Synaptic Transmission - Clinical Tree

LEARNING OBJECTIVES

Upon completion of this chapter, the student should be able to answer the following questions:

1
What are the characteristics of electrical synapses?
2
What are the specializations found in the presynaptic and postsynaptic elements of a chemical synapse?
3
What sequence of events connect the arrival of the action potential at the presynaptic terminal to the entry of calcium?
4
What sequence of events connect the entry of calcium at the presynaptic terminal to release of neurotransmitter?
5
What is the quantal hypothesis of synaptic transmission, and how does the presence of miniature end plate potentials support this hypothesis?
6
Why is the reversal potential of a typical EPSP near 0 mV?
7
What distinguishes EPSPs and IPSPs in terms of underlying ionic conductances, effect on membrane potential, and neuronal firing probability?
8
How does an IPSP still inhibit a neuron when its reversal potential is equal to or more positive than the neuron’s resting potential?
9
What are the mechanisms by which synaptic effects can change over time?
10
What are the criteria for determining a substance is a neurotransmitter, and what are the major excitatory and inhibitory neurotransmitters?
11
What are the major classes of neurotransmitter receptors?

Synaptic transmission is the major process by which electrical signals are transferred between cells within the nervous system (or between neurons and muscle cells, or sensory receptors). Within the nervous system, synaptic transmission is usually conceived of as an interaction between two neurons that occurs in a point-to-point manner at specialized junctions called synapses. There are two main classes of synapses: electrical and chemical. However, as the list of chemical neurotransmitters has grown, and as understanding of their mechanisms of action has increased, the definition and conception of what constitutes synaptic transmission has had to be refined and expanded. We no longer think of synaptic transmission as a process that involves only neurons, but now realize that glia form an important element of the synaptic transmission, and that signaling occurs between neurons and glia. Moreover, in some cases, neurotransmitter released at a synapse will act over a widespread territory ( extrasynaptic transmission ) rather than just at the synapse from which it is released. In this chapter we first describe the classic conception of synaptic transmission (electrical and chemical), then introduce some of the nontraditional neurotransmitters, and discuss how they affect chemical communication between cells in the nervous system.

Electrical Synapses

Although their existence in the mammalian central nervous system (CNS) has been known for a long time, electrical synapses, or gap junctions, between neurons were thought to be of relatively little importance for the functioning of the adult mammalian CNS. Only recently has it become apparent that these synapses are quite common and that they may underlie important neuronal functions.

An electrical synapse is effectively a low-resistance pathway between cells that allows current to flow directly from one cell to another and, more generally, allows the exchange of small molecules between cells. Electrical synapses are present in the CNS of animals from invertebrates to mammals. They are present between glial cells as well as between neurons. Electrical coupling of neurons has been demonstrated for most brain regions, including the inferior olive, cerebellum, spinal cord, neocortex, thalamus, hippocampus, olfactory bulb, retina, and striatum.

A gap junction is the morphological correlate of an electrical synapse. These junctions are plaque-like structures in which the plasma membranes of the coupled cells become closely apposed (the intercellular space narrows to ≈ 3 nm) and filled with electron-dense material ( Fig. 6.1 ). Freeze-fracture electron micrographs of gap junctions display regular arrays of intramembrane particles that correspond to proteins that form the intercellular channels connecting the cells. The typical channel diameter is large (1–2 nm), thus making it permeable not only to ions but also other small molecules up to approximately 1 kDa in size.

AT THE CELLULAR LEVEL

Each gap junction channel is formed by two hemichannels (called connexons ), one contributed by each cell. Each connexon, in turn, is a hexamer of connexin protein subunits, which are encoded for by a gene family of at least 21 different members in mammals. (A second family of proteins that form gap junctions, the pannexins, has also been identified.) Gap junctions formed by different connexins have distinct biophysical properties (gating and conductance) and cellular distributions. Although at least 10 connexin types are expressed in the CNS, connexin 36 (connexins are named according to their molecular weight; thus, the number refers to the approximate molecular weight of the connexin in kilodaltons) is the major neuronal connexin in the adult CNS. Other connexin types found in the CNS form gap junctions between glial cells or are primarily expressed transiently during development.

Fig. 6.1, Gap junction structure. A, Schematic view of the gap junction showing narrowing of the intercellular space to 3.5 nm at the junction. The gap junction has multiple channels, with each channel formed by two connexon hemichannels. Each connexon in turn comprises six connexin subunits. B, Electron micrograph of part of a complex synaptic arrangement called a glomerulus that is found in the inferior olive and some other CNS regions. Two dendritic spines are coupled by a gap junction (small black arrows). An axon terminal packed with synaptic vesicles fills the upper right part of the panel. Large arrowheads point to the electron-dense material that marks the active zones. Black dots are immunogold labeling for GABA, thus identifying this terminal as GABAergic. Red arrowheads point to synaptic vesicles.

Electrical synapses are fast (essentially no synaptic delay) and bidirectional (i.e., current generated in either cell can flow across the gap junction to influence the other cell). In addition, they act as low-pass filters. That is, slow electrical events are preferentially transmitted compared to fast signals, such as action potentials. Gap junctions are particularly abundant during neogenesis and appear to play an important role in forming functional neuronal networks in the neocortex and thalamus. Another important role for neuronal gap junctions appears to be synchronization of network activity. For example, the activity of inferior olivary neurons is normally synchronized but becomes uncorrelated when pharmacological blockers of gap junctions are injected into the inferior olive. It also appears that the patterns of electrical coupling by gap junctions may be highly specific. For example, neocortical interneurons almost exclusively couple to interneurons of the same type. This specific gap junction–coupling pattern suggests that multiple, independent, electrically coupled networks of interneurons may coexist across the neocortex.

