Medical Physiology, 3rd Edition

Mechanisms of Synaptic Transmission

Electrical continuity between cells is established by electrical or chemical synapses

Once the concept of bioelectricity had taken hold among physiologists of the 19th century, it became clear that the question of how electrical signals flow between cells posed a fundamental biological problem. Imagine that two cells lie side by side without any specialized device for communication between them. Furthermore, imagine that a flat, 20-µm2 membrane area of the first, or presynaptic, cell is separated—by 15 nm—from a similar area of the second, or postsynaptic, cell. In his classic book on electrophysiology, Bernard Katz calculated that a voltage signal at the presynaptic membrane would suffer 10,000-fold attenuation in the postsynaptic membrane. A similar calculation based on the geometry and cable properties of a typical nerve-muscle synapse suggests that an action potential arriving at a nerve terminal could depolarize the postsynaptic membrane by only 1 µV after crossing the synaptic gap—an attenuation of 105. Clearly, the evolution of complex multicellular organisms required the development of special synaptic mechanisms for electrical signaling to serve as a workable means of intercellular communication.

Two competing hypotheses emerged in the 19th century to explain how closely apposed cells could communicate electrically. One school of thought proposed that cells are directly linked by microscopic connecting bridges that enable electrical signals to flow directly. Other pioneering physiologists used pharmacological observations to infer that cell-to-cell transmission was chemical in nature. Ultimate resolution of this question awaited both the development of electron microscopic techniques, which permitted visualization of the intimate contact region between cells, and further studies in neurochemistry, which identified the small organic molecules that are responsible for neurotransmission. By 1960, accumulated evidence led to the general recognition that cells use both direct electrical and indirect chemical modes of transmission to communicate with one another.

The essential structural element of intercellular communication, the synapse, is a specialized point of contact between the membranes of two different but connected cells. Electrical and chemical synapses have unique morphological features, distinguishable by electron microscopy. One major distinction is the distance of separation between the two apposing cell membranes. At electrical synapses, the adjacent cell membranes are separated by ~3 nm and appear to be nearly sealed together by a plate-like structure that is a fraction of a micrometer in diameter. Freeze-fracture images of the intramembrane plane in this region reveal a cluster of closely packed intramembranous particles that represent a gap junction. As described, a gap junction corresponds to planar arrays of connexons, each of which is made up of six connexin monomers (see Fig. 6-18). The multiple connexons from apposing cells physically connect the two cells together via multiple aqueous channels.

In contrast to those of the gap junction, the apposing cell membranes of the chemical synapse are separated by a much larger distance of ~30 nm at a neuronal chemical synapse and up to 50 nm at the vertebrate nerve-muscle synapse. An additional characteristic of a chemical synapse is the presence of numerous synaptic vesicles on the side of the synapse that initiates the signal transmission, termed the presynaptic side. These vesicles are sealed, spherical membrane-bound structures that range in diameter from 40 to 200 nm and contain a high concentration of chemical neurotransmitter.

The contrasting morphological characteristics of electrical and chemical synapses underline the contrasting mechanisms by which they function (Table 8-1). Electrical synapses pass voltage changes directly from one cell to another across the low-resistance continuity that is provided by the connexon channels. On the other hand, chemical synapses link two cells by the diffusion of a chemical transmitter across the large gap separating them. Key steps in chemical neurotransmission include release of transmitter from synaptic vesicles into the synaptic space, diffusion of transmitter across the cleft of the synapse, and activation of the postsynaptic cell by binding of transmitter to a specific receptor protein on the postsynaptic cell membrane.

TABLE 8-1

Summary of Properties of Electrical and Chemical Synapses

   

CHEMICAL

 

ELECTRICAL

IONOTROPIC

METABOTROPIC

Agonist

None

e.g., ACh

e.g., ACh

Membrane protein

Connexon

Receptor/channel

Receptor/G protein

Delay in transmission

Instantaneous

~1 ms

Seconds to minutes

Direct evidence for the existence of chemical synapses actually predated the experimental confirmation of electrical synapses. The foundations of synaptic physiology can be traced to early studies of the autonomic nervous system. Early in the 1900s, researchers noted that adrenal gland extracts, which contain epinephrine (or adrenaline), elicit physiological effects (e.g., increases in heart rate and blood pressure) that are similar to those elicited by stimulation of sympathetic nerve fibers. In 1904, Thomas R. Elliot proposed that sympathetic nerves might release a substance analogous to epinephrine—later identified as norepinephrine—that would function in chemical transmission between a nerve and its target organ.

