- Email: [email protected]
Arthropod neurons and nervous system
Current Biology
Magazine Primer
Arthropod neurons and nervous system Carmen Ramona SmarandacheWellmann Arthropods are very diverse, come in many different forms with diverse adaptations, and through such diversity have populated all environmental niches on the planet. Almost 80% of the animals on planet Earth belong to this phylum. Despite their very diverse phenotypes they share fundamental similarities which were previously used to generate a phylogenetic tree. All arthropods have segmented bodies and possess jointed limbs at all or many of their body segments. An additional common feature of arthropods is their exoskeleton, made mainly of chitin and/ or sclerotin. As in a vertebrate, the nervous system of an arthropod consists of specialized cells, the neurons. The central nervous system of arthropods is segmented and can be roughly divided into the brain, located in the head at the anterior end, and the ventral nerve cord spanning from the head to the caudal end, the abdomen (Figure 1). Because of the huge morphological diversity in this phylum, it is difficult to derive generalizations. Not all features described here are found in all families of this group (please see the papers mentioned in the Further reading list for more detailed information). Still, the nervous systems of arthropods share many common features: these are often points of similarity to vertebrate nervous systems, but sometimes also points of fundamental difference. Evolutionary relationships Arthropods are divided in five main subphyla. All specimens of the subphylum of Trilobitomorpha are extinct, so I will exclude them from this description. The other four subphyla are: Chelicerata, including Arachnida (scorpions and spiders); Myriapoda, including Chilopoda (centipedes) and Diplopoda (millipedes); Crustacea, including Malacostraca (including lobster, crabs, shrimp); and Hexapoda, with Insecta (including dragonflies, R960
bees, flies) and Entognatha (spring tails). In the last decades, there have been a number of disputes about the evolutionary relationships between these subphyla and the different classes in each phylum. New methods, including genetic and molecular analyses, have helped to elucidate the phylogenies. In the Pancrustacean hypothesis, the Chelicerata and Myriapoda divide first and are not as closely related to the other two groups. Crustacea and Insecta are closely related; it seems that insects evolved from the crustacean lineage. This result is mainly based on mitochondrial gene sequences and embryogeny. The close relationships might explain why neurobiological features from these two groups exhibit remarkable similarities. Development of the arthropod nervous system The anatomy of the arthropod central nervous system is determined during embryonic development. Neuroblasts, progenitor cells of neurons, are placed at specific regions in each body segment across the rostro-caudal axis of the nerve cord and give rise to the ganglia. Within a given species, and also across phyla, there are similarities in respect to the number of neurons, and formation of tracts, neuropils, and commissures (Figure 2). The precise segmentation of arthropods is rooted in its genetic configuration. One gene family, the homeobox genes (Hox genes), is responsible for the rostro-caudal axis during the regional development in an embryo. They were first identified in Drosophila, and have now been characterized in vertebrates and in representatives of all major animal classes, including Arthropoda and Annelida. In Arthropoda (Drosophila), eight genes exist for the rostro-caudal axis formation and segmentation at specific locations on a single chromosome. The first three Hox genes on the chromosome encode specific transcription factors mediating the formation of the anterior part of the animal with three brain regions. The following four specify the different body segments across the longitudinal axis, and the last Hox gene specifies the abdominal segments. Mutations of Hox genes result in severe developmental
Current Biology 26, R937–R980, October 24, 2016 © 2016 Elsevier Ltd.
