The molecular mechanisms of neuronal synapses are similar but not identical to those of the neuromuscular junction
Chemical synapses use diffusible transmitter molecules to communicate messages between two cells. The first chemical synapse to be understood in detail was the neuromuscular junction (the nerve-muscle synapse) in vertebrate skeletal muscle, which is described in Chapter 8. In this chapter, we are concerned with the properties of the synapses that occur between neurons. We now know that all synapses share certain basic biochemical and physiological mechanisms, and thus many basic insights gained from the neuromuscular junction are also applicable to synapses in the brain. However, neuronal synapses differ from neuromuscular junctions in many important ways; they also differ widely among themselves, and it is the diverse properties of synapses that help make each part of the brain unique.
It is useful to begin by reviewing some of the mechanisms that are common to all chemical synapses (see Figs. 8-2 and 8-3). N13-1 Synaptic transmission at chemical synapses occurs in seven steps:
Steps in Synaptic Transmission
Contributed by Barry W. Connors
EFIGURE 13-1 A chemical synapse between neurons. A, The resting state before the arrival of an action potential is shown. B, The arrival of the action potential in the presynaptic neuron triggers a series of seven events.
Step 1: Neurotransmitter molecules are packaged into membranous vesicles, and the vesicles are concentrated and docked at the presynaptic terminal.
Step 2: The presynaptic membrane depolarizes, usually as the result of an action potential, although some synapses respond to graded variations of membrane potential (Vm).
Step 3: The depolarization causes voltage-gated Ca2+ channels to open and allows Ca2+ ions to flow into the terminal.
Step 4: The resulting increase in intracellular [Ca2+] triggers fusion of vesicles with the presynaptic membrane (see pp. 219–221), and the rate of transmitter release increases ~100,000-fold above baseline. The Ca2+dependence of fusion is conferred by neuron-specific protein components of the fusion apparatus called synaptotagmins. The actual fusion events are incredibly fast; each individual exocytosis requires only a fraction of a millisecond to be completed.
Step 5: The transmitter is released into the extracellular space in quantized amounts and diffuses passively across the synaptic cleft.
Step 6: Some of the transmitter molecules bind to receptors in the postsynaptic membrane, and the activated receptors trigger some postsynaptic event, usually the opening of an ion channel or the activation of a G protein–coupled signal cascade.
Step 7: Transmitter molecules diffuse away from postsynaptic receptors and are eventually cleared away by continued diffusion, enzymatic degradation, or active uptake into cells. In addition, the presynaptic machinery retrieves the membrane of the exocytosed synaptic vesicle, perhaps by endocytosis from the cell surface.
The molecular machinery of synapses is closely related to components that are universal in eukaryotic cells (see p. 37). A large set of proteins is involved in the docking and fusion of vesicles, and the proteins present in nerve terminals are remarkably similar to the ones mediating fusion and secretion in yeast. Docking and fusion of synaptic vesicles are discussed on page 219. Ligand-gated ion channels and G protein–coupled receptors (GPCRs), the receptors on the postsynaptic membrane, are also present in all eukaryotic cells and mediate processes as disparate as the recognition of nutrients and poisons, and the identification of other members of the species. Even most of the neurotransmitters themselves are simple molecules, identical or very similar to those used in general cellular metabolism. Clearly, the evolutionary roots of synaptic transmission are much older than nervous systems themselves.
Within nervous systems, however, myriad variations on the basic molecular building blocks yield synapses with wide-ranging properties. Neuronal synapses vary widely in the size of the synaptic contact, the identity of the neurotransmitter, the nature of the postsynaptic receptors, the efficiency of synaptic transmission, the mechanism used for terminating transmitter action, and the degree and modes of synaptic plasticity. Thus, the properties of neuronal synapses can be tuned to achieve the diverse functions of the brain.
A major difference between the neuromuscular junction and most neuronal synapses is the type of neurotransmitter used. All skeletal neuromuscular junctions use acetylcholine (ACh). In contrast, neuronal synapses use many transmitters. The most ubiquitous are amino acids: glutamate and aspartate excite, whereas gamma-aminobutyric acid (GABA) and glycine inhibit. Other transmitters include simple amines, such as ACh, norepinephrine, serotonin, and histamine, ubiquitous molecules such as ATP and adenosine, and a wide array of peptides.
Even more varied than the neuronal transmitters are their receptors. Whereas skeletal muscle manufactures a few modest variants of its ACh receptors, the nervous system typically has several major receptor variants for each neurotransmitter. Knowledge about the wide range of transmitters and receptors is essential to understand the chemical activity of the brain as well as the drugs that influence brain activity. For one thing, the many transmitter systems in the brain generate responses with widely varying durations that range from a few milliseconds to days (Fig. 13-2).