Subsequent studies by Otto Loewi suggested that the vagus nerve, which is parasympathetic, produces a substance responsible for depression of the heartbeat. His classic 1921 experiment is widely cited as the first definitive evidence for chemical neurotransmission. Loewi used an ingenious bioassay to test for the release of a chemical substance by the vagus nerve. He repeatedly stimulated the vagus nerve of a cannulated frog heart and observed a slowing of the heartbeat. At the same time, he collected the artificial saline that emerged from the ventricle of this overstimulated heart. When he later applied the collected fluid from the vagus-stimulated heart to a different heart, he observed that this perfusate slowed the second heart in a manner that was identical to direct vagal stimulation. He also later identified the active compound in the perfusion fluid, originally called Vagusstoff, as acetylcholine (ACh).

Efforts by Henry Dale and coworkers to understand the basis of neurotransmission between motor nerves and skeletal muscle culminated in the identification of ACh as the endogenous excitatory neurotransmitter. Thus, the inherent complexity of chemical synaptic transmission was evident from these earliest investigations, which indicated that the same neurotransmitter (ACh) could have an inhibitory action at one synapse (vagus nerve–heart) and an excitatory action at another synapse (motor nerve–skeletal muscle). For their work on nerve transmission across chemical synapses, Otto Loewi and Henry Dale received the Nobel Prize in Physiology or Medicine in 1936. imageN8-1

N8-1

Sir Henry H. Dale and Otto Loewi

Contributed by Emile Boulpaep, Walter Boron

The recognition of norepinephrine and ACh as the principal neurotransmitters of the autonomic nervous system led to the classification of peripheral nerve terminals and their synapses as either adrenergic or cholinergic, corresponding to their dependence on these two transmitters.

Dale and Loewi shared the 1936 Nobel Prize in Physiology or Medicine for their research on the chemical transmission of impulses. For more information about these investigators and the work that led to their Nobel Prize, visit http://www.nobel.se/medicine/laureates/1936/index.html (accessed October 2014).

Electrical synapses directly link the cytoplasm of adjacent cells

Whereas overwhelming support for chemical synaptic transmission accumulated in the first half of the 20th century, the first direct evidence for electrical transmission came much later from electrophysiological recordings of a crayfish nerve preparation. In 1959, Furshpan and Potter used two pairs of stimulating and recording electrodes to show that depolarization of a presynaptic nerve fiber (the crayfish abdominal nerve) resulted in excitation of a postsynaptic nerve cell (the motor nerve to the tail muscle) with virtually no time delay. In contrast, chemical synapses exhibit a characteristic delay of ~1 ms in the postsynaptic voltage signal after excitation of the presynaptic cell. The demonstration of an electrical synapse between two nerve membranes highlighted an important functional difference between electrical and chemical synapses—immediate signal propagation (electrical) versus briefly delayed communication (chemical) via the junction.

An electrical synapse is a true structural connection formed by connexon channels of gap junctions that link the cytoplasm of two cells (Fig. 8-1). These channels thus provide a low-resistance path for electrotonic current flow and allow voltage signals to flow with little attenuation and no delay between two or more coupled cells. Many types of gap junctions pass electrical current with equal efficiency in both directions—these are termed reciprocal synapses. In other words, the current passing through the gap junction is ohmic; it varies linearly with the transjunctional voltage (i.e., the Vm difference between the two cells). However, the crayfish synapse described by Furshpan and Potter allows depolarizing current to pass readily in only one direction, from the presynaptic cell to the postsynaptic cell. Such electrical synapses are called rectifying synapses to indicate that the underlying junctional conductance is voltage dependent. Studies of cloned and expressed connexins have shown that the voltage dependence of electrical synapses arises from unique gating properties of different connexin isoforms. Some isoforms are voltage dependent; others are voltage independent. Intrinsic rectification can also be altered by the formation of a gap junction that is composed of two hemichannels, each made up of a different connexin monomer. Such hybrid connexins are called heterotypic channels.

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FIGURE 8-1 An electrical synapse. An electrical synapse consists of one or more gap junction channels permeable to small ions and molecules (see Fig. 6-18).

Chemical synapses use neurotransmitters to provide electrical continuity between adjacent cells

By their very nature, chemical synapses are inherently rectifying or polarized. They propagate current in one direction: from the presynaptic cell that releases the transmitter to the postsynaptic cell that contains the receptors that recognize and bind the transmitter. However, the essentially vectorial nature of chemical synaptic transmission belies the possibility that the postsynaptic cell can influence synapse formation or transmitter release by the presynaptic cell. Studies of synapse development and regulation have shown that postsynaptic cells also play an active role in synapse formation. In the CNS, postsynaptic cells may also produce retrograde signaling molecules, such as nitric oxide (see pp. 315–317), that diffuse back into the presynaptic terminal and modulate the strength of the synaptic connection. Furthermore, the presynaptic membrane at many synapses has receptors that may either inhibit or facilitate the release of transmitter. Thus, chemical synapses should be considered a unidirectional pathway for signal propagation that can be modulated by bidirectional chemical communication between two interacting cells.

