Spinal Cord Motor Systems

Spinal Cord Motor Systems

1. Movement type

2. Force

   • Properties of motor units
     • innervation number
       • Small
         • Finer control
       • Large
         • fewer for peak
     • contraction speed
       • Fast twitch
       • Slow twitch
     • maximal force
       • +
       • -
     • Fatigability
       • Fast Fatigue
       • Fast intermediate
       • Fast resistant
       • Slow (all resistant)
   • Rate of mn AP
     • summation
       • greater forces
   • Number motor units recruited
     • more units = more force
     • Which recruited?
       • Voltage= current times resistance
         • More resistance means more voltage
           • small
             • first
             • high resistance
           • large
           • not specific connectivity
   • Motor unit

3. Amyotrophic lateral sclerosis

   • Less force at maximal firing
   • Fewer motor units
   • Fatigue resistant
   • PROBABLY slow twitch "trying" to compensate for loss of fast twitch
     • fast twitch more vulnerable

4. Reflexes

   • Stretch reflex
     • Spindle
       • 1a
         • primary
           • length and rate change
       • changes in length
       • II
         • Secondary
           • length only
       • Gamma motor neurons
         • intrafusal fibers
           • regulate sensitivity
     • Monosynaptic
       • 1a afferents
         • homonymous
           • within same muscle
         • synergistic
           • around same joint
             • co-contract
               • "stiffen"
                 • prepare to contract
     • polysynaptic
       • inhibitory interneurons
         • antagonistic
     • gamma
       • increase sensitivity spindle
         • compensate contraction
         • maintain 1a firing
       • intrafusal
   • Tension reflex
     • Golgi tendon organ
       • between tendon and muscle fibers
         • stretched by load/muscle contraction
           • distinguish tension/force
     • 1b
       • tension
         • activate 1b
           • 1b interneuron
             • inhibit homonymous
               • decrease tension
                 • decrease 1b
                   • control amount of force by muscle
   • cutaneous reflex
     • flexion-withdrawal
       • painful
         • nociceptors
           • excitatory acivate flexor mn
             • withdrawal from stimulus
           • inhibit extensor
             • flexion away from stim
               • force dependent on intensity of stimulus
       • polysynaptic
     • crossed extensor
       • excite extensors
       • inhibit flexor
         • opposite leg
           • support
       • commissural
   • Ib afferent facilitation of locomotion
     • state-dependent reflex reversal
     • rest stay at rest, motion stay in motion
     • rest
       • excitation
         • inhibitory
           • ankle extensor inhib
     • locomotor
       • excitation
         • premotor excitatory
           • ankle muscles
   • 1b inhib interneurons
     • cutaneous, joint, 1b
     • descending
     • CPG
       • rhythm
         • frequency
       • pattern
         • sequential activation
         • L/R
         • hop
         • undulation
       • mammalian
         • mesencephalic locomotor regin (MLR)
           • issue command
           • Long propriospinal pathways (LPNs)
             • connect limb half centers on both sides of cord
             • flexor and extensor half centers generate rhythmic pattern

5. supraspinal

   • speed of movement
   • turn
   • avoid obstacles
   • adjust to environment
   • adjust changes in body
     • position
     • size
     • shape
     • load
   • new movement patterns
     • ie swimming

6. SC injury separates

   • descending commands
   • monoaminergic and peptidergic input
   • ascending sensory info
   • repair
     • regrow axons
     • use spared pathways
     • activate CPGs or reflex pathways below injury
       • chemicals
       • electrical
       • neurons or progenitors

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