Voluntary actions
A voluntary action is one you decide to perform. It is started by the conscious part of the brain. Writing, walking to the door and picking up a cup are all voluntary — you can choose whether and when to do them. Impulses travel from the brain along motor neurones to skeletal muscles, which contract to produce the movement you intended. With practice, many voluntary actions such as riding a bicycle become smoother and need less conscious effort.
Involuntary actions
An involuntary action happens automatically, without conscious control. The heartbeat, breathing, peristalsis in the gut and the narrowing of the pupil in bright light are involuntary. Many are controlled by the spinal cord or by the autonomic nerves and continue even while you sleep.
Key idea
Voluntary = started by conscious brain, under your control. Involuntary = automatic, not consciously controlled.
The reflex action and reflex arc
A reflex action is a fast, automatic, protective response to a stimulus. Because speed matters, the impulse does not have to travel up to the conscious brain first; it takes a short route through the spinal cord called the reflex arc:
Stimulus → receptor → sensory neurone → relay neurone (spinal cord) → motor neurone → effector → response.
The brain is informed of what happened, but the response is already under way. This short pathway is why reflexes are so quick and help protect the body from harm.
Example
Stepping on a sharp stone: pain receptors in the foot send an impulse along a sensory neurone to the spinal cord, a relay neurone passes it to a motor neurone, and the leg muscle contracts to lift the foot — all before you consciously feel the pain.
Remember
Reflex actions are involuntary and coordinated mainly by the spinal cord, making them faster than voluntary actions started by the brain.