EVOLUTION HIGHLIGHTS: GENERAL ORGANIZING PRINCIPLES localized rostrally where they receive information from specialized sensory receptors in the front of the animal as it swims Bilateral symmetry, centralization, and cephalization are three cardinal organizational trends in nervous system evolution Int Flatworms are the simplest animals with an abun dant, clearly distinct third neuron division, internet rons, which are interpolated between sensory and motor neurons(Fig. 2.6). As already noted, Cajal rec- ognized some atypical interneurons that apparently lack distinguishable dendrites and axon (amacrine neurons, or more precisely, amacrine interneurons However, most interneurons have recognizable den drites and axon and so presumably transmit informa tion down the axon in only one direction, toward its terminals. They are typical neurons conforming to the functional polarity rule ne consequence of adding a third "layer"of neurons to the nervous system is simply to increase diffusely throughout the body wall of the animal. This drawing convergence and divergence of information process- shows maturation of the nerve net in a hydra bud, starting near the ing, and thus the capacity for response complexity base and finishing near the tentacles. Refer to McConnell(1932)an There are. however. three other critical functions Koizumi(2002) interneurons subserve. They can act as excitatory or al networks em can a Cephalic ct as pattern detectors and generators ganglia between sensory and motor neurons, and they can be pacemakers if they generate intrinsic rhythmical activity Invertebrate ventral CNs tumulus Transverse nerve cord G Axons FIGURE 2.5 The nervous system of the planarian, a flatworm, tal and transverse nerve cords associated with ntralization, and two fused cephalic ganglia in the rostral end associated with cephalization. Centralization and cephalization probably are related to the flatworm's bilateral symmetry and abilit IllelLLLD swim forward rapidly. Refer to Lentz(1968). Reproduced with FIGURE 2.6 Invertebrate ganglia( G)usually display two neuron permission from Yale University Press classes: motor neurons(m)and interneurons (i), both typically uni- lar, with dendrites arising from a single axon. Here neuronal cell dies are arranged peripherally and synapses occur in a central rion called the innervate motoneurons and interneurons but not effectors(e). Arrows show the usual direction of information flow L NEUROSCIENCE
I. NEUROSCIENCE Tentacles Base localized rostrally where they receive information from specialized sensory receptors in the front of the animal as it swims. Bilateral symmetry, centralization, and cephalization are three cardinal organizational trends in nervous system evolution. Interneurons Flatworms are the simplest animals with an abundant, clearly distinct third neuron division, interneurons, which are interpolated between sensory and motor neurons (Fig. 2.6). As already noted, Cajal recognized some atypical interneurons that apparently lack distinguishable dendrites and axon (amacrine neurons, or more precisely, amacrine interneurons). However, most interneurons have recognizable dendrites and axon and so presumably transmit information down the axon in only one direction, toward its terminals. They are typical neurons conforming to the functional polarity rule. One consequence of adding a third “layer” of neurons to the nervous system is simply to increase convergence and divergence of information processing, and thus the capacity for response complexity. There are, however, three other critical functions interneurons subserve. They can act as excitatory or inhibitory “switches” in neuronal networks, assemblies of them can act as pattern detectors and generators between sensory and motor neurons, and they can be pacemakers if they generate intrinsic rhythmical activity patterns. FIGURE 2.4 The nerve net of hydra, a simple cnidarian, is spread diffusely throughout the body wall of the animal. This drawing shows maturation of the nerve net in a hydra bud, starting near the base and fi nishing near the tentacles. Refer to McConnell (1932) and Koizumi (2002). Cephalic ganglia Longitudinal nerve cord Transverse nerve cord FIGURE 2.5 The nervous system of the planarian, a fl atworm, includes longitudinal and transverse nerve cords associated with centralization, and two fused cephalic ganglia in the rostral end associated with cephalization. Centralization and cephalization probably are related to the fl atworm’s bilateral symmetry and ability to swim forward rapidly. Refer to Lentz (1968). Reproduced with permission from Yale University Press. Invertebrate ventral CNS G G stimulus s m m i e e e e Axons Dendrites FIGURE 2.6 Invertebrate ganglia (G) usually display two neuron classes: motor neurons (m) and interneurons (i), both typically unipolar, with dendrites arising from a single axon. Here neuronal cell bodies are arranged peripherally and synapses occur in a central region called the neuropil. Sensory neurons (s) usually innervate motoneurons and interneurons but not effectors (e). Arrows show the usual direction of information fl ow. EVOLUTION HIGHLIGHTS: GENERAL ORGANIZING PRINCIPLES 19
