Types of skeletal muscle

Classification

A. Slow-type muscle fibres

• Uncommon.
• Can be found mixed with twitch-type muscle fibres in extra-ocular muscles
• Response to suxamethonium differently
• No propagated AP
• Do not respond rapidly and require repeated stimulation
• Produce tonic contraction rather than twitch response

B. Twitch-type muscle fibres

Slow fatigue-resistant fibres (I)

aka red muscle fibres

• Respond slowly but does not fatigue easily
• Maintenance of posture
• Red colour because of high content of myoglobin

Fast fatigue-resistant fibres (IIA)

• Also present in red muscles
• Uncommon in humans

Fast fatigable fibres (IIB)

aka white muscle fibres

• Fine skilled movement
• Also present in extraocular muscles

Comparison of slow fatigue-resistance fibres and fast fatigable fibres

Slow fatigue-resistant (I)

• High myoglobin, mitochondria, dense capillary supply
  --> high oxidative capacity
• Aerobic metabolism
• Lower glycolytic capacity
• Myosin ATPase: slow
• Contraction onset: slow
• Contraction duration: long (up to 100ms)
• Strong, sustained contractions, e.g. posture

Fast fatigable (IIB)

• High glycolytic activity
• Low oxidative capacity
• Myosin ATPase: fast
• Contraction onset: fast
• Contraction duration: short (down to 7.5ms)
• Fine precise movement, e.g. hand muscles