The nervous system performs its functions as neurons (nerve cells that transmit nerve impulses) innervate muscle structures. For a muscle to contract, the nervous system must first receive sensory inputs. During this process, sensory neurons take nerve impulses from sensory receptors in the skin and other organs to the central nervous system (i.e. brain and spinal cord) in response to external and internal stimuli. For example, to open a book by arm muscle contraction, continual sensory input to the central nervous system from the eyes and hands must be made to inform the position of the book. In a neuron, the dendrites (short extensions) receive signals from sensory receptors or other neurons, and the signal pass to the cell body before reaching an axon (portion of a neuron that conducts nerve impulses). Nerve impulses from sensory receptors (distal end of the long axon of a sensory neuron...