Читать книгу The Mind-Body Cure - Bal Pawa - Страница 40

The nervous system uses a system of nerve cells, or neurons, to take up, process, and transmit information through electrical and chemical signals (Figure 2.4). The brain alone contains about 100 million neurons and 100 trillion connections.² Each of these cells, which is about a tenth the diameter of a human hair, has three parts. The dendrites are treelike branches that receive input from other neurons. The cell body is attached to the dendrites and contains the DNA of the cell. The axons are wires of various lengths that carry electrical impulses at high speeds toward dendrites of other neurons. These impulses—which carry either excitatory (encourages neurons to fire) or inhibitory (discourages neurons from firing) messages—are transmitted across a gap called a synapse.

A neuron looks like a thin tree. It consists of a nucleus, which is embedded within a larger structure known as the cell body. From the cell body, branches called dendrites sprout. The axon resembles a tree trunk, which then splits into root-like structures. At the tips of these roots are the synaptic terminals. The space where the synaptic terminal communicates with another neuron's dendrite is called the synapse.

Figure 2.4. Anatomy of a neuron

Science reveals that neurons that “fire together” in a repeated pattern establish permanent neural pathways: they “wire together.” The communication between neurons is carried out by a number of neurotransmitters. Some cause arousal in the brain, others drowsiness; some focus your attention, others are associated with memory and mood. Over time, exposure to chronic levels of stress hormones changes our neural chemistry and the size and shape of parts of the brain.

Chronic stress changes our neural chemistry by altering neurotransmitters such as serotonin and dopamine. Ordinarily, serotonin is an inhibitory neurotransmitter that stabilizes our mood and balances the number of excitatory neurotransmitters being fired into our brain. Dopamine is usually the “feel-good” chemical that can be both an inhibitory and excitatory neurotransmitter. If our dopamine levels are balanced, our body can reduce any symptoms of anxiety, depression, and stress. But when we are anxious and under chronic stress, the brain begins to produce an overabundance of fear-related neurotransmitters, such as adrenaline and norepinephrine, and fewer neurotransmitters associated with happiness and relaxation, such as dopamine and serotonin. To compensate, the brain creates more receptors to handle the extra fear neurotransmitters and fewer serotonin receptors, which it deems less necessary. The result is that we feel anxious more often than we feel relaxed. Some people can live in a perpetual fight-or-flight reaction, while others have such extreme anxiety they develop a panic disorder, which may require medication.

Chronic stress can also directly change the shape and size of certain areas in the brain. In neuroscience, we call this ability of the brain to change in shape and size neuroplasticity, and it can be both positive and negative. Long-term exposure to the stress hormone cortisol appears to cause brain neurons to shrink and interferes with their ability to send and receive information. As a result, the hippocampus becomes smaller—stress actually shrinks our brain! On the one hand, stress affects our ability to retrieve information, pay attention, or stay focused. On the other hand, the amygdala, the center for the fear reaction, becomes enlarged in chronically stressed people. We become more anxious and more fearful, and those neurons then start to wire together. For example, using MRI scans we can see that the shape and size of various brain structures are different in individuals who have post-traumatic stress disorder (PTSD).