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Voluntary Movement
Ch. 14: Supplemental Images
Skeletal Motor Unit: The basics
• Upper motor neuron: Neurons that supply input to lower motor neurons.
• Lower motor neuron: neuron that innervates muscles, including skeletal muscle fibers
involved in movement.
1. Alpha motor neuron: Aα fiber innervates extrafusal muscle fibers
2. Gamma motor neurons: Aγ fibers innervates intrafusal muscle fibers
• Motor Unit: one lower motor neuron and all muscle fibers that it innervates.
1. Innervation ratio: Extraocular =1:10; Hand = 1:100; Gastrocemius in knee = 1:2000
2. Proportional to size of muscle
3. Trade off between control and total force
• Extrafusal muscle fibers:
1. Workhorse fibers – all of the force of muscle
2. ACh at neuromuscular endplate: endplate potential that leads to “All-or-none” fiber contraction.
3. Sherrington (1925) “ Final common pathway”
a. Lower motor neuron influenced by many sources
b. Sums influences on neuron to determine if Action Potential occurs, releasing ACh at endplate
• Grading a muscle contraction
1. Number of motor units activated
2. Rate of action potentials of a motor neuron (rate modulation): higher rate = more force
Hierarchy of Movement
•
Motor Unit
•
Reflexes:
– Monosynaptic reflex arc (e.g. patellar)
– Polysynaptic reflexes (e.g. cross-extensor reflex)
•
Rhythmic Motor Patterns:
– once initiated, the sequence of relatively stereotyped repetitive movements
can continue in automatic manner (e.g., walking)
– combines characteristics of reflexes with voluntary (initiation,
termination) movement.
– central pattern generators (Ch 13)
•
Voluntary movement:
– purposeful, goal-drected movement
– Often learned movements which improve with practice and requires less
attention, e.g., combing hair, swinging a bat, driving a car, swimming,
using eating utensils, etc, etc, etc….
– Cortical control over movement.
Voluntary movement
Organization of voluntary movement – general sequence
• Planning, strategy:
– Purpose/motivation
– Order of movement (each set of movement prepares body for next set)
• Direction
• Rate of movement
• Strength
• Timing
–One movement relative to another
–Duration of each movement
–Phasic (transient, discrete movements)
–Static (e.g., stabilize joints)
•
•
•
•
•
•
Initiate (start) movement
Postural adjustments (+ feedback)
Medial limb movement (+ feedback)
Distal limb (+feedback)
Adjustments at EVERY point
Stop movement
Voluntary movement (cont.):
hierarchical organization
Non-proprioceptive sensory input
1
1
2
8
3
3
3
7
3.5
3.5
4
5, 6
Proprioceptive
Sensory input
7
General Organization of Spinal Cord
Ascending pathways
Lateral spinothalamic tract
Descending pathways
14.2
Primary Motor Cortex: M1/Area 4/Precentral gyrus
• Amount of force
– Different cells for extensors, flexors
– Cells are active BEFORE contraction
– AP rate increases and decreases as load
increases or decreases.
• Direction of movement depends on
population of cells
– Individual cells have directional preference
– Population vectors
– Requires feedback for ongoing corrections
• Minor discrepancies –modifies cells
within population
• Major discrepancies - changes
population of cells
• ALL cells are modified by
– Other cortices
– Basal ganglia
– Cerebellum
14.14
14.7
Premotor Motor Cortex: Area 6
• Proximal and axial muscles – initial
phases of orienting body and limbs
– Set-related neurons
• Preparation of motor response: cells
activated when subject is given information
to perform a task with specific location
• Relatively simple tasks.
• Cells active before execution of movement,
continues until movement is complete.