Finally, although electrical synapses are generally regarded as relatively simple and static in comparison to chemical synapses, they may actually be fairly dynamic entities. For example, the properties of electrical synapses can be modulated by several factors, including voltage, ligands, intracellular pH, and [Ca ⁺⁺ ]. Moreover, they are subject to regulation by neurotransmitter-mediated G protein–coupled receptor activation, and connexins (the protein subunits that form a gap junction, see At The Cellular Level) contain sites for phosphorylation. These factors can change the coupling between cells by causing changes in single-channel conductance, formation of new gap junctions, or removal of existing ones.

Chemical Synapses

Chemical synaptic transmission was first demonstrated between the vagus nerve and the heart by a simple experiment by Otto Loewi. The vagus nerve of a frog was stimulated to slow the heart rate down while the solution perfusing the heart was collected. This solution was then used to perfuse a second heart, whose beating then also slowed, demonstrating that the vagal nerve stimulation had caused a chemical to be released into the solution. The chemical responsible was found to be acetylcholine, which we now know is also a neurotransmitter at the neuromuscular junction and at other synapses in the peripheral and central nervous systems.

Unlike the situation at electrical synapses, at chemical synapses there is no direct communication between the cytoplasm of the two cells. Instead the cell membranes are separated by a synaptic cleft of some 20 nm, and interaction between the cells occurs via chemical intermediaries known as neurotransmitters. Chemical synapses are generally unidirectional, and thus one can refer to the presynaptic and postsynaptic elements that are diagramed in Fig. 6.2 . The presynaptic element is often the terminal portion of an axon and is packed with small vesicles whose exact shape and size vary with the neurotransmitter they contain. In addition, the presynaptic membrane apposed to the postsynaptic element has regions, known as active zones , of electron-dense material that corresponds to the proteins involved in transmitter release (see Fig. 6.1 B ). Moreover, mitochondria and rough endoplasmic reticulum are typically found in the presynaptic terminal. The postsynaptic membrane is also characterized by electron-dense material, which in this case corresponds to the receptors for the neurotransmitter and second messenger molecules engaged by neurotransmitter receptor activation.

Fig. 6.2, Schematic of a chemical synaptic terminal releasing all three main classes of neurotransmitter. For each, the mechanisms of release, sites of action, and mechanisms for termination of activity are shown. Real synapses release transmitter from one or more classes.

Chemical synapses occur between different parts of neurons. Traditionally, focus has been placed on synapses formed by an axon onto the dendrites or soma of a second cell ( axodendritic or axosomatic synapses ), and our description will be based primarily on such synapses. However, there are many additional types of chemical synapses, such as axoaxonic (axon to axon), dendrodendritic (dendrite to dendrite), and dendrosomatic (dendrite to soma). Furthermore, complex synaptic arrangements are possible, such as mixed synapses, in which cells form both electrical and chemical synapses with each other; serial synapses, in which an axoaxonic synapse is made onto the axon terminal and influences the efficacy of that terminal’s synapse with yet a third element; and reciprocal synapses, in which both cells can release transmitter to influence the other. Fig. 6.1 B shows a complex synaptic arrangement called a glomerulus that involves both chemical and electrical synapses among the participating elements.

Much of what we know about chemical synapses comes from the study of two classic preparations, the frog neuromuscular junction (the synapse from a motor neuron onto a muscle fiber) and the squid giant synapse (the synapse from a second-order neuron onto third-order neurons that innervate the muscle of the squid’s mantle; i.e., the motor neurons whose axons were used to characterize the conductances underlying the action potential [see Chapter 5 ]). The principles governing transmission at these synapses mostly apply to “classic” synaptic transmission within the mammalian CNS (see the section Neurotransmitters). Synaptic transmission at a chemical synapse may be summarized as follows: (1) Synaptic transmission is initiated by arrival of the action potential at the presynaptic terminal. (2) The action potential depolarizes the terminal, which causes Ca ⁺⁺ channels to open. (3) The subsequent rise in [Ca ⁺⁺ ] within the terminal triggers the fusion of vesicles containing neurotransmitter with the plasma membrane. (4) The transmitter is then expelled into the synaptic cleft, diffuses across it, and binds to specific receptors on the postsynaptic membrane. (5) Binding of transmitter to receptors then causes the opening (or, less often, the closing) of ion channels in the postsynaptic membrane, which in turn results in changes in the potential and resistance of the postsynaptic membrane that alter the excitability of the cell.

The changes in membrane potential of the postsynaptic cell are termed excitatory and inhibitory postsynaptic potentials ( EPSPs and IPSPs ) ( Fig. 6.3 ), depending on whether they increase or decrease, respectively, the cell’s excitability, which can be defined as its probability of firing action potentials. The transmitter typically acts for only a very short time (milliseconds) because reuptake and degradation mechanisms rapidly clear the transmitter from the synaptic cleft.

Fig. 6.3, IPSPs and EPSPs recorded with a microelectrode in a cat spinal motor neuron in response to stimulation of appropriate peripheral afferent fibers. Forty traces are superimposed. Note that these IPSPs are hyperpolarizing, but in some cases IPSPs can be depolarizing—see text for an explanation.

