The Somatosensory System
CH7 Blumenfield
By: Laurence Poliquin-Lasnier
R2 neurology
Outline
• Sensory neuron
• Main somatosensory pathways
– Posterior column-medial lemniscus
– Spinothalamic tract
•
•
•
•
•
Somatosensory cortex
Central modulation of pain
Thalamus
Spinal cord syndromes
Bladder, bowel and sexual function
Main somatosensory pathways
• Posterior column-medial lemniscus
– Proprioception, vibration, fine
discriminative touch
• Spinothalamic tract
– Pain, temperature, crude touch
• Via unipolar sensory neuron
Unipolar sensory neuron
Sensory neuron fiber types
Name
A- (I)
A-β (II)
A-δ (III)
C (IV)
Diameter
Myelinated
(µm)
13-20
6-12
Yes
Yes
Receptors
Sensory mdality
Muscle spindle, golgi
tendon organ
Proprioception
Muscle spindle
Proprioception,
Meissner’s corpuscule
Superficial touch
Merkel’s receptor
Superficial touch
Pacinian corpuscule
Deep touch, vibration
Ruffini endings
Deep touch, vibration
Hair receptor
Touch, vibration
1-5
Yes
Bare nerve ending
Pain, temperature
(cool)
0.2-1.5
No
Bare nerve ending
Pain, temperature
(warm), itch
Sensory neuron
• Sensory neuron cell body located in
dorsal root ganglia
• A peripheral region innervated by sensory
fibers from a single nerve root =
dermatome
Outline
• Sensory neuron
• Main somatosensory pathways
– Posterior column-medial lemniscus
– Spinothalamic tract
•
•
•
•
•
Somatosensory cortex
Central modulation of pain
Thalamus
Spinal cord syndromes
Bladder, bowel and sexual function
Posterior column-Medial lemniscus
Posterior column
-Medial lemniscus
• Large myelinated
axons
• Proprioception,
vibration, fine
touch
Posterior column-Medial lemniscus
Ascend through:
• Gracile Fasciculus: legs + lower trunk
• Cuneate Fasciculus: arms, neck, trunk above
T6
• 1st order sensory neuron synapses synapse
onto 2nd order neurons in the nucleus gracilis
and nucleus cuneatus at th level of the medulla
• Axons of these 2nd order neurons decussate as
internal arcuate fibers and form the medial
lemniscus on the other side of the medulla
Posterior column-Medial lemniscus
• 2nd order neurons synapse into the ventral
posterior lateral (VPL) nucleus of the
thalamus
• 3rd order neurons then project to the
posterior limb of the internal capsule to
reach the primary somatosensory cortex
in the post-central gyrus
Sensory homunculus
Outline
• Sensory neuron
• Main somatosensory pathways
– Posterior column-medial lemniscus
– Spinothalamic tract
•
•
•
•
•
Somatosensory cortex
Central modulation of pain
Thalamus
Spinal cord syndromes
Bladder, bowel and sexual function
Spinothalamic tract
Spinothalamic
Tract
• Small diameter
• Unmyelinated
• Pain and
temperature
Spinothalamic tract
• Enter spinal cord via dorsal root ganglia
• 1st order neuron synapse in the grey matter of the
dorsal horn marginal zone (lamina1) and deeper in the
dorsal horn (lamina 5)
• Some axon collaterals ascend or descend for a few
segments in lissauer tract before entering the central
gray
• 2nd order neuron cross over in the spinal cord anterior
commissure to ascend in the anterolateral white matter
• It takes 2-3 spinal segments for the decussating fibers
to reach the opposite side ( so sensory level of spinal
cord lesion starts a few levels below the lesion)
Spinothalamic tract
• Anterolateral pathway reaches medulla
• Run between the olives and the inferior
cerebellar peduncles
• Enters pontine tegmentum
• 2nd order neuron synapses in the thalamus to 3rd
order neuron
• 3rd order neuron to somatosensory cortex in the
postcentral gyrus
• Secondary somatosensory association cortex in
parietal operculum (somatotopic organization)
and association area in posterior parietal lobule
Anterolateral pathway: 3 tracts
• Spinothalamic (I, V)