2. BASIC PLAN OF THE NERVOUS SYSTEM BOX 2.2 CAJAL: ICONOCLAST TO ICON Santiago Ramon y Cajal (1852-1934)is considered by and are thus structurally independent units, which was many people to be the founder of modern neurosci- finally proven when the electron microscope was used in ence-a peer of Darwin and Pasteur in nineteenth- the 1950s. This concept became known as the neuron century biology. He was born in the tiny Spanish village doctrine of Petilla de Aragon on May 1, 1852, and as related in Cajal's second major conceptual achievement was the his delightful autobiography, he was somewhat mischie- theory of functional polarity, which stated that the den vous as a child and determined to become an artist, rites and cell bodies of neurons receive information much to the consternation of his father, a respected local whereas the single axon with its collaterals transmits physician. However, he eventually entered the Univer- information to the other cells. This rule allows prediction sity of Zaragoza and received a medical degree in 1873. of information flow direction through neural circuits As a professor of anatomy at Zaragoza his interests were based on the morphology or shape of individual neurons mostly in bacteriology( the nineteenth-century equiva- forming them, and it was the cornerstone of Charles lent of molecular biology today in terms of an exciting Sherrington,'s(1906)revolutionary physiological analysis biological frontier)until 1887, when he visited Madrid at of mammalian reflex organization. Recent evidence that age 35 and first saw through the microscope histological many dendrites transmit an action potential or grad sections of brain tissue treated with the Golgi method, potential in the retrograde direction would not violate the which had been introduced in 1873. Although very few tenants of the functional polarity theory unless the poten- workers had used this technique, Cajal saw immediately tial led to altered membrane potentials in the associated that it offered great hope in solving the most vexing presynaptic axon--and if this were the case the"den- problem of nineteenth-century neuroscience: how do drite"would be classed instead as an amacrine prod nerve cell interact with each other? This realization gal-(see text) vanized and directed the rest of his scientific life, which Around the close of the nineteenth century, Cajal made was extremely productive in terms of originality, scope, a remarkable series of discoveries at the cellular level. In and accuracy. addition to the two concepts outlined earlier, they include hortly after Jacob of axon termination in the adult CNS(1888) Rudolf virchow proposed the cell theory in the late 1830s, (2)the dendritic spine(1888),(3)the first diagrams of Joseph von Gerlach, Sr and Otto Deiters suggested that reflex pathways based on the neuron doctrine and func- nerve tissue was special in the sense that nerve cells are tional polarity(1890),(4)the axonal growth cone(1890), not independent units but instead form a continuous syn-(5)the chemotactic theory of synapse specificity(1892). cytium or reticular net(Fig. 2.2A). This concept was later and (6)the hypothesis that learning could be based on the refined by Camillo Golgi who, based on the use of his selective strengthening of synapses(1895) silver chromate method, concluded that axons of nerve In one of the great ironies in the history of neurosci- cells form a continuous reticular net, whereas in contrast ence, Cajal and Golgi shared the Nobel Prize for Medicine dendrites do not anastomose but instead serve a nutritive in 1906 though they had used the same technique to elab- role, much like the roots of a tree. Using the same tech- orate fundamentally different views on nervous system nique, Cajal almost immediately arrived at the opposite organization! The meeting in Stockholm may not have conclusion, based first on his examination of the cerebel- diminished the great personal friction between them. I lum, and later of virtually all other parts of the nervous 1931 Cajal wrote: " What a cruel irony of fate to pair like system. In short, he proposed that neurons interact by Siamese twins united by the shoulders, scientific adver- way of contact or contiguity rather than by continuity, saries of such contrasting characters. bar y this definition the vast majority of vertebrate projection interneurons send a longer axon to a different in neurons are interneurons. So it is useful at the gray matter region or ganglion, although it may also outset to recognize two broad interneuron categories generate local axon collaterals local and projection. Local interneurons, or local circuit The omnidirectional information flow typical of cni- neurons, have an axon that remains confined to a dis- darian nerve nets is unusual in the rest of the animal tinguishable gray matter region or ganglion, whereas kingdom, where most neurons are functionally polar- I. NEUROSCIENCI
