- Define the nervous system and its components
- Define the parts of a neuron and their functions
- Understand the role of neurotransmitters in relaying messages across neurons
- List some important neurotransmitters and their functions


[SLIDE 1]

The nervous system is a system of nerves involved in thought processes, heartbeat, visual–motor coordination, and so on. The nervous system consists of the brain, the spinal cord, and other parts that make it possible for us to receive information from the world outside and to act on it. It is made up of cells, most of which are neurons.

Within our nervous system lies a complex system of nerve cells, otherwise known as neurons. Neurons are specialized cells of the nervous system that conduct impulses. Neurons can be visualized as having branches, trunks, and roots, similar to trees. Many nerve cells lie alongside one another, similar to a thicket of trees. But neurons can also lie end to end, with their roots intertwined with the branches of the neurons that lie below.

Neurons receive messages from a number of sources such as light, other neurons, and pressure on the skin, and they can pass these messages along. We are born with more than 100 billion neurons, most of which are in the brain.


[SLIDE 2]

The nervous system also contains glial cells. Glial cells remove dead neurons and waste products from the nervous system, nourish and insulate neurons, form a fatty, insulating substance called myelin, and play a role in neural transmission of messages.

But neurons are the main focus of the nervous system. The messages transmitted by neurons account for phenomena ranging from the perception of an itch from a mosquito bite to the coordination of a skier's vision and muscles to the composition of a concerto to the solution of an algebraic equation.


[SLIDE 3]

Neurons vary according to their functions and their location. Neurons in the brain may be only a fraction of an inch in length; whereas neurons in the legs can be several feet long. Most neurons include a cell body, dendrites, and an axon. The cell body contains the core or nucleus of the cell. The nucleus uses oxygen and nutrients to generate the energy needed to carry out the work of the cell. Dendrites extend like roots from the cell body to received incoming messages.


[SLIDE 4]

As we just covered, neurons consist of a cell body, dendrites, and an axon. Messages enter neurons through dendrites, are transmitted along the trunk-like axon, and then are sent from axon terminal buttons to muscles, glands, and other neurons. Axons are very thin, but those that carry messages from the toes to the spinal cord extend several feet in length. Like tree trunks, axons can branch in different directions. Axons end in small, bulb-shaped structures called axon terminals or terminal buttons.

Neurons carry messages in one direction only: from the dendrites or cell body through the axon to the axon terminals. The messages are then transmitted from the terminal buttons to other neurons, muscles, or glands. As a child matures, the axons of neurons become longer and the dendrites and terminals proliferate, creating vast interconnected networks for the transmission of complex messages. The number of glial cells also increases as the nervous system develops, contributing to its dense appearance.


[SLIDE 5]

The axons of many neurons are wrapped tightly with white myelin, which gives them the appearance of a string of sausages under the microscope. The fat insulates the axon from electrically charged atoms, or ions, found in the fluids that surround the nervous system. The myelin sheath minimizes leakage of the electrical current being carried along the axon, thereby allowing messages to be conducted more efficiently.

Myelination is part of the maturation process that leads to a child's ability to crawl and walk during the first year. In other words, infants are not physiologically ready to use visual–motor coordination until the coating process reaches a certain level. In people with the disease multiple sclerosis, myelin is replaced with a hard, fibrous tissue that throws off the timing of nerve impulses and disrupts muscular control.


[SLIDE 6]

If you step on a thorn, the sensation is registered by receptors or sensory neurons near the surface of your skin. This information is then transmitted to the spinal cord and brain through sensory neurons, or afferent neurons, which can be as long as two to three feet in length. In the brain, subsequent messages might be conveyed by associative neurons that are only a few thousandths of an inch long. You experience the pain through this process, and motor neurons, or efferent neurons, send messages to your foot so that you withdraw it from the thorn causing you pain.

Efferent neurons transmit messages from the brain or spinal cord to muscles and glands. Other efferent neurons stimulate glands so that your heart beats more rapidly; you may also start sweating and have other reactions to the pain. A handy way of distinguishing these words that sound almost the SAME is to use the letters of the word SAME to remember: "Sensory is to Afferent as Motor is to Efferent." 


