Читать книгу Overcoming Internet Addiction For Dummies - David N. Greenfield - Страница 39

Neurons are essentially the central part of the electrochemical connection process that makes up our nervous system; most of what we’re talking about with addiction involves the central nervous system, where neurons do their job. The central nervous system includes the brain and spinal cord. A neuron is made up of three major parts, as shown in Figure 2-2:

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FIGURE 2-2: The major parts of a neuron.

 Soma, or cell body

 Dendrites, which are the connecting strands that allow all neurons to interconnect

 Axons, which are the fibers that act like electrical cables, connecting cell bodies to their dendritic terminals

Obviously, this is an oversimplified explanation, but suffice it to say that the power of the human brain derives not only from its constituent lobes and functional centers, but from how they all interconnect and communicate.

Neurotransmitters are the chemicals that allow neurons to communicate and connect with each other. Many neurotransmitters are in the central and peripheral nervous system, the major ones being dopamine, serotonin, norepinephrine, GABA, acetylcholine, glycine, and glutamate. The three that we frequently deal with in psychiatry, psychopharmacology, and addiction medicine are dopamine, serotonin, and norepinephrine, although we have recently begun targeting and integrating some of the other neurotransmitters as well.

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FIGURE 2-3: A neuron firing.

The way in which neurons connect is quite amazing and is a combination of electricity and chemistry. Without getting into the weeds here, each neuron is typed to a specific neurotransmitter, and for the most part only uses that neurotransmitter for firing its respective neurons. Here’s how it works (see Figure 2-3):

 An electrical signal moves through an axon (the electrical cable I mention earlier), and this signal travelling along the biological wire is called propagation. Some unique aspects of the axon help facilitate this propagation and make it as fast and efficient as possible; a special superfast coating exists along the axon called myelin, and this myelin sheath allows for a faster electrical transmission.Nature also uses other tricks to keep nerve signals moving along. Many axons also have built-in speed bumps called nodes of Ranvier (shown in Figure 2-2), which essentially allow the electrical signal to jump across the tips of these nodes to speed up transmission. This is in part what allows our nervous system to operate at lighting speed, and this leaping is called saltatory conduction.

 That electrical signal comes barreling down the axon to a gate (actually, a gap) called a synapse. When it reaches the synapse (or gap), a seemingly magical thing occurs: The electrical signal is converted to a chemical one by facilitating an action potential that commands the presynaptic membrane (one end of the axon) to release its store of the associated neurotransmitter. This transmitter is then taken up (the term is uptake) into the post-synaptic receptor and then converted back to an electrical signal; this nerve conduction process occurs millions of times a second. (See Figure 2-4.)

 The leftover neurotransmitter that is not taken up is then either biodegraded (absorbed) or taken back up (reuptake) by the pre-synaptic receptor for later use.

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FIGURE 2-4: Inside a synapse.

Antidepressants, particularly selective serotonin reuptake inhibitors (SSRI), prevent the reuptake of serotonin. This allows more serotonin to be available, which, in theory, can elevate mood and help alleviate depression.

The king of neurotransmitters related to addiction is dopamine. Although other neurotransmitters are undoubtedly involved with addiction, dopamine constitutes the lion’s share of the pleasure drive we see in addiction. Internet screen use elevates dopamine reception at the post-synaptic receptor in the nucleus accumbens, and over time there is an up-regulation of these post-synaptic receptors to manage the increasing level of dopamine present in this area of the brain. See the nearby sidebar “The role of up-regulation and down-regulation in addiction” for details.