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Motor Neurons
Dendrite
receives and
integrates
information
Soma
(body)
Axon
transmits information
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
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
Precision of 2 Muscles
1st muscle
2nd muscle
10,000 fibers
100 motor neurons
10,000 fibers
200 motor neurons
100 motor units
100 fibers/mu
200 motor units
50 fibers/mu
less precision
more precision
Muscle
Number of
Muscle
Fibers
Number of
MU’s
Mean
Number of
Fibers Per
MU
27,100
1,100
25
Brachioradialias >129,200
330
>410
First Lumbrical
10,000
100
110
Tibialias
Anterior
250,000
450
600
Gastrocnemius
(medial head)
1,120,000
580
2,000
Platysma
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)
EMG
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”
Control of Tension
• excitation of each motor unit is an all-ornothing 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)
Stimulation vs Activation
Voltage
Voltage
threshold
NOTHING
ALL
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)
Rate Coding
summation (B) - the
overall effect of added
stimuli
tetanus (C) - sustained maximal
tension due to high frequency
stimulation
Sensory Receptors
•Sensory neurons provide feedback on
the characteristics of the muscle or
other tissues.
2 neuromuscular proprioceptors:
MUSCLE SPINDLES &
GOLGI TENDON ORGANS
Muscle Spindles
• location:
– interspersed throughout
muscle belly
• responds to:
– muscle length
– muscle velocity
• causes:
– autogenic facilitation
– reciprocal inhibition
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.
Golgi Tendon Organ
• location:
– near the muscle-tendon
junction
Muscle Fibers
GTO
• responds to:
– muscle tension
• causes:
– autogenic inhibition
– antagonistic facilitation
tendon
GOLGI TENDON ORGAN
“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
BALLISTIC
activate muscle
spindles which
elicits a stretch
reflex
STATIC
if static position
achieved slowly then
can minimize muscle
spindle response
if held for sufficiently
long period (~30s) then
can elicit GTO
response
may result in
tearing a muscle
STATIC BETTER THAN BALLISTIC
Spindle response: minimal if performed slowly
GTO response: active stretch of hip extensors
causes GTO to relax hip extensors and to
activate the hip flexors
motive force: actions of the hip flexors
consequences: no negatives -- limited ROM
limits possibility of injury and exercise
antagonists
ACTIVE
STRETCH
Spindle response: minimal if performed slowly
GTO response: passive stretch of hip extensors
causes GTO to relax hip extensors
motive force: external force
consequences: no direct control of ROM thus
may exceed physiological limits and induce
muscle damage
PASSIVE
STRETCH
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
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.