[SLIDE 7]

In the 18th century, the Italian physiologist Luigi Galvani, who lived from 1737 to 1798, conducted a shocking experiment in a rainstorm. He and his wife connected lightning rods to the heads of dissected frogs whose legs were connected by wires to a well of water. When lightning struck, the frogs' muscles contracted. Galvani was demonstrating that the messages, which we call neural impulses, are electrochemical in nature.

These impulses travel within neurons at somewhere between 2 miles per hour in nonmyelinated neurons, to 225 miles per hour in myelinated neurons. Distances in the body are short, so a message will travel from a toe to the brain in perhaps 1/50th of a second.

Chemical changes take place within neurons that cause an electrical charge to be transmitted along their lengths. Neurons and body fluids contain ions -- positively or negatively charged atoms. In a resting state, negatively-charged chloride ions are plentiful within the neuron, contributing to an overall negative charge in relation to the outside. The difference in electrical charge readies, or polarizes, a neuron for firing by creating an internal negative charge in relation to the body fluid outside the cell membrane. 


[SLIDE 8]

The conduction of the neural impulse along the length of a neuron is what is meant by the term, "firing." Like a rifle sending bullets through a barrel, a neuron sends neurotransmitters down an axon. Some neurons fire in less than 1/1,000th of a second. When they fire, neurons transmit messages to other neurons, muscles, or glands.

However, neurons will not fire unless the incoming messages combine to reach a certain strength, which is defined as the threshold at which a neuron will fire. A weak message may cause a temporary shift in electrical charge at some point along the cell membrane, but this charge will dissipate if the neuron is not stimulated to its threshold.

Every time a neuron fires, it transmits an impulse of the same strength. This occurrence is known as the all-or-none principle. That is, either a neuron fires or it does not. Neurons fire more often when they have been stimulated by larger numbers of other neurons. Stronger stimuli cause more frequent firing, but again, the strength of each firing remains the same.

For a few thousandths of a second after firing, a neuron is in a refractory period, which means it is insensitive to messages from other neurons and will not fire. This period is a time of recovery during which sodium is prevented from passing through the neuronal membrane. Because such periods of recovery might occur hundreds of times per second, it seems a rapid recovery and a short rest indeed.


[SLIDE 9]

A neuron relays its message to another neuron across a junction called a synapse. A synapse consists of an axon terminal from the transmitting neuron, a dendrite, or the body of a receiving neuron, and a fluid-filled gap between the two that is called the synaptic cleft, as illustrated in the figure above.

Although the neural impulse is electrical, it does not jump across the synaptic cleft like a spark. Instead, when a nerve impulse reaches a synapse, axon terminals release chemicals into the synaptic cleft. Sacs called synaptic vesicles in the axon terminals contain neurotransmitters, which are the chemical keys to communication. Researchers have identified a few dozen of these chemicals to date. In the next few slides, we will review several of these that are of the greatest interest to psychologists. 


[SLIDE 10]

When a neural impulse, or action potential, reaches the axon terminal, the vesicles release varying amounts of neurotransmitters into the synaptic cleft. From there, they influence the receiving neuron. Each kind of neurotransmitter has a unique chemical structure, and each can fit into a specifically tailored receptor site.

Once released, not all molecules of a neurotransmitter find their way into receptor sites of other neurons. The loose neurotransmitters are usually either broken down or reabsorbed by the axon terminal. This is a process known as reuptake. Some neurotransmitters act to excite other neurons, or to cause other neurons to fire. Other neurotransmitters inhibit receiving neurons, or to prevent the neurons from firing. The sum of the stimulation, either excitatory and inhibitory, determines whether a neuron will fire.

Neurotransmitters are involved in physical processes such as muscle contraction and psychological processes like thoughts and emotions. Excesses or deficiencies of neurotransmitters have been linked to psychological disorders such as depression and schizophrenia. 


[SLIDE 11]

There are various neurotransmitters that are of interest to psychologists. Here, we will discuss acetylcholine, dopamine, norepinephrine, serotonin, GABA, and endorphins.