defects in anterior-posterior formation, segmental formation and body polarity. Hox genes are extremely conserved throughout the animal kingdom. In vertebrates, Hox genes have been duplicated and are found on four different chromosomes with slight variations. Even though segmentation in vertebrates is not as visible as in arthropods, the Hox genes still play a crucial role in the development of the different CNS portions and body. In a similar manner to arthropod segmentation, the first three genes are responsible for the development of the three brain regions: prosencephalon, mesencephalon and rhombencephalon. The last gene specifies the caudal region of the spinal cord. One of the major differences between arthropods and vertebrates is the position of the nerve cord. In arthropods, the neuronal cord is located ventrally, in vertebrates dorsally. But it seems like this is not such a big difference because ventral in arthropods seems to be equivalent to dorsal in vertebrates. The same genetic factors, molecules, and embryological features are found in vertebrates and invertebrates, as they derive from the same ancestor. The inversion of the dorso-ventral body axis comes about as a result of gastrulation during embryonic development. Arthropods are protostomes; their mouth develops from the initial invagination side (the blastopore); vertebrates are deuterostomes, and their mouth develops secondarily. Because of this invagination, the neuroblasts in invertebrates turn ventrally, while the neural tube (progenitor cells) in vertebrates is located dorsally. Signaling gradients of the morphogen Bone Morphogenetic Protein (BMP) are necessary for formation of the dorsoventral axis in all bilateral animals, both for the invagination as well as patterning of the CNS. The BMP4 gene is a dorsal identity marker, encoding a transcription factor primarily found at the peripheral part of the embryo, while the gene Shh (Sonic hedgehog) encodes a transcription factor found ventrally. The BMP4 and Shh proteins influence cell formation in the vertebrate spinal cord and the arthropod ventral cord. To sum up: the Hox genes are responsible for developmental specification along the anterior-posterior
Current Biology
Magazine axis, and dorso-ventral organization involves a gradient of two different transcription factors encoded by the genes BMP4 and Shh. In arthropods and vertebrates, axonal growth is guided by proteins encoded by the same set of genes, Netrins and Slits. Thus, although interesting the nervous system of an arthropod ends up being ventral and that of a vertebrate ends up being dorsal, the genetic regulation of their development shows fundamental similarities. Neuroanatomy The blueprint of the arthropod CNS consists of a dorsal cephalic ganglion, the ‘brain’, followed by a chain of ventral ganglia, the ventral cord (Figure 1). In each metameric segment, bilateral ganglia are fused at the midline and their lateral nerves extend into the body segment and appendages (Figure 1A). The segmental ganglia are connected to each other through axon bundles, or connectives. During larval development in crustaceans, the bilateral symmetric ganglia are not completely fused at the midline and a ladder-like structure, as in annelids, is visible. In each segment, axons cross the midline in commissures to connect the hemiganglia. In all other arthropods, the ganglia are tightly fused at the midline, while commissures are still present but hidden in the layer of the ganglion (Figure 2). One characteristic which is clearly visible in most arthropods is the heavily segmented body. In evolutionarily older phylae, like myriapods, each segment is similar (metameric) to the others and carries pairs of appendages (one or two on each side); only head and tail segments are specialized. The body segmentation in other arthropods is more diverse. There is a clear distinction between thorax and abdomen. The appendages at the thorax level are the walking legs, or periopods, and in insects even wings. The abdomen’s morphology can be even more distinct: it can be either equipped with specialized appendages, known as pleopods, as seen in long tailed crustaceans, or without appendages (insects), or even compressed (crabs, spiders). The segmentation is preserved internally. The brain of an arthropod lies in the head (dorsally) and consists of three pairs of ganglia: the protocerebrum
Ci
Bi
A
b
b
ch
Di
pp
b
b
wl1 SOG wl2 TG
wl3 wl4
SOG
TG
AG
AG
Cii
b AG
Bii b
Dii wl1
chn ppn
wl1
b
wl1 wl2
wl2
wl3
wl3
wl2 wl3
wl4 wl4 Current Biology
Figure 1. Morphology of the arthropod CNS. Schematic of arthropod CNS: (A) Myriapoda; (B) Insecta; (C) Crustacea; (D) Chelicerata. The brain (b) is located anteriorly in the head. The ventral nerve cord is a chain of segmental ganglia, some compressed. (A) In a centipede, ganglia are metameric and connected via the paired connectives. The ventral nerve cord of some insects, such as dragonflies (Bi), and crustaceans, such as crayfish (Ci), are spatially distributed; only the subesophageal ganglion (SOG) and the terminal ganglion are compressed. The CNS of evolutionarily younger insects, such as fruit flies (Bii), and of brachyurian crabs (Cii) is compressed. In insects the brain is fused to the SOG; and the thoracic ganglia form with the abdominal ganglia a thoracic mass. In crabs the subesophageal, the thoracic and the abdominal ganglia are compressed into a subesophageal mass, the thoracic ganglion. (D) Chelicerate CNS is strongly compressed. The first three neuromers form the brain, or syncerebrum. It lies dorsal to the fused subesophageal and thoracic segments and form together the synganglion. (Di) The abdominal ganglia of a scorpion are spatially distributed. (Dii) In arachnids, the abdominal segments are compressed into the synganglion. Abbreviations: b, brain, when fused with the following segments and positioned above of others shown in grey; SOG, subesophageal ganglion; TG, thoracic ganglion; AG, abdominal ganglion; ch, cheliceral; pp, pedipalp; wl 1–4, walking leg nerves 1–4. Figures adapted from: (Bi) Wendler (1999) with permission of Springer, (Ci) Storch and Welsch (2009), with permission of Springer; (Cii) Thomson (1916); (Di) Preprinted by permission from Macmillan Publishers Ltd: Nature Tanaka et al. (2013); (Dii) Adapted from Lehmann et al. (2016) by permission of Oxford University Press.
