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.