Stimulus and response: how we react to the world around us

Why do we need the nervous system?

Let us make a little guess about you.

Right now you may remember that you are thirsty—and then proceed to make yourself a tasty cup of tea or coffee.

That’s your nervous system in action. 

Go on, help yourself if you need a drink. We’ll wait. When you’re ready—scroll down to learn more. 

While we’re at it, let us guess a couple more things about you.

• When you pour the boiling water making your tea—you are probably careful not to burn yourself.
• When you’re hungry, you look for something to eat. 
• If it’s summer and the outside is hot—you’d pick a t-shirt over a heavy coat to wear.
• When you go to sleep, you sometimes see dreams.

Were we correct? Most of us, humans, have these things in common—because that’s how our nervous system works.

To be precise: the thirst, the reflexes to avoid boiling water, the hunger, the reaction to the changing temperature, the dreams we see when sleeping—all of these are reactions of our body, processed and executed by electrical and chemical signals to and from the nerve cells.

The nervous system is in charge of every organ and system in the body; it determines our every psychological and physiological reaction.

All in all, the nervous system does excellent work for us! It really controls most of what we do and who we are.

So, can you guess which parts of our lives have something to do with the nervous system?

1. Thoughts and actions. 
2. Dreams and emotions.
3. Reactions and reflexes.

The nervous system handles all of that through its functions!

When something has changed in the environment or inside you, and you can notice it — it is known as a stimulus. The main function of the nervous system is to react to various stimuli in different ways.

We can break this reaction down into three steps:

Let’s use another example. Imagine your morning run along a route you use every day and know very well.

Suddenly, you notice a puddle in the middle of the road!

In a moment, your legs turn as if by themselves, and you alter your way to run past the puddle. What happened exactly?

In a nutshell, the process goes like this: 

• Through senses, you notice that something has changed—for example, you see a puddle in the middle of the road. That is the sensory input.
• Your nervous system processes the input and makes a decision: Should you stop? Run through as if nothing has changed? Jump over? That is the integration.
• Certain body parts move to react to the change: your body turns to the side and your legs carry you past the puddle. That is the motor output.

Many living creatures have nervous systems. Elephants, cats, birds, fish, even worms and hydras have it. What makes us, humans, so different?

Among all animals, the human nervous system is the most developed and complex. Humans can think, analyze and make decisions—and this sets us apart.

The levels of organization

Our nervous system is highly organized because we have so much to notice, analyze and react to. The cycle of input-integration-output repeats over and over, many times per second.

Handling so many various stimuli on different levels, the nervous system is organized into subdivisions. The two main subdivisions are thecentral nervous system and the peripheral nervous system.

Imagine the brain as the main office of a large international corporation. It has many departments and employees of its own but also oversees the network of regional branches. 

These “regional branches” (which we call the spinal cord) gather information from and send orders to the small local offices (receptors and nerves) and deliver it upstairs. However, there are some issues the branches can deal with by themselves, not disturbing the main office.

Thus, the brain and the spinal cord form the central nervous system. It is the control center that ultimately comes up with every decision for every action your body takes: for example, getting around the puddle.

The peripheral nervous system includes nerves (long fibers) and ganglia (neuron clusters) that ensure effective communication between the central nervous system and the rest of the body.

The function of the nervous system is to make the body react to changes in the environment. The critical steps of every such reaction are sensory input, integration, motor output. The nervous system is organized into two parts: the central and the peripheral. It needs to handle the stimuli on various levels and manage the body’s reactions effectively.

The central nervous system

As you may have noticed, different parts of the nervous system have different functions.

In general, the higher the part is located, the more complex its functions are. 

The highest in the body (and in the order of hierarchy) is thebrain: it coordinates reflexes, movement, general actions and reactions, and the activity of every organ system. The spinal cord is the lower part of the central nervous system. It manages the work of separate groups of muscles and organs. 

To review the parts one by one, let’s also start from the top. 

The skull protects the brain like a robust case. 

The skull bones are very rigid, and the brain tissue is soft—like a jelly. For it not to get damaged with every head movement, the brain is protected by several layers of membranes and is doused in cerebrospinal fluid. This liquid maintains the right pressure inside the skull, keeping the brain stable and nourished. The average adult human brain weighs 1300–1400 grams. Some people think that the mass of the brain determines the smarts of the person. It is a myth. In history, there have been different cases of geniuses with much smaller or much bigger brains. For example, the brain of a Russian novelist Ivan Turgenev was 2021 grams in weight; a French writer Anatole France had his Nobel-prize-winning brain measure only 1017 grams.

The brain is quite hard to study—scientists usually obtain information about specific parts of the brain through examining brain lesions and trauma.

There are many different parts of the brain, small and big, each in charge of its specific function, from problem-solving to maintaining posture.  Course block

The nervous cells that make up the brain can be seen only with a microscope—but the nerve fibers going through are big enough to be visible. 

If we slice the brain through the middle, the contours of gray and white areas (they are actually called gray matter and white matter) resemble a butterfly. Gray matter is mostly neuron cells, and white matter contains glial cells and axons. 

The downstairs of the brainstem is the spinal cord. Its main function is to connect the brain with the rest of the body.

It’s a long structure that looks like a tube 35–45 cm long (depending on the person’s height). It’s divided into31 segments, each having a pair of nerves connecting to specific muscles or areas of the body.  When the embryo starts to develop, the neural tube is one of the first things to appear. Soon the top part of the tube will develop into the brain, and the rest of it will become the spinal cord.

The peripheral nervous system

While the brain and the spinal cord are responsible for decision-making and management, the input and the output rely upon the peripheral nervous system.

Sensory organs (ears, eyes) and muscles in our heads and necks are connected directly to the brain with the 12 pairs of cranial nerves.

The nerves in the peripheral nervous system often come in pairs, working in different directions. 

Sensory nerves take note of the stimuli (like the puddle you were trying to avoid when running) and deliver it to the brain. Motor nerves deliver the brain’s response to the muscles (like that in your legs) and glands. The sensory nerves are also called afferent, and the motor nerves are called efferent. It is easy to remember which is which—because the information arrives (starting with an ’a’) through the afferent nerves and exits through the efferent ones.

If we look at the motor function from a slightly different point of view, we can divide the peripheral nervous system into somatic and autonomic systems.

The somatic nervous system controls the voluntary movement of the skeletal muscles—that is, the muscles attached to the skeleton that need specific instructions from the brain to move the body around.

The autonomic nervous system is in charge of involuntary yet vital processes like breathing, heart beating, and bowel movements.  It has further subdivisions: the sympathetic and the parasympathetic systems.

Imagine a huge dragon suddenly flying right before you. As you see it, your sympathetic nervous system kicks in. It makes your heart pump the blood faster, raises your blood pressure, and kickstarts the adrenaline rush through your veins. It prepares you for battle, activating the “fight-or-flight” response.

After you had successfully slain the dragon and saved the kingdom, your organism would relax and return back to normal thanks to the parasympathetic nervous system. The brain is the core of the nervous system, handling most of the analysis and decision-making. The spinal cord connects it to the rest of the body. The brain and the spinal cord both make up the central nervous system. The peripheral nervous system is the network of nerves that run through the whole body. These nerves often come in pairs — one sensory and one motor.

The nervous tissue

The brain, the spinal cord, the nerves, and all other parts of the nervous system are made up of nervous tissue.

Up to 80% of this tissue are neurons—tiny nerve cells. 

The other 20% are glial cells—the supporting cells which vary in function and location. They can glue neurons together and support them, help draw nutrients from blood, and coat the neurons in myelin to make signals go faster.