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Lesson 5 | Teacher's Guide

Neurons and Neurotransmitters

"Teachers: Nikki neuron reminds you to check the standards tables in the front of the toolkit to see which ones apply for this lesson and your subject area."


The Nervous System: Command Central1,2

The main job of the nervous system is to serve as the communication center for the body. Nerve cells, also called neurons, are its basic building blocks. Since the job of neurons is to transmit signals to control every bodily and mental function (including our moods), it is imperative that they function properly.3,4,5

The nervous system is made up of many other kinds of cells besides neurons. In fact, most cells in the brain are not neurons but glial cells, which provide structural support and nutrients to the neurons and are essential for the health of the nervous system.6 The focus of this lesson, however, will be the neuron because of its central role in the communication processes within the brain and between the brain and the rest of the nervous system.

Communication occurs as one neuron sends information to another, using chemicals called neurotransmitters. All bodily functions, behavior, and thought originate with these neurological processes.

Thus mental health and illness are inextricably connected to neuron and neurotransmitter function. It is, therefore, important to understand something about how neurons and neurotransmitters work if we are to understand depression.7

The Neuron and Synapse8

The neuron is a specialized cell that is found in the central nervous system (the brain and spinal cord). Some neurons in the spinal cord project to muscles to control movement, and others receive input from the senses. Neurons in the brain have various functions, including processing sensory input, initiating and fine tuning body movement, thinking (cognitive functions), and converting thoughts into speech. Neurons communicate with each other through a complex web of projections – some that are designed to receive information from other neurons (dendrites), and some that send signals to other nearby or distant neurons (axons).

Picture the neuron’s body as a tree trunk that maintains all the functions that keep the neuron alive and functioning, its dendrites as branches of the tree, and its axon as the roots of the tree that store and release neurotransmitters. The axons of some neurons are myelinated, which means that they are covered by a fatty substance called myelin. Myelin acts as a sort of insulator for the axon, and the axon functions as an electrical wire for transmitting electrical impulses along the neuron. The myelin’s insulating qualities make the axon transmit this electrical impulse faster and more efficiently. When an electrical impulse reaches the end of the axon, it causes release of chemical substances called neurotransmitters, which are manufactured in the axon. Terminal buttons on the ends of the axons release these neurotransmitter molecules into the synaptic gap to be taken up by a dendrite of an adjacent neuron.

The junction where the cell membranes of an axon and the adjacent dendrite meet is called the synapse, and the space between neurons is called the synaptic gap (or synaptic cleft). A structure in the axon called a reuptake pump can take back excess neurotransmitter chemicals from the synapse and bring them back into the axon. The receiving neuron’s dendrite contains receptors, sometimes called receptor sites, that take in the neurotransmitter molecules that were released by the sending neuron.

Neurotransmitters These chemicals, which are stored in the neural axons, transmit information within the brain and from the brain to the rest of the body. There are more than 300 known neurotransmitters, including serotonin and norepinephrine, which play a major role in regulation of mood, sleep and appetite. Serotonin and norepinephrine are in a class of neurotransmitters called monoamines, which are synthesized by the body from amino acids, a basic element of protein. The endorphins, another type of neurotransmitter, are in the class known as peptides (small protein fragments, each made up of a chain of amino acids). They are chemically and functionally similar to heroin and the painkillers morphine and codeine, which are all derivatives of opium. Many diseases have been traced to abnormalities in the production or functioning of certain neurotransmitters, including Parkinson’s disease and depression.

Neurons and Electrochemical Messages

The flow of information in the nervous system from one neuron to another involves the release of neurotransmitters by a neuron into the synaptic gap. From there, the chemical “message” is picked up by receptor sites on the postsynaptic membrane of an adjacent neuron, which relays it to another neuron, and so on. A neuron is only able to send its neurotransmitters to another neuron when the electrical charges of both cells are at the right level. A complex interaction of neurotransmitter molecules between two cells produces the electrical charge or “potential” of the cells that is needed. The terminal buttons on the end of an axon usually must release neurotransmitters several times before this threshold is reached, which causes the cell to “fire,” sending the impulse message along its body to the synapse, much like electricity conducted along an electric wire.

But the process is actually much more complex. A single dendrite “connects” to many adjacent cells at once, which means a cell can get many “incoming calls” at the same time. Neuroscientists believe that this competing information is essentially summed up by a cell, which then “decides” how to respond. To add even more complexity to this process, some cells and neurotransmitters send inhibitory messages that lower the chance that the neuron will reach a critical threshold and fire, whereas others send excitatory messages that move the neuron toward its critical level for firing.
Whether or not a neuron’s firing threshold is reached can thus be affected not only by the sum of the neurons that send messages to the receiving neuron, but also by the relative number of excitatory or inhibitory impulses received.

Neurotransmitter Reuptake and Depression
9

When a neurotransmitter is released into the synaptic cleft, it eventually reaches a receptor site on the receiving neuron’s dendrite, but transmission of an impulse between cells is a complex and delicate process. A sufficient amount of a neurotransmitter is needed, but if too much stays in a synapse for too long, the receptor sites become saturated. Some of the excess molecules simply diffuse away from the synaptic cleft, but many are removed from the cleft by the reuptake pump through a process called reuptake. Enzymes break down the built-up neurotransmitters as part of the normal function of the neurons to keep them in a homeostatic, or stable, state. However, the reuptake process can sometimes malfunction and interfere with the normal flow of “messages” by taking away too much of the neurotransmitters. This can have the effect of “turning off” the signal from the releasing neuron.

