Stepping Up
Tracing a Nerve Impulse
In order to step up one step, a conscious impulse would first originate in the cerebral cortex, and then be translated into an electrical impulse in the spinal cord. The electrical impulse originates in the dendrite of one neuron. It is then carried through to the axon, where it uses chemical messengers to leap across the gap or synapse that connects it to a dendrite of the next neuron in the chain, and so on until it reaches the spinal cord. It is here that the impulse is broken up into the constituent muscle movements that will complete the desired action, and separate impulses are sent from the spinal cord to the muscles of the leg necessary for stepping up via motor neurons.
This chain of nerve impulses is possible because of the resting potential of the individual nerve cells. With positively charged sodium on the outside of the cell and positively charged potassium on the inside, the cells are polarized, and remain negatively charged on the inside. An impulse opens "gates" in the membrane that allow the positive sodium ions to rush in, which pushes the impulses along by moving electrons. When the impulse passes, the sodium moves out again and the nerve cells basically resets itself back to its resting potential.
When the nerve impulses reach the muscles they are meant to move, a similar use of ions inside and outside the muscle fibers occurs. In this case, sodium and potassium are used to spread the impulse across the muscle, but it is the influx of calcium into muscle fibers that reacts with the tropomyosin present in the cells, causing the fibers to contract, which pulls on the bones of the skeletons creating movement. The specific leg muscles necessary for stepping up in a step are the biceps femoris, which flexes the knee and extends the hip, the pectneus, which flexes the hip and raises the leg, and the quadriceps which extends the leg.
