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4.7: Neurons- Structure and Function

  • Page ID
    139782
    • Todd LaMarr
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    The Structure of Neurons

    The human brain is arguably the most complex of all biological systems. The adult brain is composed of more than 100 billion neurons (Pakkenberg and Gundersen 1997). Neurons are the information processing cells in the brain (see Figure \(\PageIndex{1}\)). There are many different kinds of neurons that vary in their size and shape as well as in their function. Neurons make connections with other neurons to form the information processing networks that are responsible for all of our thoughts, sensations, feelings and actions. Since each neuron can make connections with more than 1,000 other neurons, the adult brain is estimated to have more than 60 trillion neuronal connections! [1]

    Structures of a nueron labeled from end to end include dendrites, cell membrane, cell body, azon, node of renvier, myelin sheath and synapse
    Figure \(\PageIndex{1}\): Structure of a neuron. ([2])

    Populations of neurons are connected to one another by fibers that extend from cell bodies of the individual neurons. There are two kinds of connecting fibers, dendrites and axons. Dendrites are arrays of short fibers that look like the branches of a tree. They extend only a short distance away from the neuron cell body. Their main function is to receive the electrochemical input signals from other neurons. Axons are long connecting fibers that extend over long distances and make connections with other neurons, often at the dendrites. Axons act a little like telephone wires in that they are responsible for sending electrochemical signals to neurons located in distant locations. Bundles of individual axons from many different neurons within one region of the brain form fiber tracts that extend to, and make connections with, groups of neurons in other regions of the brain forming the information processing networks. Axons are wrapped in a fatty substance called myelin that, like insulation on a telephone wire, makes the transmission of electrochemical signals between regions efficient. Myelin is white in appearance, thus fiber pathways of the brain are often referred to as “white matter”, or “white matter pathways”. [3]

    Neurons communicate with each other through “chemical signaling,” a process in which they exchange chemicals called “neurotransmitters. This is how it happens: an action potential is an electrical impulse that travels through a neuron, triggering it to release neurotransmitters into a small gap found in between two neurons, called a synapse. Then, the dendrites of the neuron on the other side of the synapse takes up the neurotransmitters, which then generates an action potential that travels along the axon of this second neuron. This process continues in several neurons that are connected to each other. [5]

    While most of the brain’s 100 to 200 billion neurons are present at birth, they are not fully mature during infancy and toddlerhood. As they mature, neurons establish connections between each other. Synapses, the connections between neurons, undergo a period of transient exuberance or temporary dramatic growth. There is a proliferation of these synapses during the first two years so that by age 2, a single neuron might have thousands of connections. Figure \(\PageIndex{2}\) shows neurons and their connections from an area within the frontal lobe of the cortex. Image A (on the left of Figure \(\PageIndex{2}\)) is from the brain of a one month old infant and image B is from the brain of a six year old. As we are born with most of the neurons we will ever have, the difference between these two images is not a difference in the quantity of neurons, rather the difference between them is the quantity and quality of connections.

    Dramatic increase in neuronal connection demonstrated for 1 year old and 6 year old
    Figure \(\PageIndex{2}\):Change in neuronal connections from 1 year old to 6 years old.[6]

    After the dramatic increase in synapses, the neural pathways that are not used will be eliminated thereby making those that are used much stronger. This process of elimination is called pruning. Experience will shape which of these connections are maintained and which of these are pruned. Ultimately, about 40 percent of the early connections will be lost (Webb, Monk, and Nelson, 2001). This activity is occurring primarily in the cortex or the thin outer covering of the brain involved in voluntary activity and thinking. The prefrontal cortex, located behind our forehead, continues to grow and mature throughout childhood and experiences an additional growth spurt during

    adolescence. It is the last part of the brain to mature and will eventually comprise 85 percent of the brain’s weight. As the prefrontal cortex matures, the child is increasingly able to regulate or control emotions, to think hypothetically, strategize, and have better judgment. Of course, this is not fully accomplished in infancy and toddlerhood, but continues throughout childhood and into adulthood (Uytun, 2018). [7]

    Myelination

    Another significant change occurring in the central nervous system is the development of myelin, a coating of fatty tissues around the axon of the neuron. Myelin helps insulate the nerve cell and speed the rate of transmission of impulses from one cell to another. This increase enhances the building of neural pathways and improves coordination and control of movement and thought processes. During infancy, myelination progresses rapidly, with increasing numbers of axons acquiring myelin sheaths. This corresponds with the development of cognitive and motor skills, including language comprehension, speech acquisition, sensory processing, reaching/grasping and crawling/walking. The development of myelin continues into adolescence but is most dramatic during the first several years of life. [8] [9]


    [1] Stiles & Jernigan (2010). The basics of brain development. Neuropsychology review, 20(4), 327-348. CC by 2.0

    [2] Image from “Physical Growth and Brain Development in Infancy” by Tera Jones for Lumen Learning is licensed by CC by 4.0.

    [3] Stiles & Jernigan (2010). The basics of brain development. Neuropsychology Review, 20(4), 327-348. CC by 2.0

    [5] Kaur et al., (2019). Using magnets to stimulate the brain helps people with depression. Frontiers for Young Minds, 7, 1-8. CC by 4.0

    [6] DeFelipe (2013). Going to school to sculpt the brain. Frontiers for Young Minds. CC by 4.0

    [7]Physical Growth and Brain Development in Infancy” by Tera Jones for Lumen Learning is licensed by CC by 4.0.

    [8]Brain Development” by: Nicole Arduini-Van Hoose provided by Hudson Valley Community College. CC BY-NC-SA 4.0

    [9]Physical Growth and Brain Development in Infancy” by Tera Jones for Lumen Learning is licensed by CC by 4.0.


    This page titled 4.7: Neurons- Structure and Function is shared under a mixed 4.0 license and was authored, remixed, and/or curated by Todd LaMarr.