5 anxiety disorders

NORMAL HUMAN NERVOUS SYSTEM 13a kneecap is struck with a reflex hammer and the leg kicks out, or as complicatedas the pathways (as yet poorly defined) that are active in the generation of emotionalstates.A neuron is made up of three parts: axon, cell body, and dendrites. The cell bodycontains the nucleus, and therefore the genetic material, DNA, and much of the“machinery” that allows the cell to function, to synthesize neurotransmitter moleculesto communicate with other cells, and to maintain the day-to-day housekeepingchores of the cell. The dendrites, in general, transmit information to the cell body.As such, dendrites are the “eyes” and “ears” of the neuron. The axon, in contrast,sends information downstream to neighboring cells via hundreds, even thousands,of axon terminals.Plasticity. There is another feature of the neuron that is different from other cellsin the human body. Neurons are generally not thought to be capable of dividingto make new neurons, though recent exciting research suggests that nerve celldivision (i.e., neurogenesis) does occur on a regular basis in at least some regions(e.g., the hippocampus) of the brain. In general, however, when a nerve cell dies,it is never replaced. This is why people seldom recover from paralysis after a severespinal cord injury. It is also why the accumulated death of many neurons resultsin dementias like Alzheimer’s disease. Fortunately, the CNS enjoys a tremendous“plasticity.” In other words, the brain learns which nerve cell connections tostrengthen, and which to weaken. This modifying of connections is the neuralsubstrate for learning and requires the synthesis of proteins. In a classic series ofexperiments, Bernard Agranoff and colleagues examined the role of protein synthesison learning. Briefly, they taught goldfish how to swim through a maze. Whenprotein synthesis was blocked, learning did not take place, whereas when proteinsynthesis was undisturbed, learning did take place. Such observations offer thetantalizing possibility that psychotherapy and pharmacotherapy may achieve theirtherapeutic benefit by producing similar or complementary changes in the brain.This plasticity is somewhat analogous to what happens when a houseplant is placednext to a window, and a few days later, the leaves have turned to face the light.In a similar fashion, nerve cells are constantly adapting. It happens so graduallythat you can’t really see it. But the dendrite and axon branches are continuouslyremoving old connections and establishing new connections with neighboring cells.This “pruning” begins before birth and continues throughout life.It is the process of pruning followed by the growth of new branches that explainswhy people paralyzed by a stroke or spinal cord injury may experience gradualimprovement over several months time. The establishment of new neuron-to-neuronconnections to replace those that were destroyed when the injury occurred sometimesallows the communication circuit to be repaired. In addition, there are certaintimes in life that are preprogrammed for a flurry of pruning activity. One such timeis adolescence and early adulthood, which has led some researchers to conclude thatproblems in the pruning process might trigger the emergence of schizophrenia.We’ll tell you more about that later.