06 The Motor System

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06 The Motor System
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General Overview

• 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
• Brain motor neurons
• Muscle sensory
• 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-exte
nsor 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 innervated
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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
• Parkinsons disease
• - DA neurons in substantia nigra