The succeeding sections will amplify specific points of this summary. However, it is worth mentioning at this point that some of the nonclassic types of neurotransmitters (e.g., neuropeptides, gaseous neurotransmitters, and metabotropic receptors) have required modifications of several aspects of this basic conception. (Whereas an ionotropic receptor usually contains the ion channel as an integral part of itself, a metabotropic receptor does not contain an ion channel, but instead is coupled to a G protein that initiates a second messenger cascade that can ultimately affect ion channels.) Some of the differences between classic and peptide transmitters are listed in Table 6.1 . More details on the properties of peptide and gaseous transmitters are provided in the relevant parts of the Neurotransmitters section of this chapter, and metabotropic receptors are covered in the Receptors section.

TABLE 6.1

Distinctions Between Classic Nonpeptide Neurotransmitters and Peptide Neurotransmitters

Nonpeptide Transmitters	Peptide Transmitters
Synthesized and packaged in the nerve terminal	Synthesized and packaged in the cell body; transported to the nerve terminal by fast axonal transport
Synthesized in active form	Active peptide formed when it is cleaved from a much larger polypeptide that contains several neuropeptides
Usually present in small clear vesicles	Usually present in large electron-dense vesicles
Released into a synaptic cleft	May be released some distance from the postsynaptic cell There may be no well-defined synaptic structure
Action of many terminated because of uptake by presynaptic terminals via Na ⁺ -powered active transport	Action terminated by proteolysis or by the peptide diffusing away
Typically, action has short latency and short duration (millisecond)	Action may have long latency and may persist for many seconds

Calcium Entry Is the Signal for Transmitter Release

Depolarization of the presynaptic membrane by the action potential causes voltage-gated Ca ⁺⁺ channels to open, which makes it possible for Ca ⁺⁺ to flow into the terminal and trigger the release of transmitter. However, Ca ⁺⁺ will enter the terminal only if there is a favorable electrochemical gradient to do so. Recall that it is the combination of the concentration and voltage gradients that determines the direction of ion flow through open channels. Extracellular [Ca ⁺⁺ ] is high relative to intracellular [Ca ⁺⁺ ], which favors entry into the terminal; however, during the peak of the action potential, the membrane potential is positive, and the voltage gradient opposes the entry of Ca ⁺⁺ because of its positive charge. Thus, at the peak of the action potential, relatively little Ca ⁺⁺ enters the terminal because although the membrane is highly permeable to Ca ⁺⁺ , the overall driving force is small. In fact, by using a voltage clamp, one can experimentally make the membrane potential positive and equal to the Nernst equilibrium potential for Ca ⁺⁺ . If this is done, no Ca ⁺⁺ will enter the terminal despite Ca ⁺⁺ channels being open, and as a result, no transmitter is released and no postsynaptic response is observed. This voltage is known as the suppression potential. If the membrane potential is rapidly made negative again (because of either the end of the action potential or by adjusting the voltage clamp), Ca ⁺⁺ rushes into the terminal as a result of the large driving force (which arises instantaneously on repolarization) and the high membrane permeability to Ca ⁺⁺ (which remains high because it takes the Ca ⁺⁺ channels several milliseconds to close in response to the new membrane potential), thereby resulting in release of transmitter and a postsynaptic response ( Fig. 6.4 ).

Fig. 6.4, Presynaptic Ca ++ current and its relationship to the postsynaptic response. A, Schematic of a giant squid synapse preparation. Electrodes 1 and 2 are used to voltage-clamp the presynaptic terminal and record its voltage and current. (Note that tetrodotoxin and tetraethyl ammonium were present to block Na + and K + conductance to isolate the Ca ++ conductance.) Electrode 3 records the membrane potential of the postsynaptic axon. The presynaptic terminal was voltage-clamped to increasingly more depolarized levels (blue traces). With a small depolarization (B), a small Ca ++ current starts shortly after the voltage step, continues to grow for the duration of the step (on current), and then decays exponentially after its termination (off or tail current). A larger voltage step (C) increases both the on and the off components of the Ca ++ current, and now distinct on and off responses are observed in the postsynaptic response. D, The voltage step is to the Nernst potential for Ca ++ , so there is no Ca ++ current during the step, but a large tail current and off response are observed.

Synaptic Vesicles and the Quantal Nature of Transmitter Release

How neurotransmitter is stored and how it is released are questions fundamental to synaptic transmission. Answering these questions began with two observations. The first was the discovery of small round or irregularly shaped organelles known as synaptic vesicles in presynaptic terminals by electron microscopy (see Figs. 6.1 B and 6.2 ). The second observation came from recordings of postsynaptic responses at the neuromuscular junction. Normally an action potential in a motor neuron causes a large depolarization in the postsynaptic muscle, termed an end plate potential (EPP), which is equivalent to EPSPs generated in neurons. However, under conditions of low extracellular [Ca ⁺⁺ ], the EPP amplitude is reduced (because the presynaptic Ca ⁺⁺ current is reduced, leading to a smaller rise in intracellular [Ca ⁺⁺ ], and transmitter release is proportional to [Ca ⁺⁺ ]). In this condition, the EPP is seen to fluctuate among discrete values ( Fig. 6.5 ). Moreover, small spontaneous depolarizations of the postsynaptic membrane, termed miniature end plate potentials (mEPPs), are observable. The amplitude of the mEPP (≤1 mV) corresponds to that of the smallest EPP evoked under low [Ca ⁺⁺ ], and the amplitudes of other EPPs were shown to be integral multiples of the mEPP amplitude; thus, it was proposed that each mEPP corresponded to the release of transmitter from a single vesicle, or a quanta (sometimes referred to as quantal release), and that EPPs represented the combined simultaneous release of transmitter from many vesicles.

