A reflex action often involves a very simple nervous pathway called a reflex arc. A reflex arc starts off with receptors being excited. They then send signals along a sensory neuron to your spinal cord, where the signals are passed on to a motor neuron. Reflex action is an involuntary and nearly instantaneous movement in response to a stimulus. Invertebrates, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord. The spinal cord acts as the main centre for reflex actions. The spinal cord acts as a link between spinal nerves and the brain.

Reflex actions are automatic responses to a stimulus that do not involve conscious thought or decision-making. These actions are controlled by the spinal cord rather than the brain due to several reasons.

1. Speed of response
When a stimulus triggers a reflex action, the response needs to be immediate to protect the body from harm. The spinal cord is much closer to the muscles and organs that need to respond, which means that signals can travel faster along neural pathways. In contrast, signals from the brain have to travel a longer distance to reach the same areas. Therefore, reflex actions are mediated by the spinal cord to ensure a rapid response.

2. Efficiency
Reflex actions are simple and pre-programmed responses that do not require input from the brain. By controlling these actions, the spinal cord can conserve the brain's resources for more complex tasks. This allows the body to respond quickly and efficiently to potential threats without wasting energy.

3. Protection
Reflex actions are often protective in nature, designed to prevent injury or further harm. For example, the withdrawal reflex is triggered when we touch something hot or painful. The reflex action causes us to withdraw our hand from the source of pain before we even consciously register the sensation. By controlling these protective reflexes, the spinal cord can prevent damage to the body before the brain even has a chance to react.

In conclusion, reflex actions are mediated by the spinal cord rather than the brain because they require a fast, efficient, and protective response that does not involve conscious thought or decision-making. By controlling these automatic responses, the spinal cord can ensure the body's safety and conserve the brain's resources for more complex tasks.