FIGURE 13-2 Time courses of synaptic events in the nervous system. Different transmitter systems in the brain generate responses that vary widely in how long they last in the postsynaptic cell. Note that the time axis is logarithmic. (Data from Shepherd GM: Neurobiology, 3rd ed. New York, Oxford University Press, 1994.)
Presynaptic terminals may contact neurons at the dendrite, soma, or axon and may contain both clear vesicles and dense-core granules
Chemical synapses between neurons are generally small, often <1 µm in diameter, which means that their detailed structure can be seen only with an electron microscope (Fig. 13-3); under the light microscope, brain synapses are usually visible only as swellings along or at the termination of the axons (see Fig. 13-1). These swellings are actually the silhouettes of the bouton terminals—the presynaptic terminals. Most presynaptic terminals arise from axons, and they can form synapses on virtually any part of a neuron. The contact site and direction of communication determine the way in which a synapse is named: axodendritic, axosomatic, and axoaxonic synapses (Fig. 13-4). These synapses are the most common types in the nervous system. In many cases, synapses occur on small outpockets of the dendritic membrane called spines and are termed axospinous synapses. However, not all synapses arise from axons, and dendrodendritic, somatosomatic, and even somatodendritic synapses may be found in the mammalian brain.
FIGURE 13-3 Electron micrograph of synapses in the cochlear nucleus. Three presynaptic terminals are filled with vesicles and make contact with the same postsynaptic dendrite. Postsynaptic densities (marking active zones) are indicated by arrows. (From Peters A, Palay SL, Webster HD: The Fine Structure of the Nervous System: The Neurons and Supporting Cells. Philadelphia, WB Saunders, 1976.)
FIGURE 13-4 The most common synaptic arrangements in the CNS.
Despite their differences in size, site, and shape, all synapses share one basic function: they deliver a small amount of chemical transmitter onto a circumscribed patch of postsynaptic membrane. To accomplish this task, they use certain common anatomical features, most of them familiar from discussions of the neuromuscular junction (see Chapter 8).
Synapses are polarized, which means that their two apposed sides have different structures. This polarity reflects the fact that most synapses transmit information primarily in one direction, although retrograde transmission also occurs in many synapses in one direction but not in the other (we will see that some rare exceptions do exist). The presynaptic side contains numerous clear vesicles, 40 to 50 nm in diameter, that appear empty when viewed by transmission electron microscopy. Synaptic termini may also contain large (100 to 200 nm in diameter) dense-core secretory granules that are morphologically quite similar to the secretory granules of endocrine cells. These granules contain neuropeptides; that is, peptides or small proteins that act as neurotransmitters and for which receptors exist in the postsynaptic membranes. Many of these neuropeptides are identical to substances secreted by “traditional” endocrine cells. Endocrine hormones such as adrenocorticotropic hormone, vasoactive intestinal peptide, and cholecystokinin are found in dense-core secretory granules present in the terminals of certain central and peripheral neurons.
The clear synaptic vesicles (i.e., not the dense-core granules) are anchored and shifted about by a dense network of cytoskeletal proteins. Some vesicles are clustered close to the part of the presynaptic membrane that apposes the synaptic contact; these vesicle attachment sites are called active zones. Synaptic vesicles are lined up several deep along the active zones, which are the regions of actual exocytosis. The number of active zones per synapse varies greatly (active zones are marked with arrows in the synapses in Fig. 13-3). Most synapses in the central nervous system (CNS) have relatively few active zones, often only 1 but occasionally as many as 10 or 20 (versus the hundreds in the neuromuscular junction). If we could view the presynaptic face of an active zone from the perspective of a synaptic vesicle, we would see filaments and particles projecting from the presynaptic membrane, often forming a regular hexagonal arrangement called a presynaptic grid. Specific points along the grid are thought to be the vesicle release sites.
Unlike the clear synaptic vesicles containing nonpeptide transmitters, dense-core secretory granules are distributed randomly throughout the cytoplasm of the synaptic terminus. They are not concentrated at the presynaptic density, and they do not appear to release their contents at the active zone. Although the molecular pathways that control exocytosis of the neuronal dense-core granules are still being elucidated, it appears that a rise in [Ca2+]i is a primary stimulus.