Fig. 6.5, A, Spontaneous miniature end plate potentials (mEPPs) recorded at a neuromuscular junction in a fiber of frog extensor digitorum longus. B, EPPs evoked by nerve stimulation under low-[Ca ++ ] conditions, which reduce the probability of transmitter release. Low [Ca ++ ] response in purple , intermediate [Ca ++ ] in green , and high [Ca ++ ] in red . The small-amplitude EPPs evoked under these conditions vary in amplitude in a step-like manner, where the size of the step is equal to the smallest EPP, which in turn equals the size of the mEPPs.

This linking of mEPPs and vesicles implies that each mEPP is caused by the action of many molecules of neurotransmitter binding to postsynaptic receptors. The alternative that each mEPP could be caused by a single transmitter molecule binding to and opening a single postsynaptic receptor was rejected, in part because responses smaller in amplitude than mEPPs could be generated experimentally by directly applying dilute solutions of acetylcholine to the muscle. In fact, mEPPs were calculated to be caused by the action of approximately 10,000 molecules, which corresponds well to estimates of the number of neurotransmitter molecules contained within a single vesicle.

Many additional studies have confirmed the vesicle hypothesis of neurotransmitter release. For example, biochemical studies have shown that neurotransmitter is concentrated in vesicles, and fusion of vesicles to the plasma membrane and their depletion in the terminal cytoplasm after action potentials have been shown with electron microscopic techniques.

Molecular Apparatus Underlying Vesicular Release

The small vesicles that contain classical neurotransmitters (nonpeptide) can fuse with the presynaptic membrane only at specific sites called active zones. To become competent to fuse with the presynaptic membrane at an active zone, a small vesicle must first dock at the active zone and then undergo a priming process. Once primed the vesicle can fuse and release its transmitter into the synaptic cleft in response to an increase in local cytoplasmic [Ca ⁺⁺ ]. On the order of 25 proteins may play roles in docking, priming, and fusion. Some of these proteins are cytosolic, whereas others are proteins associated with the vesicle membrane or the presynaptic plasma membrane.

As with other exocytotic processes, neurotransmitter release involves SM (sec1/Munc18-like) and SNARE ( s oluble N -ethyl maleimide–sensitive factor a ttachment protein r eceptor) proteins: v-SNARES in the vesicle membrane and t-SNARES in the (target) presynaptic plasma membrane. Zipper-like interactions between synaptobrevin (a v-SNARE) and syntaxin and SNAP-25 (both are t-SNARES) with the assistance of SM proteins bring the vesicle membrane and the presynaptic plasma membrane close together before fusion. The SNARE proteins are targets for various botulinum toxins, which disrupt synaptic transmission, thus demonstrating their critical role in this process. Nevertheless, they do not bind Ca ⁺⁺ , so another protein must be the Ca ⁺⁺ sensor that triggers the actual fusion event. Evidence indicates that a synaptotagmin protein is almost certainly the Ca ⁺⁺ sensor and, even more specifically, that the second of its two cytoplasmic domains contains the Ca ⁺⁺ binding site. Interestingly, synaptotagmins differ in their kinetics, and brain regions vary as to which synaptotagmin family member acts as the Ca ⁺⁺ sensor for vesicular fusion. Thus, differential expression of synaptotagmin genes in neurons may be a mechanism to adapt the kinetics of vesicle release and thereby tailor the specific characteristics of synaptic transmission to the functional needs of each CNS region.

Calcium channels are located in the active zone membrane at sites adjacent to the docked vesicles. When they open, a small area of high [Ca ⁺⁺ ], a microdomain is created at the active zone. This local high concentration (which lasts for less than a millisecond) allows the rapid binding of Ca ⁺⁺ to synaptotagmin, triggering the fusion of a docked vesicle and allowing release of its neurotransmitter. Despite the multiple steps involved, the process of vesicular release at a synapse is extremely rapid because of the close proximity of the molecular apparatus involved to each other. Indeed, the time from Ca ⁺⁺ influx to vesicle fusion is about 0.2 millisecond.

Synaptic Vesicles Are Recycled

During synaptic transmission, vesicles must fuse with the plasma membrane to release their contents into the synaptic cleft. However, there must be a reverse process; otherwise, it would be hard to sustain the vesicle population, the presynaptic membrane surface area would expand with each bout of synaptic transmission, and the molecular content and functionality of the presynaptic member would likely change (because, as just discussed, the protein content of the vesicle membrane is distinct from that of the terminal membrane).

There appear to be two distinct mechanisms by which vesicles are retrieved after release of their neurotransmitter content ( Fig. 6.6 ). One mechanism is the endocytotic pathway commonly found in most cell types. Clathrin-coated pits are formed in the plasma membrane, which then pinch off to form coated vesicles within the cytoplasm of the presynaptic terminal. These vesicles then lose their coat and undergo further transformations (i.e., acquire the correct complement of membrane proteins and be refilled with neurotransmitter) to become synaptic vesicles ready for release.

Fig. 6.6, Vesicle recycling pathways. Synaptic vesicles have been thought to fuse with the membrane while emptying their contents and then be recycled by forming clathrin-coated pits that are endocytosed to form coated vesicles (1 → [2 or 2′] → 3′ → 1). An alternative pathway that may allow more rapid recycling of vesicles has been proposed. This pathway, called “kiss and run,” involves only transient fusion of the vesicle to the presynaptic membrane to form a pore through which the vesicle contents may be emptied, followed by detachment of the vesicle from the membrane (1 → 2 → 3 → 4 → 5 → 1).

