What muscle tissue is
Every movement a body makes — walking, swallowing, a heartbeat, the narrowing of a blood vessel — is muscle tissue doing one thing: getting shorter, then letting go.
Muscle tissue
Muscle tissue consists of muscle cells, or muscle fibres. These fibres have the ability to contract and relax. In co-ordination, it is muscle tissue that acts as the effector — the thing that carries out the response.
Why muscle needs a blood supply and epithelium does not
Unlike epithelial tissue, muscle tissue has a good blood supply. That supply delivers oxygen and nutrients to the tissue very quickly. Contracting costs energy, and energy has to be released from food using oxygen, so a tissue that contracts constantly must have both delivered constantly.
You met the word effector in grade 10
The page on co-ordination said that nerves carry the message fast, hormones carry it slowly, and muscles actually make the response happen. This skill is the muscle half of that sentence and the next skill is the nerve half. The stimulus-and-response table you filled in then ends, every time, at a muscle.
The three kinds
Smooth muscle
In the walls of hollow organs. Works slowly and steadily, without being told.
Skeletal muscle
Attached to the skeleton. Works powerfully and on command, and tires.
Cardiac muscle
In the heart, and nowhere else. Works rhythmically and never tires.
How to tell them apart
Four questions answer it every time: what shape are the cells, are they branched, how many nuclei, and are there cross striations. The next three pages answer those for each in turn.