The postsynaptic membrane contains transmitter receptors and numerous proteins clustered in the postsynaptic density
The postsynaptic membrane lies parallel to the presynaptic membrane, and they are separated by a narrow synaptic cleft (~30 nm wide) that is filled with extracellular fluid. Transmitter molecules released from the presynaptic terminal must diffuse across the cleft to reach postsynaptic receptors. The most characteristic anatomical feature of the postsynaptic side is the postsynaptic density, a strip of granular material visible under the electron microscope on the cytoplasmic face of the membrane (see Fig. 13-3). The most important molecular feature of the postsynaptic side is the cluster of transmitter receptors embedded within the postsynaptic membrane. Staining methods that use specific antibodies, toxins, or ligands coupled to some visible tag molecule can reveal the positions of the receptors.
In >90% of all excitatory synapses in the CNS, the postsynaptic site is a dendritic spine. The ubiquity of spines implies that they serve prominent functions, but their small size (usually <1 µm long) makes their function extremely difficult to study. Spines come in a variety of shapes, and their density varies from one dendrite to another (Fig. 13-5); indeed, some central neurons have no spines. The postsynaptic density of spines (as for all central synapses) contains >30 proteins in high concentration, including transmitter receptors, protein kinases, a host of structural proteins, and proteins that are involved in endocytosis and glycolysis.
FIGURE 13-5 Dendritic spines. A, Drawings of various dendrites in the neocortex, made from Golgi-stained material. The numerous protrusions are spines. B, Electron micrograph of an axospinous synapse in the neocortex. Note that the dendritic spine (S) protrudes from the dendritic shaft (D), making contact with a presynaptic terminal. (From Feldman ML: In Peters A, Jones EG [eds]: Cerebral Cortex: Cellular Components of the Cerebral Cortex, vol 1. New York, Plenum, 1984, pp 123–200.)
Numerous functions for spines have been proposed. It may be that spines increase the opportunity for a dendrite to form synapses with nearby axons. Many hypotheses have focused on the possibility that spines isolate individual synapses from the rest of a cell. This isolation may be electrical or chemical; the narrow spine neck may reduce current flow or the diffusion of chemicals from the spine head into the dendritic shaft. Evidence suggests that the high electrical resistance of the spine neck can amplify the size of an excitatory postsynaptic potential within the spine while reducing its amplitude along the shaft of the dendrite; this mechanism may allow neurons to integrate larger numbers of synaptic inputs. Activation of some excitatory synapses allows substantial amounts of Ca2+ to enter the postsynaptic cell. Spines may compartmentalize this Ca2+, thus allowing it to rise to higher levels or preventing it from influencing other synapses on the cell. Because increases in postsynaptic [Ca2+]i are an essential trigger for many forms of long-term synaptic plasticity, an attractive but unproven possibility is that dendritic spines play an important role in the mechanisms of learning and memory.
Some transmitters are used by diffusely distributed systems of neurons to modulate the general excitability of the brain
The brain carries out many sensory, motor, and cognitive functions that require fast, specific, spatially organized neural connections and operations. Consider the detailed neural mapping that allows you to read this sentence or the precise timing required to play the piano. These functions require spatially focused networks (Fig. 13-6A).
FIGURE 13-6 Synaptic connections.
Other functions, such as falling asleep, waking up, becoming attentive, or changing mood, involve more general alterations of the brain. Several systems of neurons regulate the general excitability of the CNS. Each of these modulatory systems uses a different neurotransmitter, and the axons of each make widely dispersed, diffuse, almost meandering synaptic connections to carry a simple message to vast regions of the brain. This arrangement can be achieved by a widely divergent network (see Fig. 13-6B). The functions of the different systems are not well understood, but each appears to be essential for certain aspects of arousal, motor control, memory, mood, motivation, and metabolic state. The modulatory systems are of central importance to clinical medicine. Both the activity of psychoactive drugs and the pathological processes of most psychiatric disorders seem to involve alterations in one or more of the modulatory systems.
The brain has several modulatory systems with diffuse central connections. Although they differ in structure and function, they have certain similarities:
1. Typically, a small set of neurons (several thousand) forms the center of the system.
2. Neurons of the diffuse systems arise from the central core of the brain, most of them from the brainstem.
3. Each neuron can influence many others because each one has an axon that may contact tens of thousands of postsynaptic neurons spread widely across the brain.
4. The synapses made by some of these systems seem designed to release transmitter molecules into the extracellular fluid so that they can diffuse to many neurons rather than be confined to the vicinity of a single synaptic cleft.
