Students learn that the neuron is the functional unit of the brain. To learn how neurons convey information, students analyze a sequence of illustrations and watch an animation. They see that neurons communicate using electrical signals and chemical messengers called neurotransmitters that either stimulate or inhibit the activity of a responding neuron. Students then use the information they have gained to deduce how one neuron influences the action of another.
Neurons convey information using electrical and chemical signals.
By the end of these activities, students will
Communication between neurons is the foundation for brain function. Understanding how neurotransmission occurs is crucial to understanding how the brain processes and integrates information. Interruption of neural communication causes changes in cognitive processes and behavior.
The brain of an adult human weighs about 3 pounds and contains billions of cells. The two distinct classes of cells in the nervous system are neurons (nerve cells) and glia (glial cells).
The basic signaling unit of the nervous system is the neuron. The brain contains billions of neurons; the best estimates are that the adult human brain contains 1011 neurons. The interactions between neurons enable people to think, move, maintain homeostasis, and feel emotions. A neuron is a specialized cell that can produce different actions because of its precise connections with other neurons, sensory receptors, and muscle cells. A typical neuron has four morphologically defined regions: the cell body, dendrites, axons, and presynaptic, or axon, terminals.1,2,3
The cell body, also called the soma, is the metabolic center of the neuron. The nucleus is located in the cell body, and most of the cell’s protein synthesis occurs in the cell body.
A neuron usually has multiple processes, or fibers, called dendrites that extend from the cell body. These processes usually branch out somewhat like tree branches and serve as the main apparatus for receiving input into the neuron from other nerve cells.
The cell body also gives rise to the axon. Axons can be very long processes; in some cases, they may be up to 1 meter long. The axon is the part of the neuron that is specialized to carry messages away from the cell body and to relay messages to other cells. Some large axons are surrounded by a fatty insulating material called myelin, which enables the electrical signals to travel down the axon at higher speeds.
Near its end, the axon divides into many fine branches that have specialized swellings called axon, or presynaptic, terminals. These presynaptic terminals end in close proximity to the dendrites of another neuron. The dendrite of one neuron receives the message sent from the presynaptic terminal of another neuron.
The site where a presynaptic terminal ends in close proximity to a receiving dendrite is called the synapse. The cell that sends out information is called the presynaptic neuron, and the cell that receives the information is called the postsynaptic neuron. It is important to note that the synapse is not a physical connection between the two neurons; there is no cytoplasmic continuity between the two neurons. The intercellular space between the presynaptic and postsynaptic neurons is called the synaptic space or synaptic cleft. An average neuron forms approximately 1,000 synapses with other neurons. It has been estimated that there are more synapses in the human brain than there are stars in our galaxy. Furthermore, synaptic connections are not static. Neurons form new synapses or strengthen synaptic connections in response to life experiences. This dynamic change in neuronal connections is the basis of learning.
The brain contains another class of cells called glia. There are as many as 10 to 50 times more glial cells than neurons in the central nervous system. Glial cells are categorized as microglia or macroglia. Microglia are phagocytic cells that are mobilized after injury, infection, or disease. They are derived from macrophages and are unrelated to other cell types in the nervous system. The three types of macroglia are oligodendrocytes, astrocytes, and Schwann cells. The oligodendrocytes and Schwann cells form the myelin sheaths that insulate axons and enhance conduction of electrical signals along the axons.
Scientists know less about the functions of glial cells than they do about the functions of neurons. Glial cells fulfill a variety of functions including as
The Blood-Brain Barrier
The blood-brain barrier protects the neurons and glial cells in the brain from substances that could harm them. Endothelial cells that form the capillaries and venules make this barrier, forming impermeable tight junctions. Astrocytes surround the endothelial cells and induce them to form these junctions. Unlike blood vessels in other parts of the body that are relatively leaky to a variety of molecules, the blood-brain barrier keeps many substances, including toxins, away from the neurons and glia.
Most drugs do not get into the brain. Only drugs that are fat soluble can penetrate the blood-brain barrier. These include drugs of abuse as well as drugs that treat mental and neurological illness.
The blood-brain barrier is important for maintaining the environment of neurons in the brain, but it also presents challenges for scientists who are investigating new treatments for brain disorders. If a medication cannot get into the brain, it cannot be effective. Researchers attempt to circumvent the problems in different ways. Some techniques alter the structure of the drug to make it more lipid soluble. Other strategies attach potential therapeutic agents to molecules that pass through the blood-brain barrier, while others attempt to open the blood-brain barrier.4
The billions of neurons that make up the brain coordinate thought, behavior, homeostasis, and more. How do all these neurons pass and receive information?
