7th Grade Science Student ebook

18.1 THE NERVOUS SYSTEM

Chapter 18: Vision and Hearing

How your body responds to a stimulus

The withdrawal

reflex

Imagine you’re relaxing on the couch, watching your favorite

television show. Someone sneaks up behind you and touches the

back of your neck with a wet, frosty ice cube. Before you even have

a chance to think “who did that?” your body springs into action. The

ice cube triggers an automatic response called a withdrawal reflex

that happens without a conscious decision on your part.

Sensory and

motor nerves

A withdrawal reflex happens because nerve impulses are sent

through the nerves in your body. When an ice cube touches the

back of your neck, sensory nerves in your skin send nerve

impulses through wire-like nerve fibers to your spinal cord. In the

spinal cord, the nerve impulse is transferred to motor nerves.

Motor nerves control muscle contractions. Impulses from your

motor nerves cause the muscles in your neck and back to contract,

jerking your body away from the ice cube. All of this happens in a

split second!

withdrawal reflex - an

involuntary response to an outside

stimulus.

sensory nerves - nerves that

receive sensory stimuli, such as

how something feels.

motor nerves - nerves that

transmit signals to skeletal muscle,

causing movement.

A withdrawal reflex happens

automatically. A stimulus like cold

or hot can trigger this response.

Write a reflection about a time you

experienced a withdrawal reflex.

What caused the withdrawal reflex

(the stimulus)? How did your body

respond? How did you react

afterwards?