The main modulatory systems of the brain are distinct anatomically and biochemically. Separate systems use norepinephrine, serotonin (5-hydroxytryptamine [5-HT]), dopamine, ACh, or histamine as their neurotransmitter. They all tend to involve numerous metabotropic transmitter receptors (see p. 206). Unlike ionotropic receptors, which are themselves ion channels, metabotropic receptors are coupled to enzymes such as adenylyl cyclase or phospholipase C through G proteins. For example, the brain has 10 to 100 times more metabotropic (i.e., muscarinic) ACh receptors than ionotropic (i.e., nicotinic) ACh receptors. We briefly describe the anatomy and possible functions of each major system (Fig. 13-7).
FIGURE 13-7 Four diffusely connected systems of central neurons using modulatory transmitters. A, Neurons containing norepinephrine are located in the locus coeruleus and innervate nearly every part of the CNS. B, Neurons containing serotonin are located in two groups of raphé nuclei and project to most of the brain. C, Neurons containing dopamine are located in the substantia nigra (and these project to the striatum) and the ventral tegmental area of the midbrain (and these project to the prefrontal cortex and parts of the limbic system). D, Neurons containing ACh are located in the basal forebrain complex, which includes the septal nuclei and nucleus basalis; the neurons project to the hippocampus and the neocortex. Other ACh-containing neurons originate in the pontomesencephalotegmental cholinergic complex and project to the dorsal thalamus and part of the forebrain.
Norepinephrine-containing neurons are in the tiny locus coeruleus (from the Latin for “blue spot” because of the pigment in its cells), located bilaterally in the brainstem (see Fig. 13-7A). Each human locus coeruleus has ~12,000 neurons. Axons from the locus coeruleus innervate just about every part of the brain: the entire cerebral cortex, the thalamus and hypothalamus, the olfactory bulb, the cerebellum, the midbrain, and the spinal cord. Just one of its neurons can make >250,000 synapses, and that cell can have one axon branch in the cerebral cortex and another in the cerebellar cortex! Locus coeruleus cells seem to be involved in the regulation of attention, arousal, and sleep-wake cycles as well as in learning and memory, anxiety and pain, mood, and brain metabolism. Recordings from awake rats and monkeys in behavioral studies show that locus coeruleus neurons are best activated by new, unexpected, nonpainful sensory stimuli in the animal's environment. They are least active when the animals are not vigilant, just sitting around quietly digesting a meal. The locus coeruleus may participate in general arousal of the brain during interesting events in the outside world.
Serotonin-containing neurons are mostly clustered within the nine raphé nuclei (see Fig. 13-7B). Raphé means “ridge” or “seam” in Greek, and indeed the raphé nuclei lie to either side of the midline of the brainstem. Each nucleus projects to different regions of the brain, and together they innervate most of the CNS in the same diffuse way as the locus coeruleus neurons. Similar to neurons of the locus coeruleus, cells of the raphé nuclei fire most rapidly during wakefulness, when an animal is aroused and active. Raphé neurons are quietest during certain stages of sleep. The locus coeruleus and the raphé nuclei are part of a venerable concept called the ascending reticular activating system, which implicates the reticular “core” of the brainstem in processes that arouse and awaken the forebrain. Raphé neurons seem to be intimately involved in the control of sleep-wake cycles as well as the different stages of sleep. Serotonergic raphé neurons have also been implicated in the control of mood and certain types of emotional behavior. Many hallucinogenic drugs, such as lysergic acid diethylamide (LSD), apparently exert their effects through interaction with serotonin receptors. Serotonin may also be involved in clinical depression; some of the most effective drugs now used to treat depression (e.g., fluoxetine [Prozac]) are potent blockers of serotonin re-uptake and thus prolong its action in the brain.
Although dopamine-containing neurons are scattered throughout the CNS, two closely related groups of dopaminergic cells have characteristics of the diffuse modulatory systems (see Fig. 13-7C). One of these groups is the substantia nigra in the midbrain. Its cells project axons to the striatum, a part of the basal ganglia, and they somehow facilitate the initiation of voluntary movement. Degeneration of the dopamine-containing cells in the substantia nigra produces the progressively worsening motor dysfunction of Parkinson disease. Another set of dopaminergic neurons lies in the ventral tegmental area of the midbrain; these neurons innervate the part of the forebrain that includes the prefrontal cortex and parts of the limbic system. They have been implicated in neural systems that mediate reinforcement or reward as well as in aspects of drug addiction and psychiatric disorders, most notably schizophrenia. Members of the class of antipsychotic drugs called neuroleptics are antagonists of certain dopamine receptors.