20 2. BASIC PLAN OF THE NERVOUS SYSTEM I. NEUROSCIENCE By this defi nition the vast majority of vertebrate brain neurons are interneurons. So it is useful at the outset to recognize two broad interneuron categories: local and projection. Local interneurons, or local circuit neurons, have an axon that remains confi ned to a distinguishable gray matter region or ganglion, whereas projection interneurons send a longer axon to a different gray matter region or ganglion, although it may also generate local axon collaterals. The omnidirectional information fl ow typical of cnidarian nerve nets is unusual in the rest of the animal kingdom, where most neurons are functionally polarSantiago Ramón y Cajal (1852–1934) is considered by many people to be the founder of modern neuroscience—a peer of Darwin and Pasteur in nineteenthcentury biology. He was born in the tiny Spanish village of Petilla de Aragon on May 1, 1852, and as related in his delightful autobiography, he was somewhat mischievous as a child and determined to become an artist, much to the consternation of his father, a respected local physician. However, he eventually entered the University of Zaragoza and received a medical degree in 1873. As a professor of anatomy at Zaragoza his interests were mostly in bacteriology (the nineteenth-century equivalent of molecular biology today in terms of an exciting biological frontier) until 1887, when he visited Madrid at age 35 and fi rst saw through the microscope histological sections of brain tissue treated with the Golgi method, which had been introduced in 1873. Although very few workers had used this technique, Cajal saw immediately that it offered great hope in solving the most vexing problem of nineteenth-century neuroscience: how do nerve cell interact with each other? This realization galvanized and directed the rest of his scientifi c life, which was extremely productive in terms of originality, scope, and accuracy. Shortly after Jacob Schleiden, Theodor Schwann, and Rudolf Virchow proposed the cell theory in the late 1830s, Joseph von Gerlach, Sr. and Otto Deiters suggested that nerve tissue was special in the sense that nerve cells are not independent units but instead form a continuous syncytium or reticular net (Fig. 2.2A). This concept was later refi ned by Camillo Golgi who, based on the use of his silver chromate method, concluded that axons of nerve cells form a continuous reticular net, whereas in contrast dendrites do not anastomose but instead serve a nutritive role, much like the roots of a tree. Using the same technique, Cajal almost immediately arrived at the opposite conclusion, based fi rst on his examination of the cerebellum, and later of virtually all other parts of the nervous system. In short, he proposed that neurons interact by way of contact or contiguity rather than by continuity, and are thus structurally independent units, which was fi nally proven when the electron microscope was used in the 1950s. This concept became known as the neuron doctrine. Cajal’s second major conceptual achievement was the theory of functional polarity, which stated that the dendrites and cell bodies of neurons receive information, whereas the single axon with its collaterals transmits information to the other cells. This rule allows prediction of information fl ow direction through neural circuits based on the morphology or shape of individual neurons forming them, and it was the cornerstone of Charles Sherrington’s (1906) revolutionary physiological analysis of mammalian refl ex organization. Recent evidence that many dendrites transmit an action potential or graded potential in the retrograde direction would not violate the tenants of the functional polarity theory unless the potential led to altered membrane potentials in the associated presynaptic axon—and if this were the case the “dendrite” would be classed instead as an amacrine process (see text). Around the close of the nineteenth century, Cajal made a remarkable series of discoveries at the cellular level. In addition to the two concepts outlined earlier, they include (1) the mode of axon termination in the adult CNS (1888), (2) the dendritic spine (1888), (3) the fi rst diagrams of refl ex pathways based on the neuron doctrine and functional polarity (1890), (4) the axonal growth cone (1890), (5) the chemotactic theory of synapse specifi city (1892), and (6) the hypothesis that learning could be based on the selective strengthening of synapses (1895). In one of the great ironies in the history of neuroscience, Cajal and Golgi shared the Nobel Prize for Medicine in 1906 though they had used the same technique to elaborate fundamentally different views on nervous system organization! The meeting in Stockholm may not have diminished the great personal friction between them. In 1931 Cajal wrote: “What a cruel irony of fate to pair like Siamese twins united by the shoulders, scientifi c adversaries of such contrasting characters.” BOX 2.2 CAJAL: ICONOCLAST TO ICON
