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There are three kinds of actions that our body carries out, namely voluntary actions, involuntary actions or reflex actions and conditioned reflex actions. All three actions involve stimuli, an impulse, neurons and effectors. However they do have their differences.
A voluntary action is basically an action which you yourself initiate by your own conscious. For example, when you see a friend over at the other side of the room, you wish to attract the attention of that friend of yours, hence you may want to wave your hand and call out his name. This is done by your brain by sending impulses from it to the effectors or in this case, your biceps and triceps muscles and also your larynx, via relaying neurons, the spinal cord, synapes and motor neurons. The impulses upon reaching their respective effector muscles cause the waving of your hand and you to shout out your friend's name. This action is under the control of the will thus is known as a voluntary action.
Involuntary Actions (Reflex Actions):
A voluntary action is under the control of the will, involuntary actions as their name suggest, are total opposites of voluntary actions. In this case, your spinal cord or your brain takes total control, without your own conscious, depending on where the stimuli originate. Reflex actions controlled by the spinal cord, example scratching, are called spinal reflexes while those by the brain, example blinking, are called cranial reflexes. Blushing, sneezing and salivation are also reflex actions however, salivation is also known an conditioned reflex action which we will be discussing later.
What actually happens?
Let's say someone throws a ball in front of you, across your face. Firstly, your eyes pick up the stimulus which is the sudden movement of the ball in front of your face. Impulses are produced and these impulses travel along the sensory neurone toward the brain since the stimulus is picked up in the head region. As a reaction is needed in the maximum speed possible to avoid harm, there isn't any interpretation of the impulse sent and instead, the impulse passes through the relaying neurone, directly to the respective motor neurones, via synapes. Impulses upon reaching the effectors, cause the blinking of the eyes, the movement of the head towards the direction in which the ball flew from, and possibly the waving of the hand to protect the head and a shout from the larynx. This is the process of the reflex action.
A conditioned reflex action is a special kind of reflex action which results from the learning from past experiences or particular stimuli which was originally ineffective in producing the response. In other words, the are either inborn reflexes or learnt reflexes. For example, many people salivate at the smell of good food as they had learnt from past experiences that such smell are associated with good food and they have to urge to want to eat them, hence salivary glands are stimulated and saliva is produced and secreted. Reading is also a conditioned reflex, so are driving, cooking and riding a bicycle.
On to Homeostasis...