(which innervates structures such as the eyes, controlling vision); the
deutocerebrum (which innervates structures such as the antennae,
Current Biology 26, R937–R980, October 24, 2016
R961
Current Biology
Magazine Insect ganglia
Crustacean ganglia MDT LDT
Ai
Aii MG
Transversal
DMT DIT VLT mcN VIT hN d
LG
VMT MVT
IVAC
VLT
HN
B
B
vcN
MDT LDT DMT DIT DLT VIT VMT MVT
A A
LVT
v
DC1 DC3 DC4 DC6 DC2 DC5
Bi
DC1 DC4 DC2 DC3 DC5 DC6 DC7
Bii MDT DMT
MDT
C
C
Sagittal
DMT VMT MVT
dC7
VIT
d IVAC
p
a
PVC SVC
HN
AVC
PVC
DMT
p
LDT
Frontal
a
DC1 DC2
Cii
DC3
LDT
DIT
Ci
DMT DIT
v
DC1 DC2
DC4
DC3
DIT
DC4 DC5
DC6
Current Biology
Figure 2. Ganglia in insects and crustaceans. Schematics of three pairs of orthogonal sections through: (i) a thoracic ganglion of an insect; and (ii) an abdominal ganglion of a crustacean. The basic structure is strongly preserved across species and across homologous segments. Tracts of axons run rostro-caudally through the ganglia, while commissures connect the bilateral ganglia. Neuropils are the center for synaptic interaction. The color coding is preserved across the figure. (A) Transversal section at the height of the first nerve root. The tracts are layered and separated by commissures. (B) Sagittal section through the ganglia at the height of the medial dorsal tract. The numbering of the dorsal commissures is different in the two species, but they are layered in a similar fashion. (C) Frontal section in the dorsal part of the ganglia showing the tracts, and commissures in the two species. Abbreviations: MDT, medial dorsal tract; LDT, lateral dorsal tract; DMT, dorsal medial tract; DIT, dorsal intermediate tract; DLT, dorsal lateral tract; VIT, ventral intermediate tract; VMT, ventral medial tract; VLT, ventral lateral tract; MVT, medial ventral tract; LVT, lateral ventral tract. Commissures: DC 1–7, dorsal commissure 1–7; AVC, anterior ventral commissure; PVC, posterior ventral commissure. Neuropils: hN/HN, horseshoe neuropil; lVAC, lateral part of ventral association center; mcN, medial coarse neuropil; vcN, ventral coarse neuropil; LN, lateral neuropil. Giant axons: MG, medial giant; LG, lateral giant. Figures adapted from: (Ai) Goldammer et al. (2012); (Bi) after Kittman et al. (1991); (Ci) after Gregory (1974); (Aii,Bii,Cii) Elson (1996) and Skinner (1985).
controlling chemosensation and tactile sensation); and the tritocerebrum (which, for example, integrates sensory information from protocerebrum and deutocerebrum). The brain’s main function is to assimilate sensory information, process it, select the appropriate behavioral program, and send it downstream. Homologies between the brains of different arthropod classes can provide R962
interesting insights informative for multiple types of investigation, and comparative studies are looking at structural, developmental and physiological aspects of brains of representative species in different arthropod subclasses (see the papers mentioned in the Further reading list for more details). The first of the ventrally located ganglia, the subesophageal ganglion,
Current Biology 26, R937–R980, October 24, 2016
also integrates sensory information and innervates the neck and mouthparts. In many species, this ganglion is built by a chain of compressed ganglia. Following the subesophageal ganglion are the thoracic ganglia, the abdominal ganglia, and the specialized terminal ganglion (Figure 1). Thoracic ganglia are morphologically very much alike, as are the abdominal ganglia; both arise from homologous neuromers and they are also very similar to each other. Ventral ganglia contain the neurons necessary for the movement of appendages. This includes pattern generating interneurons and motor neurons which innervate the muscles. The thoracic ganglia innervate legs and wings (walking and flying, respectively), while the abdominal ganglia innervate the pleopods (swimming, ventilation), the reproductive organs, and the anus. Sensory information from the body appendages, like proprioception and hearing, is processed in its own segment and sent to neighboring ganglia. There are some arthropods with longitudinal compression of the chain of ganglia (Figure 1Bii,Cii,D). This goes along with the body shape of the animal. The CNS of evolutionarily older insects is spatially segregated (Figure 1Bi). In flies (Figure 1Bii), the thorax and abdomen are compressed into a thoracic mass. The crustacean CNS exhibits similar diversifications: in lobster and crayfish, the CNS is a well segmented ganglia chain (Figure 1Ci); in crabs, the chain is shortened, fused, and compressed into a thoracic mass (Figure 1Cii); in chelicarates another form of compression can be found. Scorpions possess, like locusts and lobster, a chain of separated abdominal ganglia, but the anterior segments (brain and thorax) are strongly compressed and are referred to as ‘synganglion’. The protocerebrum is the most dorsal and anterior segment, immediately followed by the deutocerebrum. The CNS experiences a dorsal bend at the tritocerebrum near the esophagus. These three parts form the syncerebrum (brain). Because of the bend, it is located immediately dorsal of the fused subesophageal section of the synganglion, which is formed by the neuromers for the pedipalps and four walking legs. The CNS of spiders is strongly compressed; brain, thorax
