Parts of the Nervous System

describe the person s potential range of reaction to the environment, but it takes the environment in order to maximize that reaction.

Also because of gene/environmental interactions, there will be differences in what happens physically and mentally to identical twins. Identical twins have the same genetic makeup or the same genotype but, because of the influence of the environment, they can have slightly different appearances and can get different diseases. Fraternal twins are different. They come from different eggs and different sperm cells so they are essentially no more related than siblings.

Do the genes affect more than just physical characteristics? According to research, yes, they do. Genes seem to be able to help to determine sexual orientation, personality traits, temperament, and even the propensity toward spirituality. Temperament affects a person s risk for schizophrenia and depression, which also seem to have genetic origins. With regard to many of these psychological disorders, however, the environment also plays a role.

PARTS OF THE NERVOUS SYSTEM

The nervous system is very complex, but plays a huge role in psychology and psychiatry. It is for this reason that you need to study the different aspects of the anatomy and physiology of the nervous system. There are certain psychological diseases that have a large neurological influence so you need to understand just how the nervous system works.

Like all systems of the body, the nervous system is made from cells. There are two major types of cells in the nervous system, divided broadly into neurons and glial cells. Neurons are the main functional cells of the nervous system. They are electrically active and send signals along processes called axons and dendrites that pass a message to other neurons so that the related parts of the nervous system can work together. Figure 5 shows the basic structure of a neuron:

Figure 5.

Neurons do not work alone but need supporting cells. These supporting cells are called glial cells or glia. There are several types of supporting cells that perform different functions in the nervous system. Astrocytes are glial cells that protect and support the neurons. Schwann cells and oligodendrocytes make myelin, which speeds the transmission of the nerve impulse. Microglia are the immune system cells of the nervous system.

Neurons have a specific structure. There are 100 billion neurons in newborns. The neuron is surrounded by a semipermeable membrane. A semipermeable membrane will allow some molecules, usually small ones, to pass through while restricting other molecules, usually large ones, from getting through. There are mechanisms in place to allow the different molecule types to pass into or out of the membrane.

The main part of the neuron is called the soma or cell body. It contains all the tiny organelles inside the cell. Projections from the soma include the dendrites and axons. Dendrites tend to be shorter, greater in number, nonmyelinated, and receive impulses from other neurons (although this is not an absolute). Axons tend to be longer, often myelinated, and fewer in number. Axons tend to send impulses away from the neuron to another neuron.

At the end of each axon are many different termini or terminal buttons, which participate in making a synapse. A synapse is a tiny connection between two neurons or between a neuron and a muscle cell. Synapses are not direct connections between the two cells but involve a space between the cells called the synaptic cleft. The synaptic cleft is where the synaptic vesicles in the pre-synaptic neuron send neurotransmitters out into the space in order to be picked up by receptors on the postsynaptic cell. Figure 6 shows a synapse:

Figure 6.

Myelin is an important part of the axon of the neuron. It is a fatty substance that insulates the axon so as to speed up the transmission of the neuronal impulse. The disease called multiple sclerosis is an autoimmune disease that destroys the myelin in the body. Neurons act much more slowly without myelin, leading to a variety of nerverelated symptoms that can often end in death.

We will talk more about neurons in a minute. For now, you need to know that neurons are chemical messengers that are sent across the synaptic cleft during nerve transmission. They fit with specific receptors that only bind to one type of neurotransmitter. When the neurotransmitter gets locked onto a receptor, it will allow for a change in the postsynaptic cell that passes on to include things like electrical

activation of the postsynaptic cell. There are various methods to recycle or get rid of the neurotransmitter after its job is done. Recycling of the neuron is called reuptake”.

Neurons have a semipermeable membrane just like all cells but these are electrically active. There is what is called a resting potential, which is a difference in the electrical charge when comparing the inside and outside of the cell. When a nerve cell is activated, sodium, which is positively charged, can pass into the membrane, changing the membrane potential. Potassium, at the same time, passes out of the cell.

At some point, the change in charge across the neuron membrane reaches a certain level, called the threshold of excitation. This activates the cell and the electrical changes, which are essentially chemical changes, pass along the nerve cell. There are mechanisms in place that pump sodium out of the cell and potassium into the cell so that the membrane reaches its resting potential. Figure 7 shows the action potential being reached in the cell:

Figure 7.