Voluntary, involuntary and reflex actions
A reflex action is a sudden, automatic response, like pulling your hand from a flame. Its path, the reflex arc, passes through the spinal cord, making it fast.
Three kinds of action
| Action | Controlled by | Thinking needed? | Examples |
|---|---|---|---|
| Voluntary | Forebrain | Yes, done by choice | Writing, talking, moving a chair, clapping |
| Involuntary | Midbrain and hindbrain | No, goes on by itself | Blood pressure, salivation, vomiting (all controlled by the medulla) |
| Reflex | Mostly the spinal cord | No, sudden and automatic | Pulling the hand away from a flame, knee jerk |
Draw it: the reflex arc
The message also travels up to the brain, so you feel the pain a moment after your hand has moved.
Why reflex arcs exist
Thinking takes time. In an emergency, such as touching a hot iron, waiting for the brain to think would cause more harm. So the nerve that detects the danger is joined directly to the nerve that moves the muscle, inside the spinal cord. The response is quick and automatic. The information still reaches the brain, which then makes us aware of what happened. Animals have reflex arcs because the thinking process of the brain is not fast enough for such moments.
Reflex action vs walking
How to draw the reflex arc
- Draw a cross-section of the spinal cord as an oval, with a butterfly-shaped grey area in the middle.
- Draw a receptor in the skin of the hand and a sensory neuron running from it into the spinal cord.
- Inside the spinal cord, draw a short relay neuron joining the sensory neuron to a motor neuron.
- Draw the motor neuron running out to a muscle in the arm; label the muscle effector.
- Add arrows from receptor to effector to show the path of the impulse, and label every part.
When a drop of hot oil splashes from the kadai while frying puris, your hand jerks away before you think: that is a reflex. You feel the sting a moment later, when the message reaches your brain.
2 marks: “What is a reflex action? Give two examples.” A sudden, automatic response to a stimulus without thinking, e.g. pulling the hand back from a flame, the knee jerk.
3 marks: “Draw a flow chart of the reflex arc and name its parts. Why does it pass through the spinal cord?” Receptor → sensory neuron → relay neuron in the spinal cord → motor neuron → effector; the spinal cord gives a quick response without waiting for the brain to think.
2 marks: “How is a reflex action different from walking?” A reflex is automatic and controlled mainly by the spinal cord; walking is voluntary, controlled by the forebrain and balanced by the cerebellum.
Parts of the reflex arc: Ram Sees Red, Moves Elbow: Receptor, Sensory neuron, Relay neuron, Motor neuron, Effector.