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Animal Systems
Animal Systems

The Nervous System

It's Electric

As the captain of a space ship, you've been put in charge of exploring the final frontier. After an enjoyable lunch of astronaut macaroni and cheese, an alien aircraft materializes out of nowhere and begins attacking. You begin barking out orders on the bridge, directing your crew to leap into action.

Reacting to an outside threat, controlling resources and directing a team working together. That's a mission of a spaceship crew, but it's also the job of the nervous system.

The nervous system is another way our bodies communicate. It's pretty similar to the endocrine system, but in the nervous system, chemical messages are sent along nerves instead of through the blood. The nervous system coordinates all of our actions, both voluntary and involuntary. As your eyes dart across this page, your nervous system is in charge. Even when you put your Thai leftovers in the microwave for lunch, your nervous system is in charge. Keep that in mind the next time you take a whiff of that week-old Pad Thai and try to figure out if it's past the point of no return.

Most animals have a nervous system, although it looks much different in a jellyfish than it does in an elephant. Jellyfish are actually brain-less animals, but don't mention it to them. They're really sensitive. Or they would be if they had brains.

Their nervous system is composed of a nerve network that allows their tentacles to feel its surroundings. These nerves are thin structures that take information from one place to another. If we accidentally brush against a jellyfish, its tentacles interpret it as a fearsome predator and respond by stinging you.

Elephants, on the other hand, do have a brain. It is true that they never forget. As you might expect, their nervous system is more complex than that of the jellyfish. An elephant's nervous system, like most mammals', is composed of a brain, cells, tissues, and the nerves that coordinate all of its actions, both voluntary and involuntary. Within the nervous system, the brain decides things and the nerves send those decisions to the muscles or the skin. The elephant's brain decides that a banana or two (or 38) would hit the spot right now, and sends a message via the nerves to its trunk to reach into a tree and pluck them out.

It's our brain that decides we'd much rather have pizza than leftover Thai, and tells our hands to pick up the phone and call Papa John's. Or if you are freezing cold, your brain tells your skin to make goose bumps to trap air between the skin and arm hair, to help keep warm.

The brain sends information down the spinal cord. From there, the information gets sent out through different types of nerves to control different types of activities at different parts of the body. Since it's a complex system, biologists have broken it down into smaller, more manageable parts with similar functions.

The Peripheral Nervous System

Vertebrates and invertebrates alike are blessed with a peripheral nervous system (PNS) and a central nervous system (CNS). Basically, the PNS deals with interpreting and transferring brain-talk to the rest of the body. The CNS is the brain and spinal cord, and they're able to take cues from the outside world and translate them into something our bodies understand. One can't function without the other, and both are equally important parts of the nervous system.

You can think of the CNS as the general leading the army. After his troops survey the battlefield and report back, he evaluates the scene, maybe strokes his mustache, and makes a decision about what his troops should do. The PNS are the troops, surveying the land, taking those orders, and putting them into action. Without a general (CNS), the troops (PNS) have no idea what to do. And without troops, a general is just some weirdo in a funny suit screaming at no one in particular.

The PNS is divided into the somatic and the autonomic systems that control our conscious and unconscious actions, respectively. Both communicate with the brain, and receive directions from it as well. The somatic system has two parts: the sensory nerves and the motor nerves. The sensory ones actually sense stuff going on in the environment (when the troops survey the land) and report that information back up to the brain. The motor nerves receive messages from the brain on how the body should act (like when the general tells the troops to jump into action).

Imagine a day where all chores and tasks have been completed, and there's nothing to worry about. You're curled up on the couch with a magazine, a snack, and a cuddly puppy resting next to you. But suddenly, out of nowhere a rhinoceros comes charging down the steps. Yes, it's quite the surprise and catches you completely off guard.
Rhinoceros. Activate the sympathetic system and grab that cuddly puppy when you see this guy coming down the steps.

After the PNS, via the sensory nerves, has sent this scary information to the brain, the brain dictates what it wants to do through the PNS using motor nerves. An obvious reaction to seeing a rhinoceros coming down the steps is to pick up the equally frightened puppy and high tail it out the door. To do that, your brain sent messages through the motor nerves of the PNS's somatic system to your arm and leg muscles.



Somatic System. In the peripheral nervous system, the sensory neurons send information to the brain, whereas the motor neurons take information from the brain, and send it to the rest of the body.