• Does NOT appear to encode fine detailed
movement
• Stimulus triggered movement
– Spontaneous movement
• Cell AP responses are modified by
– Other motor cortices
– Basal ganglia
– Cerebellum
14.7
Descending pathways: Reticulospinal pathways
14.6
14.5
•
Posture and antigravity muscles of limbs
– Pontine: Enhances antigravity reflexes by facilitating extensors of legs to
maintain length and tension of muscles (e.g., vestibular).
– Medullary: Inhibits antigravity reflexes (opposite effect) to allow change as
needed (e.g., to begin movement).
– Cortex keeps these two antagonistic systems in balance.
Descending pathways: Vestibulospinal and tectospinal
14.4
•Posture of head and neck
– Vestibulospinal: Several vestibular nuclei in brain stem keep head balanced
during movement and turns head as needed.
– Tectospinal: Helps coordinate eye movement (superior colliculus function)
with head movement (tectospinal).
Descending pathways: Corticospinal and rubrospinal tracts
14.3
Fine Movement of Arms and Fingers
– Corticospinal (pyramidal): Contralateral Areas 4 and 6. 80-85%
decussate (cross) at posterior medulla. Terminate on alpha motor neurons
and interneurons – mostly flexor activation.
– Rubrospinal: Complements corticospinal pathways.
Basal Ganglia Terminology
• “Basal Ganglia” includes all of the
following – Caudate nucleus
– Putamen
– Globus pallidus (palidum) - external,
internal
– Subthalamus
– Substantia nigra
• Subgroupings
– “Striatum” = caudate nucleus + putamen
– “Lenticular nucleus” = putamen + globus
pallidus
– “Corpus striatum” = caudate n. +
lenticular n. + internal capsule (crosshatch of gray and white fibers)
• Direct Pathway
• Indirect Pathways
Direct Pathway (Motor Loop)
• Flow of information through basal ganglia for point-topoint adjustments in execution of ongoing motor signals
Motor cortices →
putamen →
globus pallidus →
VA\VL →
motor cortices
No Movement
no AP
no AP
high AP = tonic inh.
no AP
unaltered
Ongoing
movement
Topographic
increase in AP
Specific
+AP
specific - AP =
release of inh.
Specific
+AP
Specific +AP
Other
Input
+
+
-
Enhanced
or modified
movement
- - + - --
Selected
excitation that
modifies
motor signals
Selected release
of inhibition
Basal Ganglia: Indirect Motor Loops
Ventrolateral n.
14.12
Substantia Nigra
Combined +/-
Subthalamus
Excites selected
cells of GP which
INCREASES
inhibition of VL
n. of thalamus
(and decreases
behavior)
Posterior and Anterior Spinocerebellar Pathways
Human upright position
Cerebellum (Latin="little brain")
• Functions
– Coordinating reflexive and voluntary movement, particularly if
• Movements are rapid
• Require accurate aiming and timing
• Automatic, not at level of awareness
– Maintain equilibrium
– Generating and planning movements
– Some motor learning (Classical conditioning; well-practiced, automatic responses)
– Cognitive functions
• Behavioral Examples
– Playing instruments
– Athletic skills
– Typing
– Writing
– Speaking
Cortical Zones (Nolte, 1993)
Medial Zone:
Vermis (“worm”-spinocerebellum)
Vestibular input/postural control
Deep nuclei
Lateral & Intermediate zones
Similar but not same circuitry.
Output through deep nuclei
Superior cerebellar peduncles
(brachium conjunctivum)
VL/VA n. of thalamus
Motor cortices
Flocculonodular node
“Vestibulocerebellum”
Vestibular input/eye movement
Deep nuclei
Deep nuclei (Nolte, 1999)
•
All output from cerebellar cortex
goes through deep nuclei.
•
Nuclei:
– Fastigial
– Interposed nuclei
• Globose n.
• Emboliform n.
– Dentate
Principal Cerebellar - Cortical Relationship
•
Per caption: principal circuit
by which deep nuclei
influence cortically
controlled movement.
•
PD: Pyramidal Decussation
•
RST: Rubrospinal tract