The third major part of the neuron is the axon, coming out of the soma like a hose. The axon carries the output messages of a neuron (nerve impulses) along its length to its axon terminals (axon endings). There is only one axon per neuron, although it can branch into multiple axon terminals. In a typical neuron, the root end of the axon emerges out of the soma at a small swelling called the axon hillock. Between the axon hillock and the first segment of the axon is where the nerve impulse is first generated (see discussion of the action potential, the nerve impulse, that follows below).

Neuron Structure The main thing that makes neurons special and differentiates them from other cells in the body is that they have many extensions of their cell membranes, generally referred to as processes. Neurons are usually described as having one, and only one, axon—a fiber that emerges from the cell body and projects to target cells. That single axon can branch repeatedly to communicate with many target cells. It is the axon that propagates the nerve impulse (also called an action potential), which is communicated to one or more cells. The other processes of the neuron are dendrites, which receive information from other neurons across specialized areas called synapses. The dendrites are usually highly branched processes, providing locations for other neurons to communicate with the cell body. Information flows through a neuron from the dendrites, across the cell body, and down the axon. Figure 4.1.54.1.5\PageIndex{5} shows the structure of a typical neuron. The main parts of a neuron are labeled in the figure and described below. Figure 4.1.54.1.5\PageIndex{5}: Neuron. A somatic motor neuron (in the CNS) with labeled structures: cell membrane, dendrites, cell body (soma), axon, axon hillock, node of Ranvier, myelin sheath, axon terminal, and synaptic end bulbs (also called axon terminal buttons). The oligodendrocyte shown to the side of the axon is a glial cell that forms the myelin sheath surrounding the axon. The cell body (or soma; soma = "body") is the part of a neuron that contains the nucleus (shown as an oval structure in the center of the cell body, but not labeled) and most of the major organelles. The cell body is usually quite compact, and may not be much wider than the nucleus. The cell membrane is the structure that surrounds all the surfaces of the cell (including the dendrites and axon) and separates the inside of the cell from the outside of the cell. Dendrites are thin structures that are extensions of the cell body. Their function is to receive messages (excitatory and inhibitory post-synaptic potentials, EPSPs/IPSPs- see the nervous system communication chapter) from other cells and carry them to the cell body. A neuron may have many dendrites, and each dendrite may branch repeatedly to form a dendrite “tree” with more than 1,000 dendritic branches. Dendritic spines (small extensions on the surface of the dendritic branches) further increase surface area for receiving messages, allowing a given neuron to communicate with thousands of other cells. The axon is a long, thin extension of the cell body. It transmits nerve impulses away from the cell body and toward other cells. The axon hillock is a small bulge found at the base of motor neuron axons. The nerve impulse (or action potential) starts from the axon hillock. The axon branches at the end, forming the axon terminal. Branches of the axon terminal end in axon terminal buttons (also called axon endings, synaptic end bulbs, synaptic buttons/boutons, bouton terminaux, etc.) These are the points where the message is transmitted to other cells (via the release of chemicals called neurotransmitters), often to the dendrites of other neurons. A small gap called a synapse (also called a synaptic gap or synaptic cleft) is located between the end of the axon terminal and the surface of the receiving cell. An axon may branch hundreds of times, but there is never more than one axon per neuron. Many axons (especially the long axons of nerves in the peripheral nervous system) are covered by sections of myelin (also called the myelin sheath). The myelin sheath is composed of lipid layers that surround the axon. Myelin is a very good electrical insulator, like the plastic or rubber that encases an electrical cord. Axons that are covered by sections of myelin are called myelinated, whereas axons without myelin sheaths are called unmyelinated. Regularly spaced gaps between sections of myelin occur along the axon. (The gaps are actually much further apart than is shown in the figure- it is necessary to shrink the distance to fit all the structures in a diagram!) These gaps are called nodes of Ranvier, and they allow the transmission of nerve impulses along the axon. Nerve impulses jump from node to node in a process called saltatory conduction, allowing nerve impulses to travel along the axon very rapidly. The oligodendrocyte shown in the figure is a glial cell that produces myelin sheaths in the central nervous system (brain and spinal cord)- see the Glia section below.

