Brain

The number of neurons in the human brain is vast—many billions (although glial cells, which provide various support functions, are even more numerous). A thread-like axon (nerve fiber) extends from the neuron, often branching repeatedly, to provide functional connections to other neurons located at its endings (terminals), sometimes at remote locations within the nervous system. Electrical impulses (action potentials) are generated in or near a neuron's cell body and travel outward along the axons, much as telegraph impulses are sent from an operator, traversing wires to receiving destinations (other neurons). A unique feature of neurons that greatly increases the number of axon terminals that can contact them are dendrites—elaborate tree-like arrays emanating from the cell body. The evolution of extensive dendritic trees, coupled with branching axons, has led to the development of neural "wiring diagrams" of enormous complexity. The situation is further complicated by the properties of synapses, the sites where dendrites and axon terminals "communicate." For the most part, the communication between neurons uses chemical neurotransmitters that are typically stored in the axon terminals. Packets of neurotransmitter molecules are released by mechanisms associated with the arrival of nerve impulses generated by the axons' parent neuron. The neurotransmitter quickly diffuses across the narrow synaptic cleft to reach a dendrite or cell body of the target neuron. The neurotransmitter molecules find their way to synaptic receptors, specialized sites in or on the receiving neuron that react with the neurotransmitter. The reaction between neurotransmitter and synaptic receptors alters some physiological properties of the receiving neuron, changing its activity and output. This neuron is, of course, connected to other neurons via the synapses made by its own axon terminals, which are in turn affected. And so on.

A number of different types of neurotransmitters are used by the nervous system, such as acetylcholine, dopamine, serotonin, and many others. Moreover, for a particular neurotransmitter there can be a variety of receptor types on one receiving neuron or another, so that the same neurotransmitter can produce different effects. The large number of possible permutations of neurotransmitter and receptors confer yet another layer of complexity upon the brain.

All of these highly varied, interacting aspects of neural circuits somehow work together in a manner that miraculously allows the nervous system to accomplish its basic functions. By the same token, they provide many "targets" for deleterious age-related changes. Reviewing some basics of the nervous system structure and function and their neurogerontological implications will help impart a sense of the mischief that aging can visit upon the nervous system. Then, after addressing a few of the basic functions, some possible ways by which age-related changes in the nervous system might be modified for the better will be outlined.