Current Biology
Morphology of arthropod ganglia Arthropod CNSs are very precisely determined during development. Each ganglion has a predefined structure that is genetically specified. Neurons follow the paths laid out by the pioneer neurons, the neuroblasts, which are guided by the Netrin and Slit systems. Neuroblasts agglomerate at certain points along the connectives and therefore build the ganglia. In all ganglia, the cell bodies of the neurons are mostly located in the outer layer; the ganglia are connected to each other by axons which run in the connectives. The core of each ganglion consists of several tracts, commissures and neuropils (Figure 2). The tracts are axon bundles which cross each ganglion from anterior to posterior. They are the extension of the connectives, which come from neighboring segments, or the axons of neurons which enter the connective from the home ganglion. The tracts lie in specific dorsal or ventral layers and differ in their morphological structure, so that they are identifiable. The commissures are the paths through which axons and primary neurites cross the ganglion from one side to the other; they are topologically and morphologically specific regions and can be identified. The neuropil regions in each ganglion are the place where neurons connect synaptically to each other. Sensory neuropils are mostly in the ventral medial region; neuropils for neurons responsible for locomotion are located laterally. The pattern of longitudinal tracts and commissures within a ventral ganglion is especially conserved in insects and crustaceans, and between thoracic and abdominal segments in a given species (Figure 2). Neuronal anatomy At first glance, there are striking differences in the morphology of neurons in vertebrates and invertebrates. Arthropod neurons are mostly monopolar (Figure 3A), not multipolar like vertebrate neurons (Figure 3B). Their soma is rarely involved in the integration of synaptic processes — if
Ai
Aii
Aiii
Ci
Cii
na nl 2-4
1
2
ncr 100µm
Insect
Bi
Bii
Biii
Motor neuron pools
and abdominal segments fuse to the synganglion (Figure 1D). The CNS of vertebrates is also segmented, even when this is not clearly visible.
Identified interneuron
Magazine
Di
Curstacean
Dii
Current Biology
Figure 3. Morphology of neuron types in invertebrate and vertebrate CNSs. Schematics of invertebrate (A) and vertebrate (B) neurons. The color code is preserved across the figure: red, dendrites; yellow, soma; cyan, axon; blue, primary neurite; orange, spike initiation zone/axon hillock; green synaptic boutons. (Ai) Bipolar sensory neuron of invertebrate. (Aii) Monopolar invertebrate interneurons. The dendritic tree is the point of integration and transfer of information. (Aiii) Monopolar invertebrate motor neurons. (Bi) Unipolar or pseudounipolar sensory neurons of vertebrates. (Bii) Two types of multipolar vertebrate interneurons. (Biii) Multipolar motor neuron of vertebrates. (C) Example of an identified interneuron in (i) stick insects, E4, and (ii) crayfish, Commissural Interneuron 1. Both project through a homolog commissure to the contralateral side. (D) Invertebrate motor neuron pools in (i) stick insects and in (ii) crayfish. Figures adapted from: (Ci) Büschges and Wolf (1995); (Di) Goldammer et al. (2012); (Dii) Mulloney and Smarandache-Wellmann (2013).
activity is recorded from them it is mainly due to retrograde propagation from the dendritic branches and axons, although there are exceptions. From each soma, a thin primary neurite (Figure 3Aii, Aiii, D) projects towards a neuropil. In the neuropil, the neurite increases in diameter, branches out and forms many dendritic arborizations; these arborizations are also referred to as integrative segments and correspond to the soma membrane of a typical vertebrate neuron. On the dendrites, the neurons can receive synaptic inputs or make synaptic contacts to neighboring cells. These contacts are presumably not randomly spread over the whole dendritic tree but cluster in specific regions. Synaptic information is integrated in the dendrites and adds up to generate an action potential at one of the spike initiation zones (somewhere on the primary neurite, usually near the zone where the integrative segment becomes the axon). Only when an action potential is generated is the information actively propagated along the axon towards the periphery, the muscles or other segments. Electrotonic propagation of neuronal activity — the passive spread of charge within a neuron — is restricted to the segmental ganglion.
