The nervous system is composed of billions of specialized cells called neurons. Efficient communication between these cells is crucial to the normal functioning of the central and peripheral nervous systems. In this section we will investigate the way in which the unique morphology and biochemistry of neurons makes such communication possible. Show
The cell body, or soma, of a neuron is like that of any other cell, containing mitochondria, ribosomes, a nucleus, and other essential organelles. Extending from the cell membrane, however, is a system of dendritic branches which serve as receptor sites for information sent from other neurons. If the dendrites receive a strong enough signal from a neighboring nerve cell, or from several neighboring nerve cells, the resting electrical potential of the receptor cell's membrane becomes depolarized. Regenerating itself, this electrical signal travels down the cell's axon, a specialized extension from the cell body which ranges from a few hundred micrometers in some nerve cells, to over a meter in length in others. This wave of depolarization along the axon is called an action potential. Most axons are covered by myelin, a fatty substance that serves as an insulator and thus greatly enhances the speed of an action potential. In between each sheath of myelin is an exposed portion of the axon called a node of Ranvier. It is in these uninsulated areas that the actual flow of ions along the axon takes place. The end of the axon branches off into several terminals. Each axon terminal is highly specialized to pass along action potentials to adjacent neurons, or target tissue, in the neural pathway. Some cells communicate this information via electrical synapses. In such cases, the action potential simply travels from one cell to the next through specialized channels, called gap junctions, which connect the two cells. Most cells, however, communicate via chemical synapses. Such cells are separated by a space called a synaptic cleft and thus cannot transmit action potentials directly. Instead, chemicals called neurotransmitters are used to communicate the signal from one cell to the next. Some neurotransmitters are excitatory and depolarize the next cell, increasing the probability that an action potential will be fired. Others are inhibitory, causing the membrane of the next cell to hyperpolarize, thus decreasing the probability of that the next neuron will fire an action potential. The process by which this information is communicated is called synaptic transmission and can be broken down into four steps. First, the neurotransmitter must be synthesized and stored in vesicles so that when an action potential arrives at the nerve ending, the cell is ready to pass it along to the next neuron. Next, when an action potential does arrive at the terminal, the neurotransmitter must be quickly and efficiently released from the terminal and into the synaptic cleft. The neurotransmitter must then be recognized by selective receptors on the postsynaptic cell so that it can pass along the signal and initiate another action potential. Or, in some cases, the receptors act to block the signals of other neurons also connecting to that postsynaptic neuron. After its recognition by the receptor, the neurotransmitter must be inactivated so that it does not continually occupy the receptor sites of the postsynaptic cell. Inactivation of the neurotransmitter avoids constant stimulation of the postsynaptic cell, while at the same time freeing up the receptor sites so that they can receive additional neurotransmitter molecules, should another action potential arrive. Most neurotransmitters are specific for the kind of information that they are used to convey. As a result, a certain neurotransmitter may be more highly concentrated in one area of the brain than it is in another. In addition, the same neurotransmitter may elicit a variety of different responses based on the type of tissue being targeted and which other neurotransmitters, if any, are co-released. The integral role of neurotransmitters on the normal functioning of the brain makes it clear to see how an imbalance in any one of these chemicals could very possibly have serious clinical implications for an individual. Whether due to genetics, drug use, the aging process, or other various causes, biological disfunction at any of the four steps of synaptic transmission often leads to such imbalances and is the ultimately source of conditions such as schizophrenia, Parkinson's disease, and Alzheimer's disease. The causes and characteristics of these conditions and others will be studied more closely are as we focus specifically on the four steps of synaptic transmission, and trace the actions of several important neurotransmitters. © Williams College Neuroscience, 1998 Learning Objectives
Neurons and Glial CellsThe information below was adapted from OpenStax Biology 35.1 and Khan Academy AP Biology The neuron and nervous system. All Khan Academy content is available for free at www.khanacademy.org The nervous system is made up of neurons, the specialized cells that can receive and transmit chemical or electrical signals, and glia, the cells that provide support functions for the neurons. A neuron can be compared to an electrical wire: it transmits a signal from one place to another. Glia can be compared to the workers at the electric company who make sure wires go to the right places, maintain the wires, and take down wires that are broken. Recent evidence suggests that glia may also assist in some of the signaling functions of neurons. Neurons communicate via both electrical signals and chemical signals. The electrical signals are action potentials, which transmit the information from one of a neuron to the other; the chemical signals are neurotransmitters, which transmit the information from one neuron to the next. An action potential is a rapid, temporary change in membrane potential (electrical charge), and it is caused by sodium rushing to a neuron and potassium rushing out. Neurotransmitters are chemical messengers which are released from one neuron as a result of an action potential; they cause a rapid, temporary change in the membrane potential of the adjacent neuron to initiate an action potential in that neuron. Parts of a NeuronLike other cells, each neuron has a cell body (or soma) that contains a nucleus and other cellular components. Neurons also contain unique structures, dendrites and axons, for receiving and sending the electrical signals that make neuronal communication possible:
 Neurons contain organelles common to many other cells, such as a nucleus and mitochondria. They also have more specialized structures, including dendrites and axons. Image credit: OpenStax Biology It is important to note that a single neuron does not act alone: neuronal communication depends on the connections that neurons make with one another (as well as with other cells, like muscle cells). Dendrites from a single neuron may receive synaptic contact from many other neurons. For example, dendrites from a neurons in the cerebellum of the brain are thought to receive contact from as many as 200,000 other neurons. GliaWhile glia are often thought of as the supporting cast of the nervous system, the number of glial cells in the brain actually outnumbers the number of neurons by a factor of ten. Neurons would be unable to function without the vital roles that are fulfilled by these glial cells. Glia guide developing neurons to their destinations, buffer ions and chemicals that would otherwise harm neurons, provide myelin sheaths around axons, and modulate communication between nerve cells. When glia do not function properly, the result can be disastrous; most brain tumors are caused by mutations in glia. There are several different types of glia with different functions. They include:
Glial cells support neurons and maintain their environment. Glial cells of the (a) central nervous system include oligodendrocytes, astrocytes, ependymal cells, and microglial cells. Oligodendrocytes form the myelin sheath around axons. Astrocytes provide nutrients to neurons, maintain their extracellular environment, and provide structural support. Microglia scavenge pathogens and dead cells. Ependymal cells produce cerebrospinal fluid that cushions the neurons. Glial cells of the (b) peripheral nervous system include Schwann cells, which form the myelin sheath, and satellite cells, which provide nutrients and structural support to neurons. Image credit: OpenStax Biology Communication Between NeuronsThe information below was adapted from OpenStax Biology 35.2 and OpenStax Anatomy & Physiology 3.1 All functions performed by the nervous system – from a simple motor reflex to more advanced functions like making a memory or a decision – require neurons to communicate with one another. While humans use words and body language to communicate, neurons use electrical and chemical signals. Just like a person in a committee, one neuron usually receives and synthesizes messages from multiple other neurons before “making the decision” to send the message on to other neurons. Neurons communicate via both electrical and chemical signals. A neuron receives input from other neurons and, if this input is strong enough, the neuron will send the signal to downstream neurons. Transmission of a signal between neurons is generally carried by a chemical called a neurotransmitter. Transmission of a signal within a neuron (from dendrite to axon terminal) is carried by a brief reversal of the resting membrane potential called an action potential. This communication is possible because each neuron has a charged cellular membrane (a voltage difference between the inside and the outside), and the charge of this membrane can change in response to neurotransmitter molecules released from other neurons and environmental stimuli. The three general phenomena required for communication between neurons are:
We’ll discuss each of these three components in turn. 1. The Resting PotentialThe lipid bilayer membrane that surrounds a neuron is impermeable to charged molecules or ions. To enter or exit the neuron, ions must pass through special proteins called ion channels that span the membrane and regulate the relative concentrations of different ions inside and outside the cell. Cells can use energy to preferentially move certain ions either inside or outside of the membrane, setting up a difference in ion charge across the membrane, where one side is relatively more negative and the other side is relatively more positive. The difference in total charge between the inside and outside of the cell is called the membrane potential. The membrane potential of a neuron at rest is negatively charged: the inside of a cell is approximately 70 millivolts more negative than the outside (-70 mV, note that this number varies by neuron type and by species). This voltage is called the resting membrane potential; it is caused by differences in the concentrations of ions inside and outside the cell. The resting potential is established and maintained by two main processes: an ATP-powered ion channel called the sodium-potassium pump, and a passive ion channel called the potassium leak channel. The sodium-potassium pump, which is also called Na+/K+ ATPase, transports sodium out of a cell while moving potassium into the cell. The Na+/K+ pump is an important ion pump found in the membranes of many types of cells. These pumps are particularly abundant in nerve cells, which are constantly pumping out sodium ions and pulling in potassium ions to maintain an electrical gradient across their cell membranes. An electrical gradient is a difference in electrical charge across a space. In the case of nerve cells, for example, the electrical gradient exists between the inside and outside of the cell, with the inside being negatively-charged (at around -70 mV) relative to the outside. The negative electrical gradient is maintained because each Na+/K+ pump moves three Na+ ions out of the cell and two K+ ions into the cell for each ATP molecule that is used. This process is so important for nerve cells that it accounts for the majority of their ATP usage. Powered by ATP, the sodium-potassium pump moves sodium and potassium ions in opposite directions, each against its concentration gradient. In a single cycle of the pump, three sodium ions are extruded from and two potassium ions are imported into the cell. Image credit: OpenStax Anatomy & Physiology. In addition to the sodium potassium pump, neurons possess potassium leak channels and sodium leak channels that allow the two cations to diffuse down their concentration gradient. However, the neurons have far more potassium leakage channels than sodium leakage channels. Therefore, potassium diffuses out of the cell at a much faster rate than sodium leaks in. Because more cations are leaving the cell than are entering, this causes the interior of the cell to be negatively charged relative to the outside of the cell. Thus the combined effects of the sodium-potassium pump and the potassium leak channels is that the interior of the cell is more negative than the outside of the cell. It should also be noted that chloride ions (Cl–) tend to accumulate outside of the cell because they are repelled by negatively-charged proteins within the cytoplasm. The resting membrane potential is a result of different concentrations inside and outside the cell.