Evidence for a second, more rapid recycling mechanism has been obtained (see Fig. 6.6 ). It involves transient fusion of the vesicle to the synaptic membrane and has been called “kiss and run.” In this case, fusion of the vesicle with the synaptic membrane leads to the formation of a pore through which the transmitter is expelled, but there is no wholesale collapse of the vesicle into the membrane. Instead, the duration of the fusion is very brief, after which the vesicle detaches from the plasma membrane and reseals itself. Thus, the vesicle membrane retains its molecular identity. Its contents can then simply be replenished, thereby making the vesicle ready for use again.

The relative importance of these two mechanisms is still being debated. However, at central synapses, which tend to be small and contain relatively few vesicles in comparison to the neuromuscular junction, the rapid time course of the kiss-and-run mechanism may help avoid the problem of vesicle depletion and the consequent failure of synaptic transmission during periods of high activity (many neurons in the CNS can show firing rates of several hundred hertz, and a few types of neuron can fire at rates of ≈ 1000 Hz).

Postsynaptic Potentials

Following vesicle fusion the neurotransmitter molecules are released and diffuse across the synaptic cleft (a very rapid process) and bind to receptors on the postsynaptic membrane. This binding leads to the opening (or less often the closing) of ion channels. These channels are termed ligand-gated because their opening and closing are primarily controlled by the binding of neurotransmitter. This mechanism can be contrasted with that of the voltage-gated channels underlying the action potential, whose opening and closing are determined by the membrane potential. Some channels, most notably the NMDA ( N -methyl- d -aspartate) channel, are both ligand- and voltage-gated.

The focus of this section will be on ionotropic receptors, which underlie fast synaptic transmission due to the ion channel being part of the receptor protein. Metabotropic receptors, which initiate “slow” synaptic transmission, are receptors that act on ion channels indirectly through second-messenger cascades (see the section Receptors for details). Despite the differing time courses, many of the same basic principles apply to both types of postsynaptic potentials (PSPs).

EPSPs (Excitatory PostSynaptic Potentials). As stated earlier, the binding of neurotransmitter generally changes the membrane potential of the postsynaptic cell, and these changes are referred to as EPSPs when they increase the excitability of the neuron and IPSPs when they inhibit the neuron from firing action potentials. EPSPs are always depolarizing potentials, and IPSPs are usually hyperpolarizing.

Once a ligand-gated channel is open, the direction of current flow through it is determined by the electrochemical driving force for the permeant ion(s). It turns out that the pores of most channels that underlie EPSPs are relatively large and therefore allow passage of most cations with similar ease. As an example, consider the acetylcholine-gated channel that is opened at the neuromuscular junction. Na ⁺ and K ⁺ are the major cations present (Na ⁺ extracellularly and K ⁺ intracellularly); therefore, the net current through the channel is approximately the sum of the Na ⁺ and K ⁺ currents (I _net = I _Na + I _K ). Recall that the current through a channel from a particular ion is dependent on two factors: the conductance of the channel to the ion and the driving force on the ion. This relationship is expressed by the equation

I_{x} = g_{x} \times (V_{m} - E_{x})

$I_{x} = g_{x} \times (V_{m} - E_{x})$

where g _x is the conductance of the channel to ion x, V _m is the membrane potential, and E _x is the Nernst equilibrium potential for ion x. In this case g _x is similar for Na ⁺ and K ⁺ , so the main determinant of net current is the relative driving forces (V _m − E _x ). If the membrane is at its resting potential (typically around −70 mV), there is a strong driving force (V _m − E _Na ) for Na ⁺ to enter the cell because this potential is far from the Na ⁺ Nernst potential (about +55 mV), whereas there is only a small driving force for K ⁺ to leave the cell because V _m is close to the K ⁺ Nernst potential (about −90 mV). Thus, if acetylcholine-gated channels open when the membrane is at its resting potential, a large inward Na ⁺ current and a small outward K ⁺ current will flow through the acetylcholine channel, thereby resulting in a net inward current, which acts to depolarize the membrane.

The net inward current that results from opening such channels is called the excitatory postsynaptic current (EPSC). Fig. 6.7 A contrasts the time course of the EPSC and the resulting EPSP for fast synaptic transmission. The EPSC is much shorter (≈1–2 milliseconds in duration) and corresponds to the time the channels are actually open. The short duration of the EPSC is due to the fact that the released neurotransmitter remains in the synaptic cleft for only a short while before being either enzymatically degraded or taken up by either glia or neurons. Binding and unbinding of a neurotransmitter to its receptor take place rapidly, so once its concentration falls in the cleft, the postsynaptic receptor channels rapidly close as well and terminate the EPSC. Note how the end of the EPSC corresponds to the peak of the EPSP, which is followed by a long tail. The duration of the tail and the rate of the decay in EPSP amplitude reflect the passive membrane properties of the cell (i.e., its resistance-capacitance [RC] properties) (see Chapter 5 ). In slow synaptic transmission, the duration of the EPSP reflects the activation and deactivation of biochemical processes more than the membrane properties. The long duration of even fast EPSPs (relative to EPSCs and action potentials) is functionally important because it allows EPSPs to overlap and thereby summate. Such summation is central to the integrative properties of neurons (see the next section, Synaptic Integration).

