Stimulus and Response
Year 9 Science
Stimulus
Any change in an organism’s environment that
causes the organism to react.
(“cause”)
Stimuli (singular) can be:
•External
•Internal
Response
How an organism reacts to a stimulus
(“effect”)
Responses can be:
•Voluntary
•Involuntary
Response
Stimulus
Touch a hot surface...
...pull back your hand
Hear a loud bang...
...flinch/yell out
Raise your body temp...
...start to sweat
Feel cold...
...put on something warm
Stimulus-Response Model
Stimulus
Receptor
(senses the
stimulus)
Messenger
(nerve cells
carry a signal
to give a
response)
Effector
(causes a
response)
Response
Stimulus-Response Example
Stimulus
(bright light)
Receptor
(light
sensitive
cells in eye)
Messenger
(nerve cells
carry the
signal to the
eye)
Effector
(ciliary
muscles)
Response
(pupil
contracts)
Learned Responses
Some responses are instinctual
(we are born with them)
Other responses can be learned.
A famous demonstration of learned response is known as “Pavlov’s Dog”
•A dog is given an external stimulus (the bell) during meals
•The dog learns to associate the bell with food and gives the same response
Involuntary responses
Some responses happen without any thought
whatsoever.
These are known as involuntary responses.
Can you think of others?
Reflex arc – fast!
A reflex arc is a fast response that occurs automatically, without any
thought. These are the fastest responses in your body
For example, when you
touch something hot, you
pull your hand away before
you “think” about it being
hot.
The stimulus (heat) is
relayed along the sensory
neuron through the spinal
cord to the effector which
makes you move your hand.
Homeostasis
Homeostasis: the maintenance of a stable internal environment in spite of external or internal
environmental changes.
The body is constantly working to maintain homeostasis inside us by coordination of our body
systems.
Blood Glucose Maintenance
Feedback loops
The body maintains homeostasis by responding in one of two ways:
•Positive feedback – a response that enhances the effect of a stimulus
(rare – production of oxytocin during labour to increase contractions)
•Negative feedback – a response that counteracts the effect of a stimulus
(these are the most common in the body)