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Somatosensory & Motor Systems
• The Reflex Arc
• Hierarchy, Somatotopy and Functional Segregation
• Somatosensory Syndromes
–
–
–
–
Hemianesthesia
Thalamic Pain
Asterognosia
Gerstmann Syndrome
• Motor Syndromes
–
–
–
–
Hemiplegia/Hemiparesis
Apraxia
Ataxia
Basal Ganglia Disorders
The reflex arc
• Neurons carry a message from
stimulated receptors
to the correct effectors
• A sensory neuron carries
the message from the receptor
to the CNS
• A motor neuron carries the
message from the CNS to the effector.
Knee-jerk
Reflex
• Simple stimulusresponse
• ‘Closed loop’ (not
influenced by other
input)
The reflex arc
• In most reflex arcs
the sensory neuron
connects to motor
neurons through
association neurons
(interneurons) in
the CNS.
• Interneurons make
the correct connection
between the sensory
neuron and the motor
neuron (integration)
Cortico-Spinal
Inhibition
via Interneurons
•
•
Interneurons provide descending input
from outside the ‘closed loop’
Permit modulation (fine tuning)
of ‘all-or-none’response of
competing flexor-extensor
muscles
The Central Nervous System is Just
One Big Interneuron
Afferent
Pathways
Efferent
Pathways
Hierarchical Organization
• Sensory system
– lower level features are integrated into higher level ‘objects’
– objects & self are integrated into meaningful environmental ‘scene’
• Motor system
– ‘goal’ is subdivided into ‘subroutines’
– ‘subroutines’ are expressed as coordinated sequence of individual
muscle movements
• Sensori-motor ‘cross-talk’ ensures smooth
execution of movement, based on continuous
sensory updates
General Motors and the Central Nervous System
Sensory System As ‘Market Research’
Self in Relation to
Objects in
Environment
(Tertiary Sensory)
Objects in
Environment
(Secondary
Sensory)
Safe
Object Features
(Primary Sensory)
Marketable Car
Headlights
Bright
Engine
Style
Speed
Dashboard
Power Economy Style
Info.
General Motors and the Central Nervous System
Motor System As ‘Production’
Dorsolateral
Pre-Frontal
Cortex
Secondary
Motor Ass.
Cortex
Primary
Motor
Cortex
Overall Design
Headlight
Design
Bulb
Basal Ganglia
Cerebellum
Cover Reflector
Headlight
Engine
Design
Axle
Piston
Engine
Car
Dashboard
Design
Valve
Meter
Lights
Dashboard
Somatosensory & Motor Disorders
• Reflect breakdown at different levels of the
– sensory hierarchy
• primary sensation
• integration of sensations into percepts
• integration of percepts into ‘environments’
– motor hierarchy
• primary motor movement
• skilled movements
• initiation, coordination and cessation of skilled movements
for goal directed behavior
• Pathognomonic sign – a sign whose presence means that a
particular disease is present beyond any doubt
• lateralized deficit in species-wide, normative behavior always
associated with lateralized and localized pathology
Somatopic Organization of
Tactile Perception
Somatopic Organization of
Tactile Perception
• Somatosensory Homunculus
Functional
Segregation
• Somatic sensation is
functionally segregated
at level of spinal cord
Cross here
Cross here
Somatosensory Disorders
• Hemianesthesia (primary sensory loss) –
– inability to register tactile information
– Test – sensory threshold (light touch, pinprick, heat/cold, limb
movement)
– Location of lesion
• anywhere from peripheral
nerve to post-central gyrus
• Loss at dermatome reveals
level of injury in spine
• Modality dissociation
(hapsis/proprioception vs.
