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Biology 463 - Neurobiology
Topic 14
Spinal Control of
Movement
Lange
Introduction
Motor Programs
– Motor system: Muscles and neurons that control muscles
– Role: Generation of coordinated movements
– Parts of motor control
• Spinal cord coordinated muscle contraction
• Brain motor programs in spinal cord
The Somatic & Autonomic Motor Systems
Types of Muscles
– Smooth:
digestive tract,
arteries, related
structures
– Striated:
Cardiac (heart)
– skeletal (bulk of
body muscle
mass)
Lower Motor Neurons
Graded Control of Muscle Contraction by Alpha Motor Neurons
– Varying firing rate of motor neurons
– Recruit additional synergistic motor units
Tetanus – as painted by Sir Charles Bell in 1809.
Lower Motor Neurons
Types of Motor Units
– Red muscle fibers: Large number of
mitochondria and enzymes, slow to contract,
can sustain contraction
– White muscle fibers: Few mitochondria,
anaerobic metabolism, contract and fatigue
rapidly
– Fast motor units: Rapidly fatiguing white
fibers
– Slow motor units: Slowly fatiguing red fibers
Figure 9.7a
Excitation-Contraction Coupling
Muscle Contraction
– Alpha motor neurons
release ACh
– ACh produces large EPSP in
muscle fiber
– EPSP evokes muscle action
potential
– Action potential triggers
Ca2+ release
– Fiber contracts
– Ca2+ reuptake
– Fiber relaxes
Excitation-Contraction Coupling
The Molecular Basis of Muscle Contraction
– Z lines: Division of myofibril into segments by disks
– Thin filaments: Series of bristles
– Thick filaments: Between and among thin filaments
– Sliding-filament model:
• Binding of Ca2+ to troponin causes myosin to bind to
action
• Myosin heads pivot, cause filaments to slide
Figure 9.3a–c
Figure 9.3c–e
Figure 9.3d
Figure 9.4
Figure 9.6
Figure 9.11
Figure 9.12
Spinal Control of Motor Units
Sensory feedback from muscle spindles - stretch receptor
Spinal Control of Motor Units
The Myotatic Reflex
Spinal Control of Motor Units
Two Types of Muscle Fiber
– Extrafusal fibers:
Innervated by alpha motor
neurons
– Intrafusal fibers: Innervated
by gamma motor neurons
Spinal Control of Motor Units
Golgi Tendon Organs
– Additional proprioceptive input - acts like strain gauge monitors muscle tension
Spinal Control of Motor Units
Golgi Tendon Organs
– Spindles in parallel with fibers; Golgi tendon organs in
series with fibers
Spinal Control of Motor Units
Excitatory Input
– Crossed-extensor
reflex: Activation
of extensor
muscles and
inhibition of
flexors on
opposite side
Interneurons
+
+
–
Afferent
fiber
+
+
–
Efferent
fibers
Efferent
fibers
Extensor
inhibited
Arm movements
Flexor
stimulated
Key:
+ Excitatory synapse
– Inhibitory synapse
Right arm
(site of stimulus)
Left arm (site of
reciprocal activation)
Flexor
inhibited
Extensor
stimulated
Information gleaned about nicotinic ACh
receptors utilzed the electric organs of
electric eels because of their high
concentration of the Ach receptor. The
Disease Myasthenia gravis is an
autoimmune disease where the body's
immune system has damaged receptors on
your muscles causing long term weakness
and eventual, premature death.
Individual showing classic, early signs of
Myasthenia gravis
END.