Skin, Bones and Muscles—Organs of Mechanical Protection and Locomotion

Skin, Bones and Muscles—Organs of Mechanical Protection and Locomotion

The skin that covers the body, the supporting skeleton and the muscles that move the bones are three organ systems, that is, groups of organs that together perform some important vital function.

The integumentary, skeletal and muscular systems function independently of each other, but since they all provide protection to the body and determine its shape and symmetry, we will consider them in one chapter.

In humans and most animals, the ability to move is ensured by the presence of specialized contractile cells - muscle fibers. Man and most vertebrates are extremely muscular creatures; Almost half of the human body's weight is muscle tissue.

In vertebrates, three types of muscles have developed to perform various types of movements: skeletal muscles, attached to the bones of the skeleton and causing them to move, cardiac muscle, thanks to which the heart can contract, dispersing blood through the vascular system, and smooth muscles, which form the walls of the digestive tract and some other internal organs and ensuring the movement of their contents. All three types of muscles have the ability to contract when stimulated, and usually this stimulation is communicated to the muscle fiber through a nerve. Cardiac muscle and smooth muscle can contract in the absence of nerve stimulation; therefore, the heart and digestive tract function almost normally even after all the nerves leading to them are cut.

On the contrary, cutting or blocking the nerves going to the skeletal muscle leads to its complete paralysis. During the first weeks, the muscle is still able to respond to artificial stimulation, for example, to an electric shock applied to the skin covering it, but this ability gradually disappears.

Curare, the main component of the poison that South American Indians used to poison their arrows, blocks the junction of nerve fibers with muscle fibers, making the transmission of impulses impossible. This produces the same result as cutting the nerves to all the skeletal muscles of the body. However, the muscles of a curarized animal are still capable of responding to direct electrical stimulation; this shows that the muscles have "independent excitability" and do not necessarily need to receive a stimulus from the nerve.