Mechanism

In the spring of 1900, a group of Greek divers near the island of Antikythera put on their helmets and headed down into the sea. When they surfaced, one held in his hand a corroded bronze object. Initial observations, followed by X-ray imaging and CT scans decades later revealed that it contained interlocking precision gears made of bronze. The gears had a precise mathematical relationship that described the movements of the sun, moon, and all the planets the Greeks would have known. Today, historians of technology know the object as the world’s first mechanism.

This ancient machine exemplifies the way engineers think of mechanisms—as a system of mechanical components that work together to achieve a specific objective. From bearings to gears, pulleys to springs, bearings to actuators, mechanisms harness power to generate a specific type of force and movement. Often the term refers to a larger process in a complex machine, such as the steering mechanism of a car.

For biologists, it’s different. A biological mechanism may involve physical or mechanical movement, but then again, it may not. When we move, for example, our skeletal muscles contract and shorten, creating tension that propels our arms, hands, legs and feet to move. In this way we can walk, run, reach or jump. Similarly, when the left ventricle of the heart contracts, it generates pressure, which pumps blood outward to the rest of the body.

But more often than not, when biologists refer to a mechanism, they’re referring to biochemical changes rather than mechanical ones. For example, a neuron in the brain signals its neighbor by releasing a substance called a neurotransmitter from its tip. The neurotransmitter is diffused across the synapse to the nearby tip of a neighboring neuron, where a receptor binds it. That triggers the receiving neuron to generate an electrical signal that propagates the message. No mechanical forces are involved, and there’s no physical movement, but to a biologist, it’s clearly a mechanism.

A biological mechanism may also involve interactions between cells and tissues that cause a change in tissue function. Rheumatoid arthritis, for example, develops when one type of immune cell signals another using biochemicals called cytokines. This in turn causes certain enzymes to break down joint tissue. The process involves cellular and biochemical changes rather than mechanical ones, but it, too, is a biological mechanism.