Muscles move on commands from the brain. Single nerve cells in the spinal cord, called motor neurons, are the only way the brain connects to muscles. When a motor neuron inside the spinal cord fires, an impulse goes out from it to the muscles on a long, very thin extension of that single cell called an axon. When the impulse travels down the axon to the muscle, a chemical is released at its ending. Muscles are made of long fibers connected to each other longways by a ratchet mechanism, the kind of mechanism that allows the two parts of an extension ladder to slide past each other and then lock in a certain position. When the chemical impulse from the motor neuron hits the muscle, it causes to muscle fibers to rachet past each other, overlapping each other more, so that the muscle gets shorter and fatter. When the impulses from the nerves stop, the muscle fibers slide back to their original positions.
Each motor neuron connects to just one muscle, say the bicep on the front of your upper arm that lifts your forearm, or to the triceps, the one on the back that extends your forearm. But when you move, you never think, “I’d like to contract my bicep two inches and relax my tricep two inches” — instead you think, “I’d like to put this cake in my mouth!” How does the brain translate from the general idea to lift something to your mouth to specific commands to muscles? It does it in stages. In the cerebral cortex, the commands in the neurons there represent coordinated movements – like pick up the cake, hit the ball, salute. The cortex then connects to a sort of console in the spinal cord that overlays the motor neurons. This console lays out arm position in space, up-down, left-right. Each desired arm position then is read out as a collection of specific commands to each motor neuron and muscle.
son Will; daughter Laura
West Middle School