Early childhood is not only a period of physical growth; it is also a time of mental development related to changes in the anatomy, physiology, and chemistry of the nervous system that influence mental health throughout life. Cognitive abilities associated with learning and memory, reasoning, problem solving, and developing relationships continue to emerge during childhood. Brain development is more rapid during this critical or sensitive period than at any other, with more than 700 neural connections created each second. Herein, complex gene–environment interactions serve to increase the number of possible contacts between neurons, as they hone their adult synaptic properties and excitability. Many weak connections form to different neuronal targets; subsequently, they undergo remodeling in which most connections vanish and a few stable connections remain. These structural changes (or plasticity) may be crucial for the development of mature neural networks that support emotional, cognitive, and social behavior. The challenge for psychology has been to integrate findings from genetics and environmental (social, biological, chemical) factors into the study of personality and our understanding of the emergence of mental illness. These studies have demonstrated that common DNA sequence variation and rare mutations account for only a small fraction (1%–2%) of the total risk for inheritance of personality traits and mental disorders (Dick, Riley, & Kendler, 2010; Gershon, Alliey-Rodriguez, & Liu, 2011). Additionally, studies that have attempted to examine the mechanisms and conditions under which DNA sequence variation influences brain development and function have been confounded by complex cause-and-effect relationships (Petronis, 2010). Epigenetics has the potential to provide answers to these important questions. It refers to the transmission of observable characteristics (phenotype) in terms of gene expression in the absence of changes in DNA sequence (Waddington, 1942; Wolffe & Matzke, 1999). The advent of advanced techniques to study the distributions of regulators of gene expression throughout the genome led to the collective description of the “epigenome.” In contrast to the genome sequence, which is static and the same in almost all cells, the epigenome is highly dynamic, differing among cell types, tissues, and brain regions (Gregg et al., 2010). Recent studies have provided insights into epigenetic regulation of developmental pathways in response to a range of external environmental factors (Dolinoy, Weidman, & Jirtle, 2007). These environmental factors during early childhood and adolescence can cause changes in expression of genes conferring risk of mental health and chronic physical conditions. Thus, the examination of genetic–epigenetic–environment interactions from a developmental perspective may determine the nature of gene misregulation in psychological disorders. This module will provide an overview of the main components of the epigenome and review themes in recent epigenetic research that have relevance for psychology, to form the biological basis for the interplay between environmental signals and the genome in the regulation of individual differences in physiology, emotion, cognition, and behavior.

Basics of Evolutionary Theory Evolution simply means change over time. Many think of evolution as the development of traits and behaviors that allow us to survive this “dog-eat-dog” world, like strong leg muscles to run fast, or fists to punch and defend ourselves. However, physical survival is only important if it eventually contributes to successful reproduction. That is, even if you live to be a 100-year-old, if you fail to mate and produce children, your genes will die with your body. Thus, reproductive success, not survival success, is the engine of evolution by natural selection. Every mating success by one person means the loss of a mating opportunity for another. Yet every living human being is an evolutionary success story. Each of us is descended from a long and unbroken line of ancestors who triumphed over others in the struggle to survive (at least long enough to mate) and reproduce. However, in order for our genes to endure over time—to survive harsh climates, to defeat predators—we have inherited adaptive, psychological processes designed to ensure success. At the broadest level, we can think of organisms, including humans, as having two large classes of adaptations—or traits and behaviors that evolved over time to increase our reproductive success. The first class of adaptations are called survival adaptations: mechanisms that helped our ancestors handle the “hostile forces of nature.” For example, in order to survive very hot temperatures, we developed sweat glands to cool ourselves. In order to survive very cold temperatures, we developed shivering mechanisms (the speedy contraction and expansion of muscles to produce warmth). Other examples of survival adaptations include developing a craving for fats and sugars, encouraging us to seek out particular foods rich in fats and sugars that keep us going longer during food shortages. Some threats, such as snakes, spiders, darkness, heights, and strangers, often produce fear in us, which encourages us to avoid them and thereby stay safe. These are also examples of survival adaptations. However, all of these adaptations are for physical survival, whereas the second class of adaptations are for reproduction, and help us compete for mates. These adaptations are described in an evolutionary theory proposed by Charles Darwin, called sexual selection theory.