It’s easy to avoid thinking about how your muscles work because it can be an overwhelming thought. How complex they must be to be capable of lifting heavy things and also finite tasks like writing! And even MORE mind-boggling is the system that connects your brain to each and every muscle cell in your body. How in the cornbread hell can a simple thought turn into an intent, and then turn into an action? How can you go from thinking about scratching your head, to intending to scratch your head, to scratching your head? It is confounding.
Your brain has highways reaching out to all of your skeletal muscles. These highways are your nerves. Most of them exit your skull and travel along your spine in that clump called the spinal cord. From there, they branch out to your muscles. A single one of these nerve cells is called a motor neuron, and is not a blob of a cell like you usually think of. It has a body and a long skinny “stem” called an axon. They’re long and skinny because- well, they’re highways! In fact, the longest single neuron you have runs from the base of your spine to your toes- up to a meter long. Think of the motor neurons a giraffe has!
|Figure 1: A single neuron.|
These highway nerve cells are uniquely structured for carrying signals- chemo-electric signals- from your brain to your muscle cells. Your brain tells your muscles what to do via your nerve signals. But these signals are not “encoded” with information- one signal does not vary from the next, nor do they vary in strength. It’s kind of like binary code- it’s either a 1 or a 0. In this case, there’s either a signal, or there’s not. Your neuron fires, or it doesn't. One single signal is called an action potential.
An action potential is basically a change in voltage of the membrane of a nerve cell. It’s like this wave of voltage that travels along one nerve cell, setting off the one after that, and the one after that, and the one after that, and- well, you get the picture. But these nerve cells- their membranes in particular- are specially built for carrying action potentials.
Alright y’all. Time to put your science panties on. It’s about to get real up in here.
The inside of the cell is negatively charged, in respect to the outside of the cell. Positively charged potassium (K) and sodium (Na) ions are floating around, both inside and outside of the cell. There are little pumps in the membrane are constantly pumping these ions against their concentration gradients in and out of the cell to maintain a high concentration of K and low concentration of Na inside the cell- which maintains an internal charge of about -70 millivolts. This is called resting potential.
|Figure 2: The basic set up of the axon of a neuron: The movement of Na+ and K+ ions through channels and a pump|
across a the membrane cause depolarization, repolarization, and hyperpolarization of the cell membrane (aka an action potential)
Now, let’s say the neuron’s dendrites become excited enough to set off an action potential. Little voltage-dependent sodium channels will open up, allowing sodium ions to diffuse along their gradient- into the cell. This influx of positively charged ions raises the inside voltage. This part is called depolarization, since the cell is becoming, well, depolarized.
Once the inside of the cell reaches about 40 mV (notice, that number is now positive!), sodium channels close and their neighboring voltage-dependent potassium channels open. Then, the potassium ions have their turn to diffuse along their gradient- out of the cell. This exodus of positively charged ions then causes the intracellular voltage to drop- bringing it back down into the negatives. These potassium channels are a little sluggish, though, and take a while to close. The cell hyperpolarizes a little past the original -70 mV, but is corrected quickly by the trusty Na/K pumps.
This process happens as a wave, traveling down the axon of a neuron. To help the process, a material called myelin wraps around the axon (Fig.1- the white bead looking things). Myelin is white (that’s why the neural tissue of your brain is white… white matter!) and is an insulator. This allows the action potential to skip right through the insulated bits, speeding up transmission to lightening fast speeds. That time you almost dropped your phone in the toilet but caught it right in time? Yeah, those lightening fast reflexes were thanks to myelin.
Once a neuron propogates an action potential all the way down to the terminal buttons, it sends chemical signals (neurotransmitters) out to the next neuron in line and excites its little dendrites- starting the process over. This occurs all the way down to the intended muscle site. And there, the muscle must take this action potential and turn it into kinetic motion. And kinetic motion is WAY more exciting than ion diffusion- so, stay tuned!
Now, I know that if you’re reading this part, you’ve read this entire blog post. And for that, faithful reader, you deserve A TREAT. To claim your treat, simply leave a comment stating your name, and I will write you a poem. And yes, it will hurt my feelings if no one takes me up on my poetry offer. FYI.