Neuromuscular Characteristics by nikeborome


    Powers CH 7
                Nervous System
   Control of internal environment
    – Nv system / endocrine system – regulate hormone release

   Voluntary control of movement

   Programming of spinal cord reflexes

   Assimilation of experiences necessary for memory
    and learning
                    Nervous System
   CNS
    – Brain
    – SC
           Ascending Tracts
           Descending Tracts

   PNS
    – Sensory neurons / afferent nv fibers
    – Motor neurons / efferent nv fibers
   Functional unit of NS
    – Cell body

    – Dendrites
         Afferent – receives electrical
          impulses toward cell body

    – Axon
         Efferent – carries electrical
          message away from cell
          body / toward another
          neuron of effector organ

    – Myelination / Schwann cells

    – Nodes of Ranvier
                                           Lg diameter myelinated axons =
                                           most rapid impulse conduction
                  Electrical Activity
   Neurons
    – Excitable tissue
        Irritability
           – Dendrites and cell body
             respond to stimulus, convert
             to neural impulse
        Conductivity
           – Transmit electrical activity
             along axon

        Saltatory Conduction
                    Action Potential
   Depolarization
    – Stimulus of sufficient strength
      reaches neuron membrane

    – Sodium gates open, allowing
      sodium ions to diffuse into neuron
      (making inside cell increasingly +)

    – Depolarization above “critical value”
      (threshold) causes sodium gates to
      open wide – AP (nerve impulse is
    Chemical messengers
      – Acetylcholine (skeletal muscle)
             Released across synapse
             Bind to post-synaptic receptor

      – Graded Depolarizations
             Caused by excitatory
             Create EPSPs

    If sufficient amounts of NT is released, post-
    synaptic neuron (or muscle) is depolarized to
    “threshold” and AP is generated
Achieved via:

   Temporal Summation
    – Increase rate of EPSPs from 1 pre-synaptic neuron

   Spatial Summation
    – EPSPs from several pre-synaptic inputs

        Depends on ratio of EPSPs to IPSPs
        Increase in IPSPs = hyperpolarization
Muscle Physiology
    Powers CH 8
         Muscles and Tendons
   Muscle
       Contracts to produce joint

   Tendon
       Connects muscle to bone

          O, I, F??
    Characteristics of Muscle Tissue

   Irritability
         Excitable to motor neuron stimulation

   Contractility
         Capacity to shorten to 50% resting length

   Extensibility
         Stretches beyond resting length when other muscles act
          across a joint
         Muscle extensibility limited by its own connective tissue

   Elasticity
         Ability to return to resting length
    Structure of Muscle Tissue

   Muscle groups contained in Fascia
   Muscle belly wrapped in Epimysium (transfers contractile
    force through tendon to bone)
   Perimysium covers muscle fiber bundles (fascicles)
   Endomysium covers muscle fiber
Muscle Contraction
            Muscle Contraction
    Runs the length of muscle fiber
    Contains sarcomeres (contractile units) in series
    Nerve action potential across sarcolemma stimulates calcium
     release, triggering actin – myosin cross-bridging in sarcomere
                 Muscle Contraction
   Action Potential (signal to contract
    ms) transmitted down motor nerve
   AP carried across synapse by
    neurotransmitter (Acetylcholine)
   AP reaches all muscles in motor
    unit at NM Junction of each muscle
   AP spreads over Sarcolemma –
    muscle cell depolarizes
   Calcium released into Myofibril
   Excitation-contraction coupling
    occurs as Myosin-Actin x-bridges
    are formed
                                           Powers CH 8… A Closer Look 8.2
       Sliding Filament Theory
   Myosin-actin protein filaments form x-bridges in presence of

   Myosin pulls actin to center of sarcomere, detaches, then moves
    to next site
     – Shortening of many sarcomeres, myofibrils, and fibers develops
       tension running through muscle and tendon transferred to bone
         Mechanical Muscle Model
   Contractile Component (CC)
         Converts nerve stimulation of a
          muscle into force

   Series Elastic Component (PEC)
         Represents elasticity of all elastic
          elements in series with force-
          generating CC (Tendon??)

