12 PAIRS - MESSAGES belonging to one of the 4 categories:
-Special Sensory Impulses - smell, taste, vision and hearing
-General sensory Impulses - Pain, touch, temperature, deep muscle sense, pressure, and vibrations
-Somatic motor Impulses - voluntary control of skeletal muscles
- Visceral Motor Impulses - involuntary control of glands and involuntary muscles ( cardiac and smooth muscles).