Neurons convey information by transmitting messages to other neurons or other types of cells, such as muscles. The following discussion focuses on how one neuron communicates with another neuron. Neurons employ electrical signals to relay information from one part of the neuron to another. The neuron converts the electrical signal to a chemical signal in order to pass the information to another neuron. The target neuron then converts the message back to an electrical impulse to continue the process.
Within a single neuron, information is conducted via electrical signaling. When a neuron is stimulated, an electrical impulse, called an action potential, moves along the neuron axon.5 Action potentials enable signals to travel very rapidly along the neuron fiber. Action potentials last less than 2 milliseconds (1 millisecond = 0.001 second), and the fastest action potentials can travel the length of a football field in 1 second. Action potentials result from the flow of ions across the neuronal cell membrane. Neurons, like all cells, maintain a balance of ions inside the cell that differs from the balance outside the cell. This uneven distribution of ions creates an electrical potential across the cell membrane. This is called the resting membrane potential. In humans, the resting membrane potential ranges from –40 millivolts (mV) to –80 mV, with –65 mV as an average resting membrane potential. The resting membrane potential is, by convention, assigned a negative number because the inside of the neuron is more negatively charged than the outside of the neuron. This negative charge results from the unequal distribution of sodium ions (Na+), potassium ions (K+), chloride ions (Cl–), and other organic ions. The resting membrane potential is maintained by an energy-dependent Na+-K+ pump that keeps Na+ levels low inside the neuron and K+ levels high inside the neuron. In addition, the neuronal membrane is more permeable to K+ than it is to Na+, so K+ tends to leak out of the cell more readily than Na+ diffuses into the cell.
A stimulus occurring at the cell body starts an electrical change that travels like a wave over the length of the neuron. This electrical change, the action potential, results from a change in the permeability of the neuronal membrane. Sodium ions rush into the neuron, and the inside of the cell becomes more positive. The Na+-K+ pump then restores the balance of sodium and potassium to resting levels. However, the influx of Na+ ions in one area of the neuron fiber starts a similar change in the adjoining segment, and the impulse moves from the cell body toward the axon terminal. Action potentials are an all-or-none phenomenon. Regardless of the stimuli, the amplitude and duration of an action potential are the same. The action potential either occurs or it doesn’t. The response of the neuron to an action potential depends on how many action potentials it transmits and their frequency.
Electrical signals carry information within a single neuron. Communication between neurons (with a few exceptions in mammals) is a chemical process. When the neuron is stimulated, the electrical signal (action potential) travels down the axon to the axon terminals. When the electrical signal reaches the end of the axon, it triggers a series of chemical changes in the axon terminal. Calcium ions (Ca++) flow into the axon terminal, which then initiates the release of neurotransmitters. A neurotransmitter is a molecule that is released from a neuron to relay information to another cell. Neurotransmitter molecules are stored in membranous sacs called vesicles in the axon terminal. Each vesicle contains thousands of molecules of a given neurotransmitter. For neurons to release their neurotransmitter, the vesicles fuse with the neuronal membrane and then release their contents, the neurotransmitter, via exocytosis. The neurotransmitter molecules are released into the synaptic space and diffuse across the synaptic space to the postsynaptic neuron. A neurotransmitter molecule can then bind to a special receptor on the membrane of the postsynaptic neuron. Receptors are membrane proteins that are able to bind a specific chemical substance, such as a neurotransmitter. For example, the dopamine receptor binds the neurotransmitter dopamine but does not bind other neurotransmitters such as serotonin. The interaction of a receptor and neurotransmitter can be thought of as a lock-and-key for regulating neuronal function. Just as a key fits only a specific lock, a neurotransmitter only binds with high affinity to a specific receptor. The chemical binding of neurotransmitter and receptor initiates changes in the postsynaptic neuron that may facilitate or inhibit an action potential in the postsynaptic neuron. If it does trigger an action potential, the communication process continues.
After a neurotransmitter molecule binds to its receptor on the postsynaptic neuron, it comes off (is released from) the receptor and diffuses back into the synaptic space. The released neurotransmitter, as well as any neurotransmitter that did not bind to a receptor, is either degraded by enzymes in the synaptic cleft or taken back up into the presynaptic axon terminal by active transport through a transporter or reuptake pump. Once the neurotransmitter is back inside the axon terminal, it is either destroyed or repackaged into new vesicles that may be released the next time an electrical impulse reaches the axon terminal. Different neurotransmitters are inactivated in different ways.
