《医学生理学 Medical Physiology》参考教材资料（PDF电子书籍）Fundamental Neuroscience, Third edition, 2008

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CHAPTER Basic plan of the nervous system INTRODUCTION because they start with the simplest condition-and the human brain is far and away the most complex The brain often is compared with a computer these object we know of, with its roughly 100 billion neurons and 100 trillion axonal connections between them. One days. True, the brain is a computer, but it is a very remarkable conclusion emerging from these two per- special kind of computer-a biological computer that has evolved by natural selection over hundreds of mil spectives is that nerve cells in all animals-from jelly lions of years and countless generations. Furthermore, fish to humans--are basically the same in terms of cell it has no obvious design features in common with biology; what changes most during ontogeny and phy- logeny is the arrangement of nerve cells into functional to ique organ that thinks and feels, generates behe human-engineered computers. Instead, the brain is a circuits: the architecture of the nervous system. The al interactions with the environment, keeps bodily bricks"or"legos"are similar, but the buildings physiology relatively stable, and enables reproduction constructed with them can vary tremendously in size of the species-its most important role from evolu- and functionality. An ultimate goal of neuroscience tion's grand perspective. And for strictly personal may be to understand the human brain, but remark reasons the brain is the most precious thing we have lyzing"lower "animals and early embryos. The other reflected in Rene Descartes's famous seventeenth equally remarkable conclusion is that all vertebrates, century aphorism, "I think therefore I am. from fish to humans, share a common basic plan of the Aristotle first emphasized that structure and fund nervous system, with the same major parts and func- tional systems tion are inextricably intertwined, two sides of the same coin, with structure providing obvious physical con- straints on function. Just think about the difference between a hammer and a saw. Unfortunately, as knowledge becomes more and more specialized, there EVOLUTION HIGHLIGHTS: GENERAL is a tendency to analyze the structure, function, an ORGANIZING PRINCIPLES chemistry of the nervous system from different, some times even isolated, perspectives. The main theme of Protists and the simplest multicellular animals this chapter is the basic structure-function organiza- (sponges)display ingestive, defensive, reproductive, tion of the nervous system: what are the parts and how and other behaviors without any nervous system are they interconnected into functional systems? In whatsoever, raising the question: what is the adaptive other words, what are the organizing principles-the value of adding a nervous system to an organism? We basic design features-of its circuitry? will now examine key structure-function correlates of Evolution and development are two approaches nervous system organization in animals with relatively often used to understand biological complexity simple body plans and behaviors Neuroscience. Third Edition 2008,2003,1999 Elsevier Inc

CHAPTER 2 Basic Plan of the Nervous System INTRODUCTION The brain often is compared with a computer these days. True, the brain is a computer, but it is a very special kind of computer—a biological computer that has evolved by natural selection over hundreds of mil￾lions of years and countless generations. Furthermore, it has no obvious design features in common with human-engineered computers. Instead, the brain is a unique organ that thinks and feels, generates behav￾ioral interactions with the environment, keeps bodily physiology relatively stable, and enables reproduction of the species—its most important role from evolu￾tion’s grand perspective. And for strictly personal reasons the brain is the most precious thing we have simply because it is the organ of consciousness, as refl ected in René Descartes’s famous seventeenth century aphorism, “I think therefore I am.” Aristotle fi rst emphasized that structure and func￾tion are inextricably intertwined, two sides of the same coin, with structure providing obvious physical con￾straints on function. Just think about the difference between a hammer and a saw. Unfortunately, as knowledge becomes more and more specialized, there is a tendency to analyze the structure, function, and chemistry of the nervous system from different, some￾times even isolated, perspectives. The main theme of this chapter is the basic structure-function organiza￾tion of the nervous system: what are the parts and how are they interconnected into functional systems? In other words, what are the organizing principles—the basic design features—of its circuitry? Evolution and development are two approaches often used to understand biological complexity— because they start with the simplest condition—and the human brain is far and away the most complex object we know of, with its roughly 100 billion neurons and 100 trillion axonal connections between them. One remarkable conclusion emerging from these two per￾spectives is