【Science: 8th Grade】How Animal Behavior Works: The Secrets of Stimulus and Response
Hello there! Our theme this time is "How animal behavior works."
"Have you ever touched something hot and pulled your hand away before you even realized it?" We’ve all been there!
The truth is, an exchange of information is happening inside our bodies at lightning speed. It might feel a little tricky at first, but keep in mind that we’re talking about your own body, so just take it one step at a time. Don't worry—if you focus on the key points, you'll definitely understand it!
1. "Sensory Organs" That Receive Stimuli
How do we know what’s going on around us?
Changes in the environment—like light, sound, and smells—are called stimuli. The special parts of our body that detect these are called sensory organs.
(1) The Eye (Receiving light stimuli)
The eye works much like a camera.
・Iris: Adjusts the amount of light entering the eye.
・Lens: Focuses the image.
・Retina: Contains cells that receive light stimuli. It’s where the image is captured.
(2) The Ear (Receiving sound stimuli)
The ear captures vibrations in the air.
・Eardrum: The first part to vibrate when it receives sound waves.
・Ear ossicles: Amplify the vibrations and transmit them inward.
・Cochlea: Converts vibrations into electrical signals and sends them to the nerves.
【Key Point】
Other sensory organs include the nose (smell), tongue (taste), and skin (temperature and pain). What they all have in common is their role: "receiving outside stimuli and converting them into signals."
2. The "Nervous System" That Transmits Signals
Where do those signals go once they’re received? A network of nerves is responsible for delivering them.
(1) Central Nervous System
The control center for the entire body. It acts as the command tower that makes decisions.
・Brain
・Spinal cord
(2) Peripheral Nervous System
Nerves that branch out from the central nervous system to the rest of the body.
・Sensory nerves: Send signals from sensory organs to the brain or spinal cord.
・Motor nerves: Carry commands from the brain or spinal cord to the muscles.
【Fun Fact】
Nerve signals travel through the body as fast as electricity. It’s almost like having a high-speed internet network woven throughout your body!
3. The Path from Stimulus to Reaction
There are two main patterns in the flow from receiving a stimulus to the body moving.
① Voluntary Reaction (Conscious)
This is when you think in your head, "Oh, a ball is coming! I'm going to catch it," and then move.
(Path) Stimulus → Sensory organ → Sensory nerve → Brain → Spinal cord → Motor nerve → Muscle → Reaction
Since the brain has to process "what to do," it takes a little bit of time.
② Reflex
This is the reaction where your hand moves before you even think, like when you touch something hot.
(Path) Stimulus → Sensory organ → Sensory nerve → Spinal cord → Motor nerve → Muscle → Reaction
Because the command comes directly from the spinal cord without going through the brain, the reaction time is incredibly short. This is your body's built-in defense system to protect you from danger.
【Common Misconception!】
You might wonder, "If a reflex doesn't go through the brain, does the brain never know about it?" Actually, after the reflex happens, the signal is sent to the brain, and that's when you feel the "Ouch!" "Moving first, feeling later" is the hallmark of a reflex.
4. "Bones and Muscles" That Move the Body
Once the command is received, it’s the job of the muscles and bones to make the move.
(1) Joints and Muscles
The places where bones connect are called joints. Muscles are attached across these joints.
(2) How Bending and Straightening the Arm Works
Muscles have a unique property: "They can contract (shorten), but they cannot push themselves to stretch." Because of this, two muscles must work in pairs.
・When bending the arm: The inner muscle contracts, and the outer muscle relaxes.
・When straightening the arm: The outer muscle contracts, and the inner muscle relaxes.
【Key Point】
The tough parts at the ends of muscles that attach to bones are called tendons. A very famous one is the Achilles tendon!
Summary: Key Terms for this Lesson
・Sensory organs: Organs that receive stimuli (eyes, ears, nose, tongue, skin, etc.).
・Central nervous system: The command center (brain, spinal cord).
・Reflex: An unconscious, lightning-fast reaction to a stimulus to protect the body.
・How muscles work: They move bones by contracting. They always work in sets with an opposing muscle.
Great job! Now you can see how our bodies are made of many parts working together in harmony.
Make sure to memorize the test-favorite points, like how "reflex pathways do not pass through the brain!"
It might seem complicated at first, but try bending and straightening your own arm while thinking about it—it makes the concept much easier to visualize!