EVOLUTION HIGHLIGHTS: GENERAL ORGANIZING PRINCIPLES ized with information flowing through neural circuits ental nerves sequentially from dendrites and cell body to axon and axon terminals. However most invertebrate motoneu erebral ganglia rons and interneurons are unipolar: a single process, the axon, extends from the cell body. Dendrites branch from the axons in the center of a ganglion-entering the neuropil-where most synapses are formed(Fig 2.6). In vertebrates most neurons are multipolar, with several dendrites, plus an axon extending from the cell body or a dendrite. Features of simple nervous systems are preserved throughout evolution. For example, the part of nervous system in the wall of the human gastrointestinal tract (the enteric nervous system) has many features of a Ventral nerve cord highly refined nerve net, and a"layer"of amacrine interneurons is found in the human retina and olfac- FIGURE 2.7 Nervous system organization in the rostral end of an annelid worm a ventral ord with more or less distinct tory bulb ganglia connects with a fused pair of cerebral ganglia(brain)dorsal to the pharynx(part of the digestive tract). Note nerves extending A Segmented Ventral Nerve Cord Typifies from ventral nerve cord and cerebral ganglia. Refer to Brusca and Brusca(1990) Annelids and Arthropod Annelid worms and arthropods have even more bral column and bony skull, with the notochord complex body plans and behaviors than flatworms, reduced to a series of cartilaginous cushions(discs) partly because of segmentation. Body segments between or within the vertebrae. The vertebrate nerve (metameres)are repeated serially along the body's cord is tremendously expanded, thickened, and folded rostrocaudal axis, and presumably share a common to form the brain and spinal cord(the central nervous underlying genetic developmental program, although system, CNS terminal differentiation(adult structure) may vary The vertebrate nervous system's basic parts are This strategy allows for more complex body plans revealed in the lancelet(amphioxus), a simple, nonver- (including the nervous system)to evolve without a tebrate chordate(subphylum Cephalochordata). The linear or exponential increase in genetic material. lancelet is a slender, fish-like filter-feeder living half Annelids and all the more complex invertebrates buried in the sand of shallow, tropical marine waters hare another characteristic feature, a ventral nerv (Fig. 2.9). The body is stiffened by a notochord, and cord with a pair of ganglia(or a single fused ganglion) dorsal nerve cord runs the length of the body, generat each segment, and longitudinal axon bundles ing segmental nerves innervating muscles and organs. between ganglia in adjacent segments(Fig. 2.7). Locomotor behavior(swimming) is produced by alter Transverse nerves also extend from each ganglion nately contracting right and left segmental muscles to sensory structures and muscles in the same (myotomes). Without a notochord these contraction segment. would shorten the animal rather than generate forward propulsive force The Basic Plan of the vertebrate nervous Ithough typical vertebrate brain regions are not System Is Found in lancelets obvious rostrally in the lancelet nerve cord,genes specifying early vertebrate head embryogenesis also Vertebrates are a subphylum of the Chordates and are expressed rostrally in the lancelet body. Thus, are the most complex of all animals in terms of struc some components of the molecular program specify ture and behavior. They share a basic body plan where ing modern vertebrate head development apparently common organ systems are arranged in a relatively were present early in chordate evolution(Holland and trict anatomical relationship with one another(Box Takahashi, 2005) 2.3 and Figs. 2.8, 2.9). Like other chordates, vertebrates display two key features during some part of their life: a cartilaginous rod, the notochord, extending dorsally along the body, and above it a hollow dorsal nerve cord The cniderian nerve net displays most of the basic In most vertebrates the notochord's body stiffening cellular features of nervous system organization, and protective functions are supplanted by the verte- including convergence and divergence of sensory and L NEUROSCIENCE