The process of chemical transmission can be summarized by the following series of steps (Fig. 8-2):

Step 1: Neurotransmitter molecules are packaged into synaptic vesicles. Vesicular transporters concentrate neurotransmitters inside the vesicle using the energy of an H+ electrochemical gradient.

Step 2: An action potential, which involves voltage-gated Na+ and K+ channels (see pp. 176–177), arrives at the presynaptic nerve terminal.

Step 3: Depolarization opens voltage-gated Ca2+ channels, which allows Ca2+ to enter the presynaptic terminal.

Step 4: The increase in intracellular Ca2+ concentration ([Ca2+]i) triggers the fusion of synaptic vesicles with the presynaptic membrane. As a result, packets (quanta) of transmitter molecules are released into the synaptic cleft.

Step 5: The transmitter molecules diffuse across the synaptic cleft and bind to specific receptors on the membrane of the postsynaptic cell.

Step 6: The binding of transmitter activates the receptor, which in turn activates the postsynaptic cell.

Step 7: The process is terminated by (a) enzymatic destruction of the transmitter (e.g., hydrolysis of ACh by acetylcholinesterase), (b) uptake of transmitter into the presynaptic nerve terminal or into other cells by Na+-dependent transport systems, or (c) diffusion of the transmitter molecules away from the synapse.

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FIGURE 8-2 A chemical synapse. Synaptic transmission at a chemical synapse can be thought of as occurring in seven steps as shown.

The molecular nature of chemical synapses permits enormous diversity in functional specialization and regulation. Functional diversity occurs at the level of the transmitter substance, receptor protein, postsynaptic response, and subsequent electrical and biochemical processes. Many different small molecules are known—or proposed—to serve as neurotransmitters (see pp. 314–322). These molecules include both small organic molecules, such as norepinephrine, ACh, serotonin (5-hydroxytryptamine [5-HT]), glutamate, gamma-aminobutyric acid (GABA), and glycine, and peptides such as endorphins and enkephalins.

Neurotransmitters can activate ionotropic or metabotropic receptors

Neurotransmitter receptors transduce information by two molecular mechanisms: some are ligand-gated ion channels and others are G protein–coupled receptors (see pp. 51–66). Several neurotransmitter molecules—such as ACh, glutamate, serotonin, GABA, and glycine—serve as ligands (agonists) for both types of receptors. Agonist-gated receptors that are also ion channels are known as ionotropic receptors. Receptors coupled to G proteins are called metabotropic receptors because their activation initiates a metabolic process involving GTP.

Ionotropic and metabotropic receptors determine the ultimate functional response to transmitter release. Activation of an ionotropic receptor causes rapid opening of ion channels. This channel activation, in turn, results in depolarization or hyperpolarization of the postsynaptic membrane, depending on the ionic selectivity of the conductance change. Activation of a metabotropic G protein–linked receptor results in the production of active α and βγ subunits, which initiate a wide variety of cellular responses by direct interaction with either ion channel proteins or other second-messenger effector proteins. By their very nature, ionotropic receptors mediate fast ionic synaptic responses that occur on a millisecond time scale, whereas metabotropic receptors mediate slow, biochemically mediated synaptic responses in the range of seconds to minutes.

Figure 8-3 compares the basic processes mediated by two prototypic ACh receptors (AChRs): (1) the ACh-activated ion channel at the neuromuscular junction of skeletal muscle, an ionotropic receptor also known as the nicotinic AChR (see Fig. 8-3A), and (2) the G protein–linked AChR at the atrial parasympathetic synapse of the heart, a metabotropic receptor also known as the muscarinic AChR (see Fig. 8-3B). The nicotinic versus muscarinic nomenclature is pharmacological based on whether the AChR is activated by nicotine or muscarine, two natural products that behave like agonists. In the case of the ionotropic (nicotinic) receptor, opening of the AChR channel results in a transient increase in permeability to Na+ and K+, which directly produces a brief depolarization that activates the muscle fiber. In the case of the metabotropic (muscarinic) receptor, activation of the G protein–coupled receptor opens an inward-rectifier K+ channel, or GIRK (see pp. 197–198) via βγ subunits released from an activated heterotrimeric G protein. Enhanced opening of these GIRKs produces membrane hyperpolarization and leads to inhibition of cardiac excitation (see p. 492). These two receptor types provide the molecular explanation for the puzzling observations of early physiologists that ACh (Vagusstoff) activates skeletal muscle but inhibits heart muscle.

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FIGURE 8-3 Ionotropic and metabotropic AChRs. A, Nicotinic AChR (ionotropic), which is a ligand-gated channel on the postsynaptic membrane. In a skeletal muscle, the end result is muscle contraction. B, Muscarinic AChR (metabotropic), which is coupled to a heterotrimeric G protein. In a cardiac muscle, the end result is decreased heart rate. Note that the presynaptic release of ACh is similar here and in A.