1. Neuroscience: Exploring the Brain, 3e
Chapter 13: Spinal Control of Movement

2. 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

3. The Somatic Motor System
Types of Muscles
Smooth: digestive tract, arteries, related structures
Striated: Cardiac (heart) and skeletal (bulk of body muscle mass)

4. Lower Motor Neurons
Lower motor neuron: in ventral horn of spinal cord

5. Lower Motor Neurons
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

6. Lower Motor Neurons 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

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

8. Lower Motor Neurons
Inputs to Alpha Motor Neurons

9. 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

10. Lower Motor Neurons Neuromuscular Matchmaking
Crossed Innervation Experiment: John Eccles
Switch nerve input - switch in muscle phenotype (physical characteristics)

11. 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

12. Spinal Control of Motor Units
Sensory feedback from muscle spindles - stretch receptor

13. Spinal Control of Motor Units
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

14. Spinal Control of Motor Units
The Myotatic Reflex (Cont’d)

15. Spinal Control of Motor Units
Two Types of Muscle Fiber
Extrafusal fibers: Innervated by alpha motor neurons
Intrafusal fibers: Innervated by gamma motor neurons

16. 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

17. Spinal Control of Motor Units
Golgi Tendon Organs
Additional proprioceptive input - acts like strain gauge - monitors muscle tension

18. Spinal Control of Motor Units
Golgi Tendon Organs
Spindles in parallel with fibers; Golgi tendon organs in series with fibers

19. Spinal Control of Motor Units
Golgi Tendon Organs
Reverse myotatic reflex function: Regulate muscle tension within optimal range

20. Spinal Control of Motor Units
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

21. Spinal Control of Motor Units
Spinal Interneurons
Synaptic inputs to spinal interneurons:
Primary sensory axons
Descending axons from brain
Collaterals of lower motor neuron axons
Other interneurons

22. Spinal Control of Motor Units
Inhibitory Input
Reciprocal inhibition: Contraction of one muscle set accompanied by relaxation of antagonist muscle
Example: Myotatic reflex

23. Spinal Control of Motor Units
Excitatory Input
Flexor reflex: Complex reflex arc used to withdraw limb from aversive stimulus

24. Spinal Control of Motor Units
Excitatory Input
Crossed-extensor reflex: Activation of extensor muscles and inhibition of flexors on opposite side

25. Spinal Control of Motor Units
Generating Spinal Motor Programs for Walking
Circuitry for walking resides in spinal cord
Requires central pattern generators

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

27. Spinal Control of Motor Units
Possible Circuit for Rhythmic Alternating Activity

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

29. Dogfish Swimming: Reafferent modulation of CPG Rhythm.
Tail movement

30. Concluding Remarks Spinal control of movement
Different levels of analysis
Sensation and movement linked
Direct feedback
Intricate network of circuits

31. End of Presentation

32. Lower Motor Neurons
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

33. Excitation-Contraction Coupling
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 Ca2+ to troponin causes myosin to bind to action
Myosin heads pivot, cause filaments to slide

34. Excitation-Contraction Coupling
Sliding-filament Model of Muscle Contraction

35. 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