Download 06 Motor Systems

06 The Motor System
Psychology 355
General Overview
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Somatic Motor System
Muscles and Motor Neurons
Excitation-Contraction Coupling
Spinal Reflexes
Spinal Motor Programs
Descending Control of Movement
Diseases Affecting the Motor System
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Somatic Motor System
Brain versus Spinal Cord Contributions
•Primary Motor Cortex
•Basal Ganglia
•Cerebellum
•Descending Pathways
•Ventral Horn
•Motor Nerves
Muscles
•Smooth
•Striated
•Groups (Axial, Proximal, Distal)
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Somatic Motor System
Muscles
•Smooth
•Striated
•Groups
(Axial, Proximal, Distal)
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Somatic Motor System
“Motor unit” concept
“Final Common Pathway” &
Behavior
Neuromuscular junction
Feedback loops & reflexes
“Descending control”
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Somatic Motor System
“Motor unit” concept
•Muscle composed of muscle fibers
•Alpha motor neurons innervates
multiple fibers
•Group of alpha motor neurons
control a muscle
•Graded contraction by rate coding
and recruitment.
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Somatic Motor System
Alpha motor neurons receive
three major classes of input:
1) Brain motor neurons
2) Muscle sensory
3) Spinal interneurons
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Excitation-Contraction Coupling
Muscle contraction
•Alpha motor neurons release Ach
•ACh produces large EPSP in muscle fibers (via
nicotinic Ach receptors
•EPSP evokes action potential
•Action potential (excitation) triggers Ca2+
release, leads to fiber contraction
•Relaxation, Ca2+ levels lowered by organelle
reuptake
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Excitation-Contraction Coupling
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Excitation-Contraction Coupling
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Spinal Reflexes
Sensory feedback from muscle
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Spinal Reflexes
The Myotatic Reflex
•Stretch reflex: Muscle pulled tendency to pull back
•Feedback loop
•Discharge rate of sensory axons: Related to muscle length
•Monosynaptic
•Example: knee-jerk reflex
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Spinal Reflexes
The Myotatic Reflex
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Spinal Reflexes
Gamma Motor Neurons
•Muscle spindle
•Intrafusal fibers: gamma
•Extrafusal fibers: alpha
•Gamma feedback loop
provides more control
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Spinal Reflexes
Reverse myotatic reflex
•Regulate muscle tension
•Golgi tendon organs
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Spinal Reflexes
Reciprocal inhibition
Contraction of one muscle set
accompanied by relaxation of
antagonist muscle
Flexor reflex: Complex
reflex arc used to
withdraw limb from
aversive stimulus
Crossed-extensor
reflex: Activation of
extensor muscles and
inhibition of flexors on
opposite side
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Spinal Motor Programs
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Descending Control of Movement
The brain influences activity of the spinal cord
Voluntary movements
Hierarchy of controls
Highest level: Strategy
Middle level: Tactics
Lowest level: Execution
Sensorimotor system
Sensory information: Used by motor system
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Descending Control of Movement
Axons from brain descend along two major pathways
•Lateral Pathways
•Ventromedial Pathways
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Descending Control of Movement
Corticospinal and Rubrospinal Systems
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Descending Control of Movement
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Descending Control of Movement
Area 4 = “Primary motor cortex”
Area 6 = “Higher motor area” (Penfield)
Lateral region  Premotor area (PMA)
Medial region  Supplementary motor area (SMA)
Motor maps in PMA and SMA
Similar functions; different groups of muscles
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Descending Control of Movement
The Contributions of Posterior
Parietal and Prefrontal Cortex
•Represent highest levels
of motor control
• Decisions made
about actions and their
outcome
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Diseases Affecting the Motor System
Amyotrophic Lateral Sclerosis (ALS)
-motor neuron disease
Duchenne Muscular Dystrophy
-dystrophin deficit
Myasthenia Gravis
-autoimmune ACh receptors
Parkinson’s disease
- DA neurons in substantia nigra
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