Anatomy and Physiology / Unit 5: The Muscular System
5.2 Muscle Fiber Anatomy
Notes
A skeletal muscle fiber is one long multinucleate cell. Its membrane is the sarcolemma and its cytoplasm is sarcoplasm; the storage organelle for calcium is the sarcoplasmic reticulum (SR).
- A fiber is filled with myofibrils, chains of contractile units called sarcomeres (Z disc to Z disc).
- Thick filaments are made of myosin; thin filaments are made of actin with troponin and tropomyosin. Overlap of these filaments makes the striped (striated) pattern.
- Neuromuscular junction: a motor neuron releases acetylcholine (ACh) onto the sarcolemma. A motor unit is one motor neuron plus all the fibers it controls.
- Fine control (eye, fingers) uses small motor units; powerful movements (thigh) use large ones.
Practice
The functional contractile unit of a muscle is the:
Sarcomeres arranged end to end make up each myofibril.
Which neurotransmitter is released at the neuromuscular junction?
ACh binds to receptors on the sarcolemma and starts muscle excitation.
The thin filament is made mainly of:
Actin forms the thin filaments; myosin forms the thick ones.
Ready for more? Take the Unit 5 test.