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The Nervous System Consists of Billions CHAPTER 13 SYNAPSES he nervous system consists of billions possess a swelling at their ends. The of neurons, each one an individual boutons, whether from myelinated or Tcell, receiving signals from some unmyelinated fibers, are always found in other cells and generating signals of its own close proximity to a soma, axon, or dendrite to be sent to other cells. We consider here of another cell or to another bouton. how these signals are transferred from one At the site of the termination of a fiber, neuron to another. In most cases, there is a the specializations of the terminal and the gap between neurons that must be bridged in cell it contacts are called collectively a order for transmission to continue synapse. Synapses on somas are termed throughout the nervous system. In some axosomatic synapses; those on dendrites are cases, there appears to be no gap between termed axodendritic; and those on other axons or boutons are axoaxonic. The fiber's bouton is called the presynaptic element and the structure it contacts is the postsynaptic element. A schematic diagram of a synapse is shown in Figure 13- 1A. The pre- and postsynaptic elements are separated by a space 15-200 nm wide, known as the synaptic cleft. At the synapse, the membrane of the postsynaptic Figure 13-1A. A schematic diagram of a synapse indicating elementthe pre- is slightly thickened, and there is and postsynaptic elements with synaptic specializations. the neurons. Transmission between cells connected in this way is believed to occur by purely electrical events. How these two kinds of transmission occur is the subject of this chapter. Anatomy of a synapse. If we follow a primary afferent fiber along its course, we find that the fiber may branch many times, becoming smaller each time. If the fiber is myelinated, then near its Figure 13-1B. A electron micrograph a synapse termination the myelin disappears and there showing the pre- and postsynaptic elements with synaptic specializations. Note the accumulation of is a swelling of the axon, called the bouton mitochondria and presence of synaptic vesicles in terminaux, end bulb, terminal, synaptic the presynaptic membrane. The actual points of knob, or just bouton; with this swelling, the synaptic contact are indicated by the arrows. fiber simply ends. Unmyelinated fibers also 13-1 often an accumulation of some electron- inactivation of the released dense (appears dark in electron micrographs) transmitter substance, i.e., it must be material near the thickened membrane. On degraded or taken-up again by the the presynaptic side, there is normally an terminal, accumulation of mitochondria in the bouton 4. A synaptic action must be identified and, in electron micrographs, a large number for the substance, and local of spherical or irregularly shaped structures application must produce effects are seen near the synaptic region. These are "exactly" like those of synaptically called synaptic vesicles. All of these released transmitter substance, structures except the synaptic cleft (you’d 5. Drugs must produce similar effects need a higher power to see that) are visible upon actions of the substance and in the electron micrograph of Figure 13-1B. natural transmitter substance. It is believed that transmission from one No one of these criteria is sufficient by cell to another at a synapse like the one just itself to define a transmitter substance. For described (a chemical synapse, as opposed example, a compound, present in nerve to an electrical synapse) is accomplished by terminals, could not be a transmitter release of a substance, the transmitter substance unless it was released and unless it substance, from the synaptic vesicles into influenced the postsynaptic cell. It is also the synaptic cleft by the process of exocyto- possible that a compound serves as a sis. The identity of the transmitter substance transmitter substance in one neuron, but is unknown for most synapses. Many serves a different purpose in another one. candidate transmitter substances have been Table 13-1 contains a list of some of the suggested; however, we can be certain a substances that have been suggested as substance is a transmitter substance only if it chemical transmitter substances, their meets all of the following criteria: presumed actions2, locations of highest 1. The substance must occur naturally concentration within the central nervous in presynaptic terminals, and either system, modes of action (we will have more the precursors and enzymes for its to say about this later), and agents that block formation or an adequate, specific their actions. transport system for its uptake into It is difficult to establish the validity of the terminal must exist, all five criteria at synapses within the central 2. The substance must be released from nervous system because the cells involved the terminals by nerve stimulation, can rarely be seen; because only small 3. A mechanism must exist for rapid1 1 How rapidly the transmitter substance must be hydrolyzed or removed from the 2 Some care should be exercised in synaptic is determined by the duration of attributing single actions to transmitter action of the natural transmitter substance. substances. Acetylcholine, for example, is As we shall see, some transmitter substances usually thought of as excitatory, but there have only short actions; others act for long are some cases in which it is known to be times; some do both at different places. inhibitory. 