Spinal Control of Movement Lesson 19

Anatomy n Ventral Spinal Cord l Topographic organization n Alpha motor neurons n Spinal interneurons n Striate muscle l extrafusal fibers ~ Proximal limbs Distal limbs

Alpha Motor Neurons n Or lower motor neurons (Class I) l Cell body in ventral horn l Emerge from ventral root n Innervate extrafusal fibers n Uninterrupted to muscle fibers l final common pathway n Only excitatory input to muscles Inhibition at spinal cord ~

Dorsal Ventral Extrafusal Fibers ACh Alpha Motor neuron

Input to Alpha Motor Neurons n 3 sources only 1. DRG neurons l sensory neurons (proprioception) l feedback from muscle spindles 2. Upper motor neurons l primarily from M1 3. Spinal interneurons l largest input (excitatory & inhibitory) l generation of motor programs ~

Inputs to Alpha Motor Neurons Dorsal Ventral Spinal interneurons DRG Sensory neurons Upper motor neurons - M1

Neuromuscular Junction n Synapse between neuron & effector n Cholinergic (ACh) l nicotinic receptors n Motor end-plate l postsynaptic membrane l folds packed with receptors increased surface area ~

Motor end-plate Terminal Button Muscle Fiber

n Motor Units l Single alpha motor neuron and all the muscle fibers that it innervates l 1:3 to 1:100 l fewer fibers  finer control n Motor Pool l all alpha motor neurons that innervate a single muscle ~ Neuromuscular Organization

Graded Control of Muscle Contraction n Highly reliable synapse 1 presynaptic AP  1 postsynaptic AP  1 twitch (contract/relax) n temporal summation  tension & sustained contraction n Recruitment  # motor units   tension l order: smallest  largest ~

Extrafusal Muscle Fibers n Striate muscle n Force for limb movements l flexion - closes joint l extension - opens joint n Contract or relax ~

Muscle Contraction n AP generated in muscle fiber (cell) l Ca++ released from internal stores n Muscle fiber contracts l continues while Ca++ & ATP available n Relaxation l Ca++ sequestered by active transport ~

Movement of Limbs n Flexors and extensors are ANTAGONISTIC l muscles and are reciprocally innervated n Limb flexion l flexors excited & extensors inhibited n Limb extension l extensors excited & flexors inhibited n Disynaptic inhibition ~

Dorsal Ventral Alpha Motor neurons + Upper Motor Neurons

Withdrawal Reflex n Flexion l remove limb from noxious stimulus n Polysynaptic reflex l sensory neuron l interneurons l motor neuron n 2 or more synapses l slower than monosynaptic ~

R Polysynaptic withdrawal reflex - + +

Generation of Rhythmic Motor Patterns

Central Pattern Generators n Half-center Model l alternating activity in flexor & extensor n Step-cycle has 2 phases l swing phase foot off ground & flexing upward l stance phase foot planted & leg extending n Each limb has own pattern generator ~

Half-center Model   + + Tonic input Flexor Extensor

Rhythmic Patterns: Sensory Feedback n Not necessary for locomotion l but slower, less coordinated n Stumble correction reaction l during swing phase l tactile stimulus on dorsal foot  flexion n Reflex reversal l override during extension l flexion would cause collapse ~