that nerve cells in all animals—from jelly- fi sh to humans—are basically the same in terms of cell biology; what changes most during ontogeny and phy￾logeny is the arrangement of nerve cells into functional circuits: the architecture of the nervous system. The “bricks” or “legos” are similar, but the “buildings” constructed with them can vary tremendously in size and functionality. An ultimate goal of neuroscience may be to understand the human brain, but remark￾able progress can nevertheless be made through ana￾lyzing “lower” animals and early embryos. The other equally remarkable conclusion is that all vertebrates, from fi sh to humans, share a common basic plan of the nervous system, with the same major parts and func￾tional systems. EVOLUTION HIGHLIGHTS: GENERAL ORGANIZING PRINCIPLES Protists and the simplest multicellular animals (sponges) display ingestive, defensive, reproductive, and other behaviors without any nervous system whatsoever, raising the question: what is the adaptive value of adding a nervous system to an organism? We will now examine key structure-function correlates of nervous system organization in animals with relatively simple body plans and behaviors. Fundamental Neuroscience, Third Edition 15 © 2008, 2003, 1999 Elsevier Inc

2. BASIC PLAN OF THE NERVOUS SYSTEM The Nerve Net Is the Simplest Type of body wall pores. These specialized cells are called Nervous System independent effectors because their contraction is evoked y stimuli like stretch or environmental ch emicals s. In his provocative 1919 book, The Elementary Nervous acting directly on the plasma membrane of individual System, George Parker outlined a reasonable scenario cell for how nervous systems evolved. An updated version The first animal phylum with a nervous system was begins with the first multicellular animals-similar to the Cnidaria, which includes jellyfish, corals, ane- modern-day sponges-that emerged over half a billion mones, and the elegantly simple hydra. In contrast years ago. They are seemingly amorphous animals to sponges, hydra locomote and show active feeding that spend their adult lives immobile, submerged in behavior(Fig. 2. 1 ). These behaviors are coordinated water. Their relatively simple behavior is mediated and mediated by a nervous system, a network of spe- largely by a set of primitive smooth muscle cell cialized units or cells called nerve cells or neurons(Box (myocyte)sphincters allowing water flow through 2.1) FIGURE 2.1 Locomotor behavior in hydra resembles a series of somersaults, as shown in the sequence beginning on the left. The tiny black dot in the region between the tentacles in the figure at the far right is the animal s mouth. Ingestive(feeding) behavior involves guiding food particles into the mouth with coordinated tentacle movements. BOX 2.1 THE NEURON DOCTRINE The cell theory, which states that all organisms are proponents of the neuron doctrine raged for decades. composed of individual cells, was developed around the Over the years, the validity of the neuron doctrine has middle of the nineteenth century by Mattias Schleiden been supported by a wealth of accumulated data. Never- and Theodor Schwann. However, this unitary vision of theless, the reticularist view is not entirely incorrect, the cellular nature of life was not immediately applied to because some neurons do act syncytially via specialized the nervous system, as most biologists at the time believed intercellular gap junctions, a feature that is more promi- in the cytoplasmic continuity of nervous system cells. nent during embryogenesis Later in the century the most prominent advocate of this In 1897, Charles Sherrington postulated that neurons reticularist view was Camillo Golgi, who proposed that establish functional contact with each other and with axons entering the spinal cord actually fuse with other other cell types via a theoretical structure he called the axons(Fig. 2.2A). The reticularist view was challenged synapse( Greek synaptein, to fasten together). It was not ost thoroughly by Santiago Ramon y Cajal, a founder until 50 years later that the structural existence of syn of contemporary neuroscience and without doubt the apses was demonstrated by electron microscopy(see Fig greatest observer of neuronal architecture. In beautifully 3.3). The synaptic complex is built around an adhesive written and carefully reasoned deductive arguments, junction, and in this and other respects the complex is Cajal presented us with what is now known as the neuron quite similar to the desmosome and the adherens junc doctrine. This great concept in essence states that the cell tions of epithelia. In fact, similarities in ultrastructure theory applies to the nervous system: each neuron is an between the adherens junction and the synaptic complex individual entity, the basic unit of neural circuitry(Fig. of central nervous tissue were noted even in early electron 2. 2B). The acrimonious debate between reticularists and microscopic studies(see Peters et al., 1991) I. NEUROSCIENCI