The axons of motor neurons leave each ganglion through lateral nerve roots, which project to muscles (Figure 3D). In invertebrates, innervation of muscle fibers by motor neurons is polyneuronal and multiterminal, whereas in vertebrates it is mononeuronal and monoterminal. Furthermore, in invertebrates, one motor neuron is not restricted to one muscle fiber, but rather innervates multiple muscle fibers. There is a significant difference between vertebrates and invertebrates in the synaptic information the muscle fibers receive: The primary excitatory neurotransmitter of motor neurons in vertebrates is acetylcholine (ACh); in contrast, invertebrates use glutamate. Invertebrate muscle fibers also receive inhibitory synaptic input, mediated by the transmitter -amino-butyric acid (GABA) at their neuromuscular junction which modifies muscle contraction in a very precise manner. The soma of sensory neurons in arthropods is mostly located in the periphery. The bipolar morphology (Figure 3Ai) enables them to obtain sensory information with their dendrites. They send the information via the axon to terminals in the ganglion’s sensory neuropils. Interestingly, these sensory neurons use acetylcholine as excitatory neurotransmitters. Somototopy is
Current Biology 26, R937–R980, October 24, 2016
R963
Current Biology
Magazine also preserved in arthropods; sense organs along appendages project to different targets in the neuropils. The primary neurons in the visual system of arthropods use serotonin as transmitter; in contrast the primary neurons in the visual system of vertebrates use glutamate. Rhythmic neural activity in all animals is driven by neuronal networks, mostly central pattern generators. In arthropods, each segmental ganglion is formed by a small number of cells, where local neurons play an important role. In many systems, these neurons have been identified physiologically and morphologically. They are mostly unique in each hemiganglion, but are repeated in the CNS segments of a given species. Connections between these identified local neurons have been characterized and studied in some detail. Usually they are contained to one hemisegment, but there are some examples of interneurons that project to the contralateral hemiganglion through the commissures (Figure 3C). Interneurons have distinct properties; some of them are even non-spiking. They lack the ability to produce action potentials and drive the postsynaptic neurons via graded transmitter release. This mode of information transfer is extremely fine-tuned and can relay more information than the all-ornothing principle of action potentials. The potential attenuation due to electrotonic propagation is their only downside; presumably it is due to the short distance between neurons in an invertebrate ganglion that non-spiking neurons even exist. In vertebrates, pattern generating neurons or premotor interneurons are always spiking neurons because they send their axons over longer distances. Examples of nonspiking cells in vertebrates are bipolar and amacrine cells in the retina, which transmit information over short distances. Voltage-gated ion channels The processing and propagation of information by neurons is made possible by the cells’ electrical properties. Voltage-gated ion channels are one of the key components of the molecular machinery underlying the biophysical properties of neurons. They belong to a large superfamily of proteins which have a similar architecture, R964
with four subunits per channel, each subunit having two transmembrane helices. Ion channels of this superfamily are found in both invertebrates and vertebrates. Despite their structural similarities, these ion channels are highly specialized. Fast transient sodium channels (NaV) are ubiquitous in the animal kingdom, with a key role in the generation of action potentials; no significant variation in the functioning of these channels has been found to date. Two NaV channel genes are expressed in arthropods, NaV1 and NaV2, while vertebrates produce several copies of the NaV1 type. Voltage-dependent potassium channels are necessary for the repolarization of membrane potentials in all phyla and are the largest and most diverse class of channel proteins. They all let only K+ ions cross through the pore, but their voltage sensitivity and opening duration varies between the different types. Chloride channels are also ubiquitous in all neuron types, their function is not completely understood, but it seems that they complement the potassium leak current in helping to maintain the resting potential, and play a major role at the inhibitory synapses. Calcium channels show as well a great diversity and are encoded by several different genes. Calcium currents depolarize the neuron. The inflow of calcium through the voltagegated ion channels also has indirect hyperpolarizing effects by opening calcium-dependent potassium channels or triggering a whole set of second messenger processes. Synaptic connections All neurons are polarized cells: they gather information via their dendrites and project it to other cells where they make synaptic connections. In both arthropods and vertebrates, the synapses can be either chemical or electrical. Chemical synapses have in all systems a similar morphology and function. Vesicles filled with the neuronal transmitter are located in the synaptic bouton and fuse with the membrane to release the transmitter. These molecules bind to specific receptors in the postsynaptic membrane in order to open ionic channels. The same types of low molecular weight transmitters are found in vertebrates and