   Parallel Elastic Component (SEC)
         Represents elastic behavior of
          muscles when CC is not producing
         Allows muscle to recoil when
          elongated w/o active contraction
                Generating Force
   Motor Unit
    – Motor neuron and all muscle fibers it innervates

    “All or None Principle”
      When a motor unit is sufficiently activated (neurally) all muscles
      belonging to it will contract
                 Force Production
   Recruiting larger motor units

   Increasing frequency (conduction velocity) of neural
    stimulus (AP) to excite motor unit

    Twitch – Steppe - Tetanus

          Spatially (size of stimulus)
          Temporally (rate of stimulus)
Contractile Properties of Skeletal Muscle

Performance Characteristics:
   Maximal force production
    – F/x-sectional area

   Speed of contraction
    – Vmax - Rate of x-bridge cycling

   Muscle fiber efficiency (economy = less energy to perform work)
    – ATP used / force production
            Muscle Fiber Type
   Slow Twitch (Type I)
        Slow contraction times but fatigue-resistant
        Oxidative
           – Large capillary network surrounding the fiber
           – High myoglobin content (shuttles O2 from cell membrane
             to mitochondria)
           – Large # of mitochondria / oxidative enzymes

                Lower Vmax
                Lower specific tension
                Greater efficiency

                                          Powers CH8: Table 8.1
               Muscle Fiber Type
   Fast Twitch (Type IIx and Type IIa)
        Type IIx – fast glycolytic
        Type IIa – fast oxidative-glycolytic (hybrid) – Adaptable
           – Rapid force production but easily fatigued
           – Rich in glycolytic enzymes / limited aerobic capacity

                 High Myosin ATPase = high Vmax
                 Higher specific tension
                 Lower efficiency (high ATPase activity = greater energy
                  expenditure per unit of work)

           – Variability??:
                Sport selection?
                Purpose of muscle?
                                                 Powers CH8: Table 8.1
              Muscle Fiber Type
Percentage of respective fiber types contained in skeletal
  muscle influenced by:

   Genetics
   Blood levels of hormones
   Exercise habits

        No apparent sex or age differences in fiber concentrations
        Sedentary = 47-53% slow twitch (endurance athletes ~ 75%
         ST, power athletes ~ 75% FT)
                                           Powers CH8: Table 8.2
             Muscle Attachment

Attached directly to bone

Attached via tendon or aponeurosis
   – Tendon – extension of non-contractile muscle coverings

       Tension transmitted to bone through tendon
       Tendon woven by mineralized fibrocartilage into lamellar bone
       Blends with periosteum and subchondral bone
          – Sharpey’s fibers?
              Origin / Insertion?
   Muscle pulls with equal force on both attached ends

   One bone moves because of greater mass or fixating
    muscle force

    – Trapezius
    – Gluteus medius
    – Psoas major
       Functional Characteristics
   Produce movement

   Fixate / Stabilize

   Dynamic Joint Stabilization
        Sensorimotor Control

– Feed forward mechanisms
   Cortically driven motor program for coordinated task execution
   Experience-based preparatory muscle activity

– Feedback mechanisms
   Reactive muscle activity
   Refined motor program based on afferent information
      – Joint position sense (proprioception)
      – Joint motion sense (kinesthesia)
      – Muscle tension

     Joint mechanoreceptors sense change in joint position
     and rate of change and activate muscles reflexively
                    Role of Muscles
   Prime mover / Agonist

   Antagonist
    – Relax to permit motion
    – Act eccentrically to brake motion

   Stabilizers / Neutralizers
    – Stabilizers – must be fatigue-resistant
      (RC, Gmed)
    – Neutralizers – guide / refine motion
           Net Muscle Actions
   Isometric
        Internal Force = External Force