I. NEUROSCIENCE ized with information fl owing through neural circuits sequentially from dendrites and cell body to axon and axon terminals. However, most invertebrate motoneurons and interneurons are unipolar: a single process, the axon, extends from the cell body. Dendrites branch from the axons in the center of a ganglion—entering the neuropil—where most synapses are formed (Fig. 2.6). In vertebrates most neurons are multipolar, with several dendrites, plus an axon extending from the cell body or a dendrite. Features of simple nervous systems are preserved throughout evolution. For example, the part of nervous system in the wall of the human gastrointestinal tract (the enteric nervous system) has many features of a highly refi ned nerve net, and a “layer” of amacrine interneurons is found in the human retina and olfactory bulb. A Segmented Ventral Nerve Cord Typifi es Annelids and Arthropods Annelid worms and arthropods have even more complex body plans and behaviors than fl atworms, partly because of segmentation. Body segments (metameres) are repeated serially along the body’s rostrocaudal axis, and presumably share a common underlying genetic developmental program, although terminal differentiation (adult structure) may vary. This strategy allows for more complex body plans (including the nervous system) to evolve without a linear or exponential increase in genetic material. Annelids and all the more complex invertebrates share another characteristic feature, a ventral nerve cord with a pair of ganglia (or a single fused ganglion) in each segment, and longitudinal axon bundles between ganglia in adjacent segments (Fig. 2.7). Transverse nerves also extend from each ganglion to sensory structures and muscles in the same segment. The Basic Plan of the Vertebrate Nervous System Is Found in Lancelets Vertebrates are a subphylum of the Chordates and are the most complex of all animals in terms of structure and behavior. They share a basic body plan where common organ systems are arranged in a relatively strict anatomical relationship with one another (Box 2.3 and Figs. 2.8, 2.9). Like other chordates, vertebrates display two key features during some part of their life: a cartilaginous rod, the notochord, extending dorsally along the body, and above it a hollow dorsal nerve cord. In most vertebrates the notochord’s body stiffening and protective functions are supplanted by the vertebral column and bony skull, with the notochord reduced to a series of cartilaginous cushions (discs) between or within the vertebrae. The vertebrate nerve cord is tremendously expanded, thickened, and folded to form the brain and spinal cord (the central nervous system, CNS). The vertebrate nervous system’s basic parts are revealed in the lancelet (amphioxus), a simple, nonvertebrate chordate (subphylum Cephalochordata). The lancelet is a slender, fi sh-like fi lter-feeder living half buried in the sand of shallow, tropical marine waters (Fig. 2.9). The body is stiffened by a notochord, and a dorsal nerve cord runs the length of the body, generating segmental nerves innervating muscles and organs. Locomotor behavior (swimming) is produced by alternately contracting right and left segmental muscles (myotomes). Without a notochord these contractions would shorten the animal rather than generate forward propulsive force. Although typical vertebrate brain regions are not obvious rostrally in the lancelet nerve cord, genes specifying early vertebrate head embryogenesis also are expressed rostrally in the lancelet body. Thus, some components of the molecular program specifying modern vertebrate head development apparently were present early in chordate evolution (Holland and Takahashi, 2005). Summary The cniderian nerve net displays most of the basic cellular features of nervous system organization, including convergence and divergence of sensory and Pharynx Mouth Ventral nerve cord Segmental nerves Cerebral ganglia FIGURE 2.7 Nervous system organization in the rostral end of an annelid worm. A ventral nerve cord with more or less distinct ganglia connects with a fused pair of cerebral ganglia (brain) dorsal to the pharynx (part of the digestive tract). Note nerves extending from ventral nerve cord and cerebral ganglia. Refer to Brusca and Brusca (1990). EVOLUTION HIGHLIGHTS: GENERAL ORGANIZING PRINCIPLES 21
