Answer
Labeling the muscle contraction diagram:
a. Myosin head of myosin-actin crossbridge
b. Myosin myofilament
c, actin myofilament
d. Z line
e. H. band
f. A. band
g. I band
Work Step by Step
This is an illustration of a relaxed skeletal muscle sarcomere. :
A muscle fiber is called a myofiber. The following are the parts of a muscle cell ( myofiber):the plasma membrane of the myofiber is called the sarcolemma; the cytoplasm is called the sarcoplasm ; the smooth endoplasmic reticulum is called the sarcoplasmic reticulum. The sarcolemma encloses hundreds of myofibrils within the myofiber--myofibrils are the contractile parts of muscle fibers. ; they run the entire length of muscle fibers. Myofilaments are finer units within myofibrils: there are two kinds of myofilaments-- myosin and actin myofilaments.
Acetylcholine (ACH) released by nerve stimuli cause calcium ions (Ca++) to be released by the sarcolemma. .Calcium ions bind to troponin on the myosin myofilament and this causes a change in shape of the tropomyosin threads on the myosin filament. This exposes the actin biding site and enables the myosin and the actin myofilaments to bind to form crossbridges. ATP molecules are also bound to myosin heads. When these hydrolyze, energy is released. The myosin filament uses this energy to pull the actin filament towards the middle of the sarcomere-- the contractile unit of the myofibril (muscle)
Myofibrils are divided into sarcomeres : Z lines ( Dark) mark the boundaries(of sarcomeres). I bands are light ( clear) areas internal to each Z line --they contain only actin filaments.
An A band is the dark central area in the middle of each sarcomere ; it contains overlapping myosin and actin filaments. Within each A band is the H band ; in an uncontracted sarcomere the H band lacks actin myofilament and contains only myosin filaments.
When a sarcomere contracts, the myofilaments retain their lengths.: the actin filaments from each side approach one another as they slide past the myosin myofilaments . The results are that the the I bands narrow; the Z lines move inward; the H band is almost obliterated and the sarcomere shortens
This movement of actin filaments relative to myosin filaments is called the sliding filament model of muscle contraction.