– Discriminative aspects of pain, location, intensity
– Synapse on VPL (different area than DCML),
relay to specific SSC target (Brodmann 3,1,2)
• Spinoreticular (VI, VII, VIII)
– Emotional and arousal aspects of pain
– Reticular formation projects to intralaminar
thalamic nuclei (centromedian), which then
project diffusely to the entire cerebral cortex
(behavioural arousal)
• Spinomesencephalic (I, V)
– To periaqueductal grey and superior colliculi
– Pain modulation
Outline
• Sensory neuron
• Main somatosensory pathways
– Posterior column-medial lemniscus
– Spinothalamic tract
•
•
•
•
•
Somatosensory cortex
Central modulation of pain
Thalamus
Spinal cord syndromes
Bladder, bowel and sexual function
Central modulation of pain
• Gate control theory
• Sensory input from large diameter non pain A-β fibers
reduce pain transmission through the dorsal horn
• Periaqueductal gray receives input from: hypothalamus,
amygdala, cortex
• Inhibits pain transmission in the dorsal horn via relay in
rostral ventral medulla (RVM)
• RVM includes serotonergic neurons of the raphe nuclei
that project to the spinal cord and modulate pain
• RVM sends input (via substance P) to the locus
ceruleus to spinal cord dorsal horn (via NE)
Central modulation of pain
• Opiate receptors and endogenous opiate
peptides located at key points in the pain
modulatory pathways
– Enkephalin and dynorphin -> PAG,
RVM, dorsal column
– β-endorphin -> hypothalamus
Outline
• Sensory neuron
• Main somatosensory pathways
– Posterior column-medial lemniscus
– Spinothalamic tract
•
•
•
•
•
Somatosensory cortex
Central modulation of pain
Thalamus
Spinal cord syndromes
Bladder, bowel and sexual function
Thalamus
• Major sensory relay station
• Deep gray matter structure part of the
diencephalon
• Convey different types of input to the
cortex
–
–
–
–
Sensory
Motor from cerebellum and basal ganglia
Limbic
Modulatory inputs involved in aroual and
sleep-wake cycle
Thalamus
• Divided by internal medullary lamina (a Y
shaped structure) into:
– Medial nuclear group
– Lateral nuclear group
– Anterior nuclear group
• Nuclei within internal medulary lamina
called intralaminar nuclei
Thalamus
Thalamus
• 3 categories of nuclei:
– Relay nuclei
– Intralaminar nuclei
– Reticular nucleus
Thalamus: Relay nuclei
• Lie mainly in lateral thalamus
• All primary sensory modalities have relays
in the lateral thalamus en route to their
specific cortical target, with one exception
-> olfaction
• Reciprocal innervation w/ cortex
Thalamus: Relay nuclei -> Lateral
nuclear group
Relay Nucleus
In
Out
Function
VPL
Medial lemniscus,
spinothalamic
Somatosensory cortex
Somatosensory spinal
input
VPM
Trigeminal lemniscus,
trigeminothalamic tract, taste
Somatosensory cortex,
taste
Somatosensory CN input
and taste
LGN
Retina
Primary visual cortex
Vision
MGN
Inferior colliculus
Primary auditory cortex
Audition
VL
Internal GP, deep cerebellar
nucleii, SN (ParsR)
Motor, premotor and
supplementary motor
Relays BG and cerebellar
inputs to cortex
VA
SN (ParsR), internal GP, deep
cerebellar nucleii
WIDESPREAD to frontal
lobe -> prefrontal,
premotor, motor,
supplementary motor
Relays BG and cerebellar
inputs to cortex
Pulvinar
Tectum (extrageniculate visual
pathway), other sensory input
P-T-O association
Behaviour orientation
toward relevant visual and
other stimuli
Lateral dorsal
w/ anterior nuclei
Lateral posterior
w/ pulvinar
Midbrain reticular formation
Widespread to cortex
Maintain alert, conscious
Thalamus: Relay nuclei -> other
groups
Relay Nucleus
IN
Out
Function
Medial group
Mediodorsal/dor
somedial
Amgdala,
Frontal cortex
olfactory cortex,
limbic cortex, BG
Limbic
pathways, major
relay to frontal
cortex