This video describes the role of the sodium/potassium pump and potassium leak channels in establishing and maintaining the membrane resting potential: 2. The Action PotentialWhen we talk about neurons “firing” or being “active,” we’re talking about the action potential: a brief, positive change in the membrane potential along a neuron’s axon. When an action potential occurs, the neuron sends the signal to the next neuron in the communication chain, and, if an action potential also occurs in the next neuron, then the signal will continue being transmitted. What causes an action potential? When a neuron receives a signal from another neuron (in the form of neurotransmitters, for most neurons), the signal causes a change in the membrane potential on the receiving neuron. The signal causes opening or closing of voltage-gated ion channels, channels that open or close in response to changes in the membrane voltage. The opening of voltage-gated ion channels causes the membrane to undergo either a hyperpolarization, where the membrane potential increases in magnitude (becomes more negative) or a depolarization, where the membrane potential decreases in magnitude (becomes more positive). Whether the membrane undergoes a hyperpolarization or a depolarization depends on the type of voltage-gated ion channel that opened. Not all depolarizations result in an action potential. The signal must cause a depolarization that is large enough in magnitude to overcome the threshold potential, or the specific voltage that the membrane must reach for an an action potential to occur. The threshold potential is usually about -55 mV, compared to the resting potential of about -70 mV. If the threshold potential is reached, then an action potential is initiated at the axon hillock in the following stages:
These steps are illustrated here: The formation of an action potential can be divided into five steps: (1) A stimulus from a sensory cell or another neuron causes the target cell to depolarize toward the threshold potential. (2) If the threshold of excitation is reached, all Na+ channels open and the membrane depolarizes. (3) At the peak action potential, K+ channels open and K+ begins to leave the cell. At the same time, Na+ channels close. (4) The membrane becomes hyperpolarized as K+ ions continue to leave the cell. The hyperpolarized membrane is in a refractory period and cannot fire. (5) The K+ channels close and the Na+/K+ transporter restores the resting potential. Image credit: OpenStax Biology. There are a few important universal features of action potentials:
The video below provides a discussion of voltage-gated ion channels: Here is a more detailed discussion of the action potential trace: And an overview of action potential propagation: As noted above, the magnitude or speed of the action potential for a given neuron never varies; however, some neurons have faster action potentials than others. In invertebrates, this difference is often due to axon diameter, where larger axons have faster conduction of action potentials. In vertebrates, this difference is typically due to myelination of the neuron’s axon. Myelin acts as an insulator that prevents current from leaving the axon; this increases the speed of action potential conduction. The nodes of Ranvier, illustrated below, are gaps in the myelin sheath along the axon. These unmyelinated spaces are about one micrometer long and contain voltage gated Na+ and K+ channels. Flow of ions through these channels, particularly the Na+ channels, regenerates the action potential over and over again along the axon. This “jumping” of the action potential from one node to the next is called saltatory conduction. Nodes of Ranvier also save energy for the neuron since the channels only need to be present at the nodes and not along the entire axon. Nodes of Ranvier are gaps in myelin coverage along axons. Nodes contain voltage-gated K+ and Na+ channels. Action potentials travel down the axon by jumping from one node to the next. Image credit: OpenStax Biology 3. The Chemical Synapse and NeurotransmittersNeurons are not in direct physical contact with each other, but instead come into very close proximity at a structure called the synapse. The neuron sending a signal to the next is called the presynaptic neuron, and the neuron receiving a signal is called the postsynaptic neuron, shown here: Chemical transmission involves release of chemical messengers known as neurotransmitters. Neurotransmitters carry information from the pre-synaptic (sending) neuron to the post-synaptic (receiving) cell. Image credit: Khan Academy https://www.khanacademy.org/science/biology/ap-biology/human-biology/neuron-nervous-system/a/the-synapse There is a small gap between the two neurons called the synaptic cleft, where neurotransmitters are released by the presynaptic neuron to transmit the signal to the postsynaptic neuron, shown here: Inside the axon terminal of a sending cell are many synaptic vesicles. These are membrane-bound spheres filled with neurotransmitter molecules. There is a small gap between the axon terminal of the presynaptic neuron and the membrane of the postsynaptic cell, and this gap is called the synaptic cleft. Image credit: Khan Academy https://www.khanacademy.org/science/biology/ap-biology/human-biology/neuron-nervous-system/a/the-synapse How does synaptic transmission work? Once the action potential reaches the end of the axon, it propagates into the pre-synaptic terminal where the following events occur in sequence:
This process is illustrated below: Communication at chemical synapses requires release of neurotransmitters. When the presynaptic membrane is depolarized, voltage-gated Ca2+ channels open and allow Ca2+ to enter the cell. The calcium entry causes synaptic vesicles to fuse with the membrane and release neurotransmitter molecules into the synaptic cleft. The neurotransmitter diffuses across the synaptic cleft and binds to ligand-gated ion channels in the postsynaptic membrane, resulting in a localized depolarization or hyperpolarization of the postsynaptic neuron. Image credit: Khan Academy https://www.khanacademy.org/science/biology/ap-biology/human-biology/neuron-nervous-system/a/the-synapse
Once neurotransmission has occurred, the neurotransmitter must be removed from the synaptic cleft so the postsynaptic membrane can “reset” and be ready to receive another signal. This can be accomplished in three ways:
This video walks through the process of signal communication across a chemical synapse: While action potentials are “all-or-nothing,” as noted above, EPSPs and IPSPs are graded; they vary in magnitude of depolarization or hyperpolarization, as illustrated below: Graded potentials are temporary changes in the membrane voltage, the characteristics of which depend on the size of the stimulus. Some types of stimuli cause depolarization of the membrane, whereas others cause hyperpolarization. It depends on the specific ion channels that are activated in the cell membrane. Image credit: OpenStax Anatomy & Physiology Often a single EPSP is not strong enough to induce an action potential in the postsynaptic neuron on its own, and multiple presynaptic inputs must create EPSPs around the same time for the postsynaptic neuron to be sufficiently depolarized to fire an action potential. This process is called summation and occurs at the axon hillock, as illustrated below. In addition, each neuron often has inputs from many presynaptic neuron – some excitatory and some inhibitory – so IPSPs can cancel out EPSPs and vice versa. It is the net change in postsynaptic membrane voltage that determines whether the postsynaptic cell has reached its threshold of excitation needed to fire an action potential. Together, synaptic summation and the threshold for excitation act as a filter so that random “noise” in the system is not transmitted as important information. A single neuron can receive both excitatory and inhibitory inputs from multiple neurons, resulting in local membrane depolarization (EPSP input) and hyperpolarization (IPSP input). All these inputs are added together at the axon hillock. If the EPSPs are strong enough to overcome the IPSPs and reach the threshold of excitation, the neuron will fire. Image credit: OpenStax Biology This video, added after the IKE was opened, provides an overview of summation in time and space: Here are two final videos to help you put this all together (in a more engaging way than any of the videos above). Note that these videos do not provide any new information, but they may help you better integrate all the information previously discussed:
What is the order in which a neuron receives and transmits information?The dendrites of neurons receive information from sensory receptors or other neurons. This information is then passed down to the cell body and on to the axon. Once the information has arrived at the axon, it travels down the length of the axon in the form of an electrical signal known as an action potential.
What is the correct order of cell body dendrites axon terminal buttons synapse?The correct answer is (B) Dendrite, Cell body, Axon, Axon terminal.
When a neuron receives a message in what order does the message travel through the various parts of a neuron?A message travels from the dendrites through the cell body and to the end of the axon. into the synapse. The neurotransmitters carry the message with them into the synapse. The synapse is the space between the axon of one neuron and the dendrites of another neuron.
What is important for carrying messages received by dendrites to other neurons?An axon is a long single fiber that transmits messages from the cell body to the dendrites of other neurons or to other body tissues, such as muscles. A protective covering called the myelin sheath, covers most neurons. Myelin insulates the axon and helps nerve signals travel faster and farther.
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Which of the following steps occurs immediately after dendrites receive a message?
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