Fig. 6.7, Properties of EPSPs. A, Time course of a fast EPSP compared with that of the underlying EPSC. In many cases, such as this one, the EPSC is much shorter than the EPSP; however, sometimes the EPSC can have a fairly extensive tail. B, Intracellularly recorded EPSPs at different levels of depolarization. EPSPs were evoked in motor neurons by stimulation of Ia afferents. The number to the left of each trace indicates the membrane potential induced by injection of current through the electrode. At initial membrane potentials of −42 and −60 mV, the EPSP triggered an action potential. At more depolarized levels, Na + channels are inactivated, so no spike occurs. C, To determine the EPSP reversal potential, the initial membrane potential is plotted against the size of the EPSP (ΔV).

Normally an EPSP depolarizes the membrane, and if this depolarization reaches threshold, an action potential is generated. However, consider what happens if the channels underlying the action potential are blocked and the membrane of the postsynaptic cell is experimentally depolarized by injecting current through an intracellular electrode. Because the membrane potential is now more positive, the driving force for Na ⁺ is decreased and that for K ⁺ increased. If the synapse is activated at this point, the net current through the receptor channel (the EPSC) will be smaller because of changes in the relative driving force. This implies that if the membrane potential is depolarized enough, there will be a point at which the Na ⁺ and K ⁺ currents through the channel are equal and opposite, and thus there is no net current and no EPSP. If the membrane is depolarized beyond this point, there is a net outward current through the receptor channels and the membrane will hyperpolarize (i.e., the EPSP will be negative). Thus, the potential at which there is no EPSP (or EPSC) is known as the reversal potential. For excitatory synapses, the reversal potential is usually around 0 mV (±10 mV), depending on the synapse (see Fig. 6.7B, C ).

It is worth noting that a reversal potential is a key criterion for demonstrating the chemical-gated as opposed to the voltage-gated nature of a synaptic response because currents through voltage-gated channels do not reverse, except at the Nernst potential of the ion for which they are selective (and then only if the channel is open at that potential). Consequently, beyond a certain membrane potential, no current will flow through voltage-gated channels because they will be closed. In contrast, ligand-gated channels can be opened at any membrane potential and thus can always have a net current flow through them, except at one specific voltage, the reversal potential.

IPSPs (Inhibitory PostSynaptic Potentials). Like EPSPs, IPSPs are triggered by the binding of neurotransmitter to receptors on the postsynaptic membrane and typically involve an increase in membrane permeability as a result of the opening of ligand-gated channels. They differ in that IPSP channels are permeable to only a single ionic species, either Cl ⁻ or K ⁺ . Thus, IPSPs will have a reversal potential equal to the Nernst potential of the ion carrying the underlying current. Typically, the Nernst potential for these ions is somewhat negative relative to the resting potential, so when IPSP channels open, there is an outward flow of current through them that results in hyperpolarization of the membrane (see Fig. 6.3 ).

However, in some cells, activation of an inhibitory synapse may produce no change in potential (if the membrane potential equals the Nernst potential for Cl ⁻ or K ⁺ ) or may actually result in a small depolarization. Nevertheless, in both these cases, the reversal potential for the IPSP is still negative with regard to the threshold for eliciting an action potential (otherwise it would increase the probability of the cell spiking and by definition be an EPSP). It may seem counterintuitive that something that depolarizes the membrane can still be considered inhibitory, but if it decreases the probability of spiking, then it is indeed inhibitory (a further explanation is given in the Synaptic Integration section).

In sum, starting from the resting membrane potential, EPSPs are always depolarizing, IPSPs can be either depolarizing or hyperpolarizing, and a hyperpolarizing potential is always an IPSP. Thus, the key distinction between inhibitory and excitatory synapses (and IPSPs and EPSPs) is how they affect the probability of the cell firing an action potential: EPSPs increase the probability, whereas IPSPs decrease the probability.

Safety factor. Synapses between cells vary in strength and thus in the size of the PSP generated in the postsynaptic cell. Many factors determine synaptic strength, including the size and number of synaptic contacts between two cells, its activity level and past history, and the probability of vesicle fusion for the synapse. For excitatory synapses at the neuromuscular junction, the strength of the synapse may be quantified by what is known as its safety factor (the ratio of postsynaptic depolarization amplitude to the amplitude needed to reach the threshold to trigger an action potential). The neuromuscular junction has a high safety factor. When a motor neuron action potential triggers release of neurotransmitter at the neuromuscular junction, an end plate potential (EPP; the equivalent of an EPSP in a neuron) is generated in the muscle fiber. The EPP is so large that under normal circumstances it depolarizes the sarcolemma well above the action potential threshold and thus always triggers a spike, leading to contraction of the muscle cell. A high safety factor makes sense for the neuromuscular junction because each muscle cell is contacted by only a single motor neuron, and if that motor neuron is firing, the nervous system has basically made the decision to contract that muscle. In certain diseases of the neuromuscular junction, such as myasthenia gravis and Lambert-Eaton syndrome, the EPPs are reduced such that the safety factor can fall below 1, and thus the EPPs sometimes fail to trigger action potentials in the muscle fibers, leading to weakness.

In contrast to the neuromuscular junction, most CNS synapses require summation of EPSPs, due to repetitive activation of a single synapse or multiple active synapses, to trigger an action potential in the postsynaptic neuron. This summation process is at the core of synaptic integration, which is taken up in the next section.

