The correct answer is Nervous system.The basic unit of the nervous system is a nerve cell or neuron. The human brain contains about 100 billion neurons.Neurons, or nerve cells, are the main structural and functional units of the nervous system. Every neuron consists of a body (soma) and a number of processes (neurites).The nervous system determines that it is a highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. Important Points Together with the endocrine system, the nervous system is responsible for regulating and maintaining homeostasisA neuron is the structural and functional unit of the brain.The longest cell in the human body is the neuron.The nervous system consists of the brain, spinal cord, sensory organs, and all of the nerves that connect these organs with the rest of the body. Together, these organs are responsible for the control of the body and communication among its parts.