1. 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 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 neuron: in ventral horn of spinal cord
Lower Motor Neurons
Lower motor neuron: in ventral horn of spinal cord
The axons of the motor neurons bundle together to form the ventral roots.
Together with the sensory (dorsal root, we form a spinal nerve.

5. 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

6. 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

7. Motor unit: Motor neuron and all the muscle fibers it innervates
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
Alpha motor neurons trigger the generation of force by muscles… creating a motor unit.
Motor neuron pool is the cluster of alpha motor neurons innervating a single muscle group.

8. Graded Control of Muscle Contraction by Alpha Motor Neurons
Lower Motor Neurons
Graded Control of Muscle Contraction by Alpha Motor Neurons
Varying firing rate of motor neurons
Recruit additional synergistic motor units
Graded control related to muscle twitch, summation, tetany.

9. Tetanus – as painted by Sir Charles Bell in 1809.

10. 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

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

12. Figure 9.7a

13. 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

14. Excitation-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

15. Figure 9.3a–c

16. Figure 9.3c–e

17. Figure 9.3d

18. Figure 9.4

19. Figure 9.6

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

24. Figure 9.11

25. Figure 9.12

26. Spinal Control of Motor Units
Sensory feedback from muscle spindles - stretch receptor
A muscle spindle is a stretch receptor… proprioreceptors detect stretch
Proprioreception is the sense of body position.

27. 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
Myotactic Reflex - Tonic contraction of the muscles in response to a stretching force, due to stimulation of muscle proprioceptors. Also called deep tendon reflex, stretch reflex.

28. Spinal Control of Motor Units
The Myotatic Reflex
1a sensory neurons are closely related to the myotatic reflex

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

30. 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

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

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

33. 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

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

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

36. Interneurons + + – + + –
Efferent
fibers
Afferent
fiber
Efferent
fibers
Extensor
inhibited
Flexor
inhibited
Flexor
stimulated
Flexes
Arm movements
Extensor
stimulated
Extends
Key:
+ Excitatory synapse
– Inhibitory synapse
Right arm
(site of stimulus)
Left arm (site of
reciprocal activation)

37. 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