Myofibril
Myofibrilla
Also known as: myofibrils
A rod-like contractile organelle, ≈1–2 µm in diameter, made of thousands of sarcomeres joined end-to-end. It runs the full length of the muscle fibre.
Overview
Myofibrils fill most of the muscle fibre. Each is a chain of sarcomeres in series, so a myofibril shortens by the sum of the small shortenings of all its sarcomeres. A 10 cm fibre contains roughly 40,000–50,000 sarcomeres in each myofibril. Neighbouring myofibrils are held in register at the Z discs by desmin intermediate filaments. Their bands therefore line up across the fibre and produce the striations seen under the light microscope.
Each myofibril is wrapped in a sleeve of sarcoplasmic reticulum, with T-tubules encircling it at every A–I junction. Every sarcomere therefore receives the Ca²⁺ signal almost at the same moment. Mitochondria are packed between the myofibrils. At the sarcolemma, Z discs anchor to costameres (rib-like adhesion complexes) that pass force sideways to the extracellular matrix.
Parts & subdivisions
- Repeating contractile units, ≈2–2.5 µm long at rest.
- Desmin networkIntermediate filaments that link neighbouring Z discs and anchor them to costameres and the nucleus.
Anatomy
- Histology
Appears as fine longitudinal striations in longitudinal sections and as dots in transverse sections. Under the electron microscope, the A, I, H, M and Z elements are clearly seen.
Function
- Produces shortening and force by summing sarcomere contractions in series
- Keeps the striations in register across the fibre
Compiled from standard anatomy and physiology references — see the Muscular System sources. For education only; not medical advice.