16 2. BASIC PLAN OF THE NERVOUS SYSTEM I. NEUROSCIENCE The Nerve Net Is the Simplest Type of Nervous System In his provocative 1919 book, The Elementary Nervous System, George Parker outlined a reasonable scenario for how nervous systems evolved. An updated version begins with the fi rst multicellular animals—similar to modern-day sponges—that emerged over half a billion years ago. They are seemingly amorphous animals that spend their adult lives immobile, submerged in water. Their relatively simple behavior is mediated largely by a set of primitive smooth muscle cell (myocyte) sphincters allowing water fl ow through body wall pores. These specialized cells are called independent effectors because their contraction is evoked by stimuli like stretch or environmental chemicals acting directly on the plasma membrane of individual cells. The fi rst animal phylum with a nervous system was the Cnidaria, which includes jellyfi sh, corals, ane￾mones, and the elegantly simple hydra. In contrast to sponges, hydra locomote and show active feeding behavior (Fig. 2.1). These behaviors are coordinated and mediated by a nervous system, a network of spe￾cialized units or cells called nerve cells or neurons (Box 2.1). The cell theory, which states that all organisms are composed of individual cells, was developed around the middle of the nineteenth century by Mattias Schleiden and Theodor Schwann. However, this unitary vision of the cellular nature of life was not immediately applied to the nervous system, as most biologists at the time believed in the cytoplasmic continuity of nervous system cells. Later in the century the most prominent advocate of this reticularist view was Camillo Golgi, who proposed that axons entering the spinal cord actually fuse with other axons (Fig. 2.2A). The reticularist view was challenged most thoroughly by Santiago Ramón y Cajal, a founder of contemporary neuroscience and without doubt the greatest observer of neuronal architecture. In beautifully written and carefully reasoned deductive arguments, Cajal presented us with what is now known as the neuron doctrine. This great concept in essence states that the cell theory applies to the nervous system: each neuron is an individual entity, the basic unit of neural circuitry (Fig. 2.2B). The acrimonious debate between reticularists and proponents of the neuron doctrine raged for decades. Over the years, the validity of the neuron doctrine has been supported by a wealth of accumulated data. Never￾theless, the reticularist view is not entirely incorrect, because some neurons do act syncytially via specialized intercellular gap junctions, a feature that is more promi￾nent during embryogenesis. In 1897, Charles Sherrington postulated that neurons establish functional contact with each other and with other cell types via a theoretical structure he called the synapse (Greek synaptein, to fasten together). It was not until 50 years later that the structural existence of syn￾apses was demonstrated by electron microscopy (see Fig. 3.3). The synaptic complex is built around an adhesive junction, and in this and other respects the complex is quite similar to the desmosome and the adherens junc￾tions of epithelia. In fact, similarities in ultrastructure between the adherens junction and the synaptic complex of central nervous tissue were noted even in early electron microscopic studies (see Peters et al., 1991). BOX 2.1 THE NEURON DOCTRINE FIGURE 2.1 Locomotor behavior in hydra resembles a series of somersaults, as shown in the sequence beginning on the left. The tiny black dot in the region between the tentacles in the fi gure at the far right is the animal’s mouth. Ingestive (feeding) behavior involves guiding food particles into the mouth with coordinated tentacle movements

EVOLUTION HIGHLIGHTS: GENERAL ORGANIZING PRINCIPLES Sensory ectoderm, and perhaps the first to evolve were sensory Hydra's body wall is simple, with an outer ectoderm neurons. One cytoplasmic extension of these bipolar layer contacting the external environment, an inner cells facing the external environment became special endoderm layer facing the body cavity's internal envi- ized to detect stimuli much weaker than those activat- ronment and promoting digestion and waste elimina ing independent effectors, whereas the other pole tion, and a vague middle or meso layer in between became specialized to transmit information about Neurons probably differentiated initially from the these stimuli to a group of independent effectors(Fig 3). Experimental evidence indicates that sensory neurons provide four major selective advantages in evolution: Faster effector cell responses Stronger behavioral responses because multiple effector cells are influenced Sensory neurons responding to different stimulus modalities can be distributed strategically in different body The bipolar shape of sensory neurons is fundamen- tally important. The prototypical theory about neural circuit organization was presented by Santiago Ramon y Cajal in his classic"bible"of structural neuroscience, The Histology of the Nervous System in Man and verte- 术 brates(1909-1911). According to the cornerstone func tional polarity theory, information normally flows in one direction through most neurons, and thus through most neural circuits-from dendrites and cell body, the input or receptive parts of the neuron, to a single axon, the output or effector part. In other words, most FIGURE 2.2 Two competing views: The nervous system as a neurons have two classes of processes: one or mor eticulum or the neuron doctrine. (A)Proponents of the reticular dendrites detecting inputs, and a single axon conduct theory believed that neurons are physically continuous with one ing an output that can influence multiple cells through another, forming an uninterrupted network.