Spinal Control
of Movement
Lesson 20
Spinal Mechanisms Of Movement
Ventral Spinal Cord
 motor neurons
 Striate muscle
 voluntary movement & reflexes
 Feedback
 sensory cells in muscle
 propioception
 safety mechanism
 postural maintenance ~

Spinal Cord Circuits
Output: motor neurons
 Ventral Horns
 muscle contraction
 Input: sensory neurons
 Dorsal Horns
 feedback
 Integration
 interneurons ~

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 ~

Dorsal
+
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
Upper motor
neurons - M1
DRG
Dorsal
Sensory
neurons
Spinal
interneurons
Ventral
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 ~

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 ~

Neuromuscular Junction
Synapse between neuron & effector
 Cholinergic (ACh)
 nicotinic receptors
 Motor end-plate
 postsynaptic membrane
 folds packed with receptor ~

Motor
end-plate
Terminal Button
Muscle Fiber
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 ~

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 ~

Dorsal
Upper Motor
Neurons
+
Ventral
+
Alpha
Motor neurons
+
Motor Units & Motor Pools
Motor Unit
 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) ~

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 ~

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
Golgi Tendon Organ
Gauges muscle tension
 Stretch receptor
 safety mechanism
 controlled contraction
 Inhibits alpha motor neurons
 disynaptic inhibition ~

Dorsal
GTO
Inhibits alpha
motor neuron
-
Ventral
+
Monosynaptic Stretch Reflex
Sensory neuron  alpha motor neurons
 monosynaptic excitation
 disynaptic inhibition
 e.g., Knee jerk reflex
 Postural adjustments
 Muscle tonus ~

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 ~

Dorsal
+
M
S
-
+
Ventral
+
++