If neurotransmitter levels are too low (according to one hypothesis, this is what causes depression), then one way to increase the levels is to block the reuptake process. Some antidepressants such as serotonin reuptake inhibitors (SSRIs), as well as some drugs of abuse, do this by inhibiting reuptake for a particular neurotransmitter so that more of the neurotransmitter is present in the synapse and available to the receiving neurons.
The process described above occurs in every neuron in the brain. Every thought or action involves a specific circuit of neurons – one is responsible for lifting your hand, another for speaking, yet another for feeling anger. So a lot of things must happen correctly for everything in the brain to work the way it should. It is amazing how well it all works, when you consider the complexity of the process (not to mention that all the cells involved originated from a single fertilized egg). Even a malfunction on the cellular level can cause circuits to not work, affecting the body in many ways. When a person is depressed, it is likely that the problem can be traced to circuits that are involved in mood regulation, probably associated with abnormal functioning of serotonin or norepinephrine transmission.

Each time an electrochemical impulse flows through a network of neurons, those connections are strengthened, making it more likely that that pathway will be used again. But with new ways of thinking and behaving, new connections between cells form and others used less often selectively die off or are “pruned.” This process takes place relatively slowly and requires many repetitions. The capacity to generate neural connections is called “synaptic plasticity,”10 which is critical to the adaptation of neurons and the entire central nervous system to the changing environment. This plasticity and pruning seem to be how the environment, i.e., learning through experience, impacts one’s neurology.

What is a “Chemical Imbalance”?11

Often heard in the popular press, the term “chemical imbalance” refers to an excess or deficit of neurotransmitters that is presumed to cause mental illness. It is important to keep in mind that behavior can be either a cause or an effect. Neural changes can lead to new ways of behaving, but behavior can also lead to neural changes through pruning and plasticity. Therefore, it is inaccurate to think of brain chemicals and their activities – or malfunction – as the only determinants of behavior.

A chemical imbalance in the brain could be caused by abnormalities in:

■ Synthesis (manufacture) of neurotransmitters

■ Release of neurotransmitters

■ Metabolism (breakdown) of neurotransmitters

■ Quantity of synaptic vesicles located on the sending neuron that release neurotransmitters

■ Quantity of neurotransmitter receptors located on the receiving neuron that receive neurotransmitters

■ Reuptake of neurotransmitters

■ Function of receptors, i.e. changes in a neuron’s response to neurotransmitters
Any of these biological processes, if gone awry, can cause behavioral changes, but it is important to remember that an abnormality on the cellular level actually may be in response to changes on the behavioral level. In the quest for understanding mental illness, the best model is one that is integrative, taking into account the complex interaction and mutual influence of the many biological processes, cognitive events and environmental factors involved in brain function.

Web Resources

Encyclopedia.com.
http://www.encyclopedia.com
A search on brain function or anatomy yields lots of kid-friendly information.

Neuroscience for Kids
http://www.soton.ac.uk/`jrc3/chudler/neurok.html
A phenomenal site on neuroscience and much more for kids – resources, activities, question & answer hotline to team of neuroscientists.

Brain & Mind: Electronic Magazine on Neuroscience
http://www.epub.org.br/cm/home_i.htm
Dozens of articles on neuroscience and behavior. See Neurons: Our Internal Galaxy Issue 7, 1998 for slides and text on nerve cell anatomy and function.

Brain power
www.pacsci.org/education/sow/brainpower/home.html
A wonderful traveling outreach program designed for students in grades 5-8 (but also appropriate for high school kids) that explores the physical, sensory and behavioral brain, the biology of drug dependency and the effects drugs have on the brain. Developed by the Pacific Science Center and the Group Health Cooperative.

References

1. Neurotransmitters. Encyclopedia.com website.
Available at http://www.encyclopedia.com/html/n1/neurotr.asp
Accessed March 31, 2002.

2. Nervous system anatomy and function. Encyclopedia.com website. Available at http://www.encyclopedia.com/html/section/nervousanatomyandfunction.asp
Accessed July 26, 2002.

3. Neuroscience for kids. Available at http://faculty.washington.edu/chudler/neurok.html. Accessed February 14, 2006.

4. Coon D. Introduction to Psychology: Gateways to Mind and Behavior. Belmont, CA: Wadsworth; 2001.

5. Neil’s Neuronets. Virtual Ventures website.
Available at http://vv.carleton.ca/~neil/neural. Accessed July 26, 2002.

6. Neurons: our internal galaxy. Available at http://www.epub.org.br/cm/n07/fundamentos/neuron/rostoi.htm
Accessed October 9, 2002.

7. Koslow et al, eds. The Neuroscience of Mental Health II. Rockville, MD: National Institutes of Health/National Institute of Mental Health/Department of Health and Human Services; 1995.

8. Pinel JP. Biopsychology. 3rd ed. Boston, Mass: Allyn and Bacon; 1997.

9. Nemeroff C. The neurobiology of depression. Scientific American; June, 1998:42-50.

10. Jeffery KJ, Reid IC. Modifiable neuronal connections: an overview for psychiatrists. American Journal of Psychiatry. 1997;154:156-164.

11. Stahl SM. Essential Psychopharmacology: Neuroscientific Basis and Practical Applications. New York, NY: Cambridge University Press; 1996.