The sensory neuron (afferent neuron) transmits nerve ’mpuls~s from the receptor to the CNS. Its dendrite ends on the receptors.

The association neuron (intermediate neuron) connects the sensory neuron to the motor neuron and also the neurons in the CNS.

TALKING ABOUT TIHE SPINAL CORD... The spinal cord is actually a whole bunch of nerve fibers grouped together. The length of the spinal cord is around 50 cm and it is enclosed in the vertebral column. It serves as the intermediate point where impulses travel to and from the brain. Also the spinal cord is the main, control center for the involuntary reflex actions. That means, the spinal cord can make orders to cause actions without receiving instructions from the brain. 
	
TEST YOUR REFLEX. 
Sit down and cross your legs. Slightly hit your crossed leg just below the kneecap. Does it jerk up? Information can be sent as fast as 298 km/hr in your nervous system!  With its weight of merely 1.2 kg  to 1.4 kg, the brain contains 10,000,000,000 nerve cells! 	Furthermore, the left hemisphere of the brain controls the right side of the body, and the right hemisphere of the brain controls the left side of the body. The gray matter, which covers the outer layer of the cerebrum, is similar to a piece of irregularly folded cloth which is more than 1 metre long (comparable to the length of an ironing board!).