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Key Ideas

  • A reflex is the simplest circuit in the nervous system.
  • Once triggered by a stimulus, a reflex goes to completion and causes a response, such as a muscle reaction. Some reflexes are easier to control than others.
  • The five parts of a reflex arc are the sensor, sensory neuron, control center, motor neuron, and muscle.


In the previous sections students learned about components of the nervous system. In this section they investigate a nerve circuit. They learn that reflexes are simple circuits in the nervous system that result in a response. Students identify reflexes, which are an important part of behavior and serve as protection from harm.



\checkmark demonstrate and explain the five-part sequence of a reflex arc.

\checkmark distinguish between a motor neuron and a sensory neuron.

\checkmark explain the role of an inhibitory interneuron in a reflex.


reflex, stimulus

Student Materials

Activity 4-1: How Fast Is Your Reaction Time?

  • Activity Report
  • 20 \ cm metric ruler

Teacher Materials

Activity 4-1: How Fast Is Your Reaction Time?

  • Activity Report Answer Key

Advance Preparation

See Activity 4-1 in the Student Edition

Activity 4-1: How Fast Is Your Reaction Time

  • Gather 20 \ cm rulers.

Interdisciplinary Connections

Math Use strings of holiday minilights set to sequence mode to illustrate the nerve impulse traveling down the axon.

Image Attributions




6 , 7 , 8

Date Created:

Feb 23, 2012

Last Modified:

Apr 29, 2014
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