When a sensory receptor is stimulated, signals pass from it along a sensory neuron to the spinal cord. The message travels out of the spinal cord along a motor neuron to the effector organ (e.g., a muscle or a gland), which acts accordingly. Such a pathway is called a reflex arc. The sensory nerve fibers of a spinal nerve do not enter the spinal cord at the same place as the motor nerve fibers of the same nerve leave the spinal cord. Sensory fibers enter the dorsal part of the spinal cord while motor fibers leave the ventral part. Sensory fibers are said to have dorsal roots and motor fibers have ventral roots. The cell bodies of sensory nerves are outside the spinal cord forming a swollen mass called a dorsal root ganglion. See also reflex.