(B)In contrast, the euron doctrine regards each neuron an individual entity commu- branching or collateralization. At least in early develop- dicating with target cells by way of contiguity rather than continuity, mental stages, all sensory neurons have this funda across an appropriate intercellular gap. Adapted from Caja mental bipolar shape, and over the course of evolution (1909-1911). they have become specialized to detect a remarkable A stimulus FIGURE 2.3 Activation of effector cells(e)in simple animals. (A) Sponges lack a nervous system; stimuli act directly on effector cells, are thus called independent effectors. (B) In cniderians, bipola ry neurons(s)differentiate in the ectoderm. The sensory neuron rocess detects stimuli and is thus a dendrite. The inner process of some sensory neurons transmits information to effector cel is thus an axon. Because this type of sensory neuron innervates effector cells directly, it is v a sensorimotor n( C)Most cnidaria neurons send their axon to motoneurons(m), which in turn send an axon to effector cells. Cniderian urons may also have lateral pre with other motoneurons, and these processes typically conduct information in either direction cesses. Arrows show the direction of information flow. L NEUROSCIENCE

I. NEUROSCIENCE A B FIGURE 2.2 Two competing views: The nervous system as a reticulum or the neuron doctrine. (A) Proponents of the reticular theory believed that neurons are physically continuous with one another, forming an uninterrupted network. (B) In contrast, the neuron doctrine regards each neuron an individual entity commu￾nicating with target cells by way of contiguity rather than continuity, across an appropriate intercellular gap. Adapted from Cajal (1909–1911). A B C e e e e e e e s s m m stimulus FIGURE 2.3 Activation of effector cells (e) in simple animals. (A) Sponges lack a nervous system; stimuli act directly on effector cells, which are thus called independent effectors. (B) In cniderians, bipolar sensory neurons (s) differentiate in the ectoderm. The sensory neuron outer process detects stimuli and is thus a dendrite. The inner process of some sensory neurons transmits information directly to effector cells and is thus an axon. Because this type of sensory neuron innervates effector cells directly, it is actually a sensorimotor neuron. (C) Most cniderian sensory neurons send their axon to motoneurons (m), which in turn send an axon to effector cells. Cniderian motoneurons may also have lateral processes with other motoneurons, and these processes typically conduct information in either direction (and are thus amacrine pro￾cesses). Arrows show the direction of information fl ow. Sensory Neurons Hydra’s body wall is simple, with an outer ectoderm layer contacting the external environment, an inner endoderm layer facing the body cavity’s internal envi￾ronment and promoting digestion and waste elimina￾tion, and a vague middle or meso layer in between. Neurons probably differentiated initially from the ectoderm, and perhaps the fi rst to evolve were sensory neurons. One cytoplasmic extension of these bipolar cells facing the external environment became special￾ized to detect stimuli much weaker than those activat￾ing independent effectors, whereas the other pole became specialized to transmit information about these stimuli to a group of independent effectors (Fig. 2.3). Experimental evidence indicates that sensory neurons provide four major selective advantages in evolution: • Increased stimulus sensitivity • Faster effector cell responses • Stronger behavioral responses because multiple effector cells are infl uenced • Sensory neurons responding to different stimulus modalities can be distributed strategically in different body regions The bipolar shape of sensory neurons is fundamen￾tally important. The prototypical theory about neural circuit organization was presented by Santiago Ramón y Cajal in his classic “bible” of structural neuroscience, The Histology of the Nervous System in Man and Verte￾brates (1909–1911). According to the cornerstone func￾tional polarity theory, information normally fl ows in one direction through most neurons, and thus through most neural circuits—from dendrites and cell body, the input or receptive parts of the neuron, to a single axon, the output or effector part. In other words, most neurons have two classes of processes: one or more dendrites detecting inputs, and a single axon conduct￾ing an output that can infl uence multiple cells through branching or collateralization. At least in early develop￾mental stages, all sensory neurons have this funda￾mental bipolar shape, and over the course of evolution they have become specialized to detect a remarkable EVOLUTION HIGHLIGHTS: GENERAL ORGANIZING PRINCIPLES 17

