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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 ~
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 ~
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 ~
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 ~
Golgi Tendon Organ
Gauges muscle tension Stretch receptor
safety mechanism controlled contraction
Inhibits alpha motor neurons disynaptic inhibition ~
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