Current Biology 26, R937–R980, October 24, 2016
invertebrates: glutamate, GABA, ACh, serotonin, dopamine and octopamine (in arthropods)/noradrenaline (in vertebrates), all of which are released into the synaptic cleft. Two types of postsynaptic receptors open ionic channels in the postsynaptic membrane. The ionotropic receptors are also ion channels, so that binding of the transmitter changes the conformation and ions pass through the membrane. The metabotropic receptors activate G-proteins to directly or indirectly open ion channels in the membrane for ion channels to pass, as well as influencing the electrical and biochemical properties of the postsynaptic cell. The same variety of ionotropic and metabotropic receptors can be found in both arthropods and vertebrates. Even if the receptors function in the same way, they do not necessarily open the same type of ion channels; the particular effect of the transmitter can therefore vary between different systems. Gap junctions are channels between cells that promote the conductance of ions and small molecules between neurons and form the electrical synapses between cells. Electrical synapses can be found in vertebrate and invertebrate nervous systems. They can be located anywhere along the neuron. Their proteins form a hexameric hemichannel and establish a channel only when the membranes of two neurons are in very close proximity. In invertebrates, gap junctions are made by members of the innexin protein family; in vertebrates, by contrast, the hemichannels are formed by the evolutionarily unrelated connexins. The innexins in invertebrates are homologous to a third gap junction protein family, the pannexins, which are also found in vertebrates. Some electric synapses are simple channels through which small molecules (>1 kDa) are exchanged and cross in both directions. Other electrical synapses are more complex: in the rectifying type, for example, ions can only flow in one direction. Electrical synapses play an important role in synchronizing or desynchronizing neuronal networks. Rectifying synapses can even change the synaptic strength between neuronal networks and can themselves be modulated by changes in the postsynaptic membrane potential.
Current Biology
Magazine Conclusions The fact that the CNSs of diverse members of the arthropod phyla are essentially similar and show clear differences from vertebrate CNSs has never been disputed. Nevertheless, it is interesting to see how many features are shared between the nervous systems of arthropods and vertebrates. Their development is driven by genes and transcription factors that are similar, indeed homologous. Neurons build a nervous system and have similar ionic channel compositions in arthropods and vertebrates. In both cases, neurons communicate with each other through synapses, chemical or electrical, and these are even built by the products of homologous genes. They use the same transmitters. And even the segmentation of the CNS (brain and nerve cord) has a similar overall organization. The differences are in the details, particularly in features that are important for nervous system specializations. In vertebrates, the need for fast propagation of action potentials is supported by the myelination of axons. The vertebrate brain is generally larger and more complex (with exceptions such as the very large brains of some cephalopods), and responsible for higher cognitive functions. Motor neurons and sensory neurons can put a similar set of neurotransmitters to different uses. And gastrulation generates a different position for the nerve cord, ventral versus dorsal, though this may not be a fundamental difference. The overwhelming similarities between the nervous system, and especially the neuronal network structure, argue that work on invertebrates will continue to provide general insights, and the case for research on invertebrate species is strengthened by their experimental tractability and the powerful tools for genetic analysis available, particularly for the fruit fly Drosophila. And of course, invertebrates are fascinating in their own right. FURTHER READING Arendt, D., and Nübler-Jung, K. (1999). Comparison of early nerve cord development in insects and vertebrates. Development 126, 2309–2325. Burrows, M. (1996). The Neurobiology of an Insect Brain (Oxford; New York: Oxford University Press)
