Spinal Control of Movement

Lesson 19

Anatomy Ventral Spinal Cord

Topographic organization Alpha motor neurons Spinal interneurons Striate muscle

extrafusal fibers ~

Proximallimbs

Distallimbs

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

Ventral

Extrafusal Fibers

ACh Alpha Motor neuron

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

Dorsal

Ventral

Spinal interneurons

DRG

Sensory neurons

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

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 ~

Neuromuscular Organization

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

Ventral

+

-

+

+ Alpha Motor neurons

+

Upper Motor Neurons

Withdrawal Reflex

Flexion remove limb from noxious stimulus

Polysynaptic reflex sensory neuron interneurons motor neuron

2 or more synapses slower than monosynaptic ~

R

+

+

+

Polysynaptic withdrawal reflex

-

+

+

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

+

+

++

+

+

+

+

Tonic input

Flexor

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 ~