Transcript Spinal Mechanisms of Movement
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 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 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 Ventral Inhibits alpha motor neuron + GTO
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 -
+
Ventral + M S