Motor Neurons
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Transcript of Motor Neurons
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Dendrite
Soma (body)
Axon
receives and integrates information
Motor Neurons
transmits information
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Motor Unit
A motor unit is composed of a motor neuron and all of the muscle fibers it innervates
It is the smallest functional unit of muscular shortening
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Motor Unit (cont)
• each muscle has many motor units (m.u.)
• # of fibers in a m.u. is dependent on the precision of movement required of that muscle (average: 100-200 fibers per m.u.)– more precision is obtained with more neurons– 100 to 2000 motor neurons per muscle
• # of m.u.’s in a muscle decreases in the elderly
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Precision of 2 Muscles
1st muscle 2nd muscle10,000 fibers 10,000 fibers
100 motor neurons 200 motor neurons
100 motor units 200 motor units
100 fibers/mu 50 fibers/mu
less precision more precision
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Muscle Number ofMuscleFibers
Number ofMU’s
MeanNumber ofFibers PerMU
Platysma 27,100 1,100 25
Brachioradialias >129,200 330 >410
First Lumbrical 10,000 100 110
TibialiasAnterior
250,000 450 600
Gastrocnemius(medial head)
1,120,000 580 2,000
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Neuromuscular Control• a motor nerve action potential stimulates the release
of acetylcholine (ACh) from the nerve ending
• ACh binds to the muscle fiber which causes depolarization and results in the release of calcium ions from the sarcoplasmic reticulum (5 ms)
• the calcium ions permit the actin-myosin interaction, which produces force
• the contraction stops when the calcium ions are removed by a pumping action (100 ms)
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EMG
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Electromechanical Delay• electromechanical delay - stimulation
begins before force is developed– it is thought that this is the time necessary to “take up
the slack in the SEC”
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Control of Tension
• excitation of each motor unit is an all-or-nothing event
• increased tension can be accomplished by:– increasing the # of stimulated motor units
(recruitment)– increasing the stimulation rate of the active
motor units (rate coding)
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Stimulation vs ActivationV
olta
ge
Vol
tage
threshold
ALLNOTHING
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Recruitment
• each motor unit has a stimulation threshold at which it will begin to produce force
• small motor units have a lower threshold than large motor unit, therefore they are recruited first (size principle)
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Rate Codingsummation (B) - the overall effect of added stimuli
tetanus (C) - sustained maximal tension due to high frequency stimulation
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Sensory Receptors
•Sensory neurons provide feedback on the characteristics of the muscle or other tissues.
2 neuromuscular proprioceptors:
MUSCLE SPINDLES & GOLGI TENDON ORGANS
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Muscle Spindles• location:
– interspersed throughout muscle belly
• responds to:– muscle length– muscle velocity
• causes:– autogenic facilitation – reciprocal inhibition
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Stretch Reflex
• The muscle spindle is responsible for the stretch reflex.
• As a muscle is rapidly stretched, the muscle spindle responds by facilitation of the same muscle and inhibition of the antagonistic muscle.
• This reflex can be seen in the patellar tendon tap.
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Golgi Tendon Organ
• location:– near the muscle-tendon
junction
• responds to:– muscle tension
• causes:– autogenic inhibition– antagonistic facilitation
tendon
Muscle Fibers
GTO
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GOLGI TENDON ORGAN
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“My Little GTO”• possibly the critical determinant to maximal
lifting levels in weight training
• may also be responsible for uncoordinated responses in untrained individuals
• response is adapted through training
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STATICBALLISTICactivate musclespindles whichelicits a stretch reflex
may result intearing a muscle
if static positionachieved slowly thencan minimize musclespindle response
if held for sufficientlylong period (~30s) then can elicit GTO
response
STATIC BETTER THAN BALLISTIC
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ACTIVESTRETCH
Spindle response: minimal if performed slowly
GTO response: active stretch of hip extensorscauses GTO to relax hip extensors and toactivate the hip flexors
motive force: actions of the hip flexors
consequences: no negatives -- limited ROMlimits possibility of injury and exerciseantagonists
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PASSIVESTRETCH
Spindle response: minimal if performed slowly
GTO response: passive stretch of hip extensorscauses GTO to relax hip extensors
motive force: external force
consequences: no direct control of ROM thusmay exceed physiological limits and inducemuscle damage
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Stretching
• Proprioceptive Neuromuscular Facilitation• PNF
– alternating contraction - relaxation of agonist & antagonist muscles
– takes advantage of the response of the proprioceptors– e.g. hamstrings
• passive static stretch of hams - relax• active maximal concentric action of hams - relax• repeat
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Plyometric Training
Plyometric training consists of exercises that rapidly stretch a muscle followed immediately by a contraction. They improve power output in the muscle by:
Neurological Influences: rapidly stretching of the muscle, which excites the motoneurons via the stretch reflex.
Structural Influences: involving elastic energy from the stretch-shortening cycle.