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There are two main types of actions, namely, (i) voluntary action and (ii) involuntary action.
1.1 VOLUNTARY ACTION
Response is under conscious control of the brain.
Same stimulus may produce DIFFERENT responses.
Response is determined by the cerebrum.
Pathway of nerve impulses:
cerebrum / brain --> relay neurone in spinal cord --> motor neurone --> effector
1.2 INVOLUNTARY ACTION
Response is NOT under conscious control of the brain.
Same stimulus always produces the SAME response.
Response is not determined by the spinal cord or the brain.
An example of an involuntary action is a reflex action.
For eg, withdrawal of the hand upon touching a hot object accidentally.
EG 1
WITHDRAWAL OF THE HAND UPON ACCIDENTALLY TOUCHING A HOT OBJECT
1 When the hand accidentally touches a hot object (ie boiling kettle), the temperature receptors on the skin detect the increase in temperature.
2 Nerve impulses are generated which are then transmitted along the sensory neurone.
3 Nerve impulses travel across the first synapse from the sensory neurone to the relay neurone at the spinal cord.
4 Nerve impulses then travel across a second synapse from the relay neurone to the motor neurone.
5 Nerve impulses travel along the motor neurone.
6 Nerve impulses are sent to the effector, ie biceps muscles of the arm.
7 Upon receiving the nerve impulses, biceps muscles contract causing withdrawal of the hand from the hot object.
EG 2 KNEE JERK REFLEX
1 A tap is applied at the tendon.
2 Stretch receptor is stimulated. Nerve impulses are generated.
3 Nerve impulses travel along the sensory neurone to the spinal cord.
4 Nerve impulses travel across a synapse directly from the sensory neurone to the motor neurone. The knee jerk reflex is an example of a monosynaptic reflex arc.
5 Nerve impulses travel along the motor neurone to the effector which is the thigh muscles.
6 Upon receiving the nerve impulses, the effector ie. the thigh muscles contract.
7 Knee jerks.
Activity
URL: http://youtube.com/watch?v=Y5nj3ZfeYDQ
After watching the video, answer the following question.
What are the three cells that make up the reflex arc?
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