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The striated and involuntary muscle of the vertebrate heart.
The girl is taking a walk, using her voluntary control of her skeletal muscles to move her body down the path. She decides to climb a hill and can feel the involuntary racing of her heart as she climbs the rugged slope. At the top of the hill, she takes a deep drink of water, which will eventually lead to an involuntary stretching of her bladder as she heads home.
Muscle tissue is a soft tissue, and is one of the four fundamental types of tissue present in animals. There are three types of muscle tissue recognized in vertebrates . Skeletal muscle, or voluntary muscle, is anchored to bone by tendons, or by aponeuroses at a few places, and is used to effect skeletal movement in activities such as locomotion and maintaining posture. Though this postural control is generally maintained as an unconscious reflex, the muscles responsible react to conscious control like non-postural muscles. An average adult male is made up of 42% of skeletal muscle and an average adult female is made up of 36%, expressed as a percentage of body mass.
Smooth muscle, or involuntary muscle, is found within the walls of organs and structures such as the esophagus, stomach, intestines, bronchi, uterus, urethra, bladder, blood vessels, and the arrector pili in the skin, in which it controls the erection of body hair. Unlike skeletal muscle, smooth muscle is not under conscious control. Cardiac muscle is also an involuntary muscle but is more akin in structure to skeletal muscle, and is found only in the heart.
Cardiac and skeletal muscles are striated, in that they contain sarcomeres and are packed into highly regular, repeating arrangements of bundles; smooth muscle has neither attribute. While skeletal muscles are arranged in regular, parallel bundles, cardiac muscle connects at branching, irregular angles, called intercalated discs. Striated muscle contracts and relaxes in short, intense bursts, whereas smooth muscle sustains longer or even near-permanent contractions.
The density of mammalian skeletal muscle tissue is about 1.06 kg/liter. This can be contrasted with the density of adipose tissue (fat), which is 0.9196 kg/liter. This makes muscle tissue approximately 15% denser than fat tissue.