Download Bear: Neuroscience

Bear: Neuroscience:
Exploring the Brain 3e
• Chapter 13: Spinal Control of Movement
Christopher Reeve
C1-C2 level injury
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The Hierarchical Control of Movement
Fig 10-2
Many components, but functions as a WHOLE
• 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 Motor System
• Types of Muscles
– Smooth:
digestive tract, arteries,
related structures
– Striated:
Cardiac (heart – sort of)
and skeletal (bulk of
body muscle mass)
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The Somatic Motor System
• Somatic Musculature
– Axial muscles: Trunk movement
– Proximal muscles: Shoulder, elbow, pelvis,
knee movement
– Distal muscles: Hands, feet, digits (fingers
and toes) movement
The Somatic Motor System
• The Lower Motor
Neuron
– Lower motor
neuron: Innervated
by ventral horn of
spinal cord
– Upper motor
neuron: Supplies
input to the spinal
cord
Spinal cord injuries
•Motorneurons
below the injury remain intact.
•Motor cortex commands do not reach muscles and muscles atrophy.
•Electrodes can artificially activate muscles and prevent atrophy
UPPER MOTOR NEURON SYNDROME DAMAGE TO DESENDING PATHWAYS
Damage to the pathways driving the motor neurons
•Spasticity
•
TONE AND REFLEXES INCREASED
Spastic cerebral palsy for example
LOWER MOTOR NEURON SYNDROME - DAMAGE DIRECT TO MOTOR
NEURONS
Diseases or lesions at the level of the motorneuron or its axon
•Atrophy- loss of muscle volume
DECREASED TONE AND REFLEXES
Poliomyelitis for example
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The Somatic Motor System
• Alpha Motor Neurons
Final Common Pathway
• Motor unit – one motor neuron
and muscle fibers
• Pool – single muscle
– Two kinds of lower motor neurons
• Alpha
• Gamma (maintains muscle tension)
Guillain Barre syndrome
(ghee yan bah ray)
• Syndrome not disease (unclear what disease)
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Paralysis (can be total)
Attacks Schwann cells, then axons
Autoimmune
Similar to MS in CNS
70% recovery! Why????
Following vaccine (rabies, swine flu)
1 case per million 1 death per 20 million
(normal?)
The Somatic Motor System
• Graded Control of Muscle Contraction by Alpha
Motor Neurons
– Varying firing rate of motor neurons
• (temporal summation)
– Recruit additional synergistic motor units
– Recruit smallest first, largest last (why small fine
movements are not possible under great load)
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The Somatic Motor System
• THREE Inputs to Alpha Motor Neurons
Feedback
on
muscle
length
(dorsal
root
ganglia)
The Somatic Motor System
• 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 (but
POWERFUL - escape)
– Fast motor units: Rapidly fatiguing white fibers
– Slow motor units: Slowly fatiguing red fibers
The Somatic Motor System
• PLASTIC
• Neuromuscular Matchmaking
– Experiment: John Eccles
• Are muscle properties due to innervating nerve characteristics?
• Alternate nerve input
– Switch in muscle phenotype (physical characteristics)
ACTIVITY
– Hypertrophy: Exaggerated growth of muscle fibers
– Atrophy: Degeneration of muscle fibers
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FAST twitch (fatigue rapidly – white) SLOW twitch (fatigue slow – red)
Together “calf muscle”
charley horse
Forced change in input – switch phenotype (physical characteristics)
30-60/sec bursts - 10-20/sec steady
Excitation-Contraction Coupling
• Muscle contraction
– Alpha motor neurons release Ach
• Innervate muscle fibers
– 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
Excitation-Contraction Coupling
• Muscle
Fiber
Structure
Myofibrils
contractile fibers
Sarcoplasmic
reticulum
(endoplasmic)
CALCIUM
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Excitation-Contraction Coupling
• The Molecular Basis of
Muscle Contraction
– Z lines: Division of myofibril
into segments by disks
– Sarcomere: Two Z lines and
myofibril
– CONTRACTILE UNIT
Thin filaments ACTIN: Series of bristles
Thick filaments MYOSIN:
Between/among thin filaments
Myosin sites blocked by troponin
Sliding-filament model:
Binding of Ca2+ to troponin causes
myosin to bind to actin
Myosin heads pivot, cause filaments to
slide
RELAXED/CONTRACTED
twitch
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Excitation-Contraction Coupling
• Steps in Excitation-Contraction Coupling
– Excitation: Action potential, ACh release,
EPSP, action potential in muscle fiber,
depolarization
– Contraction: Ca2+, myosin binds actin,
myosin pivots and disengages, cycle
continues IF Ca2+ and ATP present
– Relaxation: EPSP end, resting potential, Ca2+
by ATP driven pump, myosin binding actin
covered
Steps in Excitation-Contraction Coupling
page 436
Rigor mortis
• No ATP (myosin uses ATP to disengage)
• Calcium pumps are driven by ATP
• Somewhat permanent myosin/actin binding
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Excitation-Contraction Coupling
• Spinal Control of Motor Units
– First source: Sensory feedback from muscle
Excitation-Contraction Coupling
• The Myotatic Reflex
– Stretch reflex: Muscle pulled tendency to
pull back
– Feedback loop
– Discharge rate of sensory axons: Related to
muscle length
– Monosynaptic (one synapse sensory/motor)
– Example: knee-jerk reflex
• The Myotatic Reflex (kneecap tendon
stretches quad muscle, triggers contraction)
WHY FASTER?