2. BASIC PLAN OF THE NERVOUS SYSTEM BOX 2.3 ANATOMICAL RELATIONSHIPS IN THE VERTEBRATE BODY To describe the physical relationships between struc- however, this is rarely the case, which leads to a certain tures in the nervous system and the rest of the vertebrate degree of ambiguity, as is obvious when looking at the body it is best to use terms that accurately and unambigu- fish, frog, cat, and human bodies shown in Fig. 2.8. ously describe the position of a given structure in three The problem is especially difficult in human anatomy dimensions. The major axis of the body is the rostrocaudal where use of an idiosyncratic terminology has a long axis, which extends along the length of the animal from ingrained tradition. The basic principles are much easier the rostrum(beak) to the cauda(tail)(Fig. 2. 8)as well as the to illustrate than to describe in writing(see Fig. 2.8),but length of the embryonic neural plate and neural tube(see one major source of confusion in the human brain is Figs. 2.10 and 2.12). A second axis, the orthogonal dorso- related to the fact that the rostrocaudal axis makes a 90 ventral axis, is vertical and runs from the dorsum(back)to degree bend in the midbrain region(unlike in rodents and the ventrum(belly). Finally, the third perpendicular axis, carnivores, for example, where the axis is relatively the mediolateral axis, is horizontal and runs from the straight). The other source of confusion is simply the dif- midline(medial)to the lateral margin of the animal ferent names that are used. For example, in human (lateral). Unfortunately, the rostrocaudal axis undergoes anatomy the spinal cord has anterior and posterior horns, complex bending during embryogenesis, and the bending and posterior root ganglia, whereas in other mammals pattern is unique to each species. It would be ideal if the they usually are referred to as ventral and dorsal horns, three cardinal axes were used in a topologically accurate and dorsal root ganglia. The merits of a uniformly applied way, say with reference to the body as it might appear nomenclature based on comparative structural principles with a"straightened out"rostrocaudal axis. In practice, seem obvious motor information. In more complex bilaterally sym- graphic neuroanatomical nomenclature(Swanson metrical invertebrates, neurons and axons tend to 2000a) aggregate in ganglia, nerve cords, and nerves(central ization), and there is a greater concentration of neurons Nervous System Regionalization Begins in the and sensory organs in the bodys rostral end (cephali- Neural Plate zation). Segmented invertebrates have a ventral nerve cord that includes a bilateral pair of ganglia(or single During embryogenesis the CNs develops as a fused ganglion) in each segment. The Primitive chor- hollow cylinder (neural tube)from a topologically flat date, lancelet, displays the basic nervous system sheet of cells(neural plate), by a process of neurulation organization characteristic of vertebrates, including Chapter 14). Here we simply consider macroscopic mammals and humans structural changes during the transformation. The neural plate is a spoon-shaped differentiation of the trilaminar embryonic disc's one-cell-thick ecto- DEVELOPMENT REVEALS BASIC dermal layer(Fig. 2.10). Its wide end lies rostrally and VERTEBRATE PARTS becomes the brain whereas the narrow end lies cau- dally and becomes the spinal cord-the two major CNS divisions. A midline neural groove divides the One nineteenth century biology triumph was the neural plate into right and left halves, so the plate dis demonstration that early stages of embryogenesis plays three cardinal morphogenetic features: polarity, are fundamentally the same in all vertebrates. The bilateral symmetry, and regionalization. Furthermore, the CNS and heart are the first organs to differentiate neural plate differentiates from rostral to caudal, so the in the embryo and the basic cns divisions differen- brain plate regionalizes first. Signs of this include tiating early in development are also common to appearance of the optic vesicles, evaginating near the all vertebrates. The names and arrangement of these rostral end of the neural plate (in the presumptive divisions are the starting point for regional or topo- hypothalamus); a midline infundibulum evaginating I. NEUROSCIENCI
22 2. BASIC PLAN OF THE NERVOUS SYSTEM I. NEUROSCIENCE motor information. In more complex bilaterally symmetrical invertebrates, neurons and axons tend to aggregate in ganglia, nerve cords, and nerves (centralization), and there is a greater concentration of neurons and sensory organs in the body’s rostral end (cephalization). Segmented invertebrates have a ventral nerve cord that includes a bilateral pair of ganglia (or single fused ganglion) in each segment. The primitive chordate, lancelet, displays the basic nervous system organization characteristic of vertebrates, including mammals and humans. DEVELOPMENT REVEALS BASIC VERTEBRATE PARTS One nineteenth century biology triumph was the demonstration that early stages of embryogenesis are fundamentally the same in all vertebrates. The CNS and