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Dynamics of Neural Development

Neuroscientists estimate that the brain is made up of more than 80 billion neurons and trillions of glial cells that produce myelin and regulate the flow of blood, supporting the function of neural systems. As the working cell of the nervous system, the neuron receives and sends signals through electrical impulses and chemical transmissions, creating energy flows throughout the brain, forming complex circuits. As Cajal detailed in his drawings, the neuron consists of a cell body; receiving ends known as dendrites that resemble dense thickets, and long, sinuous, tubular fibers called axons that form connections with other neurons. In accord with his theory' of dynamic polarization, the neuron sends an electrical impulse down the axon. Subsequent research showed that neurotransmitters are released at the synapse, which either activate or inhibit the adjoining neuron. The synapse serves as the point of contact in a border area between neurons, connecting them to one another. Researchers believe that each neuron makes between one thousand and ten thousand connections with other neurons (Kandel, Schwartz, Jessell, Siegelbaum & Hudspeth, 2013).

Patterns of activation create synaptic connections that organize the structures and functions of neural networks. As Donald Hebb explains in his classic axiom, neurons that fire together become “associated” so that “activity' in the one facilitates activity in the other” (1949, pp. 69-70). In this sense we can think of the architecture of the brain as associational, as Freud and James had proposed toward the end of the 19th century'. Freud functionally' linked the activity' of nerve cells early' in his career as a neuroanatomist (Sulloway, 1979). James introduced “the law of association” in his account of brain function and the mind in the Principles of psychology (1890). When processes are stimulated jointly or immediately' after one another, they' proposed, the stimulus activates subsequent processes in a sequential order.

Experience shapes particular patterns of neuronal firing, and the dynamics of activation create distinctive configurations of sensation, emotion, cognition, imagery', and behavior that mediate states of self. What we register as “my' experience,” accordingly, corresponds to specific forms of neural activation. James thought of consciousness as a process, not a thing.

We understand “instantiations” as distinct patterns of neural activity' that form a functional whole (Cozolino, 2017; Siegel, 2020). Experiential conditions activate particular patterns of firing. My grandmother cared for me as a child, and we gardened together every' morning through the spring and summer. More than half a century' later, the scent of lilacs, the yellow of forsythia, the feel of damp earth, or the song of cardinals evoke the sense of her soothing voice and the encircling arms that held me. My experience of nature, relationship, and care was structured as neural networks—“instantiations”— that shape ongoing reactions and states of mind.

Experiential opportunities continue to reorganize the configurations of neural networks across the course of life, influencing states of mind and behavior, challenging earlier conceptions of the brain as fixed, programmed like a computer. The process by which experience brings about structural change in neural connections, known as Hebbian learning or long-term potentiation, is thought to be instrumental in neuroplasticity. We explore these concepts further in our review of neuroplasticity' in the following chapter.

 
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