Büschges, A., and Wolf, H. (1995). Nonspiking local interneurons in insect leg motor control.1. Common layout and species-specific response properties of femur-tibia joint control pathways in stick insect and locust. J. Neurophysiol. 73, 1843–1860. Büschges, A., Scholz, H., and El Manira, A. (2011). New moves in motor control. Curr. Biol. 21, R513–524. Elson, R.C. (1996). Neuroanatomy of a crayfish thoracic ganglion: sensory and motor roots of the walking-leg nerves and possible homologies with insects. J. Comp. Neurol. 365, 1–17. Goldammer, J., Büschges, A., and Schmidt, J. (2012). Motoneurons, DUM cells, and sensory neurons in an insect thoracic ganglion: a tracing study in the stick insect Carausius morosus. J. Comp. Neurol. 520, 230–257. Gregory, G.E. (1974). Neuroanatomy of mesothoracic ganglion of cockroach Periplaneta americana (L). 1. Roots of peripheral nerves. Phil. Trans. R. Soc. Lond. B. 267, 421–465. Kittmann, R., Dean, J., and Schmitz, J. (1991). An atlas of the thoracic ganglia in the stick insect, Carausius morosus. Phil. Trans. R. Soc. Lond. B 331, 101–121. Lehmann, D., Melzer, R.R., Hörnig, M.K., Michalik, P., Sombke, A., and Harzsch, S. (2016) Arachnida - excl. Scorpiones. In Structure and Evolution of Invertebrate Nervous Systems, Schmidt-Rhaesa, A., Harzsch, S., and Purschke, G. ed. (Oxford: Oxford University Press), pp. 453-477 Loesel, R., Wolf, H., Kenning, M., Harzsch, S., and Sombke, A. (2013). Architectural principles and evolution of the arthropod central nervous system. In Arthropod Biology and Evolution, A. Minelli, G. Boxshall, and F.G., eds. (SpringerVerlag Berlin Heidelberg), pp. 299–342. Mulloney, B., and Smarandache-Wellmann, C. (2012). Neurobiology of the crustacean swimmeret system. Prog. Neurobiol. 96, 242–267. Mulloney, B., Tschuluun, N., and Hall, W.M. (2003). Architectonics of crayfish ganglia. Microsc. Res. Tech. 60, 253–265. Orlovsky, G.N., Deliagina, T.G., and Grillner, S. (1999). Neuronal Control of Locomotion: From Mollusc to Man (Oxford: Oxford University Press). Schmidt-Rhaesa, A., Harzsch, S., and Purschke, G. (2016). Structure and Evolution of Invertebrate Nervous Systems (Oxford: Oxford University Press). Skinner, K. (1985). The structure of the fourth abdominal ganglion of the crayfish, Procambarus clarki (girard). I. Tracts in the ganglionic core. J. Comp. Neurol. 234, 168–181. Smarandache-Wellmann, C., Weller, C., Wright, T.M., Jr., and Mulloney, B. (2013). Five types of nonspiking interneurons in local patterngenerating circuits of the crayfish swimmeret system. J. Neurophysiol. 110, 344–357. Storch, V., and Welsch, U. (2009). Crustacea, Krebse. In Kükenthal - Zoologisches Praktikum. (Hamburg: Spektrum Akademischer Verlag). Strausfeld, N. J. (2012). Arthropod Brains: Evolution, Functional Elegance, and Historical Significance (Cambridge, MA: Harvard University Press). Tanaka, G., Hou, X., Ma, X., Edgecombe, G.D., and Strausfeld, N.J. (2013). Chelicerate neural ground pattern in a Cambrian great appendage arthropod. Nature 502, 364-367. Wendler G. (1999). Fortbewegung und sensomotorische Integration. In Lehrbuch der Entomologie, K. Dettmer and W. Peters, eds. (Stuttgart, Jena, Lübeck, Ulm: Gustav Fischer), pp. 229-272.
Emmy Noether Group / Institute of Zoology, University of Cologne, D50674 Cologne Germany. E-mail: [email protected]
Primer
Evolution of highly diverse forms of behavior in molluscs Binyamin Hochner1,* and David L. Glanzman2,3,* Members of the phylum Mollusca demonstrate the animal kingdom’s tremendous diversity of body morphology, size and complexity of the nervous system, as well as diversity of behavioral repertoires, ranging from very simple to highly flexible. Molluscs include Solenogastres, with their worm-like bodies and behavior (see phylogenetic tree; Figure 1); Bivalvia (mussels and clams), protected by shells and practically immobile; and the cephalopods, such as the octopus, cuttlefish and squid. The latter are strange-looking animals with nervous systems comprising up to half a billion neurons, which mediate the complex behaviors that characterize these freely moving, highly visual predators. Molluscs are undoubtedly special — their extraordinary evolutionary advance somehow managed to sidestep the acquisition of the rigid skeleton that appears essential to the evolution of other ‘successful’ phyla: the exoskeleton in ecdysozoan invertebrates and the internal skeleton in Deuterostomia, including vertebrates. A skeletal body provides stability and enables, through lever action, efficient exploitation of muscle forces for the generation of rapid movements. By contrast, molluscs must use their muscles for both body support and movement. Having a skeleton also simplifies motor control, because motor commands are limited to a rather restricted number of control variables (degrees of freedom) dictated by the number of joints. Indeed, except for cephalopods, molluscs are not renowned for their motor capabilities. The flexibility and speed of cephalopod motor behaviors have been achieved through radical changes in morphology, and the
Current Biology 26, R937–R980, October 24, 2016 © 2016 Elsevier Ltd.