pain/temperature) indicates
hemi-spinal cord lesion
ipsilateral to side that lost
hapsis/proprioception
Higher-Level Somatosensory Disorders
• Thalamic Pain –
– initial hemianesthesia, later hypersensitivity
– ‘knife-like’, ‘burning’, ‘crushing’
– increased by light touch, change in temperature or other
sensory/affective stimuli (music, fear, anger)
– Location of lesion – posterolateral thalamus
• Astereognosia/Graphesthesia
– Failure to recognize palpated objects in presence of intact
hapsis and feature identification
– Test – recognize different coins by feel, fingertip-numberwriting
– Location of lesion – contralateral parietal association cortex
Multi-modal Sensory-Integration Deficit
• Gerstmann Syndrome
– Extremely rare
– ‘Gerstmann tetrad’
• Right-left confusion
• Finger agnosia
• Dysgraphia (difficulty with writing)
• Dyscalculia (difficulty with
calculations)
– Isolated cases due to small strokes
near junction of left parietal (body
schema) and temporal (symbol
meaning) association cortex
Lateralized and
Crossed Motor
Organization
• Guided limb movements of
most interest to
neuropsychologists
• Lower motor neurons (LMNs)
synapse ipsilaterally
• 90% of upper motor neurons
(UMNs) cross at level of
medulla (pyramids)
Cross here
Upper Motor
Neurons
Lower Motor Neuron
Somatopic Organization of
Motor Function
• Motor Homunculus
Simple Motor Disorders
• Hemiparesis/hemiplegia
– difficulty/inability to move limb
– Test – grip/leg strength
– Location of lesion – anywhere along cortico-spinal tract
• if spinal cord lesion, deficit is ipsilateral to lesion
• if supra-medullary lesion, deficit is contralateral to lesion
Symptom
Lower Motor Neuron
Upper Motor Neuron
Flaccid
Spastic
Yes
No
Reflexes
Absent
Hyperactive
Babinski sign
Absent
Present
Muscle Tone
Muscle Atrophy
Higher Cortical
Motor Disorders
• Apraxia
– Inability to perform skilled movement in presence of preserved single
muscle group movements and muscle strength
– Test – imitate/carry out gestures, use objects
• Inability to imitate/differentiate gestures (salute, blow a kiss)
• Inability to carry out action to command (answer phone)
• Inappropriate use of object (comb hair with screwdriver)
• Incorrect sequencing of movements (mailing a letter)
– Cognitive mechanisms and subtypes are still debated
– Location of lesion – left hemisphere (supplementary motor, inferior
parietal lobe), corpus callosum
Disorders of Coordinated Movements
• Ataxia
– Inability to coordinate goal-directed movements in presence of
preserved muscle strength and appropriate motor programs
– Test – finger-to-nose pointing, heel-to-shin, heel-to-toe walk,
Grooved Pegboard
– Location of lesion – cerebellum (motor coordination) or dorsal
columns (proprioception)
Basal Ganglia
Lateral View
Anterior view
a.
b.
Coronal View
a.
b.
a. Caudate
Striatum
b. Putamen
c. Globus pallidus
c.
Caudate
Globus
Pallidus
Ventral View
Putamen
Axial View
Basal Ganglia Disorders
• Normally inhibits excitatory input from thalamus to motor cortex
• Regulate initiation, termination and modulation of movements
Condition
Signs/Symptoms
Etiology
Pathology
Parkinson’s
Disease
Resting tremor, slowed
movement, muscle rigidity
Unknown genetic &
environmental
interactions
Late Degeneration of
Substantia Nigra
Huntington’s
Disease
Chorea (writhing
movements), psychiatric
symptoms
Known autosomal
dominant mutation
Early Degeneration
of Caudate Nucleus
Motor Function Tests
• Grip Strength
– simple motor
strength
– Sensitive to
dysfunction of
primary motor
pathways
• Finger Tapping
– rapid rhythmic movements
– sensitive to deficits in smooth
execution of movements d/t
frontal, subcortical or
cerebellar dysfunction
Grooved
Pegboard
• Sensitive to motor deficits at
different levels in the hierarchy
– Primary motor – weakness/slowing/poor grip
– Cerebellum - incoordination
– Parietal lobe – mis-orientation of pegs (failure
to integrate visual and tactile information)