Spinal Control of Motor Units

Neuroscience: Exploring the

Brain, 3e

Chapter 13: Spinal Control of Movement

Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins

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  activate motor programs in spinal cord


The Somatic Motor System

• Types of Muscles

– Smooth: digestive tract, arteries, related structures

– Striated: Cardiac

(heart) and skeletal

(bulk of body muscle mass)


Lower Motor Neurons

•

Lower motor neuron: in ventral horn of spinal cord



• Distribution of lower motor neurons in the ventral horn

– Motor neurons controlling flexors lie dorsal to extensors

– Motor neurons controlling axial muscles lie medial to those controlling distal muscles



• Alpha Motor Neurons

– Motor unit: Motor neuron and all the muscle fibers it innervates

– Motor neuron pool: All the motor neurons that innervate a single muscle



• Graded Control of Muscle Contraction by Alpha Motor Neurons

– Varying firing rate of motor neurons

– Recruit additional synergistic motor units



• Inputs to Alpha 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



• Neuromuscular Matchmaking

– Crossed Innervation Experiment: John Eccles

• Switch nerve input - switch in muscle phenotype

(physical characteristics)


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

Ca 2+ release

– Fiber contracts

– Ca 2+ reuptake

– Fiber relaxes


Spinal Control of Motor Units

• Sensory feedback from muscle spindles - stretch receptor



• The Myotatic Reflex

– Stretch reflex: Muscle pulled  tendency to pull back

– Feedback loop

– Discharge rate of sensory axons: Related to muscle length

– Monosynaptic

– e.g., Knee-jerk reflex



• The Myotatic Reflex (Cont’d)



• Two Types of Muscle Fiber

– Extrafusal fibers:

Innervated by alpha motor neurons

– Intrafusal fibers:

Innervated by gamma motor neurons



• Gamma Loop

– Keeps spindle “on air”

– Changes set point of the myotatic feedback loop

– Additional control of alpha motor neurons and muscle contraction



•

Golgi Tendon Organs

–

Additional proprioceptive input - acts like strain gauge - monitors muscle tension



•

Golgi Tendon Organs

–

Spindles in parallel with fibers; Golgi tendon organs in series with fibers



• Golgi Tendon Organs

– Reverse myotatic reflex function: Regulate muscle tension within optimal range



• Proprioception from the joints

– Proprioceptive axons in 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

– Most receptors are rapidly adapting, bring information 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 set accompanied by relaxation of antagonist muscle

• Example: Myotatic reflex



• Excitatory Input

– Flexor reflex:

Complex reflex arc used to withdraw limb from aversive stimulus



• Excitatory Input

– Crossed-extensor reflex:

Activation of extensor muscles and inhibition of flexors on opposite side



• Generating Spinal Motor Programs for Walking

– Circuitry for walking resides in spinal cord

– Requires central pattern generators



• Rhythmic Activity in a Spinal Interneuron via NMDA

Receptors



• Possible Circuit for Rhythmic Alternating Activity


Central Pattern Generators: Pyloric Rhythm & Endogenous burster (Pacemaker) Neurons


Dogfish Swimming: Reafferent modulation of CPG Rhythm.

Tail movement


Concluding Remarks

• Spinal control of movement

– Different levels of analysis

– Sensation and movement linked

• Direct feedback

– Intricate network of circuits


End of Presentation



• Somatic Musculature and distribution of lower motor neurons in spinal cord

– Axial muscles: Trunk movement

– Proximal muscles:

Shoulder, elbow, pelvis, knee movement

– Distal muscles: Hands, feet, digits (fingers and toes) movement



• The Molecular Basis of Muscle Contraction

– Z lines: Division of myofibril into segments by disks

– Sarcomere: Two Z lines and myofibril

– Thin filaments: Series of bristles

– Thick filaments: Between and among thin filaments

– Sliding-filament model:

• Binding of Ca 2+ to troponin causes myosin to bind to action

• Myosin heads pivot, cause filaments to slide



Sliding-filament Model of Muscle Contraction



•

Steps in Excitation-Contraction Coupling

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


Spinal Control of Motor Units

Lower motor neuron: in ventral horn of spinal cord

Golgi Tendon Organs

Additional proprioceptive input - acts like strain gauge - monitors muscle tension

Golgi Tendon Organs

Spindles in parallel with fibers; Golgi tendon organs in series with fibers

Excitation-Contraction Coupling

Steps in Excitation-Contraction Coupling

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

Related documents

Products

Support

Spinal Control of Motor Units

Lower motor neuron: in ventral horn of spinal cord

Golgi Tendon Organs

Additional proprioceptive input - acts like strain gauge - monitors muscle tension

Golgi Tendon Organs

Spindles in parallel with fibers; Golgi tendon organs in series with fibers

Excitation-Contraction Coupling

Steps in Excitation-Contraction Coupling

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

Related documents

Add this document to collection(s)

Add this document to saved

Suggest us how to improve StudyLib