heart are the fi rst organs to differentiate in the embryo, and the basic CNS divisions differentiating early in development are also common to all vertebrates. The names and arrangement of these divisions are the starting point for regional or topographic neuroanatomical nomenclature (Swanson, 2000a). Nervous System Regionalization Begins in the Neural Plate During embryogenesis the CNS develops as a hollow cylinder (neural tube) from a topologically fl at sheet of cells (neural plate), by a process of neurulation (Chapter 14). Here we simply consider macroscopic structural changes during the transformation. The neural plate is a spoon-shaped differentiation of the trilaminar embryonic disc’s one-cell-thick ectodermal layer (Fig. 2.10). Its wide end lies rostrally and becomes the brain, whereas the narrow end lies caudally and becomes the spinal cord—the two major CNS divisions. A midline neural groove divides the neural plate into right and left halves, so the plate displays three cardinal morphogenetic features: polarity, bilateral symmetry, and regionalization. Furthermore, the neural plate differentiates from rostral to caudal, so the brain plate regionalizes fi rst. Signs of this include appearance of the optic vesicles, evaginating near the rostral end of the neural plate (in the presumptive hypothalamus); a midline infundibulum evaginating To describe the physical relationships between structures in the nervous system and the rest of the vertebrate body it is best to use terms that accurately and unambiguously describe the position of a given structure in three dimensions. The major axis of the body is the rostrocaudal axis, which extends along the length of the animal from the rostrum (beak) to the cauda (tail) (Fig. 2.8) as well as the length of the embryonic neural plate and neural tube (see Figs. 2.10 and 2.12). A second axis, the orthogonal dorsoventral axis, is vertical and runs from the dorsum (back) to the ventrum (belly). Finally, the third perpendicular axis, the mediolateral axis, is horizontal and runs from the midline (medial) to the lateral margin of the animal (lateral). Unfortunately, the rostrocaudal axis undergoes complex bending during embryogenesis, and the bending pattern is unique to each species. It would be ideal if the three cardinal axes were used in a topologically accurate way, say with reference to the body as it might appear with a “straightened out” rostrocaudal axis. In practice, however, this is rarely the case, which leads to a certain degree of ambiguity, as is obvious when looking at the fi sh, frog, cat, and human bodies shown in Fig. 2.8. The problem is especially diffi cult in human anatomy where use of an idiosyncratic terminology has a long, ingrained tradition. The basic principles are much easier to illustrate than to describe in writing (see Fig. 2.8), but one major source of confusion in the human brain is related to the fact that the rostrocaudal axis makes a 90 degree bend in the midbrain region (unlike in rodents and carnivores, for example, where the axis is relatively straight). The other source of confusion is simply the different names that are used. For example, in human anatomy the spinal cord has anterior and posterior horns, and posterior root ganglia, whereas in other mammals they usually are referred to as ventral and dorsal horns, and dorsal root ganglia. The merits of a uniformly applied nomenclature based on comparative structural principles seem obvious. BOX 2.3 ANATOMICAL RELATIONSHIPS IN THE VERTEBRATE BODY
DEVELOPMENT REVEALS BASIC VERTEBRATE PARTS Rostral Cauda Transverse plane nferior Frontal (transverse) Dorsal Horizontal section Anterior Rostral ostra Caudad-> FIGURE 2.8 Orientation of the vertebrate body Orientation planes for fish, quadrupeds, and bipeds are depicted. Associated with the thre cardinal planes(rostrocaudal, dorsoventral, and mediolateral) are three orthogonal planes: horizontal, sagittal, and transverse (or frontal). which are the same in all early vertebrate embryos. For more explanation, see Williams(1995). L NEUROSCIENCE
I. NEUROSCIENCE Dorsal Dorsal Posterior Ventral Ventral Anterior Caudal Caudal Posterior Rostral Rostral Anterior Superior Inferior Sagittal Horizontal Frontal plane Transverse plane Midsagittal plane Medial Lateral Frontal or transverse Horizontal section Frontal (transverse) Sagittal section <—Rostrad Caudad—> FIGURE 2.8 Orientation of the vertebrate body. Orientation planes for fi sh, quadrupeds, and bipeds are depicted. Associated with the three cardinal planes (rostrocaudal, dorsoventral, and mediolateral) are three orthogonal planes: horizontal, sagittal, and transverse (or frontal), which are the same in all early vertebrate embryos. For more explanation, see Williams (1995). DEVELOPMENT REVEALS BASIC VERTEBRATE PARTS 23