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Monosynaptic reflexes
bypass the brain???.
Myasthenia Gravis
•
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Severe muscle weakness (but fluctuates)
1 in 10,000 M & F
Autoimmune disease (Thymus?)
Attacks nicotinic Ach receptors
Blocks and degrades receptors
Treatment – inhibit AChE!! (carefully)
Treatment – suppress immune system
Duchenne Muscular Dystrophy
• Genetic – Duchenne 1 in 3500
• ONLY males, so X-linked
(single X is enough)
X region codes for protein “dystrophin”
In MD, no mRNA for this cytosketal protein
Muscles tears
WHY normal phenotype for early life?
Could virus help????? (gene therapy)
Could stem cells help?
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Excitation-Contraction Coupling
• Gamma Motor Neurons
– Muscle spindle (stretch
receptor)
• Intrafusal fibers: gamma
• Extrafusal fibers: alpha
Excitation-Contraction Coupling
• Gamma Loop
– GAMMA LOOP
• Gamma motor neuron intrafusal muscle
fiber Ia afferent axon
alpha motor neuron -> extrafusal muscle fiber
Excitation-Contraction Coupling
• Proprioception from Golgi Tendon Organs
• Golgi tendon monitors muscle tension
• (spindle muscle length)
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Excitation-Contraction Coupling
• Proprioception from the joints, too
– Proprioceptive axons in collective joint tissues
– Respond to angle, direction and velocity of
movement in a joint
– Information from joint receptors: Combined
with muscle spindle, Golgi tendon organs, skin
receptors
Excitation-Contraction Coupling
• Spinal Interneurons
– Synaptic inputs
• Primary sensory axons
(Ia)
• Descending axons from
brain
• Collaterals of lower
motor neuron axons
Excitation-Contraction Coupling
• Inhibitory Input
– Reciprocal inhibition: Contraction of one
muscle set accompanied by relaxation of
antagonist muscle
• Example: Myotatic reflex
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Excitation-Contraction Coupling
• Excitatory Input from
Interneurons too
– Flexor reflex:
– Complex reflex arc
used to withdraw limb
from aversive stimulus
Excitation-Contraction Coupling
• Excitatory Input
Crossed-extensor reflex: Activation of extensor muscles
and inhibition of flexors on opposite side
Excitation-Contraction Coupling
• The Generation of Spinal Motor Programs
for Walking
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Excitation-Contraction Coupling
• The Generation of Spinal Motor Programs
for Walking
Central pattern generators
– Membrane depolarizes
– Na & Ca in (NMDA)
– Ca activates K
channels
– K LEAVES cell
– Membrane
hyperpolarizes
– Ca stops
– K channels close
– repeat
• Spinal control of movement
– Different levels of analysis
– Sensation and movement linked
• Direct feedback
– Intricate network of circuits
EXCITE
CONTRACT
RELAX
UNDERSTAND steps on Pg 436
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ventral and lateral tracts– motor - spinal cord to muscles
cell bodies are in gray matter of spinal cord
dorsal fibers – sensory, periphery to Spinal Cord
cell bodies are outside -in dorsal root ganglia
The Motor Neurons -and interneurons
located in gray matter of ventral “horn”
Lateral corticospinal tract synapses on motor neurons
that move muscles in limbs and digits in contralateral
side.
Ventral corticospinal tract synapses on motor neurons
of midline muscles (trunk) in ipsi side.
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