Skeletal Muscles & Movement

• 3 types of muscle:

  • skeletal
  • cardiac
  • smooth
• Anatomy skeletal muscle:

  • whole muscle > muscle fascicle > muscle fibre > myofibril > myofilaments
• Factors regarding muscle layers:

  • whole muscle = endomysium
  • muscle fascicle = perimysium
  • muscle fibre = epimysium
  • myofibrils = contractile unit
  • myofilaments = actin and myosin
• Sarcomeres:

  • within myofibrils
  • functional unit of muscle fibre
  • gives muscles striated look

  Different zones:

  • z-lines = where actin attaches
  • M-lines = where myosin attaches
• Myofilaments:

  • involved in muscle contraction
  • includes actin and myosin
  • thin myofilament = actin
  • thick myofilament = myosin
• Muscle fibre components:

  • mitochondria (between myofibrils)
  • basal lamina (wraps around individual muscle fibres)
  • sarcolemma (under basal lamina; connection between nervous system and muscle)
  • cytosol (aqueous space between myofibrils; store nutrients)
  • sarcoplasmic reticulum (surrounds myofibrils; contains calcium)
  • t-tubules (allow outside of myofibril to interact with other myofibrils on inside of cell)
• Nervous system:

  brain > brainstem > spinal cord > nerves > t-tubules
• Divisions of NS:

  • CNS
  • PNS
  • Sensory and effector
  • (effector) autonomic and somatic
  • (autonomic) sympathetic and parasympathetic
• Difference between effector and sensory (PNS):

  • efferent = signals from brain to muscle (motor neurons)
  • afferent = signals to brain
• The motorneuron:

  • one motor neuron = multiple muscle fibres
  • called: motor unit
• Neuromuscular junction:

  • where motor neuron connects with muscle fibre
• Neuromuscular transmission:

  • NMJ = where signals are transmitted
  • chemical for excitation = acetyl-choline (ACh)
  • ACh = excites sarcolemma and causes electrical impulse
  • ACh released from motor neuron; travels across cleft; binds to receptors on sarcolemma; depolarizes it
  • electrical impulse travels down t-tubules
  • causes sarcoplasmic reticulum to release calcium into cytosol
• Muscle fibre contraction:

  • ”sliding filament theory”
  • trigger = calcium
  • myosin binds to actin
  • z-lines pull together = tension
  • tension causes energy demand
  • energy demand driven by ATP (therefore “sliding filament theory” is driven by ATP)
• Twitch contraction:

  • electrical impulse travels down t-tubules and causes twitch contraction
• Length-tension relationship:

  • depends on = crossbridges

  Curved graph:

  • decrease CBs + increase shortening = decrease force (1)
  • increase CBs + increase shortening = increase force (2,3,4)
  • increase CBs + decrease shortening = decrease force (5)
• Contractile response:

  • series of twitch contraction < unfused tetanic contraction < fused tetanic contraction
  • multiple APs = smoother movement and stronger force
  • more APs = more force
  • increase AP frequency = sustained contraction
• Force-velocity relationship:

  • increase force = decrease velocity
  • decrease force = increase velocity
  • if velocity is too high, CBs are less likely to attach (no contraction)
  • P0 = determined by number of CBs and calcium stores
  • Vmax = determined by quality of CBs and how fast we can remove calcium
• Fatigue:

  • loss in capacity of muscle to develop force and/or velocity
  • reversible by rest
  • due to interruption in chain of events: CNS > PNS > NMF > muscle fibres
• Motor units:

  • motor neuron = alpha motor neuron
  • increase operating motor units = increase contractile force
  • alpha motor neuron axon divides into as many branches as necessary to innervate all muscle fibres in that motor unit
• Motor neuron ”pool”:

  • all the alpha motor neurons that innervate a single muscle
• All-or-none principle of motor units:

  • all muscle fibres of a motor unit act together/synchronously if action potential is triggered
  • impulse either causes an action or not
  • muscle cells always act to their fullest extent
• Force of muscle action varies from minimum to maximum in 2 ways:

  1. increasing number of recruited motor units (starts force contribution)
  2. increasing frequency of motor unit discharge (ex. twitch vs tetanus contraction) (takes over force contribution)
• Motor unit characteristics:

  • size (ex. number of muscle fibres innervated; therefore: motor unit)
  • physiological properties of muscle fibres
  • biomechanical properties of muscle fibres
• Motor unit size:

  • number of alpha motor neurons that innervate a single muscle
  • therefore: how big is the motor unit pool
  • number of alpha motor neurons is equivalent to number of motor units
  • less fibres allow for more precise movement (# of fibres / # of motor neurons)
• Motor unit contraction time:

  • motor units vary in their contraction times (aka: how long it takes to generate a peak twitch force)
• Motor unit force vs contraction time:

  • faster contraction time = increase force
  • therefore: slower contraction time produces the least amount of force
• Muscle fibre type classification:

  • slow twitch = type 1
  • fast twitch = type 2a
  • fast twitch = type 2x
  • fast twitch = type 2b
• Properties for motor unit types:

  • type 1 = slow to fatigue, maintains force for long time, less muscle fibres
  • type 2a = does not maintain peak force, falls slower
  • type 2x = fast to fatigue, force decreases quickly, more muscle fibres
• Muscle fibre type classification:

  • type 1 = slow oxidative (resist muscle fatigue)
  • type 2a = fast oxidative/glycolytic (mixture)
  • type 2x = fast glycolytic (tire quickly; anaerobic)
• Staining:

  • reveals colours of different fibre types
  • this happens because of a protein structure located in myosin
  • roman numeral = type
  • small letter = isoform of myosin heavy chain
• Myosin heavy chain:

  • determine rate of CB reaction with actin and speed of muscle shortening
  • control pH sensitivity of ATP hydrolysis
  • responsible for depth of staining at various pH levels
• Type 1 characteristics:

  • oxidative capacity = high
  • glycolytic capacity = low
  • contractile speed = slow
  • fatigue resistance = high
  • motor unit/force strength = low
  • size of motor neuron = small
  • activity used for = aerobic
  • max. duration of use = hours
  • mitochondrial density = high
  • capillary density = high
  • major storage fuel = triacylglycerol
  • myosin heavy chains/human genes = MYH7a
• Type 2a characteristics:

  • oxidative capacity = moderately high
  • glycolytic capacity = high
  • contractile speed = fast
  • fatigue resistance = moderate
  • motor unit strength/force = high.
  • activity used for = long-term anaerobic
  • max. duration of use = 30 mins
  • mitochondrial density = high
  • capillary density = intermediate
  • major storage fuel = creating phosphate, glycogen
  • myosin heavy chains, human genes = MYH2
• Type 2x characteristics:

  • oxidative capacity = low
  • glycolytic capacity = high
  • contractile speed = fast
  • fatigue resistance = low
  • motor unit strength/force = high
  • size of motor neuron = large
  • activity used for = short-term anaerobic
  • max. duration of use = 5 mins
  • mitochondrial density = medium
  • capillary density = low
  • major storage fuel = creatine phosphate, glycogen
  • myosin heavy chains, human genes = MYH1
• Type 2b characteristics:

  • oxidative capacity = low
  • glycolytic capacity = high
  • contractile speed = fast
  • fatigue resistance = low
  • motor unit strength/force = high
  • size of motor neuron = large
  • activity used for = short-term anaerobic
  • max. duration of use = 1 min
  • mitochondrial density = low
  • capillary density = low
  • major storage fuels = creatine phosphate, glycogen
  • myosin heavy chains, human genes = MYH4
• Fibre type determinants:

  • fitness is based on oxygen uptake
  • if uptake is high = aerobically fit
  • type of fibre and oxygen uptake go together (ex. more oxygen uptake = more type 1 activation)
  • in aerobic sports, type 1 is used the most
• Motor unit recruitment:

  • less force production = fewer/smaller motor units
  • more force production = more/larger motor units
  • size principle = order of recruitment relates to size of alpha motor neuron and its excitability
  • recruit in same order each time
  • 1 —> 2a —> 2x —> 2b
  • 1 = most excitable
  • 2b = least excitable
  • as effort increases, impulses sent from CNS to muscle become greater, and more motor units are recruited (more APs, therefore more contraction)
• Glycogen depletion:

  • at low intensities = glycogen content of type 1 fibres decreases; all type 2 remained fairly the same (no decrease)
  • at higher intensities = glycogen content of type 1 decreased first, then type 2 began to decrease
  • as time goes on, even if power and force output stay the same, type 1 and type 2a start to deplete, and then type 2x and type 2b also start to deplete because they had to take over to maintain power and force
  • therefore, we see the greatest depletion in the first fibres recruited
  • all according to the size principle
• How to measure percent of tetanic force developed:

  • stimulate the nerve that innervates that muscle
  • increase the amplitude/intensity of current (recruit ALL muscle fibres)
  • increase frequency of stimulation until we have a fused tetanus
• Dynamic muscle contraction:

  • selective recruitment and firing pattern of all types of fibres provide the mechanism to produce a desired coordinated response
  • therefore = coordination