Acetylcholine, abbreviated as ACh, is a neurotransmitter that controls muscle contractions. It is excitatory at synapses between nerves and muscles that involve voluntary movement but inhibitory at the heart and some other locations. The effects of the poison curare highlight the functioning of ACh. Curare is extracted from plants by South American indigenous people and used in hunting. If an arrow tipped with curare pierces the skin and the poison enters the body, it prevents ACh from binding to the receptor sites on neurons. Because ACh helps muscles move, curare causes paralysis. The victim is prevented from contracting the muscles used in breathing and dies from suffocation.

ACh is normally prevalent in a part of the brain called the hippocampus, which is a structure involved in the formation of memories. When the amount of ACh available to the brain decreases, as in Alzheimer’s disease, memory formation is impaired.  


[SLIDE 12]

Dopamine is a neurotransmitter that acts in the brain and affects the ability to perceive pleasure, voluntary movement, and learning and memory. Nicotine, alcohol, and many other drugs are pleasurable because they heighten levels of dopamine.

Deficiencies of dopamine are linked to Parkinson's disease, in which people progressively lose control over their muscles. They develop muscle tremors and jerky, uncoordinated movements. The psychological disorder schizophrenia is characterized by confusion and false perceptions, and it has also been linked to dopamine. People with schizophrenia may have more receptor sites for dopamine in an area of the brain that is involved in emotional response. For this reason, they may "overutilize" the dopamine available in the brain. Overutilization is connected with hallucinations and disturbances of thought and emotion.

The phenothiazines, a group of drugs used in the treatment of schizophrenia, inhibit the action of dopamine by blocking some dopamine receptors. Because of their action, phenothiazines may have Parkinson's like side effects, which are usually lessened by lowering the dosage or prescribing other drugs.


[SLIDE 13]

Norepinephrine is a neurotransmitter whose action is similar to that of the hormone epinephrine and that may play a role in depression. It is produced largely by neurons in the brain stem and acts both as a neurotransmitter and as a hormone. It is an excightatory neurotransmitter that speeds up the heartbeat and other body processes and is involved in general arousal, learning and memory, and eating. Excesses and deficiencies of norepinephrine have been linked to mood disorders. Deficiencies of both ACh and norepinephrine particularly impair memory formation.

The stimulants cocaine and amphetamine, or speed, boost norepinephrine and dopamine production, increasing the firing of neurons and leading to persistent arousal. Amphetamines both facilitate the release of these neurotransmitters and prevent their reuptake. Cocaine also blocks reuptake.


[SLIDE 14]

Serotonin is a neurotransmitter that is involved in emotional arousal and sleep. Deficiencies of serotonin have been linked to eating disorders, alcoholism, depression, aggression, and insomnia. The drug LSD decreases the action of serotonin and is also believed to increase the utilization of dopamine, which may be the mechanism by which it produces hallucinations.


[SLIDE 15]

Gamma-aminobutyric acid, known as GABA, is an inhibitory neurotransmitter that apparently helps calm anxiety reactions. Tranquilizers and alcohol may quell anxiety by binding with GABA receptors and amplifying its effects. One class of anti-anxiety drug may also increase the sensitivity of receptor sites to GABA. Other studies link deficiencies of GABA to depression.


[SLIDE 16]

Endorphins are inhibitory neurotransmitters. The word endorphin is the contraction of endogenous morphine. Endogenous means developing from within. Thus, endorphins occur naturally in the brain and in the bloodstream and are similar to the narcotic morphine in their functions and effects. They lock into receptor sites for chemicals that transmit pain messages to the brain, thus locking out pain-causing chemicals. Endorphins may also increase our sense of competence, enhance the functioning of the immune system, and is associated with the pleasurable runner's high reported by some long-distance runners.

The combined activity of all these neurotransmitters we have discussed determines which messages will be transmitted and which ones will not. We can think of neurons as the microscopic building blocks of the nervous system. Millions upon millions of these neurons gather together to form larger, visible structures that we think of as the parts of the nervous system. We discuss those parts in the next lesson.