Chapter 13 Spinal Control of

Movement

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)

Graded Control of Muscle Contraction by Alpha Motor Neurons– Varying firing rate of motor neurons– Recruit additional synergistic motor units

Lower Motor Neurons

Tetanus – as painted by Sir Charles Bell in 1809.

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

Lower Motor Neurons

Figure 9.7a

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 CouplingExcitation-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

Excitation-Contraction CouplingExcitation-Contraction Coupling

Figure 9.3a–c

Figure 9.3c–e

Figure 9.3d

Figure 9.4

Figure 9.6

Figure 9.11

Figure 9.12

Sensory feedback from muscle spindles - stretch receptor

Spinal Control of Motor Units

The Myotatic Reflex

Spinal Control of Motor Units

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

Excitatory Input– Crossed-extensor

reflex: Activation of extensor muscles and inhibition of flexors on opposite side

Spinal Control of Motor Units

Afferentfiber

Efferentfibers

Extensorinhibited

Flexorstimulated

Right arm(site of stimulus)

Left arm (site ofreciprocal activation)

Arm movements

Interneurons

Key:+ Excitatory synapse– Inhibitory synapse

Efferentfibers

Flexorinhibited

Extensorstimulated

+

–+

–

+

+

Flexes

Extends

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