Acetylcholine is the familiar transmitter of the neuromuscular junction and the autonomic nervous system. Within the brain are two major diffuse modulatory cholinergic systems: the basal forebrain complex (which innervates the hippocampus and all of the neocortex) and the pontomesencephalotegmental cholinergic complex (which innervates the dorsal thalamus and parts of the forebrain) (see Fig. 13-7D). The functions of these systems are poorly understood, but interest has been fueled by evidence that they are involved in the regulation of general brain excitability during arousal and sleep-wake cycles as well as perhaps in learning and memory formation.
Collectively, the diffuse modulatory systems may be viewed as providing general regulation of brain function, much like the autonomic nervous system (see Chapter 14) regulates the organ systems of the body. Because their axons spread so widely within the CNS, the few modulatory neurons can have an inordinately strong influence on behavior.
Electrical synapses serve specialized functions in the mammalian nervous system
Many cells are coupled to one another through gap junctions. The large and relatively nonselective gap junction channels (see p. 165) allow ion currents to flow in both directions (in most types of gap junctions) or unidirectionally (in rare types). It follows from Ohm's law that if two cells are coupled by gap junctions and they have different membrane voltages, current will flow from one cell into the other (see Fig. 6-18C). If the first cell generates an action potential, current will flow through the gap junction channels and depolarize the second cell; this type of current flow, for example, is the basis for conduction of excitation across cardiac muscle. Such an arrangement has all the earmarks of a synapse, and indeed, when gap junctions interconnect neurons, we describe them as electrical synapses.
Electrical synapses would seem to have many advantages over chemical synapses: they are extremely fast and limited only by the time constants of the systems involved, they use relatively little metabolic energy or molecular machinery, they are highly reliable, and they can be bidirectional. Indeed, electrical synapses have now been observed in nearly every part of the mammalian CNS. They interconnect inhibitory neurons of the cerebral cortex and thalamus, excitatory neurons of the brainstem and retina, and a variety of other neurons in the hypothalamus, basal ganglia, and spinal cord. At nearly all of these sites, the gap junction protein connexin-36 (Cx36)—which is expressed exclusively in CNS neurons and β cells of the pancreas—is an essential component of the electrical synapse (see Fig. 6-18C). Glial cells in the brain express several other types of connexins. However, in all of the aforementioned sites, electrical synapses tend to be outnumbered by chemical synapses. Gap junctions universally interconnect the photoreceptors of the retina, astrocytes and other types of glia (see p. 289) throughout the CNS, and most types of cells early in development.
Why are chemical synapses, as complex and relatively slow as they are, more prevalent than electrical synapses in the mature brain? Comparative studies suggest several reasons for the predominance of chemical synapses among mammalian neurons. The first is amplification. Electrical synapses do not amplify the signal passed from one cell to the next; they can only diminish it. Therefore, if a presynaptic cell is small relative to its coupled postsynaptic cell, the current that it can generate through an electrical synapse will also be small, and thus “synaptic strength” will be low. By contrast, a small bolus of neurotransmitter from a chemical synapse can trigger an amplifying cascade of molecular events that can cause a relatively large postsynaptic change.
A second advantage of chemical synapses is their ability to either excite or inhibit postsynaptic neurons selectively. Electrical synapses are not inherently excitatory or inhibitory, although they can mediate either effect under the right circumstances. Chemical synapses can reliably inhibit by simply opening channels that are selective for ions with relatively negative equilibrium potentials; they can excite by opening channels selective for ions with equilibrium potentials positive to resting potential.
A third advantage of chemical synapses is that they can transmit information over a broad time domain. By using different transmitters, receptors, second messengers, and effectors, chemical synapses can produce a wide array of postsynaptic effects with time courses ranging from a few milliseconds to minutes and even hours. The effects of electrical synapses are generally limited to the time course of the presynaptic event.
A fourth advantage of chemical synapses is that they are champions of plasticity; their strength can be a strong function of recent neural activity, and they can therefore play a role in learning and memory, which are essential to the success of vertebrate species. Electrical synapses also display forms of long-term plasticity, although this has not been well studied in the mammalian CNS.
It might also be noted that the few perceived advantages of electrical synapses may be more apparent than real. Bidirectionality is clearly not useful in many neural circuits, and the difference in speed of transmission may be too small to matter in most cases. Electrical synapses serve important but specialized functions in the nervous system. They seem to be most prevalent in neural circuits in which speed or a high degree of synchrony is at a premium: quick-escape systems, the fine coordination of rapid eye movements, or the synchronization of neurons generating rhythmic activity. Gap junctions are also effective in diffusely spreading current through large networks of cells, which appears to be their function in photoreceptors and glia.