Different neurotransmitters fulfill different functions in the brain. Some neurotransmitters act to stimulate the firing of a postsynaptic neuron. Neurotransmitters that act this way are called excitatory neurotransmitters because they lead to changes that generate an action potential in the responding neuron.1,6 Other neurotransmitters, called inhibitory neurotransmitters, tend to block the changes that cause an action potential to be generated in the responding cell. Table 2.1 lists some of the “classical neurotransmitters” used in the body and their major functions. In addition to the so-called classical neurotransmitters, there are many other peptide transmitters, sometimes called neuromodulators. They are similar to classical neurotransmitters in the way they are stored (in vesicles) and released, but they differ in how they are inactivated. Most neurons contain multiple transmitters, often a classical one (such as dopamine) and one or more peptides (such as neurotensin or endorphins).
The postsynaptic neuron often receives and integrates both excitatory and inhibitory messages. The response of the postsynaptic cell depends on which message is stronger. Keep in mind that a single neurotransmitter molecule cannot cause an action potential in the responding neuron. An action potential occurs when many neurotransmitter molecules bind to and activate their receptors. Each interaction contributes to the membrane permeability changes that generate the resultant action potential.
|Neurotransmitter||Role in the body|
|Acetylcholine||Used by spinal cord motor neurons to cause muscle contraction and by many neurons in the brain to regulate memory. In most instances, acetylcholine is excitatory.|
|Dopamine||Produces feelings of pleasure when released by the brain reward system. Dopamine has multiple functions depending on where in the brain it acts. It is usually inhibitory.|
|GABA (gamma-aminobutyric acid)||The major inhibitory neurotransmitter in the brain. It is important in producing sleep, reducing anxiety, and forming memories.|
|Glutamate||The most common excitatory neurotransmitter in the brain. It is important in learning and memory.|
|Glycine||Used mainly by neurons in the spinal cord. It probably always acts as an inhibitory neurotransmitter.|
|Norepinephrine||Acts as a neurotransmitter and a hormone. In the peripheral nervous system, it is part of the fight-or-flight response. In the brain, it acts as a neurotransmitter regulating blood pressure and calmness. Norepinephrine is usually excitatory, but it is inhibitory in a few brain areas.|
|Serotonin||Involved in many functions including mood, appetite, and sensory perception. In the spinal cord, serotonin is inhibitory in pain pathways.|
|For the class||For each group of 3 students||For each student|
|1 transparency of Master 2.1, Anatomy of a Neuron
1 transparency of Master 2.2, Neurons Interact with Other Neurons Through Synapses
1 transparency of Master 2.4, Neurons Communicate by Neurotransmission
1 transparency of Master 2.6, Recording the Activity of a Neuron
1 transparency of Master 1.7, The Reward System (from Lesson 1)
|1 copy of Master 2.3, How Do Neurons Communicate?||1 copy of Master 2.5, Neurotransmission
1 copy of Master 2.7, Neurotransmitter Actions
1 copy of Master 2.8, Neurons in Series
|2||computers or overhead projector|
Arrange for students to have access to the Internet for Activities 2, 3, and 4, if possible.
Students may respond correctly that the areas shown in red or yellow on the PET images are made up of brain cells that are more active than the cells in other regions. Students may even be able to say that the areas represent neurons in the brain that are activated. The goal is to reinforce that the brain is made up of billions of individual cells. The areas shown in the PET images are not just large amorphous masses.
The cell body of the neuron is the metabolic center of the neuron. The nucleus is in the cell body. Most of the proteins are made in the cell body.
Neurons have specialized cell processes, or fibers, that extend from the cell body. The dendrites are branched fibrous processes specialized to receive input and carry information toward the cell body.
The axon is usually larger in diameter than the dendrites and is specialized to carry information away from the cell body. An axon may be very long. Some axons are over 1 meter long.
The presynaptic neuron is the neuron whose axon forms a synapse with the dendrite of another neuron. The presynaptic neuron sends out information.
The postsynaptic neuron is the neuron whose dendrite forms a synapse with the axon of the presynaptic neuron. The postsynaptic neuron receives information.
Note: Help students understand that there is no physical connection between the two neurons..
The cell bodies of the neurons that drugs affect are located in the ventral tegmental area (VTA). Those cells extend their axons to nerve cells in an area of the brain called the nucleus accumbens. Some nerve fibers extend to part of the frontal region of the cerebral cortex.
Next: Lesson 2 (Page 2 of 2)