Transcript Spinal Motor Control

Spinal Control of
Movement
Lesson 19
Anatomy
Ventral Spinal Cord
 Topographic organization
 Alpha motor neurons
 Spinal interneurons
 Striate muscle
 extrafusal fibers ~

Proximal
limbs
Distal
limbs
Alpha Motor Neurons
Or lower motor neurons (Class I)
 Cell body in ventral horn
 Emerge from ventral root
 Innervate extrafusal fibers
 Uninterrupted to muscle fibers
 final common pathway
 Only excitatory input to muscles
Inhibition at spinal cord ~

Dorsal
Extrafusal
Fibers
ACh
Alpha
Motor neuron
Ventral
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 ~

Inputs to Alpha
Motor Neurons
Sensory neurons
Dorsal
DRG
Spinal
interneurons
Ventral
Upper motor
neurons - M1
Neuromuscular Junction
Synapse between neuron & effector
 Cholinergic (ACh)
 nicotinic receptors
 Motor end-plate
 postsynaptic membrane
 folds packed with receptors

increased surface area ~
Motor
end-plate
Terminal Button
Muscle Fiber
Neuromuscular Organization
Motor Units
 Single alpha motor neuron and all the
muscle fibers that it innervates
 1:3 to 1:100
 fewer fibers  finer control
 Motor Pool
 all alpha motor neurons that innervate a
single muscle ~

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 ~

Extrafusal Muscle Fibers
Striate muscle
 Force for limb
movements
 flexion - closes joint
 extension - opens joint
 Contract or relax ~

Muscle Contraction
AP generated in muscle fiber (cell)
 Ca++ released from internal stores
 Muscle fiber contracts
 continues while Ca++ & ATP available
 Relaxation
 Ca++ sequestered by active transport ~

Movement of Limbs
Flexors and extensors are ANTAGONISTIC
 muscles and are reciprocally innervated
 Limb flexion
 flexors excited & extensors inhibited
 Limb extension
 extensors excited & flexors inhibited
 Disynaptic inhibition ~

Dorsal
Upper Motor
Neurons
+
Ventral
+
Alpha
Motor neurons
+
Withdrawal Reflex
Flexion
 remove limb from noxious
stimulus
 Polysynaptic reflex
 sensory neuron
 interneurons
 motor neuron
 2 or more synapses
 slower than monosynaptic ~

Polysynaptic withdrawal reflex
+
+
+ +
-
+
R
Generation of
Rhythmic Motor Patterns
Central Pattern Generators
Half-center Model
 alternating activity in flexor & extensor
 Step-cycle has 2 phases
 swing phase

foot off ground & flexing upward

stance phase
foot planted & leg extending

Each limb has own pattern generator ~
Half-center Model
Flexor
a
+
Tonic
input
+
+
+
+
a
+
+
Extensor
+
Rhythmic Patterns: Sensory Feedback
Not necessary for locomotion
 but slower, less coordinated
 Stumble correction reaction
 during swing phase
 tactile stimulus on dorsal foot  flexion
 Reflex reversal
 override during extension
 flexion would cause collapse ~