Synaptic Integration

The overall effect of a particular synapse is dependent on its location. To understand this concept fully, we must first recall that action potentials are typically generated at the initial segment of the cell because it has the highest density of voltage-gated Na ⁺ channels and therefore the lowest threshold for initiation of a spike. Thus, it is the summed amplitudes of the synaptic potentials at this point, the initial segment, that is critical for the decision to spike. EPSPs generated by synapses close to the initial segment (i.e., synapses onto the soma or proximal dendrites) will result in a larger depolarization at the initial segment than will EPSPs generated by synapses on distal dendrites ( Fig. 6.8 A single action potential in axon 2 versus 1). This is because the cell membrane is leaky and synaptic currents are generated locally at the synapse, so even if two synapses generate a local EPSC of the same size, less of the initial current will arrive at the initial segment from the more distal synapse than from the more proximal one, thereby resulting in the generation of a smaller EPSP at the initial segment by the distal synapse (see discussion of length constant in Chapter 5 ). Thus, the synapse’s spatial location in the dendritic tree is an important determinant of its efficacy. However, as already mentioned, EPSPs generated by most CNS synapses, even those in favorable positions (i.e., close to the initial segment), are too small by themselves to reach the spiking threshold in the postsynaptic cell, as illustrated in Fig. 6.8 A , where an action potential in either axon 1 (distal) or 2 (proximal) both produce EPSPs that are too small to trigger a spike. Thus, generally the summed EPSPs from multiple synapses are required to reach threshold and trigger a spike.

The requirement for multiple EPSPs to summate in order to trigger a spike is what makes the relatively long duration of EPSPs so important. Temporal summation refers to the fact that EPSPs that are separated by a latency less than their duration can sum. This is illustrated in Fig. 6.8B , where the same synapse is activated multiple times in rapid succession (axons can fire action potentials at rates well over 100 Hz); in this situation, successive EPSPs will be less than 10 milliseconds apart and therefore overlap and sum. Note the higher amplitude of the second peak.

Spatial summation refers to the fact that synaptic potentials generated by different synapses can interact. For example, in Fig. 6.8 , suppose axon 1 and 3 each fire an action potential but at widely separated times. Each produces an EPSP that depolarizes the cell but is too small to reach threshold (see Fig. 6.8 C , EPSP1, EPSP3). Instead, if both axons fire within a short enough time of each other, their effect can be additive, as shown in Fig. 6.8C (EPSP 1+3). The combined EPSP amplitude may then reach threshold and lead to spiking of the cell. If the EPSPs generated by axons 1 and 3 were simultaneous, then we would have an example of pure spatial summation. In the example shown, however, the times of the two EPSPs were slightly separated, thus we have both spatial and temporal summation present. The fact that EPSPs have a long time course (when compared with action potentials or the underlying EPSCs) facilitates both types of synaptic integration.

In the previous example, the combined EPSP was approximately the linear summation of the two individual EPSPs evoked by action potentials in axons 1 and 3. This is the case when two synapses are far apart. If the two synapses are close together, such as for axons 2 and 4 (see Fig. 6.8 D ), the summation becomes less than linear because of what is known as a shunting effect. That is, when synapse 2 is active, channels are opened in the cell membrane, which means that it is more leaky. Therefore, when synapse 4 is also active, more of its EPSC will be lost (shunted) through the dendritic membrane, and less current will be left to travel down the dendrite to the initial segment. The result is that synapse 4 causes a smaller EPSP at the initial segment than it would have generated in isolation. Nevertheless, the combined EPSP is still larger than an EPSP caused by either synapse 2 or 4 alone.

Where do IPSPs fit into synaptic integration? Whereas EPSPs add together to help bring the membrane potential up to and beyond the spiking threshold, IPSPs subtract from the membrane potential to make it more negative, and therefore, further from threshold. In deciding whether to spike, a cell adds the ongoing EPSPs and subtracts the IPSPs to determine whether the sum reaches threshold. As with an EPSP, the efficacy of an IPSP varies with its location.

In addition to subtracting algebraically from the membrane potential, IPSPs exert an inhibitory action via the shunting mechanism, just as was described earlier for EPSPs. That is, while the IPSP channels are open, they make the membrane more leaky (i.e., lower its resistance) and thereby reduce the size of EPSPs, thus making them less effective. This shunting mechanism explains how IPSPs that do not change the membrane potential—or even those that slightly depolarize it—can still decrease the excitability of the cell. An alternative way to look at this effect is to view each synapse as a device that tries to bring the membrane potential to its own equilibrium potential. Because this potential is below the action potential threshold in the case of IPSPs, IPSPs make it harder for the cell to spike.

Thus far the interaction of synaptic potentials has been presented under the assumption that the postsynaptic cell membrane is passive (i.e., it acts as though it were simply resistors and capacitors in parallel with each other). However, it is clear that the dendrites and somas of most, if not all, neurons contain active elements (i.e., gated channels) that can amplify and alter EPSPs and IPSPs. For example, a distal EPSP can have a larger-than-expected effect if the EPSP activates dendritically located voltage-gated Na ⁺ or Ca ⁺⁺ channels that boost its amplitude or even generate propagated dendritic action potentials. Another example is Ca ⁺⁺ -activated K ⁺ channels that are present in the dendrites of some neurons. These channels are activated by the influx of Ca ⁺⁺ through synaptic channels or via dendritic voltage-gated Ca ⁺⁺ channels opened by EPSPs, or by the release of Ca ⁺⁺ from smooth endoplasmic reticulum, and can cause long-lasting hyperpolarizations that effectively make the cell inexcitable for tens to hundreds of milliseconds. As a final example, there are some Ca ⁺⁺ channels that underlie a low-threshold Ca ⁺⁺ spike. These channels are normally inactive at resting membrane potentials, but the hyperpolarization that results from a large IPSP can de-inactivate them and allow them to open (and produce a spike) after termination of the IPSP. In this case “inhibition” actually increases the cell’s excitability. In sum, synaptic integration is a highly complex, nonlinear process. Nevertheless, the basic principles just described remain at its core.

