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ENGL 301: Assignment 1:3 – Definitions Shannon Benson – 38634168 2017-01-28 Introduction: The objective of this assignment is to gain experience in defining complex technical terms for an audience of non-technical readers. In particular, the intended audience for these definitions is a person who is taking an introductory level psychology class. Three types of definitions will be given for the term “neuron” from one of my core disciplines, psychology. The first of these is a parenthetical definition, which illustrates the meaning of a term by following it with a clarifying phrase in parentheses. The second is a sentence definition, which goes into more detail about the term and its features. Last is an expanded definition, which gives the reader a more elaborate understanding of the term and its technical applications by, for example, explaining the term’s history or using visual aids. Parenthetical definition: Neurons (nerve cells) comprise the body’s nervous system and transmit signals between the brain and other parts of the body. Sentence definition: Neurons are the building-blocks of the body’s nervous system. They are specialized nerve cells that communicate electrical and chemical messages between the brain, through the spinal cord, and to other parts of the body. Expanded Definition: History: The term “neuron” was first introduced in an article by German anatomist Heinrich Wilhelm Waldeyer in 1891. Waldeyer proposed neurons to be specialized cells that make up the nervous system (Anctil, 2015). Spanish anatomist Ramon y Cajal later expanded upon Waldeyer’s initial findings, developing a framework called neuron theory. Neuron theory states that neurons are the core functional units of the central nervous system, that nerve impulses are transmitted unidirectionally across gaps between individual neurons, and that neurons are composed of three major parts: the axon, dendrite, and cell body (Fishman, 2007). FIGURE 1 Anatomy of a Neuron. Source: Bill Day, www.educarer.org, 2006. Analysis of Parts: Figure 1 displays the various parts of a single neuron. The cell body or “soma” contains the nucleus and other key cellular structures such as the mitochondria and Golgi body. Short branches near the cell body called dendrites receive electrical impulses and pass them along to the cell body. The axon extends out of the cell body and acts as a cable, carrying impulses to the far ends of the neuron at points called axon terminals. The axon is encased in a fatty layer called the myelin sheath which insulates the electrical signals and increases their conduction speed along the axon. Operating Principle and Required Conditions: The conduction of impulses between neurons operates under an “all-or-none” principle. This means that the magnitude of a neuron’s response to a stimulus is independent of the strength of that stimulus. When a single stimulus is strong enough to exceed a certain threshold potential, the neuron will fire. A stimulus that substantially exceeds the threshold potential does not generate a “bigger” response; the neuron simply fires as opposed to not firing. If the stimulus does not exceed this potential, the neuron will not fire at all (Redman, 1990). Comparison and Contrast: Most neurons fall under one of three types: sensory neurons, motor neurons, and interneurons. Sensory neurons receive sensory input such as taste, smell, or touch, and carry this information to the spinal cord or to the brain for processing. Motor neurons send messages away from the brain to the spinal cord and toward muscles or glands, creating direct or indirect responses such as muscle movement. Interneurons mediate sensory and motor neurons by passing messages between them, and they also play a role in reflex responses (Jankowska, 1992). References. Anctil, M. (2015). Dawn of the neuron: the early struggles to trace the origin of nervous systems. Montreal: MQUP. Fishman, R.S. (2007). The Nobel Prize of 1906. Arch Ophthalmol., 125(5), 690-694. Jankowska, E. (1992). Interneuronal relay in spinal pathways from proprioceptors. Progress in Neurobiology, 38(4), 335-378. Redman, S. (1990). Quantal analysis of synaptic potentials in neurons of the central nervous system. Physiological Reviews, 70(1), 165-198.