Dendrite

Soma (body)

Axon

receives and integrates information

Motor Neurons

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 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

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

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-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)

Stimulation vs ActivationV

olta

ge

Vol

tage

threshold

ALLNOTHING

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 Codingsummation (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

• responds to:– muscle tension

• causes:– autogenic inhibition– antagonistic facilitation

tendon

Muscle Fibers

GTO

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

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

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

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

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.