"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."
■ Identify basic anatomy of neurons and their role in communication in the nervous system.
■ Describe the components of nerve impulse transmission via neurotransmitters.
Suggestions for Presentation of Material
■ Ask the class questions to determine what they know about how the brain works: How do nerve cells communicate with each other? How do the various brain regions communicate? What is meant by the term “chemical imbalance”?
■ Hand out pictures or display a picture of neuron and synapse for discussion.
Key Points of Discussion
■ The main job of the nervous system –communication center for the body.
■ Neurons – basic building blocks (cells) of the nervous system.
■ Parts of neurons: dendrites – receive signals; axons – send signals.
■ The flow of information in the brain is both electrical and biochemical in nature.
■ All communication in the brain and nervous system occurs by the flow of chemicals (neurotransmitters) between neurons. This chemical flow changes the electrical properties of the cell, and certain electrical conditions in the cell cause it to fire or not to fire. This is the basic process involved in all brain activities. All behavior, thoughts, and actions are mediated by the brain.
■ When an electrical impulse reaches the end of the axon, it causes release of neurotransmitters, which are transported in synaptic vesicles 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 come close together is called the synapse, and the space between neurons is called the synaptic gap (or synaptic cleft). A structure 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, which take in the neurotransmitter molecules that were released by the sending neuron.
■ A single dendrite “connects” to many adjacent cells at once, which means a cell can get many “incoming calls” at the same time. This competing information is somehow summed up by a cell, which then “decides” how to respond. 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.
■ Many diseases have been traced to abnormalities in the production and/or functioning of certain neurotransmitters, including Parkinson’s disease and depression.
■ Often heard in the popular press, the term “chemical imbalance” refers to any malfunction of the neurotransmission process that is presumed to cause mental illness. It is important to keep in mind that such neurochemical changes can lead to behavioral changes, but behavior can also lead to neural changes. Therefore it is inaccurate to think of brain chemicals and their activities – or malfunction – as the only determinants of behavior or illness.
■ The most convincing evidence from research into the biological causes of depression indicates that problems occur with the transmission of the neurotransmitters serotonin or norepinephrine, which play a major role in regulation of mood, sleep and appetite.
■ The level of a neurotransmitter can become excessive or deficient for various reasons: the amount of neurotransmitters produced, the metabolizing of the neurotransmitter, the number of nerve cells or pathways that use the neurotransmitter, etc.
■ Malfunction of the reuptake process can 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, that is what causes depression), then one way to bring the levels to normal is to block the reuptake process. Some antidepressants such as serotonin reuptake inhibitors (SSRIs) do this by inhibiting reuptake of serotonin so that more of the neurotransmitter is present in the synapse and available to the receiving neurons.
■ 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 are established, and others used less often selectively die off or are “pruned.” The capacity to generate neural connections is called “synaptic plasticity,” which is critical to the adaptation of neurons and the entire central nervous system to the changing environment.
■ In the quest for understanding about mental illness, the best model is one that is integrative, taking into account the complex interaction and mutual infl uence of the many biological processes, cognitive events and environmental factors involved in brain function.
Inhibitory and excitatory impulses
Sponsored by the UAMS College of Medicine, Department of Psychiatry’s Partners in Behavioral Health Sciences program which is made possible by support from a Science Education Partnership Award (R25 RR15976) from the National Center for Research Resources at the National Institutes of Health.