Your brain also tells the other branch of the PNS, the autonomic system, to activate certain internal unconscious processes, like increasing your blood pressure and heart rate. Just like the name suggests, the autonomic system handles the bodily functions that do not require deliberate thought and are done automatically (breathing, blinking, etc.). Depending on the type of situation you find yourself in, the sympathetic or parasympathetic part takes control. The sympathetic system responds to stressful and dangerous situations, while the parasympathetic system is in charge when all is right with the world, and you are relaxed.

To help out, remember that since "Parasympathetic" has an "r" in it, this system controls Rest and Relaxation. It decreases blood pressure and heart rate. "Sympathetic" doesn't have the "r," and it's responsible for the "fight and flight" responses in the body.

Since a rhinoceros in the house is nothing to laugh at, your sympathetic nervous system gets activated, and in response to the impending danger, heart rate and blood pressure increase. Redistributing the blood flow and energy to your muscles is really important as you to grab the poor puppy and book it out of the door. After you've slammed the door on the impending pointy doom, be sure to thank your sympathetic system for saving your butt.

Once you've called the zookeeper (or whoever you call when there's a rhinoceros in your house), and they've come to fix the situation, you can relax once again. Now that all is safe in your world, it's time for the parasympathetic system to take control. Your heart rate and blood pressure decrease and you start digesting the peanut butter and jelly sandwich you were eating.

The parasympathetic and sympathetic systems control the exact opposite actions at the exact same organs. So after the sympathetic system dilates your pupils so you can take in as much light as possible as you frantically search for an escape route, the parasympathetic system constricts them.


Peripheral Nervous System. In general, the parasympathetic side of the PNS makes the body relax. It's the sympathetic side that speeds things up to prepare for the "fight or flight" response.

The other part of the PNS sometimes gets overlooked in books. We here at Shmoop HQ like our pizza bites and ice cream bars way too much to let the enteric system be snubbed. This is the system that conveys translated messages about digestion to our digestive system, pancreas, and gallbladder. Through the enteric system, the brain controls secretion of various digestion enzymes and peristalsis, which are the muscle contractions that keep your food moving down the digestive tract.

This system is perfectly happy functioning on its own, but usually the bossy sympathetic and parasympathetic systems take control, and dictate when it can and can't work. When disaster strikes, the sympathetic system tells the enteric system to take a break and halt digestion so the energy used to break down a hamburger can be used to make those little legs run. The parasympathetic system eventually gives it the "all clear" for digestion to gear up again. It works out well, since you'll never have to go Number 2 with a grizzly bear breathing down your neck.

The Central Nervous System


Chimp Brain. The brain is an important part to the central nervous system. Image from here.

The PNS is powerful, but it can't make decisions—that's what the CNS is for. The CNS takes in whatever information the PNS sends in, and figures out what to do and how to do it (the strategy for attack). Here, we're talking about the brain and spinal cord.

We usually refer to the brain in hemispheres since one side is structurally identical to the other side. That also means that our brain has two of a lot of structures like the hippocampus (where we store memories) and the basal ganglia (it provides motor control).

Between the right and left hemispheres is a central canal, which is, we kid you not, a gap in the brain. Don't worry, it's supposed to be like that. Inside the canal is cerebrospinal fluid, which is made from blood filtrate. This cerebrospinal fluid sort of acts like blood since it flows to the rest of the brain, providing cells with valuable nutrients and taking away any metabolic wastes.


Brain architecture. The forebrain is the largest portion of the brain, and it does the majority of the sensing and deciding.

The meaty part of the brain is made up of white and gray matter. To the eye, white matter looks, well, white. It's primarily on the inside of the brain where the myelinated axons are. (Don't worry, we'll talk about axons in a few minutes. If you are just dying to know, these are the thin structures that pass information between cells.) Myelin is a compound that coats these axons, allowing their electrical signals to travel faster than a pig covered in Vaseline.

The white matter is where neuronal communication primarily takes place. It's where we learn, process emotions, and interpret information passed along by our senses. The gray matter is mostly on the surface of the brain and generally consists of cell bodies, their dendrites, and unmyelinated axons.

