Motor Unit & Physiology of Locomotion

1. Motor Unit &
Physiology of Locomotion

3. Motor Unit
Motoneuron + muscle fibers it innervates
Range in size from a few muscle fibers
(e.g. extraocular muscles)
To hundreds of fibers
(e.g. digits)
To thousands of fibers
(e.g. trunk and major limb segments)
Smaller motor units yield more refined control

5. sarcomere

7. Excitation of a Muscle Fiber
• An action potential in an alpha motoneuron axon.
• ACh is released by the axon terminal at the neuromuscular junction (synapse
between motoneuron and muscle.
• Nicotinic receptor channels open, Na+ enters and the membrane depolarizes
(EPSP).
• Voltage-gated Na+ channels open and an action potential is propagated down the
muscle fiber.
• Depolarization produces Ca2+ release from intracellular stores.

8. Contraction of a Muscle Fiber
• Ca2+ binds to tropinin.
• Myosin binding sites are exposed.
• Myosin heads bind actin.
• Myosin cross-bridges pivot.
• Myosin heads disengage at the expense (ATP-dependent).
• Recycle as long as Ca2+ and ATP are present.

9. Relaxation of a Muscle Fiber
• As EPSPs end, membrane returns to resting potential.
• Free Ca2+ is sequestered by metabolically driven pump.
• Myosin binding sites covered.

14. at rest
α-motoneuron
activation
α-, γ-motoneuron
co-activation

16. Meissner’s Merkel’s
Pacinian Ruffini’s Free nerve ending

17. Monosynaptic stretch reflex
Golgi tendon organ reflex

18. somatosensory
afferents
motor afferents

19. (Aδ)
(C)

20. Muscle Spindles (intrafusal fibers)
Golgi Tendon Organs
respond to muscle length, or stretch
insensitive to tension
1a afferents for dynamic spindles
Group II afferents for postural spindles
monosynatic stretch reflex (excitatory)
respond to muscle tension
insensitive to muscle length, or stretch
1b afferents
disynaptic reflex (inhibitory)

21. Problem of Locomotion
Degrees of freedom problem
Dozens of joints controlled by hundreds
of muscles
Muscles can contract in any combination,
in any order, for any duration, and with
any force.
Possibilities are virtually limitless, making
control of complex movements
at the level of individual muscles
impossible

22. Reciprocal inhibition between motor
units of antagonistic muscles

23. Crossed extensor reflex

24. oscillators = “legos” of movement
CPGs = what you can build with legos

25. Central Pattern Generators vs. Reflexes
Central Pattern Generators
Fast
Act as “clocks”
Flexible--<100 oscillators necessary
Reflexes
Refine timing
Respond to the unexpected
Acquisition of new skills

26. Descending motor pathways
Lateral and ventromedial pathways
Pyramidal and extra-pyramidal systems

33. Supplementary Motor Area (SMA)
important for instituting motor programs
activated prior to voluntary movement
activated during mental rehearsal
Premotor cortex
sensory guided movement
cooperatively with basal ganglia
mirror neurons
Pre-frontal cortex
decisions for actions
long-term planning

34. Thank you