2. BASIC PLAN OF THE NERVOUS SYSTEM variety of stimuli from light, temperature divide neuronal processes into three types: dendritic range of chemicals and ions, to vibration and other (input), axonal (output), or amacrine(bidirectional) mechanical deformations Adding a second layer to the nervous system has obvious adaptive advantages related to increased Motor neurons capacity for response complexity and integration. Con A second stage of differentiation or complexity in sider a stimulus to one specific part of the animal or hydra's nervous system was the addition of neurons even one sensory neuron. Its influence may radiate to between sensory and effector. They are defined as distant parts of the animal because one sensory neuron motor neurons (motoneurons) because they directly innervates multiple motoneurons, those motoneurons innervate effector cells(usually muscle or gland cells), innervate additional motoneurons, and each motoneu which in turn receive their inputs from sensory neurons ron innervates multiple effector cells-an example of (Figs. 2.2B, 2.3C). Conceptually, this provides a two- what Cajal called avalanche conduction. There may be layered nervous system: the first or top layer having great divergence between stimulus and effector cells sensory neurons and the second or bottom layer having producing a response, with the actual divergence motor neurons. In this prototypical network sensory pattern shaped by the structure-function architecture neurons project(send axon collaterals) to multiple of the nervous system: how the neurons and their motoneurons, and then each motoneuron innervates a interconnections are arranged in the body. It is easy to set of effector cells(with a motoneuron and its effector imagine how this arrangement in hydra might coordi cell set defined as a motor unif). During an animals nate the tentacles to bring a food morsel detected by normal behavior, information flow is unidirectional just one of them to the mouth, or how it might coordi- or polarized from one cell type, sensory neuron, to nate locomotion(Fig 2.1) another cell type, motoneuron, to a third cell type A second basic con structu effector. This is the basic definition of a simple reflex, arrangement is information convergence in the nervous as defined by Charles Sherrington in his cornerstone system. Just consider a particular motoneuron: it can of systems neuroscience, The Integrative Action of the receive inputs from more than one sensory neuron and Nervous System (1906) from other motoneurons as well In this hypothetical scenario(Fig. 2.3)an environ- mental stimulus detected by a sensory neurons den Nerve Nets drite is transmitted by its axon to the dendrites of a At first glance hydra's nervous system is distributed motoneuron population. Then the axon of each moto- fairly uniformly around the radially symmetrical body neuron innervates an effector cell population. This wall and tentacles 2. 4). Its essentially double- is the functional polarity rule applied to a simple layered arrangement of distributed sensory and motor two-layer, sensory-motor network mediating reflex neurons is called a nerve net. However, in certain behavior regions of the body with specialized function, like Another general feature of the hydra two-layered around the mouth and base of the tentacles, neurons nervous system has been observed: sensory neurons tend to aggregate--a tendency toward centralization do not innervate each other, whereas motoneurons do that will now be examined more carefully interact directly. Here, motoneurons have two projec- tion classes: one to effector cells and another to other motoneurons. Structurally and functionally, many of Bilateral Symmetry, Centralization, and these hydra motoneurons also have two types of Cephalization Emerge in Flatworms In contrast to cnidarians, flatworms are bilaterally effector cell population. However, the other is a process symmetrical predators with rostral(head)and caudal that contacts homologous processes from other moto-(tail)ends, and dorsal and ventral surfaces. These neurons. Interestingly, many of these" horizontal" changes in body plan and behavior are accompanied processes between motoneurons transmit information by equally important changes in nervous system orga- in either direction--either motoneuron can transmit nization. Many flatworm neurons are clustered into information to the other via these processes. This is an distinct ganglia interconnected by longitudinal and exception to the functional polarity rule and is medi- transverse axon bundles called nerve cords( Fig. 2.5) ated by reciprocal rather than the more common uni- This condensation of neural elements, or centraliza- directional synapses. Cajal (1909-1911)described tion, allows faster and thus more efficient communica- several examples of neurons that lack a clear axon(in tion between neurons because cellular material is retina, olfactory bulb, and intestine)and called them conserved and conduction times are reduced. The amacrine cells. As an extension of this it is useful to largest, most complex ganglia(cephalic ganglia)are I. NEUROSCIENCI

