fast and slow fibers

  • there are fast skeletal muscles and slow skeletal muscles

  • slow muscles use oxygen as energy source
    • makes the muscles red
    • puts it close to a blood vessel
    • slow muscles can be contracted for hours and hours
      • slow to fatigue
    • eg: standing (for a day)
    • 1-5 fibers per motor neuron
  • fast fibers get energy from glycogen
    • do not depend on oxygen for primary energy
      • stays contracted for a short period of time
      • glycogen is a limited resource
    • eg: jumping, blinking (obicularis oculi)
    • fatiguable
    • 100 fibers per motor neuron
  • there is an intermediate fast type of muscle that uses oxygen until supply runs out then uses glycogen
    • fatigue resistant fast muscles
    • 10-30 fibers per motor neuron

motorneurons types

  • skeletal muscles cannot move on their own
    • they need motorneurons
    • some muslces depend on oxygen (slow), others on glycogens (fast)
  • each types of muscles are innervate different types of neurons
    • every motor neuron innervates some number of muscle fibers
  • relatively, lots of nerves innervate the fast muscles
    • a few nerves innervate the slow muscles
    • somewhere in between for the fatigue resistent fast muscles

  • polio, for instance, kills the innervation in muscles

  • S: Slow fiber motorneurons
  • FR: Fast Fatigue Resistant fiber motorneurons
  • FF: Fast Fatiguable motorneurons

orderly recruitment

  • motorneuron activation is necessary for skeletal muscle contraction
    • type of motorneuron is the same as motorneuron unit
  • slow first, less force
    • then, fast fatigue resistant, a bit more force
    • then, fast fatigue fibers, a lot more force
  • slow muscles give endurance
  • fast muscles give strength
    • sedentary lifestyle leads to atrophy of strength
    • strength training help regain FF motor units
  • exercising once will not have a significant effect on muscle mass

stretch reflex

  • mono-synaptic reflex - the knee tap
    • the most complicated circuit
  • reflex
    • sensory input to motor output
  • muscle spindles are pockets of very thin muscles that play a sensory role among other muscles fibers
    • intrafusal fiber spindles exist among extrafusal fibers
    • neurons wrap itself around the intrafusal fibers to sense when muscles are stretched or contracted
  • there is only one synapse in the brain for the neuron the goes from this spindle to the brain
    • sensor neuron responds to stretch in intrafusal spindles
    • and actuates contraction in the surrounding extrafusal fibers via a motor neuron
    • a sensory neuron synapses directly onto a motorneuron

co-activation

  • when the extrafusal fibers contract, the intrafusal ones tend to slack and become useless
    • so the polar ends of the intrafusal are innervated by gamma motor neurons
    • to keep he intrafusal sensitive to muscle stretch, the gamma neurons contract the spindles as well

  • the regular motor neurons that innervate the extrafusal fibers are called alpha motorneurons

  • the contraction of the intrafusal spindles along with the extrafusal fibers is called alpha-gamma-co-activation
    • this makes the spindles sensitive to stretch again
    • to keep the muscle stretch stretch is online even when thee extrafusal has contracted

gamma reflex loop

  • cerebellum indirectly activates the gamma motorneuron
    • this reverse activates the alpha motorneuron
    • the cerebellum is responsible for keeping the muscle toned up
    • they stretch the intrafusal spindles to make the muscle taut
  • this loop is used for a basic level of muscle tone
    • the cerebellum modulates the sensitivity of the intrafusal spindles
  • this loop specifically activates when doing activity where we might fall over or needs additional handling
    • i.e. tight rope walking, walking in a construction site, ice, hiking etc.
    • this counts as spinal reflexes