Cell-to-cell communication among the trillions of cells that make up multicellular animals relies primarily upon the specialized tissues of the nervous and endocrine systems. These two systems are intricately connected, with the former having evolved from the latter during the past five hundred million years of animal life. The endocrine system consists of specialized ductless glands located throughout the animal body that produce and secrete hormones directly into the bloodstream. Hormones are chemical messengers that usually are composed of protein or steroid subunits. The bloodstream transports the hormones to various target body tissues, where the hormones contact cell membranes and trigger a sequence of enzyme reactions which ultimately result in the activation or inactivation of genes located on chromosomes in the cell nucleus.
A gene is a segment of a chromosome that is composed of deoxyribonucleic acid (DNA). The DNA nucleotide sequence of the gene encodes a molecule of messenger ribonucleic acid (mRNA) which, in turn, encodes a specific protein for the given gene. If the control sequence of a gene is activated, then ribonucleic acid (RNA) and protein will be produced. If the control sequence of a gene is inactivated, then RNA and protein will not be produced. Hormones target the genes in specific cells to start or stop the manufacture of certain proteins. Within cells and the entire organism, proteins perform important functions. Therefore, hormones control the pro duction of proteins by genes and, as a result, control many activities of the entire animal.
The nervous system, which in vertebrate animals has evolved to become more elaborate than the endocrine system, consists of billions of neurons (nerve cells) that conduct electrical impulses throughout the body. Neurons transmit information, contract and relax muscles, and detect pressures, temperature, and pain. Neuron networks are most dense in the brain (where there are one hundred billion neurons) and spinal cord, where much of the electrical information is centralized, relayed, and analyzed. Neurons must communicate electrical information across the gaps, or synapses, which separate them. To accomplish this goal, the transmitting neuron releases hormones called neurotransmitters, which diffuse across the synapse to the receiving neuron, thereby instructing the receiving neuron to continue or stop the conduction of the electrical message. There are many different types of neurotransmitters, just as there are many different types of regular hormones.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.