13-2 Table 13-1 Known and Putative Transmitter Substances in the Mammalian Nervous System Presumed Locations of maximum Mode of Substance action concentration action Blocking agents Acetylcholine Excitation, Interpeduncular, dorsal raphe and Ionotropic, Curare, atropine inhibition caudate nuclei, nucleus metabotropic accumbens, ventral horn of spinal (cGMP‡) cord Glycine Inhibition Spinal cord, medulla, pons Ionotropic Strychnine (-aminobutyric acid Inhibition Cerebellum, cerebral cortex, spinal Ionotropic Bicuculline cord, retina Norepinephrine Excitation, Pons, medulla Metabotropic Propanolol, inhibition (cAMP†) phentolamine Dopamine Excitation, Putamen, caudate, locus ceruleus, Metabotropic Phenoxybenzamine inhibition hypothalamus (cAMP†) Serotonin (5-HT) Excitation, Amygdala, hypothalamus, septum, ? LSD! inhibition striatum L-Glutamate Excitation Temporal cortex, basal ganglia, Ionotropic ? cerebellum, amygdala L-Aspartate Excitation Substantia nigra, occipital cortex, Ionotropic ? thalamus, cerebellum, hypothalamus Epinephrine ? Thalamus, hypothalamus Metabotropic Propanolol, (cAMP†) phentolamine Substance P? Excitation Substantia nigra, trigeminal Metabotropic ? nucleus, dorsal horn of spinal cord, (?) limbic system Enkephalins? Inhibition Globus pallidus, caudate, nucleus ? Naloxone accumbens, hypothalamus Endorphins? Inhibition Pituitary, striatum, spinal cord ? Naloxone Histamine Inhibition Hypothalamus, thalamus Metabotropic Ethanolamine, (cAMP†) butamide Others: Taurine, neurotensin, carnosine, angiotensin II, hypothalamic releasing factors, serine, proline, N-acetyl- L-aspartate, adenosine, P-tyramine, tryptamine ? Peptides † cAMP = cyclic adenosine monophosphate ‡ cGMP = cyclic guanine monophosphate ! LSD = lysergic acid diethylamide 13-3 Table 13-2 Status of Putative Transmitter Substances s n i h p r o d d i c n a E e c n i + e i r r n s y i P h t l n i e e p e u e o l t n e e n c n b i h t a a i i n e n c a o n h i l t a n m m n o m p r p t i y i t a a a e a t s e c t t o r p e k p b m y u r s c o l l s o i n e u Criterion A A N ( - G G A D S S E H 1. Present in presynaptic % % % L % % % % % % % terminals–precursors and enzymes present 2. Released from terminals by nerve % % % x % % % % ? ? % impulses 3. Rapid inactivation mechanism % % % ? ? % ? % % ? ? 4. Known action–local application mimics % % % % ? % ? ? ? ? ? natural transmitter substance 5. Pharmacologically similar to natural % % % % ? % % ? ? ? ? transmitter substance Key: % = Property demonstrated L = Labeled substance is taken up by synapses in vitro or in vivo, but it is not know that synapses normally contain it x = Exogenous, labeled substance is released, but release of endogenous substance has not been shown ? = Property not examined experimentally or results hard to interpret quantities of transmitter substance are autonomic ganglia, and at some sympathetic released and even these are rapidly degraded and all parasympathetic postganglionic if they are not protected from enzyme action; synapses, NE at most sympathetic and because it is impossible to know if the postganglionic synapses, and dopamine at putative transmitter substance is being sympathetic ganglionic synapses. Table 13- applied near the postsynaptic membrane. 2 shows the validity of all five criteria for Thus, acetylcholine (ACh), norepinephrine these substances and for (-aminobutyric (NE) and dopamine (DA) have been acid (GABA), an inhibitory transmitter identified as transmitter substances at substance in the central nervous system. synapses in the peripheral nervous system, The remaining compounds of Table 13-2 ACh at the neuromuscular junction, at all meet one or more of the five criteria. These 13-4 compounds must still be considered as accumulation of synaptic vesicles on what is putative transmitter substances though some presumed to be the presynaptic side of each people ignore the deficiencies and call them synapse. The function of this arrangement is transmitter substances. unknown. The physiology of these new synaptic arrangements has not been studied in detail yet, but it can be presumed for now that they behave like other chemical synapses. Their presence certainly broadens the possibilities for interactions between neurons in the central nervous system. Another type of synaptic arrangement that has received a great deal of attention is the electrotonic synapse or electrical synapse, the substrate for which is thought to be the gap junction. A schematic diagram of a gap junction (sometimes called a connexon) is shown in Figure 13-3. The membranes of the two cells involved come extremely close or may actually fuse together. X-ray diffraction studies suggest Figure 13-2. A reciprocal synaptic relationship between two dendrites.
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