Lec #13_Spinal Motor

8/13/2019 Lec #13_Spinal Motor

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Copyright 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins

Neuroscience: Exploring the

Brain, 3e

Chapter 13: Spinal Control of Movement


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Introduction

Motor Programs

Motor system: Muscles and neurons that controlmuscles

Role: Generation of coordinated movements

Parts of motor control

Spinal cordcoordinated muscle contraction

Brainactivate motor programs in spinal cord


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The Somatic Motor System

Types of Muscles

Smooth: digestive

tract, arteries, relatedstructures

Striated: Cardiac(heart) and skeletal(bulk of body muscle

mass)


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Lower Motor Neurons

Lower motor neuron: inventral horn of spinal

cord


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Lower Motor Neurons

Distribution of lower motorneurons in the ventralhorn

Motor neuronscontrolling flexors liedorsal to extensors

Motor neurons

controlling axialmuscles lie medial tothose controlling distalmuscles


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Lower Motor Neurons

Alpha Motor Neurons

Motor unit: Motor

neuron and all themuscle fibers itinnervates

Motor neuron pool: Allthe motor neurons that

innervate a singlemuscle


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Graded Control of Muscle Contraction by Alpha Motor Neurons

Varying firing rate of motor neurons

Recruit additional synergistic motor units

Lower Motor Neurons



Inputs to Alpha Motor NeuronsLower 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, anaerobicmetabolism, contract and fatigue rapidly

Fast motor units: Rapidly fatiguing white fibers

Slow motor units: Slowly fatiguing red fibers

Lower Motor Neurons



Neuromuscular Matchmaking

Crossed Innervation Experiment: John Eccles

Switch nerve input - switch in muscle phenotype(physical characteristics)

Lower Motor Neurons



Muscle Contraction

Alpha motor neuronsrelease ACh

ACh produces largeEPSP in muscle fiber

EPSP evokes muscleaction potential

Action potential triggersCa2+release

Fiber contracts

Ca2+reuptake

Fiber relaxes

Excitation-Contraction Coupling



Sensory feedback from muscle spindles - stretch receptor

Spinal Control of Motor Units



The Myotatic Reflex

Stretch reflex: Muscle pulledtendency to pull back

Feedback loop

Discharge rate of sensory axons: Related to musclelength

Monosynaptic

e.g., Knee-jerk reflex




The Myotatic Reflex (Contd)Spinal Control of Motor Units




Two Types of Muscle Fiber

Extrafusal fibers:Innervated by alpha motorneurons

Intrafusal fibers:Innervated by gamma

motor neurons



Spinal Control of Motor Units Gamma Loop

Keeps spindle on air

Changes set point of the myotatic feedback loop Additional control of alpha motor neurons and

muscle contraction



Spinal Control of Motor Units Golgi Tendon Organs

Additional proprioceptive input - acts like straingauge - monitors muscle tension




Spindles in parallel with fibers; Golgi tendonorgans in series with fibers




Reverse myotatic reflex function: Regulate muscletension within optimal range



Proprioception from the joints

Proprioceptive axons in joint tissues

Respond to angle, direction and velocity ofmovement in a joint

Information from joint receptors: Combined withmuscle spindle, Golgi tendon organs, skin receptors

Most receptors are rapidly adapting, bringinformation about a moving joint




Spinal Interneurons

Synaptic inputs to spinal interneurons:

Primary sensory axons

Descending axons from brain

Collaterals of lower motor neuron axons

Other interneurons




Inhibitory Input

Reciprocal inhibition: Contraction of one muscle setaccompanied by relaxation of antagonist muscle

Example: Myotatic reflex




Excitatory Input

Flexor reflex:Complex reflex arcused to withdraw limbfrom aversivestimulus



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Excitatory Input

Crossed-extensor reflex:

Activation of extensormuscles and inhibition offlexors on opposite side



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Generating Spinal Motor Programs for Walking

Circuitry for walking resides in spinal cord

Requires central pattern generators



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Spinal Control of Motor Units Rhythmic Activity in a Spinal Interneuron via NMDA

Receptors


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Possible Circuit for Rhythmic Alternating Activity



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Central Pattern Generators: Pyloric Rhythm & Endogenousburster (Pacemaker) Neurons


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Dogfish Swimming: Reafferent modulation of CPG Rhythm.

Tail

movement


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Concluding Remarks

Spinal control of movement

Different levels of analysis

Sensation and movement linked

Direct feedback

Intricate network of circuits


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End of Presentation


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Lower Motor Neurons

Somatic Musculature anddistribution of lower motorneurons in spinal cord

Axial muscles: Trunkmovement

Proximal muscles:Shoulder, elbow,

pelvis, knee movement Distal muscles: Hands,

feet, digits (fingers andtoes) movement


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The Molecular Basis of Muscle Contraction

Z lines: Division of myofibril into segments bydisks

Sarcomere: Two Z lines and myofibril Thin filaments: Series of bristles

Thick filaments: Between and among thinfilaments

Sliding-filament model:

Binding of Ca2+to troponin causes myosin tobind to action

Myosin heads pivot, cause filaments to slide



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Sliding-filament Model of Muscle Contraction


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Excitation-Contraction Coupling Steps in Excitation-Contraction Coupling

Ca+ binding to troponin allows myosin headsto bind to actin. Then myosin heads pivot,causing filaments to slide

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