The document defines key terms related to stimulus and response, including receptors, sensory neurons, relay neurons, motor neurons, synapses, and effectors. It also describes the components of a reflex arc, including how a pain withdrawal reflex is mediated by the spinal cord's grey matter. Specifically, injury stimulates pain receptors, starting an action potential in sensory neurons that relays to relay neurons in the spinal cord which coordinate the motor neuron response of muscle contraction or relaxation through the effector to remove the body from the painful stimulus. Finally, it provides two examples of how animal responses have evolved through natural selection in blackcap bird migration patterns and butterfly mate preferences.

2. Assessment Statements Obj.
E1.1 Define the terms stimulus, response and reflex. 1
Explain the role of receptors, sensory neurons, relay neurons, motor neurons,
E1.2 3
synapses and effectors in the response of animals to stimuli.
Draw and label a diagram of a reflex arc for a pain withdrawal reflex, including the
E1.3 spinal cord and its spinal nerves, the receptor cell, sensory neuron, relay neuron, 1
motor neuron and effector. Include white and grey matter, dorsal and ventral roots.
Explain how animal responses can be affected by natural selection, using two
examples.
E1.4 3
• Sylvia atricapilla (blackcap bird) migration changes
• Heliconius cydno butterflies and changes in mate preference
What connections can you make with other topics?
nerves, hormones, evolution
homeostasis
Stimulus & Response
Command terms: http://i-biology.net/ibdpbio/command-terms/ Assessment statements from: Online IB Biology Subject Guide

4. The pain withdrawal reflex is a rapid, unconscious response to
The Pain Reflex danger or injury. It is mediated by the spine’s grey matter.
A very basic overview from the BBC:
Reflex animation from: http://www.bbc.co.uk/schools/gcsebitesize/science/add_ocr_pre_2011/brain_mind/reflexactionsrev1.shtml

5. The pain withdrawal reflex is a rapid, unconscious response to
The Pain Reflex danger or injury. It is mediated by the spine’s grey matter.
(nociceptors)
Spine-mediated pain reflex arc, from: http://en.wikibooks.org/wiki/Anatomy_and_Physiology_of_Animals/Nervous_System

6. The pain withdrawal reflex is a rapid, unconscious response to
The Pain Reflex danger or injury. It is mediated by the spine’s grey matter.
(nociceptors)
Injury stimulates pain receptors (nociceptors), causing Ca2+ ions to rush
in. This depolarises the sensory neuron and starts an action potential.
http://www.vet.ed.ac.uk/animalpain/Pages/animation2.htm
Spine-mediated pain reflex arc, from: http://en.wikibooks.org/wiki/Anatomy_and_Physiology_of_Animals/Nervous_System

7. The pain withdrawal reflex is a rapid, unconscious response to
The Pain Reflex danger or injury. It is mediated by the spine’s grey matter.
(nociceptors)
propagates the AP along the
axon (review action and resting potentials!)
Injury stimulates pain receptors (nociceptors), causing Ca2+ ions to rush
in. This depolarises the sensory neuron and starts an action potential.
http://www.vet.ed.ac.uk/animalpain/Pages/animation2.htm
Spine-mediated pain reflex arc, from: http://en.wikibooks.org/wiki/Anatomy_and_Physiology_of_Animals/Nervous_System

8. The pain withdrawal reflex is a rapid, unconscious response to
The Pain Reflex danger or injury. It is mediated by the spine’s grey matter.
http://bcs.whfreeman.com/thelifewire/content/chp46/4601s.swf
Relay neurons (interneurons in the animation above) are in
the grey matter of the spine. They receive the signal from
the sensory neuron, through synaptic transmission (review
it!) and coordinate the response to pain:
• An excitatory neurotransmitter (e.g. ACh) is sent
across the next synapse to the motor neuron for
muscles to contract.
• An inhibitory NT is used to send a signal to
antagonistic (opposite) muscles to relax.
Spine-mediated pain reflex arc, from: http://en.wikibooks.org/wiki/Anatomy_and_Physiology_of_Animals/Nervous_System

9. The pain withdrawal reflex is a rapid, unconscious response to
The Pain Reflex danger or injury. It is mediated by the spine’s grey matter.
(nociceptors)
Finally, the motor neurons conduct
the AP from the relay neuron to the
effector (muscle).
• Muscles that need to contract
receive and excitatory signal.
• Muscles that need to relax
receive an inhibitory signal.
The response is rapid movement
away from the source of pain.
Spine-mediated pain reflex arc, from: http://en.wikibooks.org/wiki/Anatomy_and_Physiology_of_Animals/Nervous_System

11. A
Dorsal root ganglion (passage for neurons)
B
C
D
E
F
Ventral root ganglion (passage for neurons)
G

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