Cortex controls each activity of the body. Stimulus keeps on taking birth in the cells of cortex and come into the muscles by the fibers which come from the cells. Stimulus of the muscle keeps on transmitting into the cortex of the brain. The nerve fibers which come into the muscle from the brain are called efferent fibers. Some fibers transmit the stimulus of muscle into the brain and such fibers are called afferent fibers. The stimulus, which take birth in the skin, joint, muscle or in other organs of the body reach into the cells of cortex through nerve fiber. The stimulus of some nerves which goes into the brain by going through the fibers of nerves is called sensory nerve. The impetus which reach into the muscles by coming out from those cells which are called motor nerve. The impetus reaches into the cortex by the sensory nerve and there the stimulus takes birth.
There are sensory cells and motor cells in the spinal cord too. The sensory cells are present in the posterior horn of spinal cord and the impetus keeps on supplying thought the posterior nerve root. Motor cells are situated in the anterior horn. Impetus takes birth in it and reach into the muscles. Generally, the motor impetus reaches into the muscles after production. Consequently, the muscles do their work. Sometimes the impetus reaches into the posterior horn of spinal cord not into the brain and the impetus reaches and then it reaches to the anterior horn when he muscles from where the muscle get excitement. This process is called reflex action.