Neuoromuscular System (continued) EXS 558 Lecture #2 Wednesday September 7, 2005

Review Question #1  Which muscle structure surround the cellular contents of each muscle fiber? a.) perimysium b.) fasciculus c.) sarcolemma d.) endomysium

Review Question #2  The sarcomere, functional unit of the muscle cell, is measured from: a.) Z-disk to Z-disk b.) A-band to A-band c.) actin to myosin d.) H-zone to Z-disk

Review Question #3, 4  What type of activity would a motor unit that contains only a few muscle fibers be best suited for?  True/False: All fibers associated with a motor unit are contracted when innervated.

Review Question #5  What is released from the presynaptic side of the neuromuscular junction (NMJ) to trigger a muscular contraction? a.) adenosine triphosphate (ATP) b.) calcium (Ca 2+ ) c.) sodium d.) acetylcholine (Ach)

Review Question #6  Explain the role of calcium in the production of a muscular contraction.

Regulation of Tension Production  Motor Unit (MU) = motor neuron and all myofibers in innervates  When motor neuron delivers impulse ALL fibers maximally contract  2 mechanisms by which CNS controls muscle tension 1.) RATE CODING - ↑ or ↓ rate of firing of neuron 2.) RECRUITMENT - ↑ or ↓ # of MU’s activated

Which dominates?  With small homogenous muscle, initially rely upon recruitment (at 50% MVC, all MU’s recruited), additional force production caused by ↑ rate coding  With heterogeneous larger muscle, first see ↑ rate coding of low threshold MU’s, then see ↑ recruitment until reach 90% MVC, then ↑ rate coding to reach 100% MVC.

Size Principle of MU Recruitment   Principle of orderly recruitment states that motor units are activated in a fixed order, based on their ranking in the muscle.   Size principle states that the order of recruitment is directly related to their motor neuron size   Slow-twitch fibers, which have smaller motor neurons, are recruited before fast-twitch fibers.

Size Principle of MU Recruitment (continued)

Why do FT MU develop more force?  FT develop electrical impulses to myofibers more quickly  FT MUs have myofibers with larger CSAs  FT MUs have greater # of associated myofibers than ST MUs

Functional Classification of Muscles   Agonists—prime movers; responsible for the movement   Antagonists—oppose the agonists to prevent overstretching of them   Synergists—assist the agonists and sometimes fine-tune the direction of movement

Muscle Action

Neuromuscular Adaptations  “An understanding of the type of alterations seen with a given training program will help the coach or athlete develop the most appropriate training program and set the most realistic training goals” - Hoffman

Mechanisms of Gains in Muscle Strength  Neural Adaptations   Synchronization and recruitment of additional motor units   Coactivation of agonist and antagonist muscles   Rate coding—the firing frequency of motor units  Muscle Hypertrophy   Fiber hypertrophy   Fiber hyperplasia

Muscle Fiber Hypertrophy

Neural Effects of Muscle Performance  Bilateral deficit: maximum force produced by simultaneous contraction of both limbs less than total force produced with each limb acting singly  EMG activity ↓ when activating both limbs concurrently as compared to singly

 Why?  May be limit of neural drive from higher centers in CNS (central drive)  Bilateral deficit associated with greater EMG activity to antagonist  Pre-contraction of antagonist can ↓ recruitment of antagonists Neural Effects of Muscle Performance (continued)

Effects of Resistance Training  Early ↑ in strength not accompanied by muscle hypertrophy  Caused by neural adaptations ( ↑ EMG activity)  See ↑ in strength in untrained contralateral limb (↑ EMG activity) [cross training effect]  ↑ EMG activity indicated improved MU activation (MUA)

Effects of Resistance Training  Maximal force development requires complete MUA  For many max effort does not induce complete MUA  Training ↑ ability to reach full MUA with max voluntary effort  Training ↑ ability to keep threshold MUs activated  Training ↑ duration at which high and low threshold MUs can sustain optimal firing rates  Training ↓ decrement between tension seen with MVC and tension induced with supramaximal stimulation of nerve  Training ↑ motor neuron excitability during voluntary effort  ↑ ability to recruit MUs and disharge them at higher frequencies  Training ↑ synchronization in activation/firing rate of MUs  Does NOT ↑ max force production, but may ↑ rate of max force production

Resistance Training Fact   Once your goals for strength development have been achieved, you can reduce training frequency, intensity, or duration and still prevent losses in strength gained for at least 12 weeks. However, you must continue training with a resistance maintenance program that still provides sufficient stress to the muscles.

Effects of Endurance Training  Training causes ↑ in MU activation occurs in early stages, ↑ skill acquistion/coordination  Delays fatigue  Training causes rotation of activity among synergists and among MU of prime mover  Delays fatigue  Training ↑ consistency of firing rates of motor neurons  Finer control of muscle  Delays fatigue

Are muscle fiber type conversions possible?   Early studies showed no change in fiber type but changes in characteristics of muscle fibers   Cross-innervation studies and chronic stimulation studies demonstrate changes   Possible change from FTb to FTa, and from FTa to ST with endurance training, and FTb to FTa with resistance training   A combination of high intensity resistance training and short-interval speed work can lead to a conversion of ST to FTa fibers