Spinal cord is said to be centre of reflect action because they are:
1. Involuntary actions that involve reflex arc
2. In most of the vertebrates information does not travel directly to the brain through the reflex arc.
3. Basically it travels through the synapse and reaches the spinal cord
which acts as the centre for these actions and send an appropriate response for the stimulus.
Spinal cord is a long tube like structure enclosed by a vertebral column. It carries signal from the brain and then transfer themto different body parts.