Biochemistry
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Neurotransmitters
Neurotransmitters are chemical messengers that signal throughout the brain. Stored in vesicles in terminal buttons at the end of axons, impulses go to these terminal buttons and signal the release of the neurotransmitters. From there, the neurotransmitters exit the neuron and are released into the synaptic cleft between the terminal junction of the releasing neuron and the dendrites of the next. Neurotransmitters bind to the dendrites of the next neuron. Each neurotransmitter has a specific shape that corresponds with the shape of its receptor, enabling a lock-and-key fit. From there, responses can start. Drugs and toxins can cause problems by mimicking the shape of the neurotransmitters and binding to the receptors, either stopping the expected reaction or amplifying it. The signal is ended by reuptake of the neurotransmitters, in which the neurotransmitters are taken back into the terminal buttons to be used for the next impulse, enzyme deactivation, in which enzymes destroy the neurotransmitters by changing the shape, or autoreception, in which the amount of neurotransmitter released is controlled and the neuron can stop when the signal is no longer needed. Some examples of neurotransmitters include acetylcholine, epinephrine, norepinephrine, serotonin, and dopamine. Acetylcholine controls messages that go between neurons and muscles that cause the muscles to contract. Epinephrine and norepinephrine work together in situations when danger is detected to prepare the body for fight or flight and then restore it to normal function. Serotonin controls emotions and mood, and dopamine is known as the brain’s reward system.
White and Gray Matter
White and gray matter is named for the literal colors of the neurons seen in the brain. The white matter refers to myelin that coats the axons of neurons. Known as a myelin sheath, this is beads of fat that go down the axon to facilitate fast conducting of neurons. The gaps between the beads of myelin are known as Nodes of Ranvier, and the negative impulses cannot travel within the axon because of the polarity of the myelin. Therefore, it jumps from node to node, enabling faster travel than if it had to pass through the entire axon. Keeping in mind that the brain is full of circuits, myelin acts as the "rubber" of wires. Gray matter refers to the cell bodies and dendrites that form connections throughout the brain to make a network of circuits that connect all of the millions of neurons so that messages can be sent to any part of the brain. These are the metal part of the wire.