1. Spinal Control of Movement
Lesson 19

2. Spinal Mechanisms Of Movement
Ventral Spinal Cord
motor neurons
Striate muscle
voluntary movement & reflexes
Feedback
sensory cells in muscle
propioception
safety mechanism
postural maintenance ~

3. Spinal Cord Circuits Output: motor neurons Ventral Horns
muscle contraction
Input: sensory neurons
Dorsal Horns
feedback
Integration
interneurons ~

4. Alpha Motor Neurons Or lower motor neurons control striate muscles
Uninterrupted to muscle fibers
final common pathway
Only excitatory input to muscles
Inhibition at spinal cord ~

5. Dorsal +
Alpha
Motor neuron
Ventral

6. Input to Alpha Motor Neurons
3 sources only
1. DRG neurons
sensory neurons (proprioception)
feedback from muscle spindles
2. Upper motor neurons
primarily from M1
3. Spinal interneurons
largest input (excitatory & inhibitory)
generation of motor programs ~

7. Inputs to Alpha Motor Neurons
Upper motor
neurons - M1
DRG
Dorsal
Sensory neurons
Spinal interneurons
Ventral

8. Striate Muscle Extrafusal Muscle Fibers muscle cells
input from a motor neurons
contraction
SC inhibition  relax
Force for limb movements
flexion - closes joint
extension - opens joint ~

9. Muscle Contraction a motor neuron excitation AP in muscle fiber
Ca++ released from internal stores
Muscle fiber contracts
continues while Ca++ & ATP available
Relaxation
Ca++ sequestered by active transport ~

10. Neuromuscular Junction
Synapse between neuron & effector
Cholinergic (ACh)
nicotinic receptors
Motor end-plate
postsynaptic membrane
folds packed with receptor ~

11. Motor end-plate
Terminal Button
Muscle Fiber

12. Myasthenia Gravis Autoimmune disorder
body develops antibodies for ACh-R
weakness & rapid fatigue
Most common: women in 30s
Risk of respiratory paralysis
Treatment
AChE inhibitors
Immunosupressants ~

13. Movement of Limbs Flexors and extensors are ANTAGONISTIC
reciprocal innervation
Limb flexion
flexors excited & extensors inhibited
Limb extension
extensors excited & flexors inhibited
Disynaptic inhibition
in spinal cord ~

14. +
Dorsal
Upper Motor
Neurons - + +
Alpha
Motor neurons +
Ventral

15. Motor Units & Motor Pools
Single alpha motor neuron
& all the muscle fibers it controls
1:3 to 1:100
fewer fibers  finer control
Motor Pool
all alpha motor neurons
that control a single muscle (e.g., biceps) ~

16. Graded Control of Muscle Contraction
Highly reliable synapse
1 presynaptic AP  1 postsynaptic AP
 1 twitch (contract/relax)
Temporal summation
tension & sustained contraction
Recruitment
# motor units  tension
order: smallest  largest ~

17. Withdrawal Reflex Flexion remove limb from noxious stimulus
Polysynaptic reflex
sensory neuron
interneurons
motor neuron
2 or more synapses
slower than monosynaptic ~

18. Polysynaptic withdrawal reflex+ + + R

19. Golgi Tendon Organ Gauges muscle tension Stretch receptor
safety mechanism
controlled contraction
Inhibits alpha motor neurons
disynaptic inhibition ~

20. Dorsal +
GTO -
Inhibits alpha
motor neuron +
Ventral

21. Monosynaptic Stretch Reflex
Sensory neuron  alpha motor neurons
monosynaptic excitation
disynaptic inhibition
e.g., Knee jerk reflex
Postural adjustments
Muscle tonus ~

22. Monosynaptic Stretch Reflex
Muscle-Spindle (MS)
Muscle length detectors
Parallel w/ extrafusal fibers
Low threshold
Too little muscle tone
 tension
MS  sensory neuron  motor neuron
And inhibition of antagonistic muscle ~

23. Dorsal + M S - + + + +
Ventral