Mammilary
bodies,
hippocampal
formation
Cingulate gyrus
Limbic pathways
Amygdala,
hippocampus,
limbic cortex
Limbic pathways
Anterior group
Anterior nucleus
Midline thalamic group
Paraventricular,
paratenial,
intermediodorsal
, rhomboid,
medial ventral
Hypothalamus,
basal forebrain,
amygdala,
hippocampus
Thalamus
In
Out
Function
Intralaminar nuclei (within internal medullary lamina)
1) Rostral intralaminar
nuclei:
Central medial nucleus,
paracentral nucleus,
central lateral nucleus
Deep cerebellar
Cerebral cortex, Alert consciousness,
nuclei, GP, brainstem, striatum
motor relay for BG
ARAS, sensory
and cerebellum
pathways
2) Caudal intralaminar
nuclei:
Centromedian nucleus,
parafascicular nucleus
GP, ARAS, sensory
pathways
Cerebral cortex, Motor relay for BG
striatum
Reticular nucleus
(only one not
projecting to cortex)
Cerebral cortex,
thalamic relay and
intralaminar nuclei,
ARAS
Thalamic relay
Regulates state of
and intralaminar other thalamic nuclei
nuclei
Clinical concept: dysfunction in pain
pathways
• Negative symptom = sensory loss
• Positive symptoms = paresthesias =
added sensation
• Dysesthesia = unpleasant abnormal
sensation
• Allodynia = painful sensation provoked by
minor stimulus eg.: light touch
• Posterior column: tingling, numb, tight
band, walking on clouds
• Anterolateral: sharp, burning pain
Outline
• Sensory neuron
• Main somatosensory pathways
– Posterior column-medial lemniscus
– Spinothalamic tract
•
•
•
•
•
Somatosensory cortex
Central modulation of pain
Thalamus
Spinal cord syndromes
Bladder, bowel and sexual function
Spinal cord lesions
Spinal shock:
• Flaccid paralysis below the lesion
• Loss of DTR
• Autonomic dysfunction
– Decreased sympathetic outflow to vascular
smooth muscles -> Hypotension
– Absent sphincter tone
• Over weeks to months, spasticity and UMN signs
develop
Cord compression:
• If non-ambulatory at tx, 80% remain so
• If ambulatory at tx, 80% will remain mobile
Sensory loss: patterns and localization
• Primary somatosensory cortex
– Contralateral face, arm, leg, trunk
– Two point discrimination, extinction, stereognosis,
graphestesia
• Thalamus (VPL or VPL)
– Contralateral face, arm, leg, trunk
– Relative preservation of cortical features
• Lateral pons and medulla
• Pain and temperature
• Ipsi face and contra hemibody
• Medial medulla
– Medial lemniscus = vibration, position sense
Spinal cord syndromes
• Transverse cord syndrome
• Sensory level with loss of
all sensory modalities
• DDx: trauma, tumor, MS,
transverse myelitis
Spinal cord syndromes
• Hemicord syndrome
• “Brown-Sequard”
• Damage to lateral
corticospinal tract = ipsi UMN
weakness
• Damage to post. column = ipsi
loss of vibration and position
sense
• Damage to anterolateral system = contra pain and
temperature
• May have a strip of 1-2 segments of ipsi loss of pain
and temp caused by damage to post horn cell before
their axons have crossed over
Spinal cord syndromes
•
•
•
•
•
Central cord syndrome
Suspended sensory loss to pain and temp
Cape-like pattern if cervical cord
Suspended dermatomes if at other level
LMN deficit if damage to anterior horn cells
Spinal cord syndromes
• May get sacral sparing as spinothalamic
tract = more medial cervical region and
more lateral sacral region
• Causes of central cord syndrome:
• Spinal cord contusion, post-traumatic
syringomyelia, intrinsic spinal cord tumor
Spinal cord syndromes
• Posterior cord syndrome
• Loss of vibration and position
sense below the lesion
• May get UMN weakness if it
encroaches lateral
corticospinal tract
• Causes: trauma, extrinsic
compression, MS, Vitamin
B12 deficiency, tabes dorsalis
(tertiary syphilis), HTLV-1
Spinal cord syndromes
• Anterior cord syndrome