   Concentric
        Internal Force > External Force
        Joint action in direction of muscle shortening
        “+” work = propulsion

   Eccentric
        External Force > Internal Force
        Joint action opposite to muscle force direction
        “-” work = braking
         Stretch-Shortening Cycle
   Contractile Component
         Sarcomere – power stroke = muscle

   Elastic Component (SEC / PEC)
         Recoil effect due to change in muscle /
          tendon length and max accumulation of
          stored energy
         “Memory” adds to force output in
          concentric phase

   Neural Component (Intrafusal Fibers)
         Muscle Spindle Fibers (“Stretch Reflex”)
         Golgi Tendon Organs
       Skeletal Muscle Receptors
   Muscle Spindles
    – Thin muscle “intrafusal” fibers
    – Provide CNS with sensory info on muscle length changes
    – Stretch reflex - plyometric training

   Golgi Tendon Organs
    – Provide CNS with feedback on tension developed in muscle
    – Sensory neuron activity excites IPSPs (inhibitory disynaptic reflex)
    – Inhibitory influences may be voluntarily reduced with strength

              Eccentric – Amortization – Concentric
   Fast rate of stretch over low amplitude of motion = great return of
    elastic energy and muscle activation

   Greater benefit in Fast Twitch Fibers (myosin-actin x-bridges do not
    form quickly enough in Slow Twitch to benefit)
               Bi-Articular Muscle
   Reduces work required from single-joint muscles

   Initiates mechanical coupling of joints allowing for rapid
    release of stored elastic energy in system

   Muscle stretched across one joint releases greater
    energy in SEC / PEC at 2nd joint
         Ex: soccer kick
Factors Influencing Force Production

  Angle of Attachment
  Force-Time Characteristics
  Length-Tension Relationship
  Force-Velocity Relationship
            Angle of Attachment
   Parallel Component
        When tendon angle is acute (lying flat to
         bone), muscle force is directed along long
         axis of bone and into the joint to either
         compress or distract it.

   Rotary Component
        Force directed (tendon insertion) at right
         angle to limb being moved produces
         greatest mechanical advantage for rotary
         or translational movement.
      Force-Time Characteristics
   Time to generate maximum force and the magnitude of
    force vary with joint position

   Slack tendon delays time to max force (reflects changes
    in tendon and non-contractile tissue laxity)
    Length-Tension Relationship
   Muscle activated at slightly greater than resting
    length = max force production
    – ~120% resting length = CC producing optimal tension + PEC
       / SEC storing elastic energy adding to total tension in the unit

    – Tension at shortened lengths
         Diminished capacity due to overlapping
         Decreased contractile capacity at end ROM

    – Tension at elongated lengths
         Decreased tension due to x-bridge slippage
    Force-Velocity Relationship

   As velocity increases, force production decreases
   Optimal force generated @ Zero Velocity (large #
    of x-bridges attached)
   Concentric
    – Increased velocity of muscle shortening = greater x-bridge
      cycling rate, leaving fewer x-bridges attached at any one
      time = decreased force production

   Eccentric
    – Tension increased with speed / rate of lengthening
      (muscle is stretching as it contracts)
         Ends abruptly @ point where ms can no longer control mvmt
                 Force x Velocity

    Increased Load = Velocity
   High velocity activities at 30% max force enhances
    Fast Twitch Ms Fiber activation (4x greater peak
    power than Slow Twitch)

    – Power Training = 1/3 max force @ 1/3 max velocity
    Exercise-Induced Changes
   Hypertrophy
   Hyperplasia?
   Atrophy
    – Reduced muscle protein synthesis (initially – first 2 days)
    – Increased rate of protein breakdown
            Age-Related Changes
   Steady loss (10% total) of muscle mass from age 25–50

   Rapid decline in muscle mass after age 50
    – FT fibers lost at accelerated rate

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