Modulation of Synaptic Activity

Integration of synaptic input by a postsynaptic neuron, as described in the previous section, represents one aspect of the dynamic nature of synaptic transmission. A second aspect is that the strength of individual synapses can vary as a function of their use or activity. That is, a synapse’s current functional state reflects, to some extent, its history.

Activation of a synapse typically produces a response in the postsynaptic cell (i.e., a PSP) that will be roughly the same each time, assuming the postsynaptic cell is in a similar state. Certain patterns of synaptic activation, however, result in changes in the response to subsequent activation of the synapse. Such use-related changes may remain for short (milliseconds to seconds) or long (minutes to days) durations and may be either a potentiation or diminishment of the synapse’s strength. These changes in synaptic efficacy are a critical feature of synaptic transmissions, in part, because they underlie cognitive abilities such as learning and memory.

Paired-Pulse Facilitation

When a presynaptic axon is stimulated twice in rapid succession, it is often found that the PSP evoked by the second stimulus is larger in amplitude than the one evoked by the first ( Fig. 6.9 ). This increase is known as paired-pulse facilitation (PPF). Note PPF is distinct from temporal summation, in which two EPSPs overlap and sum to a larger response; with PPF the second EPSP itself is greater in size. If one plots the relative size of the two PSPs as a function of the time between two stimuli, the amount of increase in the second PSP will be seen to depend on the time interval. Maximal facilitation occurs at around 20 milliseconds, followed by a gradual reduction in facilitation as the interstimulus interval continues to increase; with intervals of several hundred milliseconds, the two PSPs are equal in amplitude and no facilitation is observed. Thus, PPF is a relatively rapid and short-lasting change in synaptic efficacy.

Fig. 6.9, A, Facilitation at a neuromuscular junction. EPPs at a neuromuscular junction in toad sartorius muscle were elicited by successive action potentials in the motor axon. Neuromuscular transmission was depressed by 5 mM Mg ++ and 2.1 mM curare so that action potentials did not occur. B, EPPs at a frog neuromuscular junction elicited by repetitively stimulating the motor axon at different frequencies. Note that facilitation failed to occur at the lowest frequency of stimulation (1/second) and that the degree of facilitation increased with increasing frequency of stimulation in the range of frequency used. Neuromuscular transmission was inhibited by bathing the preparation in 12 to 20 mM Mg ++ . C, Post-tetanic potentiation at a frog neuromuscular junction. The top two traces indicate control EPPs in response to single action potentials in the motor axon. Subsequent traces indicate EPPs in response to single action potentials after tetanic stimulation (50 impulses/second for 20 seconds) of the motor neuron. The time interval between the end of tetanic stimulation and the single action potential is shown on each trace. The muscle was treated with tetrodotoxin to prevent generation of action potentials.

Post-tetanic Potentiation

Post-tetanic potentiation (PTP) is similar to PPF; however, in this case the responses are compared before and after stimulation of the presynaptic neuron tetanically (tens to hundreds of stimuli at a high frequency). Such a tetanic stimulus train causes an increase in synaptic efficacy (see Fig. 6.9 C ). PTP, like PPF, is an enhancement of the postsynaptic response, but it lasts longer: tens of seconds to several minutes after the cessation of tetanic stimulation.

Numerous experiments have shown that PPF and PTP are the result of changes in the presynaptic terminal and do not generally involve a change in the sensitivity of the postsynaptic cell to transmitter. Rather, the repeated stimulation leads to an increased number of quanta of transmitter being released. This increase is thought to be due to residual amounts of Ca ⁺⁺ that remain in the presynaptic terminal after each stimulus and help potentiate subsequent release of transmitter. However, the exact mechanism or mechanisms by which this residual Ca ⁺⁺ enhances release is not yet clear. The residual Ca ⁺⁺ does not, however, appear to act simply by binding to the same sites as the Ca ⁺⁺ that enters at the active zone and directly triggers vesicle fusion in response to the action potential.

Synaptic Depression

Use of a synapse can also lead to a short-term depression in its efficacy. Most commonly, the postsynaptic cell at such a fatigued or depressed synapse responds normally to transmitter applied from a micropipette; hence, as was the case for PPF and PTP, the change is presynaptic. In general, the depression is thought to reflect depletion of the number of releasable presynaptic vesicles. Thus, short-term depression of synaptic transmission is most often and most easily seen at synapses in which the probability of release after a single stimulus is high and under conditions that favor release (i.e., high [Ca ⁺⁺ ]). A postsynaptically related cause of synaptic depression can be desensitization of the receptors in the postsynaptic membrane.

Both potentiation and depressive processes can occur at the same synapse; in general, the type of modulation observed will depend on which process dominates. This in turn can reflect stimulus parameters, local ionic conditions, and the properties of the synapse. In particular, synapses have different baseline probabilities for releasing vesicles. Synapses with a high release probability will be more likely to show poststimulus depression, whereas those with low release probability are less likely to deplete their vesicle store and thus can be facilitated more easily. Sometimes mixed responses can occur. For example, during a tetanic stimulus train a synapse may show a depressed response, but after the train the synapse can show post-tetanic facilitation once the vesicles are recycled.

You're Reading a Preview

Become a Clinical Tree membership for Full access and enjoy Unlimited articles

Become membership

If you are a member. Log in here