary sensory axons from trigeminal
nerve descending from nerve entrance in pons to synapse in
spinal V nucleus)
Decussation
VPm (also Posterior and Intralaminar thalamic nuclei)
Face area
PAIN PATHWAYS
Why more detail here?
Clinically important
Unusual features
Pain vs. Nociception
Nociception = physiological (sensory cell response to tissue damage)
Pain = psychological (experience generally associated with such stimuli)
A-delta vs. C fiber contribution
A-delta
Fast pain
High threshold mechano- and thermo-receptors
C-fibers
Slow pain
Chemoreceptors
Perfusion expt
Collect perfusate from damaged skin
Inject  pain
Bradykinin (released by tissue damage)
C-fibers as effectors
Flare in triple response
Depends on "sensory" endings
Axon reflex explanation
Stimulate distal stump of cut dorsal root  pain
(mechanism requires dermatomal overlap)
Components within spinothalamic (anterolateral)
Review main pathway
Components
Neo-spinothalamic tract
Direct inputs to VPL
Spinal origin: Marginal
Inputs: A-delta, C
Response: high thresh nociceptors
Role: Fast, localized pain
Paleo- spinothalamic
Inputs to intralaminar
Spinal origin: Proprius
Inputs: A-beta via interneurons in addition to A-delta/C
Response: wide dynamic range (respond to non-noxious as
well as noxious stim)
Role: Slow, agonizing pain
Archi- spinothalamic tract
Almost identical to Paleo
Target: RETICULAR FORMATION
What is reticular formation
Core of whole brainstem minus nuclei/tracts
Mishmash cells/fibers
Diverse roles (pain, motor, state changes [sleep,
arousal])
Not a functional unit
Reticular  intralaminar
TRACT
TARGET
Neospinothalamic
Paleospinothalamic
Archispinothalamic
VPL
Intralaminar
nuclei
Reticular
formation
 Intralaminar
SPINAL
CORD
ORIGIN
Marginal
nucleus
Nucleus
proprius
Nucleus
proprius
INPUT
A,C
RESPONSE
High threshold
nociceptors
A,C and Wide dynamic
indirect A range
A,C and Wide dynamic
indirect A range
FUNCTION
Fast, localized
pain
Slow, diffuse
pain
Slow, diffuse
pain
Central Control of Pain
Pain is different from nociceptive input
Denervation (transection of nociceptive pathways)
Often only temporary effect, may be followed by "Central
Pain" (unbearable, indescribable)
WHY?
Diffuseness of pathways?
Lissauer's tract (intersegmental smearing)
Some bilaterality of A-L system
Reticular formation (diffuse topography)
Intralaminar output to cortex (widespread)
Central control mechanisms
Counterirritation Phenomenon (peripheral)
Bash shin  rub it
Implies Class I/II fibers "inhibit" III/IV fibers
Gate control theory
Wrong but
Dorsal horn is a gate as the theory
proposed
Psychological factors
Trauma
Battlefield anecdotes
40% of trauma patients report no pain
Power of Suggestion
Hypnosis
Placebo
Periaqueductal grey (PAG) / Raphe system
Brain input to dorsal horn gate
Circuit responsible
Appearance of PAG and raphe in
sections
Modality specificity (pain only)
Clinical utility
Opiate relationship
Systemic opiates act on cord same
way
Receptors and microinfusion
Blockers of opiate or 5-HT
 block stimulation and Placebo
effects
Cut dorsolateral funiculus or raphe
 lose opiate or stim effects
Labeled Lines revisited: are DC system and AL system lines for different
submodalities?
Mixing problem
DC and AL each heterogeneous
DC and AL converge at thalamus
Answer: mixing of information more apparent than real
Single neurons usually unimodal
Clustering; for example...
Dorsal column nuclei (light touch vs. muscle stretch)
Cortex
Columns (hair, joint, pressure)
Areas (S-I has multiple representations, also S-II)
Area 3a - muscle stretch
Area 3b - slowly adapting cutaneous
Area 2 - deep pressure
So labeled lines are there, just finer grained
Integration of Submodalities (parietal cortex)
STEREOGNOSIS Hand in pocket: keys or scorpion?
Requires integration
Parietal association (areas 5,7) (one place it happens)
Inputs
Widespread somatic
Other sensory modalities
Damage  Astereognosis
This is an example of an "agnosia" (a higher order
perceptual deficit in the absence of any obvious primary
sensory deficit)
Also: disturbances in complex somatic sensory functions, "body
image"
Extinction to double simultaneous
Neglect
Dressing
Denial of deficit
Not just somatic
Neglect of other modalities, even in mental images
Cognitive
Spatial coding (have looked at what, but how about where?)
Topographic order critical
But not simple point-to-point
Distorted homunculus
Relation to two-point discrimination
Other factors at play
Innervation density
Receptive field size
Covariation of all three