Neurons and Their Jobs
The human brain is made up of billions of neurons. Each has a cell body, an axon, and many dendrites. The cell body contains a nucleus, which controls all of the cell's activities, and several other structures that perform specific functions. The axon, which is much, much narrower than the width of a human hair, extends out from the cell body and transmits messages to other neurons. Sometimes, the messages have to travel over very long distances (even up to 5 feet!). Dendrites also branch out from the cell body. They receive messages from the axons of other nerve cells. Each nerve cell is connected to thousands of other nerve cells through its axon and dendrites. Neurons are surrounded by glial cells, which support, protect, and nourish them.
Groups of neurons in the brain have special jobs. For example, some are involved with thinking, learning, and memory. Others are responsible for receiving sensory information. Still others communicate with muscles, stimulating them into action.
Several processes all have to work smoothly together for neurons to survive and stay healthy. These processes are communication, metabolism, and repair.
Communication: Sending Millions of Messages a Second
Imagine the telecommunication cables that run under our streets. All day and night, millions of telephone calls are flashing down fiber optic cables at incredible speeds, letting people strike deals, give instructions, share a laugh, or learn some news. Multiply that many-fold and that's the brain. Neurons are the great communicators, always in touch with their neighbors.
As a neuron receives messages from surrounding cells, an electrical charge, or nerve impulse, builds up. This charge travels down the axon until it reaches the end. Here, it triggers the release of chemical messengers called neurotransmitters, which move from the axon across a tiny gap to the dendrites or cell bodies of other neurons. The typical neuron has up to 15,000 of these tiny gaps, or synapses. After they move across the synapse, neurotransmitters bind to specific receptor sites on the receiving end of dendrites of the nearby neurons. They can also bind directly to cell bodies.
Once the receptors are activated, they open channels through the cell membrane into the receiving nerve cell's interior or start other processes that determine what the receiving nerve cell will do. Some neurotransmitters inhibit nerve cell function (that is, they make it less likely that the nerve cell will send an electrical signal down its axon). Other neurotransmitters stimulate nerve cells; they prime the receiving cell to become active or send an electrical signal down the axon to more neurons in the pathway.
During any one moment, millions of these signals are speeding through pathways in the brain, allowing it to receive and process information, make adjustments, and send out instructions to various parts of the body. If neurons are disconnected, they become sick and may die.
Metabolism: Turning Chemicals and Nutrients Into Energy to Keep Neurons Working
Metabolism is the process by which cells and molecules break down chemicals and nutrients to generate energy and form building blocks that make new cellular molecules like proteins. Efficient metabolism needs enough blood circulating to supply the cells with oxygen and glucose, a type of sugar. Glucose is the only source of energy usually available to the brain. Without oxygen or glucose, neurons will die.
This figure shows young and aged rat neurons at rest and with increasing duration of stimulation. When neurons are stimulated, metabolism increases. The stimulated neurons of young rats maintain calcium within normal levels. Older rats are unable to do this. High levels of calcium in old neurons may make them susceptible to dysfunction and death. The color scale is an index of cellular calcium with red indicating the highest levels.
Repair: Keeping Long-lived Neurons in Good Working Order
Unlike most cells, which have a fairly short lifespan, nerve cells, which are generated in the fetus or a short time after birth, live a long time. Brain neurons can live for up to 100 years or longer. In an adult, when neurons die because of disease or injury, theyare not usually replaced. Recent research, however, shows that in a few brain regions, new neurons can be born, even in the old brain.
To prevent their own death, living neurons must constantly maintain and remodel themselves. If cell cleanup and repair slows down or stops for any reason, the nerve cell cannot function well. Eventually, it dies.
This figure shows the effects of exercise on levels of brain-derived neurotrophic factor (BDNF) in the hippocampus of rats. Growth factors like BDNF help many neurons survive. Levels of the message that makes BDNF are much higher in exercising rats (a) than in sedentary animals (b). Exercise may promote healthy neurons in rats by causing their neurons to make more protective BDNF. Red and yellow denote the highest levels of BDNF, while green and blue denote the lowest.
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