The human body is, in its simplest observation, a vast network of cells. The human body is one of the most complex productions in the world. By looking at just one part of the body, such as the eye, the complexity of the body's composition becomes apparent. The body is made up of cells that come together to form tissues. These tissues join together to form organs. Organs coexist to form organ systems. These organ systems make up the entire organism that is man. The nervous system is the key system of the human body. Everything that happens in the body is regulated by the nerves of the nervous system. The nervous system has some basic building blocks that make it up in its entirety. Nerve cells are made up of some parts vital for accepting and sending stimuli. Near the body of the cells are small branch-like protrusions called dendrites. Dendrites are the receptors on cells that absorb stimuli sent by the previous cell. Next is the nucleus of the cell located in the center of the cell body. From the dendrites, stimuli travel along a long tail-like projection of cells called axons. At the end of the axon there are small branches with bulbs at the end called axon terminals where stimuli are sent into a space between cells called a synapse where the dendrites of another cell gather and the cycle begins again. There are two main basic cells: afferent neurons and efferent neurons. Afferent neurons act as cells that collect information or stimuli from sensory receptors and bring them back to the spinal cord. Efferent neurons do exactly the opposite. They send a triggered response from the spinal cord or brain to the appropriate muscles or cells to generate a response. Eph...... middle of paper ......and calcitonin. Thyroxine is the hormone that regulates metabolism. The effect of this hormone is an increase in metabolic rate. During this period cellular respiration increases. Works Cited No author. (2013) Brain Basics: Know Your Brain. Retrieved March 19, 2014, from http://www.ninds.nih.gov/disorders/brain_basics/know_your_brain.htmThe Franklin Institute. (2014). Blood. Retrieved March 19, 2014, from http://www.fi.edu/learn/heart/blood/blood.htmlNo author. (1998). Endocrine system. Retrieved March 19, 2014, from http://www.ama-assn.org//ama/pub/physician-resources/patient-education-materials/atlas-of-human-body/endocrine-system.pageTaylor, T (2013 ). Lungs. Retrieved March 19, 2014, from http://www.innerbody.com/anatomy/respiratory/lungs No authors. (2014). Your muscles. Retrieved March 19, 2014, from http://kidshealth.org/kid/htbw/muscles.html