Types of Movement

Movement is one of the most basic hallmarks of life, and it is shown at several different levels in the human body -- not only by the whole body walking or running, but also by individual cells and even by structures inside a cell. Four broad types of movement are recognised.

Amoeboid movement is the crawling movement shown by certain free or wandering cells of the body using pseudopodia (temporary finger-like or blunt extensions of the cell surface), formed by the streaming of cytoplasm. Macrophages and leucocytes (white blood cells) patrolling the blood and tissues move in this amoeboid fashion, and this same mechanism helps leucocytes squeeze out of blood capillaries and migrate towards a site of infection. The cytoskeletal proteins actin and myosin are involved in amoeboid movement too, showing that the molecular basis of movement is shared even between very different cell types.

Ciliary movement is produced by the coordinated, whip-like beating of many short, hair-like cilia projecting from the surface of certain cells. In the human body, the internal lining (mucosal epithelium) of tubular organs such as the trachea and the bronchi is ciliated: the beating cilia continuously sweep a thin layer of mucus, along with trapped dust particles and microbes, upward and out of the respiratory passage. In a similar way, the ciliated epithelium lining the female reproductive tract (the fallopian tube/oviduct) helps move the released ovum, and later the fertilised egg, along the tube towards the uterus.

Flagellar movement is produced by the undulating, propeller-like beating of a single long flagellum (or a small number of them) and is used mainly for locomotion of the whole cell through a fluid medium. The clearest human example is the sperm cell, which swims through the female reproductive tract using the lashing movement of its single long flagellum, propelling itself towards the ovum for fertilisation.

Muscular movement is the movement produced by the contraction and relaxation of muscles pulling on the skeleton, and it is responsible for the visible movements of the body as a whole -- walking, running, chewing, the movement of the limbs, and the movements of internal organs such as the heart and the gut. Because it is by far the most important type of movement for locomotion, the rest of this chapter is devoted to the structure of skeletal muscle, the molecular mechanism of muscle contraction, and the skeletal system against which muscles act.