put your science panties on (part 2)

 Last time, we covered the first part of the journey a neural signal takes on the way to becoming muscular movement.  We started at the brain and made it all the way down the nerve highway to a muscle. And it is here at the neuromuscular junction that you have been waiting with bated breath for the tale to continue.

When an action potential makes it to the end of the nerve road, it must make something happen in your muscles. The electro-chemical energy of the action potential must be somehow translated into kinetic energy. While your muscles are capable of creating large-scale movements like swinging a golf club, the tiny movements of the molecular structures of your muscles is where this kinetic energy is translated. So let’s get up close and personal with muscle structure.

Your muscles are like bundles of cords within bundles of cords- kind of like Russian nesting dolls- bound together by connective tissue. When you get down to the cellular unit, a single muscle cell is called a muscle fiber. A single fiber is made of smaller units called myofibrils. It is in these myofibrils that the basic contractile units of muscles are found. This basic contractile unit is called a sarcomere. Sarcomeres are made up of long proteins that slide past each other to make the muscle contract. These two stringy proteins are actin and myosin.

A relaxed sarcomere

Myosin filaments have these little bulbs hanging off called “myosin heads.” The myosin heads like to link up with the actin filaments. Through a process involving some supplementary energy molecules (holla, ATP!), the myosin heads creep along the actin filaments, bringing them closer together. Think of a little army guys (myosin heads) climbing along a rope (actin). This is muscle contraction on the tiniest level.

Contracted sarcomere

Just watch this. http://www.youtube.com/watch?v=zQocsLRm7_A&feature=fvwp

So, imagine that a whole bunch of sarcomeres are crammed into a muscle fiber. And a whole bunch of fibers are crammed into a single muscle. You’ve got tons of sarcomeres, working simultaneously, causing a whole chunk of muscle to squeeze in on itself.

Well that’s all well and good. But how is it that we’re so coordinated? I wouldn’t call Beyonce’s performance in the video for Single Lady “chunks of muscle squeezing themselves.” I mean, one muscle group is capable of a wide variety of motion, so there must be more nuance to it than  “chunks of muscle squeezing in on themselves,” right?

Right. This is where we go back to part 1- the innervation of the muscle. The complex maze of nerve fibers feeding muscle is responsible for the precise control we have over our movement. Your brain can selectively fire the pathways that feed just part or parts of a muscle. These different combinations are what make a smile different from a frown, from an inquisitive facial expression, from an embarrassed expression, from an amused expression. All use the same muscle groups, but each stimulates them in a different pattern.

The complex neural to motor pathway, to me, is one of the most intricately wired processes in all of Vertebrata, but always funnels down to one of the most basic: the little sarcomere.