The human body is extraordinarily complex and it can be intimidating to try to understand it. In fact, although we don’t truly understand all of it, we do know a lot about how it works.

And you don’t need to be a physician or have a PhD in physiology to acquire a basic understanding of how your body works and what you can do to make it as healthy as possible.

This article is part of Thorne’s “Body Basics” series and is designed to help make learning about the body less intimidating. By learning about how your body works you can make informed decisions about your health – whether you’re at home, the store, your health-care practitioner’s office, or online. Knowledge is power.

Follow the leader

Did you know that under laboratory conditions, with the proper nutrients and oxygen, a muscle cell from the heart can be removed and it will continue to beat? Amazing, yes? What’s even more amazing is that the cell’s rhythm in the dish won’t likely match the rhythm it had in the heart – because individual cardiac muscle cells have their own internal rhythm.

When the muscle cells in your heart beat at their own internal pace, instead of beating in time with the group, it causes an arrhythmia.

 In some cases this can be fatal when not corrected quickly. What this example illustrates so nicely is that while cells alone can behave in a particular way, in the context of the body your cells must coordinate with other cells to keep your body working properly.

For all the cells in the body to coordinate properly so the whole system remains healthy, it not only requires healthy individual cells capable of performing their duties, it requires that information from one cell can be shared with other cells located all over the body.

Like getting cues to driving safely from signs and signal lights along the road, cells get information about how to function properly from the cells and fluid surrounding them.

Generally, this communication can be boiled down to a combination of two parts – a signal and a receptor. Signals are like radio waves and receptors are like radio receivers; they are both specialized for a particular job, neither is valuable without the other, and when you have both you get useful communication.


Internal body signals tend to be made of molecules. Although there are other types of signals the body can use, like the light your eyes respond to and the sound waves your ears detect, these still generate molecular signals inside the body.

Even the electrical signals in your nervous system are based on the movement of molecules.

Often these are specialized molecules that act only as signals, but nearly any molecule can have signal-like effects in the body. For instance, carbohydrates in the food we eat are broken down to release glucose (blood sugar), which acts as a signal – rising glucose levels after a meal let your pancreas know when and how much insulin to release to help store some of that glucose and return blood glucose levels back to normal.

On rare occasion, some molecules (like antibodies from the immune system) can act as both a signal and a receptor!  


A few types of signals enter the cell and have a direct effect, much like radio waves enter your home for your radio to pick up directly. Steroids are a good example. Signals that can pass into the cell can interact with a receptor on the inside, or they can have a more direct effect, such as binding to DNA to change how it is used.

Most signal molecules are blocked by the cell membrane (the outer surface of the cell) and can’t enter the cell to deliver their message directly. Think of it like receiving a package – the delivery person gets the package to your front door but they can’t enter the house.

They need a way to let you know the package is there so you can come to the door to get it. For the delivery person, there is a doorbell; for the cell, there is a receptor.

Like the doorbell has a button on the outside connected to a chime on the inside, the cell’s receptor molecules typically span across the cell membrane so a portion is exposed on both the outside and inside of the cell.

When the delivery person presses the doorbell on the outside of your home a sound on the inside notifies you to check the door. Likewise, when a signal arrives at a cell it interacts with the portion of the receptor on the outside, which causes a change in the receptor that results in the signal’s message being communicated to the inside of the cell.

The cell will have different responses to the signal based on other conditions in and around it, just as you might rush to the door if you’re expecting an urgent delivery or quietly wait for a stranger to go away.

The nature of the changes triggered by the signal interacting with the receptor, and the processes involved in the cell’s response, are extremely complex and beyond the scope of this article.  


Unlike a doorbell that anyone can press to trigger the chime, the interactions of cellular signals and receptors are much more specific; more like a lock and key. Like your house key will open your door, but won’t open your neighbor’s, a signal molecule must match its receptor to activate it.

This is called “specificity” and is largely coordinated by the shape of the respective signal and receptor molecules.

Within the body specificity tends to be high – one signal type interacts with one receptor type or perhaps a couple of related receptors. For example, acetylcholine will not bind and trigger a dopamine receptor, but it can activate two different types of acetylcholine receptors (nicotinic and muscarinic).

Things get interesting when foreign molecules are introduced into the body.

It turns out that some pharmaceuticals, supplements, and recreational drugs have an effect in the body because they are similar enough in shape to a natural signal molecule that they can successfully interact with the body’s receptors.

For example, the body has receptors that are matched to a molecule called anandamide (an endocannabionoid made in the body). The THC (tetrahydrocannabinol – a phytocannabinoid made by a plant) found in marijuana has a similar enough shape to anandamide that it can bind to the same receptors and have an effect.

Another interesting fact is that signals do not always activate receptors.

Caffeine works by blocking the receptors for adenosine, thus preventing adenosine from making you feel sleepy. Similarly, morphine blocks pain signals from being transmitted from pain receptors.

Thinking back to the heart muscle cell example at the beginning, it is signals from the nervous system, from the endocrine system, and from neighboring cells that all work together to coordinate the pumping action of the heart.

Amazingly this works, usually without a hitch, for decades – and begins months before you are born! So next time you take a medication or a supplement, try to imagine how it is interacting with your body – binding receptors and acting like a signal or changing the way your body responds to signals that are already there, helping to restore balance and improve your health.

Check out other Body Basics:

Body Basics: What Are Adrenal Glands and What Do They Do?