The structure of skeletal muscle
Macroscopic structure of skeletal muscle
The belly of the muscle varies in thickness and is attached to bones via strong bands of connective tissue called tendons . Skeletal muscle is organised into bundles of muscle cells or fibres that are held together by a sheath of connective tissue. This enables the muscle cells to function together as a unit.
Each muscle fibre is a single cell with many nuclei. Around each cell is plasma membrane called the sarcolemma which contains sarcoplasm (cytoplasm). Each cell/fibre is comprised of many smaller myofibrils arranged lengthwise. Look in your textbook for a diagram showing these structures.
Microscopic structure of skeletal muscle
When viewed under a microscope myofibrils appear as banded units called sarcomeres . Each sarcomere is comprised of two kinds of myofilaments , a thin protein called actin and a thicker one called myosin . These are the units that cause muscle contraction. They overlap to form light and dark bands and it is this banding that gives skeletal muscle its striated appearance.
A sarcomere has two distinct bands – an 'I' band and an 'A' band. The 'I' band corresponds to a light area. This is where the actin proteins are located. The 'A' band is dark and corresponds to the area in which actin and myosin overlap. An 'H' zone marks the location of myosin proteins only. The boundary of each sarcomere is marked by 'Z' lines.