Changes

==='''Fibres specialised for endurance'''===

==='''Fibres specialised for endurance'''===

There are only two characteristics of a single sarcomere of mechanical significance.  The first is the intrinsic speed of contraction, already described.  The second is the ability of the sarcomere to sustain a contraction during repetitive rapid stimulation, or tetanus.  This ability has been called nonfatiguability, but might be better termed endurance.  Endurance is directly dependent on the capacity of a sarcomere to derive energy by aerobic metabolism, whether from nutrients stored within the muscle fibre (intrinsic nutrients) or carried to the fibre in the blood (extrinsic nutrients).   

There are only two characteristics of a single sarcomere of mechanical significance.  The first is the intrinsic speed of contraction, already described.  The second is the ability of the sarcomere to sustain a contraction during repetitive rapid stimulation, or tetanus.  This ability has been called nonfatiguability, but might be better termed endurance.  Endurance is directly dependent on the capacity of a sarcomere to derive energy by aerobic metabolism, whether from nutrients stored within the muscle fibre (intrinsic nutrients) or carried to the fibre in the blood (extrinsic nutrients).   

Thus a typical high endurance fibre is relatively small to allow for diffusion of oxygen and nutrients, has a rich blood supply, an intrinsic nutrient supply that must be burnt aerobically (triglyceride), a high density of mitochondria, a high activity of enzymes of the tricarboxylic acid cycle (Fig. 6.7) and a high density of oxygen carrying pigment (cytochromes and myoglobin).  Because of this last property, high endurance fibres provide the colouring of red muscles, and are sometimes called red fibres.

Thus a typical high endurance fibre is relatively small to allow for diffusion of oxygen and nutrients, has a rich blood supply, an intrinsic nutrient supply that must be burnt aerobically (triglyceride), a high density of mitochondria, a high activity of enzymes of the tricarboxylic acid cycle (Fig. 6.7) and a high density of oxygen carrying pigment (cytochromes and myoglobin).  Because of this last property, high endurance fibres provide the colouring of red muscles, and are sometimes called red fibres.

:::::'''Fig 6.7 Aerobic and anaerobic metabolism in muscle'''  

:::::'''Fig 6.7 Aerobic and anaerobic metabolism in muscle'''