PHYSIOLOGY OF NERVE AND MUSCLE CELLS lecture 12

O B J E C T I V E S :
AT THE END OF THE LECTURE YOU SHOULD BE ABLE TO
•Verify types of skeletal muscle fibers.
•Illustrate properties of skeletal muscles in intact organisms.
•Define motor unit.
•Understand the size principle.
•Recognize frequency summation.
oSkeletal muscle is a heterogeneous tissue; i.e made up of fibers that vary in myosin ATPase activity, contractile speed, and other properties.
oIn addition to the classical (slow-twitch) and (fast-twitch) fibers,
ohuman muscles have an intermediate fiber type.
SKELETAL MUSCLE FIBER TYPES:
•Many differences in the fibers stem from differences in the proteins within them. Most are encoded by multigene families:
1.Ten different isoforms of the myosin heavy chains (MHCs) have been characterized.
2.Each of the two types of light chains also have isoforms.
3.Only one form of actin, but
4.Multiple isoforms of tropomyosin and all three components of troponin.
SKELETAL MUSCLE FIBER TYPES:
•Skeletal muscles are frequently classified into two categories,
“slow” and “fast”;
each can contain a mixture of three fiber types:
Red or slow muscles: containing many type I fibers:
•These muscles respond slowly and have a long latency and long twitch duration.
•They have smaller fibers that are innervated by smaller nerves.
•They have a high level of oxidative metabolism; hence, there is a more extensive blood vessel and capillary system, greater numbers of mitochondria and larger amounts of myoglobin.
•The myoglobin gives the slow muscles their reddish (darker) appearance.
•These muscles have a high endurance and are fatigue resistant.
•They are adapted for long, slow, posture-maintaining contractions. Examples are the long muscles of the back.