how can you say that muscle is contracted?

Answer 1

Skeletal Muscle Structure.The cells of skeletal muscles are long fiber-like structures. They contain many nuclei and are subdivided into smaller structures called myofibrils. Myofibrils are composed of 2 kinds of myofilaments. The thin filaments are made of 2 strands of the protein actin and one strand of a regulatory protein coiled together. The thick filaments are staggered arrays of myosin molecules.
Units of organization of skeletal muscle. The filaments are organized into structures called sarcomeres. Sarcomeres are constructed in the following manner:
Z lines are at the borders of the sarcomere. They align in adjacent myofibrils.
I bands are areas near the edge of the sarcomere containing only thin filaments.
A bands are regions where thick and thin filaments overlap and correspond to the length of the thick filaments.
H zones are areas in the center of the A bands containing only thick filaments.
The Mechanism of Muscular contraction. Muscle contraction reduces the length of each sarcomere. The sliding filament theory is explained below.
The thin filaments ratchet across thick filaments to pull the Z lines together and shorten the sarcomere. The myofilaments themselves do not contract.
Myosin molecules on thick filaments attach to the actin on the thin filaments to form a crossbridge. The cross bridges then bend inward pulling the thin filament toward the center of the sarcomere. These cross bridges are broken and reformed further down .
Energy for cross bridge formation comes from the hydrolysis of ATP by the head region of the myosin.
Skeletal muscle contracts when stimulated by motor neurons.
Graded Contraction of Whole Muscles. are due to summation of multiple motor unit activity and wave summation.
Motor neurons usually deliver their stimuli rapidly, resulting in a smooth contraction.
A motor unit consist of a single motor neuron and all the muscle fibers it controls.
As more motor neurons are recruited by the brain, tension in the muscle progressively increases.
Duration of the muscle contraction depends on how long the concentration of calcium remains elevated.
Slow fibers have longer lasting twitches because they have less sarcoplasmic reticulum. The calcium remains in the cytoplasm longer. They have many mitochondria, rich blood supply, and myoglobin.
Fast fibers have short duration twitches and found in fast muscles for rapid, powerful contractions.

Answer 2

There are two different type of filaments that make up muscle fibers; actin and myosin. Actin are thin filaments and myosin are thicker unts. When a nerve signals a muscle to contract, the thin filaments slide past the thick filaments. This is called teh sliding-filament model- what I've given you is a very simplified version of what occurs.

Answer 3

wen the cells of the muscle contract the muscle contracts.