18 2. BASIC PLAN OF THE NERVOUS SYSTEM I. NEUROSCIENCE variety of stimuli from light, temperature, and a wide range of chemicals and ions, to vibration and other mechanical deformations. Motor Neurons A second stage of differentiation or complexity in hydra’s nervous system was the addition of neurons between sensory and effector. They are defi ned as motor neurons (motoneurons) because they directly innervate effector cells (usually muscle or gland cells), which in turn receive their inputs from sensory neurons (Figs. 2.2B, 2.3C). Conceptually, this provides a two￾layered nervous system: the fi rst or top layer having sensory neurons and the second or bottom layer having motor neurons. In this prototypical network sensory neurons project (send axon collaterals) to multiple motoneurons, and then each motoneuron innervates a set of effector cells (with a motoneuron and its effector cell set defi ned as a motor unit). During an animal’s normal behavior, information fl ow is unidirectional or polarized from one cell type, sensory neuron, to another cell type, motoneuron, to a third cell type, effector. This is the basic defi nition of a simple refl ex, as defi ned by Charles Sherrington in his cornerstone of systems neuroscience, The Integrative Action of the Nervous System (1906). In this hypothetical scenario (Fig. 2.3) an environ￾mental stimulus detected by a sensory neuron’s den￾drite is transmitted by its axon to the dendrites of a motoneuron population. Then the axon of each moto￾neuron innervates an effector cell population. This is the functional polarity rule applied to a simple two-layer, sensory-motor network mediating refl ex behavior. Another general feature of the hydra two-layered nervous system has been observed: sensory neurons do not innervate each other, whereas motoneurons do interact directly. Here, motoneurons have two projec￾tion classes: one to effector cells and another to other motoneurons. Structurally and functionally, many of these hydra motoneurons also have two types of output processes. One is a typical axon innervating an effector cell population. However, the other is a process that contacts homologous processes from other moto￾neurons. Interestingly, many of these “horizontal” processes between motoneurons transmit information in either direction—either motoneuron can transmit information to the other via these processes. This is an exception to the functional polarity rule and is medi￾ated by reciprocal rather than the more common uni￾directional synapses. Cajal (1909–1911) described several examples of neurons that lack a clear axon (in retina, olfactory bulb, and intestine) and called them amacrine cells. As an extension of this it is useful to divide neuronal processes into three types: dendritic (input), axonal (output), or amacrine (bidirectional). Adding a second layer to the nervous system has obvious adaptive advantages related to increased capacity for response complexity and integration. Con￾sider a stimulus to one specifi c part of the animal or even one sensory neuron. Its infl uence may radiate to distant parts of the animal because one sensory neuron innervates multiple motoneurons, those motoneurons innervate additional motoneurons, and each motoneu￾ron innervates multiple effector cells—an example of what Cajal called avalanche conduction. There may be great divergence between stimulus and effector cells producing a response, with the actual divergence pattern shaped by the structure–function architecture of the nervous system: how the neurons and their interconnections are arranged in the body. It is easy to imagine how this arrangement in hydra might coordi￾nate the tentacles to bring a food morsel detected by just one of them to the mouth, or how it might coordi￾nate locomotion (Fig. 2.1). A second basic consequence of this structural arrangement is information convergence in the nervous system. Just consider a particular motoneuron: it can receive inputs from more than one sensory neuron and from other motoneurons as well. Nerve Nets At fi rst glance hydra’s nervous system is distributed fairly uniformly around the radially symmetrical body wall and tentacles (Fig. 2.4). Its essentially double￾layered arrangement of distributed sensory and motor neurons is called a nerve net. However, in certain regions of the body with specialized function, like around the mouth and base of the tentacles, neurons tend to aggregate—a tendency toward centralization that will now be examined more carefully. Bilateral Symmetry, Centralization, and Cephalization Emerge in Flatworms In contrast to cnidarians, fl atworms are bilaterally symmetrical predators with rostral (head) and caudal (tail) ends, and dorsal and ventral surfaces. These changes in body plan and behavior are accompanied by equally important changes in nervous system orga￾nization. Many fl atworm neurons are clustered into distinct ganglia interconnected by longitudinal and transverse axon bundles called nerve cords (Fig. 2.5). This condensation of neural elements, or centraliza￾tion, allows faster and thus more effi cient communica￾tion between neurons because cellular material is conserved and conduction times are reduced. The largest, most complex ganglia (cephalic ganglia) are