Nervous System and Reflexes
Your nervous system is the body’s control center and communication network. It senses the world, decides what to do, and sends commands, all in a fraction of a second. Understanding its parts and how a reflex works explains everything from reading these words to yanking your hand off a hot stove.
This lesson covers the nervous system and the reflex arc.
The nervous system controls and coordinates the body using electrical signals carried by nerve cells called neurons. It has two main parts: the central nervous system (brain and spinal cord) and the peripheral nervous system (nerves throughout the body). A reflex is a fast, automatic response that protects the body without waiting for the brain to decide.
What are the parts of the nervous system?
The nervous system is divided into two parts. The central nervous system is the brain and spinal cord; it processes information and makes decisions. The peripheral nervous system is the network of nerves that carries messages between the central system and the rest of the body. The messengers are neurons, specialized cells that pass signals as tiny electrical impulses, letting information travel almost instantly.
How does a reflex work?
A reflex is an automatic response to a stimulus that happens without conscious thought, which is why it is so fast. The path a reflex signal follows is called a reflex arc. When you touch something hot, a sensory neuron carries the signal to your spinal cord, which immediately sends a command back through a motor neuron to your muscles to pull away, all before your brain has even registered the pain. Skipping the brain saves precious time and protects you from harm.
| Step | What happens |
|---|---|
| Stimulus | You touch a hot surface |
| Sensory neuron | Carries the signal to the spinal cord |
| Spinal cord | Sends an immediate command back |
| Motor neuron | Tells the muscle to pull away |
Why are reflexes automatic?
Reflexes are automatic because the signal takes a shortcut through the spinal cord instead of traveling all the way to the brain and back. That shortcut is the whole point: it protects the body from damage in situations where even a fraction of a second matters. You feel the pain a moment later, once the brain receives its copy of the message, but your hand is already safely away.
Watch: A Short Video Lesson
Save My Exams walks through this skill clearly in a few minutes. It is a helpful companion to the reading above:
A routine for nervous system questions
- Split the system: central (brain and spinal cord) versus peripheral (nerves).
- Remember neurons carry signals as electrical impulses.
- For a reflex, trace the arc: stimulus, sensory neuron, spinal cord, motor neuron, muscle.
- Note that a reflex skips the brain to save time.
- Connect the fast response to protecting the body.
Practice questions
- What two organs make up the central nervous system?
- What are the cells that carry nervous system signals called?
- What is a reflex?
- Why does a reflex not wait for the brain?
- Which neuron carries the command to the muscle?
- True or false: the peripheral nervous system includes the brain.
Answers:
- The brain and the spinal cord.
- Neurons.
- A fast, automatic response to a stimulus.
- Because the signal takes a shortcut through the spinal cord to save time and protect the body.
- The motor neuron.
- False. The brain is part of the central nervous system.
Where this fits
The nervous system coordinates the muscles and skeleton and works alongside the senses you study next. It is one of the major human body systems. Find all topics on the ASVAB General Science Learning Hub.
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