The center of the filament lacks motors it contains only the coiled-coil region of the myosins. The motors on one side of the filament are oriented in the same direction whereas the motors on the other side of the filament are oriented in the opposite direction.

The thick filaments are a bipolar array of polymerized myosin motors.

The nature of these filaments can be understood in the context of the histological landmarks of the myofibril. The individual filaments do not change in length during muscle contraction rather the thin filaments slide over the thick filaments to shorten the sarcomere.

The M-line (Mittelschiebe) bisects each A-band (and, in doing so, bisects each H-band).Įach myofibril can be understood as a series of contractile units called sarcomeres that contains two types of filaments: thick filaments, composed of myosin, and thin filaments, composed of actin.

A light band called the H-band (Heller) sits within each A-band.

The Z-lines (Zwischenschieben) bisect the I-bands.

The alternating pattern of these bands results in the striated appearance of skeletal muscle. The I-bands (isotropic in polarized light) appear light in color and the A-bands (anisotropic in polarized light) appear dark in color.

The myofibril is composed of alternating bands.The myofibril contains several important histological landmarks: The nuclei are displaced peripherally within a cross section of the sarcoplasm while a large number of longitudinal myofibrils, groups of arranged contractile proteins, occupy most of the center space. The plasma membrane of skeletal muscle is called the sarcolemma its cytoplasm is known as sarcoplasm the endoplasmic reticulum is called the sarcoplasmic reticulum.Įach muscle cell is defined by a sarcolemma and contains many nuclei along its length. Skeletal muscle cells contain similar components and structures as other cells but different terms are used to describe those components and structure in skeletal muscle cells. Skeletal muscle is specialized for rapid and forceful contraction of short duration. It is a voluntary muscle, and therefore under conscious control.

Skeletal muscle is mainly responsible for the movement of the skeleton, but is also found in organs such as the globe of the eye and the tongue.

Muscle labeling game