Nervous System

What Every EMT Needs to Know About Nerves

representation of the brain cells firing
The brain is the quintessential organ of the central nervous system. Science Photo Library - PASIEKA / Getty Images

The nervous system is an organ system that handles communication in the body. There are four types of nerve cells in the nervous system: sensory nerves, motor nerves, autonomic nerves and inter-neurons (neuron is just a fancy word for nerve cell). You can divide up all the nerves in the body into roughly two parts: the central nervous system and the peripheral nervous system.

Central Nervous System (CNS)

The central nervous system contains two organs: the brain and the spinal cord.

It has all four types of nerve cells and is the only place you can find inter-neurons. The central nervous system is insulated from the outside world pretty well. It never even touches blood. It gets its nutrients from cerebrospinal fluid, a clear liquid that bathes the brain and spinal cord.

Both organs are covered with three layers of membranes called the meninges. The meninges and cerebrospinal fluid cushion the brain to keep it from being injured by a knock on the noggin. It's possible to get an infection from viruses or bacteria in the meninges called meningitis. It's also possible to have bleeding either between the meninges and the skull (called an epidural hematoma) or in between the layers of the meninges (called a subdural hematoma). Any bleeding or infection inside the skull can put pressure on the brain and cause it to malfunction.

The central nervous system is like the guts of your computer (maybe the computer you're using to read this).

It's in there with millions of connections moving little impulses around from circuit to circuit (nerve to nerve), calculating and thinking. Your brain makes all the calculations and stores information. Your spinal cord is like a cable with lots of individual wires running to all different parts of the brain.

But the computer brain inside your laptop, like the brain inside your head, is useless all by itself. You have to be able to tell your computer what you need and see or hear what your computer is trying to tell you. You need some sort of input and output devices. Your computer uses a mouse, a touchscreen or a keyboard to sense what you want it to do. It uses a screen and speakers to react.

Your body works very similarly. You have sensory organs to send information to the brain: eyes, ears, nose, tongue and skin. To react, you have muscles that make you walk, talk, focus, wink, stick your tongue out--whatever. Your input/output devices are part of your peripheral nervous system.

Peripheral Nervous System (PNS)

The peripheral nervous system is everything connected to the central nervous system. It has motor nerves, sensory nerves and autonomic nerves. Autonomic nerves act automatically, which is a way to remember them. They are the nerves that regulate our bodies. They are the body's version of a thermostat, a clock and a smoke alarm.

They work in the background to keep us on track and healthy, but they don't take up brain power or need to be controlled.

Autonomic nerves are loosely split into either sympathetic or parasympathetic nerves.

  • Sympathetic nerves have a tendency to speed us up. They increase heart rate, breathing and blood pressure. These nerves are responsible for the Fight or Flight response.
  • Parasympathetic nerves stimulate blood flow to the gut. They slow down the heart and decrease blood pressure.

Think of the sympathetic nerves as the body's accelerator and parasympathetic nerves as the brake pedal. Your body is always stimulating both the parasympathetic side and the sympathetic side at the same time--just like my grandmother used to drive, with a foot on each pedal.

Motor nerves start from the central nervous system and go out toward the far reaches of the body. They're called motor nerves because they always end in muscles. If you think about it, the only signals your brain sends to the outside world consist of making things move. Walking, talking, fighting, running or singing all take muscles.

Sensory nerves go the other direction. They carry signals from the outside toward the central nervous system. They always start in a sensory organ: eyes, ears, nose, tongue or skin. Each of those organs has more than one type of sensory nerves--for instance, the skin can sense pressure, temperature and pain.

A Word About the Spinal Cord

The spinal cord is the connection between the central nervous system and the peripheral. It is technically part of the CNS, but it is how most of the motor and sensory nerves get to the brain. Inside the spinal cord are some of those inter-neurons mentioned above. In the brain, inter-neurons are like the microscopic switches in a computer chip, helping to make calculations and do the heavy thinking.

In the spinal cord, inter-neurons have a different function. Here they act like a planned short circuit, letting us react to some things faster than we could if the signal had to travel all the way to the brain and back. Inter-neurons in the spinal cord are responsible for reflexes--the reason you jerk back when you touch a hot pan before you even realize what happened.

Sending Signals

Nerves carry messages via signals called impulses. Like a computer the signal is binary, it's either on or off. A single nerve cell can't send a weaker signal or a stronger signal. It can change frequency--ten impulses per second, for example, or thirty--but each impulse is exactly the same.

Impulses travel along a nerve in exactly the same way as muscle cells contract, through chemistry. Nerve cells use ionized minerals (salts like calcium, potassium and sodium) to propel the impulse along. I won't get too deep into the physiology, but the body needs a proper balance of all three of these minerals for the process to work correctly. Too much or too little of any of these and neither muscles nor nerves will function properly.

Nerve cells can be pretty long, but it still takes several to reach from the tip of your finger to your spinal cord. The cells don't touch each other. Instead, the impulse is chemically sent (transmitted) from one nerve cell to the next using substances known as neurotransmitters.

Adding neurotransmitters to the bloodstream can cause nerves to send signals. For example, many of the sympathetic nerve cells mentioned above (the Fight or Flight cells) react to a neurotransmitter called adrenaline, which is released into the bloodstream from the adrenal glands when we get scared, stressed or startled.

If you have a solid grasp of how the nervous system works, it's a small leap to understanding why certain substances or medications affect us the way they do. It's also easier to understand how strokes or concussions affect the brain.

The body is a dynamic collection of chemicals constantly interacting. The nervous system is the most basic of those interactions. This is the foundation for understanding physiology as a whole.

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