The Somatosensory System - PowerPoint Presentation

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The Somatosensory SystemCH7 Blumenfield

By: Laurence Poliquin-Lasnier

R2 neurology

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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

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Main somatosensory pathways

Posterior column-medial

lemniscus

Proprioception

, vibration, fine discriminative touch

Spinothalamic

tract

Pain, temperature, crude touch

Via

unipolar

sensory neuron

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Unipolar sensory neuron

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Sensory neuron fiber types

Name

Diameter

(µ

m

)

Myelinated

Receptors

Sensory

mdality

A-

 (I)

13-20

Yes

Muscle spindle,

golgi

tendon organ

Proprioception

A-

β

(II)

6-12

Yes

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

A-

δ

(III)

1-5

Yes

Bare nerve ending

Pain, temperature (cool)

C (IV)

0.2-1.5

No

Bare nerve ending

Pain, temperature (warm), itch

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Sensory neuron

Sensory neuron cell body located in dorsal root ganglia

A peripheral region innervated by sensory fibers from a single nerve root = dermatome

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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

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Posterior column-Medial lemniscus

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Posterior column-Medial lemniscus

Large myelinated axonsProprioception, vibration, fine touch

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Posterior column-Medial lemniscus

Ascend through:

Gracile

Fasciculus: legs + lower trunk

Cuneate

Fasciculus: arms, neck, trunk above T6

1

st

order sensory neuron synapses synapse onto 2

nd

order neurons in the nucleus

gracilis

and nucleus

cuneatus

at

th

level of the medulla

Axons of these 2

nd

order neurons decussate as internal

arcuate

fibers and form the medial

lemniscus

on the other side of the medulla

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Posterior column-Medial lemniscus

2

nd

order neurons synapse into the ventral posterior lateral (VPL) nucleus of the thalamus

3

rd

order neurons then project to the posterior limb of the internal capsule to reach the primary

somatosensory

cortex in the post-central

gyrus

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Sensory homunculus

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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

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Spinothalamic tract

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SpinothalamicTract

Small diameterUnmyelinatedPain and temperature

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Spinothalamic tract

Enter spinal cord via dorsal root ganglia

1

st

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

2

nd

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)

Slide18

Spinothalamic tract

Anterolateral

pathway reaches medulla

Run between the olives and the inferior

cerebellar

peduncles

Enters

pontine

tegmentum

2

nd

order neuron synapses in the thalamus to 3

rd

order neuron

3

rd

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

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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

Slide20

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

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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)

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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

Slide23

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

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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

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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

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Thalamus

Slide27

Thalamus

3 categories of nuclei:

Relay nuclei

Intralaminar

nuclei

Reticular nucleus

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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

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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

Ventral medial

Midbrain reticular formation

Widespread to cortex

Maintain alert, conscious state

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Thalamus: Relay nuclei -> other groups

Relay Nucleus

IN

Out

Function

Medial

g

roup

Mediodorsal/dorsomedial

Amgdala

, olfactory cortex, limbic cortex, BG

Frontal cortex

Limbic pathways, major relay to frontal cortex

Anterior group

Anterior nucleus

Mammilary

bodies,

hippocampal

formation

Cingulate

gyrus

Limbic pathways

Midline thalamic group

Paraventricular

,

paratenial

,

intermediodorsal

, rhomboid, medial ventral

Hypothalamus, basal forebrain,

amygdala

, hippocampus

Amygdala

, hippocampus, limbic cortex

Limbic pathways

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Thalamus

In

Out

Function

Intralaminar

nuclei (within

internal

medullary

lamina)

1)

Rostral

intralaminar

nuclei:

Central medial nucleus,

paracentral

nucleus, central lateral nucleus

Deep

cerebellar

nuclei, GP, brainstem, ARAS, sensory pathways

Cerebral cortex, striatum

Alert consciousness, motor relay for BG and cerebellum

2)

Caudal

intralaminar

nuclei:

Centromedian

nucleus,

parafascicular

nucleus

GP, ARAS, sensory pathways

Cerebral cortex, striatum

Motor relay for BG

Reticular

nucleus

(only one not projecting to cortex)

Cerebral cortex, thalamic relay and

intralaminar

nuclei, ARAS

Thalamic relay and

intralaminar

nuclei

Regulates state of other thalamic nuclei

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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

Slide35

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

Slide36

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

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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

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Spinal cord syndromes

Transverse cord syndromeSensory level with loss of all sensory modalitiesDDx: trauma, tumor, MS, transverse myelitis

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Spinal cord syndromes

Hemicord syndrome“Brown-Sequard”Damage to lateral corticospinal tract = ipsi UMN weaknessDamage 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

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Spinal cord syndromes

Central cord syndromeSuspended sensory loss to pain and tempCape-like pattern if cervical cordSuspended dermatomes if at other levelLMN deficit if damage to anterior horn cells

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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

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Spinal cord syndromes

Posterior cord syndromeLoss of vibration and position sense below the lesionMay get UMN weakness if it encroaches lateral corticospinal tractCauses: trauma, extrinsic compression, MS, Vitamin B12 deficiency, tabes dorsalis (tertiary syphilis), HTLV-1

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Spinal cord syndromes

Anterior cord syndromeDamage to anterolateral pathway = loss of pain and temp below lesionDamage to anterior horn cell may produce LMN weakness at the level of the lesionIf larger lesion, corticospinal tract involved -> UMN weakness

Slide44

Spinal cord syndrome

Anterior spinal artery syndrome:Back of neck pain of sudden onsetRapidly progressive flaccid and areflexic paraplegiaLoss of pain and temperature to a sensory levelPreservation of JPS and vibration sensationUrinary incontinence

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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

Slide46

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

Slide47

Bladder function: detrusor reflex (voiding) and urethral reflex (storage)

Voluntary relaxation of external urethral sphincterInhibition of sympathetics to bladder neck (relaxes)Parasympathetic activation for detrusor (dome) contractionSelf-perpetuate as long as urine flowsWhen urine stops, , urethral sphincters contract triggering detrusor relaxation

Detrusor

reflex mediated by intrinsic spinal cord circuits,

pontine

micturition

center,

cerebellar

and BG pathways

Slide48

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

Slide49

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

Slide50

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

Slide51

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

Slide52

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