Specific nerves and muscles of the human body

Stepping Up One Step and Reaching Up

After making a conscious decision to step up one step, the brain starts working on autopilot. A message (via electrical impulse) is sent from the cerebellum -- which handles such practiced movements as stepping up -- down the spinal cord and along the correct periphery nerves to the leg muscles (pbs.org). Chemical transmitters in the spaces or synapses between nerve cells allow for the electrical impulse to be rapidly transmitted, to the point that action feels almost instantaneous with thought (pbs.org). This is still only the most basic process involved in stepping, however; though the interplay of the neurotransmitters is somewhat complex between nerves, it basically consists of certain positively and negatively charged atoms or ions -- especially sodium, calcium, and potassium -- moving in and out of the nerve cell membranes, creating an imbalance in charge that results in an electric current (Freudenrich 2009). When this electrical impulse hits the muscles it is targeting, things get a lot more complicated.

At the neuromuscular junction, the small gap (synapse) between the end of the motor neuron and the muscle fiber, a process similar to that which occurs between nerve cells takes place. A chemical neurotransmitter travels from the nerve cell to receptors on the muscle, where it binds and starts a reaction across the entire muscle tissue, creating an action potential in the muscle by releasing other chemical messengers that find their way into the individual cells of the muscle fiber through the T-tubules (Freudenrich 2009). In this way, a mixture of interrelated electric and chemical signals are need to transmit the impule for movement from the brain, through the nerve cells, and into the actual muscle tissue. Even in the muscle tissue, however, the line between a chemical and electric reaction is a rather thin one. The processes are too linked together in the ways they work on the human body to be considered separately -- but more on that later.

So far, the process as described could be referring to any muscle movement. There are of course very specific muscles, with specific actions, that are used when stepping up one step. The bones that must be moved are first the femur, the major bone of the thigh that attaches to the hip with a ball-and-socket joint, which in turn will draw up the tibia and fibula (the bones of the lower leg) (Cluett 2009). The muscle that accomplishes this is he quadriceps, which runs along the front of the thigh and draws it up (raising the knee and rotating the femur forward and up in the hip socket) when it contracts; though the tibia and fibula are involved in the raising of the leg, they and the calf muscles to which they are attached do not really perform any work. In raising the body up on the step, however, the calf muscles would tighten, as would the hamstring (which works in opposition to the quadriceps) in order to hold the leg straight so that it can support the body in a balanced fashion on the step (Cluett 2009).

So far, we have accomplished stepping up one step, but this is only half of the process necessary for reaching something on a high shelf. The second step, of course, is reaching up with the arm to grasp the desired object. The beginning of the process is pretty much the same -- a nerve impulse originates in the brain (possibly in the motor cortex for this more complex and less-often performed task), and then travels along the spinal cord and periphery nerves to the proper muscles (pbs.org). Again, the neuromuscular junction is the site of chemical/electrical messaging between he nerve and the muscle fiber, and the same process activates the muscle tissue.

It is worthwhile to examine exactly what process takes place in the muscle tissue once activated that actually enables movement. muscles work by contracting; at the cellular level, the muscle fibers actually cling together and shorten when activated, and are able to relax but not push in the opposite direction (Freudenrich 2009). This is why muscles often operate in oppositional pairs, so that one muscle can perform the opposite action of its partner. In order to accomplish this feat of contraction, the spreading chemical messenger causes the calcium stores located in the muscle tissue to open (Freudenrich 2009). The calcium ions flow into the cytoplasm of the muscle cells where it binds to troponin-tropomyosin molecules located in grooves on the actin filaments. These molecules normally prevent myosin from bonding with the actin, keeping the muscle from contracting. The calcium ions change the shape of the troponin, pushing the tropomyosin out of the groves and allowing the myosin fibers to connect with the actin (Freudenrich 2009). The connection of the fibers draws the muscle tissue closer together, contracting the tissue which in turn pulls on the bones of the skeleton and creates movement.