There are three different sections of the brain, all with their own functions and responsibilities. The forebrain, right up at the front of the brain, receives all sensory information from the PNS, so it's slated with the tough task of thinking and perceiving. Sights, smells, sounds, and things you touch all get pumped through the PNS and into the forebrain for processing. If that weren't enough to keep it busy, it's also the area that controls language and motor function. Within the forebrain are important areas of the brain including the cerebral cortex, cerebrum, hypothalamus, and thalamus.

The midbrain controls auditory and visual responses, and connects the forebrain with the hindbrain. This rear part of the brain is crucial for maintaining balance, coordination, and other bodily processes that maintain homeostasis. Together, the midbrain and hindbrain make up the brain stem, which is the transfer point for information between the PNS and CNS.

The spinal cord is the other big part to the CNS, and it's really just a group of nerves that runs inside the spinal column, reaching from the brain all the way down to the lower back. Think of it as the superhighway for the brain's messages, with plenty of well-marked exits along the way for signals to head off to their final destinations. These nerves connect with the parasympathetic and sympathetic systems of the PNS.

Yes, the structure is important, but we think the coolest part of the brain is actually how it communicates with other areas of the brain and the rest of the body.

The brain transmits and receives information through a process called neurotransmission. The millions of cells that do all the work are called neurons. (Since we're talking brains, you'll see a lot of "neuro" words.) When one neuron has something to say, it does so by sending electrical signals through an axon. Because lots of messages have to get passed from one brain area to another, axons are usually pretty long, and send information super-fast since they're covered in myelin. Seriously, this stuff is crazier than the Internet.

All neurons receive information through their dendrites, which are branchy structures that protrude from a cell body. Dendrites are shorter and more branched than axons (they kind of look like tree roots and branches). While they don't usually reach other brain areas, they cover a lot of ground. If you piece the puzzle together, you'll see that axons communicate with dendrites, but dendrites don't normally communicate with axons. (The nerve.) When scientists talk about nerves, they are really talking about bundles of both dendrites and axons.


Neuron. When the electrical signal travels from the cell body, down through the myelinated axon, it reaches the synapse where neurotransmission sends a message to another neuron's dendrites.

The point where an axon communicates with a dendrite is called a synapse, and that's where the neuro-magic happens: neurotransmitters are released (from the presynaptic cell) and bind to a receptor on the nearby dendrite (on the postsynaptic cell). These neurotransmitters are little chemical messengers that carry coded messages from one neuron to the other.

The brain is a control freak, and doesn't let neurons release neurotransmitters whenever they want to. Only when a cell sends an electrical signal to its axon will the axon release the neurotransmitter. Think of these chemicals as bits of classified information—it's given on a need-to-know basis.

There are many different neurotransmitters, and they all do different things in the brain. Some of the most popular ones are glutamate, gamma-aminobutyric acid (thankfully it's called GABA for short), acetylcholine, dopamine, and the different catecholamines like norepinephrine and epinephrine.

The dendrites are coated with all sorts of different receptors, and each one is specific to a certain type of neurotransmitter. For instance, dopamine can't bind to a glutamate receptor, and vice versa. Some receptors pass on an electrical signal from one cell to another through ionic gradients. These receptors are basically a bunch of Gossip Girls, making the message go viral to other cells. There are others that mediate a particular cellular response—maybe activating gene transcription or inserting new receptors into the cell membrane. (We like to call these guys bossy-pants.)

For example, if the neurotransmitter-dude "GABA" is released from a presynaptic cell, the postsynaptic receptors allow certain ions to flow across the membrane. (Typical Gossip Girl.)

But if dopamine binds to its receptors, certain internal proteins will get activated and others will shut off. Dopamine is always bossing our internal processes around. (Typical bossy-pants.)

This whole neurotransmission thing also happens outside of the brain, when the PNS sends signals to various parts of the body. For the sympathetic system, norepinephrine/epinephrine and acetylcholine are the favorite neurotransmitters. But if you favor hot tubs and relaxation, you'll learn to love acetylcholine since it's the most common in the parasympathetic system. Our favorite is the Ben and Jerry's neurotransmitter. We need more of that.

Brain Snack

Goose bumps don't just happen in response to cold temperatures. In fact, they also respond to fear, anxiety, and pleasure. Have you ever scared a cat so much that her tail gets all bushy? Feline goose bumps. Crazy, huh?
Next Page: Fitting It All Together
Previous Page: The Endocrine System

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