Spinal Control of Movement

Lesson 19

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

Ventral

+ 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

Striate Muscle

Extrafusal Muscle Fibers muscle cells input from motor neurons contraction SC inhibition relax

Force for limb movements flexion - closes joint extension - opens joint ~

Muscle Contraction

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

Ventral

+

-

+

+ Alpha Motor neurons

+

Upper Motor Neurons

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) ~

Motor Units & Motor Pools

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 ~

R

+

+

+

Polysynaptic withdrawal reflex

Golgi Tendon Organ

Gauges muscle tension Stretch receptor

safety mechanism controlled contraction

Inhibits alpha motor neurons disynaptic inhibition ~

Dorsal

Ventral

-+

GTO

+Inhibits alphamotor neuron

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 ~

Monosynaptic Stretch Reflex

Dorsal

Ventral

++

+

MS

+-

+