• Damage to anterolateral
pathway = loss of pain and
temp below lesion
• Damage to anterior horn
cell may produce LMN
weakness at the level of
the lesion
• If larger lesion,
corticospinal tract involved
-> UMN weakness
Spinal cord syndrome
Anterior spinal artery syndrome:
• Back of neck pain of sudden
onset
• Rapidly progressive flaccid and
areflexic paraplegia
• Loss of pain and temperature
to a sensory level
• Preservation of JPS and
vibration sensation
• Urinary incontinence
Outline
• Sensory neuron
• Main somatosensory pathways
– Posterior column-medial lemniscus
– Spinothalamic tract
•
•
•
•
•
Somatosensory cortex
Central modulation of pain
Thalamus
Spinal cord syndromes
Bladder, bowel and sexual function
Anatomy of bowel, bladder and sexual
function
• Complex interplay between sensory, motor (voluntary and
involuntary) and autonomic pathways at multiple levels of the
nervous system
– Frontal “micturition inhibiting area”, sensorimotor sphincter
control area, BG, vermis, pontine micturition center
• S2-S4
– Sensory (bladder, rectum, urethra, genitalia)
• Ascends via posterior & anterolateral columns
– Motor
• ant. horn cell  pelvic floor
• Onuf’s nucleus =sphincteromotor nucleus  urethral
and anal sphincters contraction
– Parasympathetics  detrusor contraction
• Sympathetics T11-L1 (intermediolateral cell column)
detrusor relaxation, bladder neck contraction
• Need bilateral pathways involved to get clinical syndrome
Bladder function: detrusor reflex
(voiding) and urethral reflex (storage)
1. Voluntary relaxation of
external urethral
sphincter
2. Inhibition of
sympathetics to bladder
neck (relaxes)
3. Parasympathetic
activation for detrusor
(dome) contraction
4. Self-perpetuate as long
as urine flows
5. When urine stops, ,
urethral sphincters
contract triggering
detrusor relaxation
Detrusor reflex mediated by intrinsic spinal cord
circuits, pontine micturition center, cerebellar and
BG pathways
Incontinence
• Lesions affecting bilateral medial frontal
micturition centers result in reflex
activation of pontine and spinal micturition
centers when the bladder is full
• Normal emptying but not under voluntary
control
• Causes of frontal type incontinence:
hydrocephalus, parasagittal meningioma,
traumatic brain injury, neurodegenerative
disorders
Incontinence
• Lesion below pontine micturition center
but above conus (S2-S4)
– Flaccid, acontractile (atonic) bladder >retention
– Evolves over months into hyperreflexic spastic
bladder -> retention 2ary dyssynergia and
feeling of urgency 2ary reflex bladder
contractions
• Peripheral nerve lesion or lesion at S2-S4
– Flaccid atonic bladder ->overflow incontinence
– Loss of parasympathetic outflow to detrusor or
loss of afferent sensory information
Bowel function
• Also mediated by medial frontal lobes
• 3 components:
– Internal smooth muscle sphincter + GI
motility (parasympathetics)
– External striated sphincter (Onuf)
– Pelvic floor muscles (S2-S4 anterior horn
cells)
• Etiologies: damage at any level
• Acute lesions flaccid sphincter and loss
of sacral PS  constipation
Sexual function
• Sensation from genitalia -> S2-S4 via
pudendal nerve
• Female:
– Parasympathetic: lubricating mucus from
bartholin gland
– Sympathetic: vaginal blood flow, secretions
• Male:
– Both sympa and parasympa control erection
– Sympa = ejaculation
Summary
• Sensory neuron: unipolar
• Main somatosensory pathways
– Posterior column-medial lemniscus (vib, position,
fine touch)
– Spinothalamic tract (pain and temp)
• Somatosensory cortex: somatotopic
• Central modulation of pain
• Thalamus
– Relay nuclei
– Intralaminar nuclei
– Reticular nucleus
• Spinal cord syndromes
• Bladder, bowel and sexual function