R965
Magazine Primer
Arthropod neurons and nervous system Carmen Ramona SmarandacheWellmann Arthropods are very diverse, come in many different forms with diverse adaptations, and through such diversity have populated all environmental niches on the planet. Almost 80% of the animals on planet Earth belong to this phylum. Despite their very diverse phenotypes they share fundamental similarities which were previously used to generate a phylogenetic tree. All arthropods have segmented bodies and possess jointed limbs at all or many of their body segments. An additional common feature of arthropods is their exoskeleton, made mainly of chitin and/ or sclerotin. As in a vertebrate, the nervous system of an arthropod consists of specialized cells, the neurons. The central nervous system of arthropods is segmented and can be roughly divided into the brain, located in the head at the anterior end, and the ventral nerve cord spanning from the head to the caudal end, the abdomen (Figure 1). Because of the huge morphological diversity in this phylum, it is difficult to derive generalizations. Not all features described here are found in all families of this group (please see the papers mentioned in the Further reading list for more detailed information). Still, the nervous systems of arthropods share many common features: these are often points of similarity to vertebrate nervous systems, but sometimes also points of fundamental difference. Evolutionary relationships Arthropods are divided in five main subphyla. All specimens of the subphylum of Trilobitomorpha are extinct, so I will exclude them from this description. The other four subphyla are: Chelicerata, including Arachnida (scorpions and spiders); Myriapoda, including Chilopoda (centipedes) and Diplopoda (millipedes); Crustacea, including Malacostraca (including lobster, crabs, shrimp); and Hexapoda, with Insecta (including dragonflies, R960
bees, flies) and Entognatha (spring tails). In the last decades, there have been a number of disputes about the evolutionary relationships between these subphyla and the different classes in each phylum. New methods, including genetic and molecular analyses, have helped to elucidate the phylogenies. In the Pancrustacean hypothesis, the Chelicerata and Myriapoda divide first and are not as closely related to the other two groups. Crustacea and Insecta are closely related; it seems that insects evolved from the crustacean lineage. This result is mainly based on mitochondrial gene sequences and embryogeny. The close relationships might explain why neurobiological features from these two groups exhibit remarkable similarities. Development of the arthropod nervous system The anatomy of the arthropod central nervous system is determined during embryonic development. Neuroblasts, progenitor cells of neurons, are placed at specific regions in each body segment across the rostro-caudal axis of the nerve cord and give rise to the ganglia. Within a given species, and also across phyla, there are similarities in respect to the number of neurons, and formation of tracts, neuropils, and commissures (Figure 2). The precise segmentation of arthropods is rooted in its genetic configuration. One gene family, the homeobox genes (Hox genes), is responsible for the rostro-caudal axis during the regional development in an embryo. They were first identified in Drosophila, and have now been characterized in vertebrates and in representatives of all major animal classes, including Arthropoda and Annelida. In Arthropoda (Drosophila), eight genes exist for the rostro-caudal axis formation and segmentation at specific locations on a single chromosome. The first three Hox genes on the chromosome encode specific transcription factors mediating the formation of the anterior part of the animal with three brain regions. The following four specify the different body segments across the longitudinal axis, and the last Hox gene specifies the abdominal segments. Mutations of Hox genes result in severe developmental
Current Biology 26, R937–R980, October 24, 2016 © 2016 Elsevier Ltd.
defects in anterior-posterior formation, segmental formation and body polarity. Hox genes are extremely conserved throughout the animal kingdom. In vertebrates, Hox genes have been duplicated and are found on four different chromosomes with slight variations. Even though segmentation in vertebrates is not as visible as in arthropods, the Hox genes still play a crucial role in the development of the different CNS portions and body. In a similar manner to arthropod segmentation, the first three genes are responsible for the development of the three brain regions: prosencephalon, mesencephalon and rhombencephalon. The last gene specifies the caudal region of the spinal cord. One of the major differences between arthropods and vertebrates is the position of the nerve cord. In arthropods, the neuronal cord is located ventrally, in vertebrates dorsally. But it seems like this is not such a big difference because ventral in arthropods seems to be equivalent to dorsal in vertebrates. The same genetic factors, molecules, and embryological features are found in vertebrates and invertebrates, as they derive from the same ancestor. The inversion of the dorso-ventral body axis comes about as a result of gastrulation during embryonic development. Arthropods are protostomes; their mouth develops from the initial invagination side (the blastopore); vertebrates are deuterostomes, and their mouth develops secondarily. Because of this invagination, the neuroblasts in invertebrates turn ventrally, while the neural tube (progenitor cells) in vertebrates is located dorsally. Signaling gradients of the morphogen Bone Morphogenetic Protein (BMP) are necessary for formation of the dorsoventral axis in all bilateral animals, both for the invagination as well as patterning of the CNS. The BMP4 gene is a dorsal identity marker, encoding a transcription factor primarily found at the peripheral part of the embryo, while the gene Shh (Sonic hedgehog) encodes a transcription factor found ventrally. The BMP4 and Shh proteins influence cell formation in the vertebrate spinal cord and the arthropod ventral cord. To sum up: the Hox genes are responsible for developmental specification along the anterior-posterior
Current Biology
Magazine axis, and dorso-ventral organization involves a gradient of two different transcription factors encoded by the genes BMP4 and Shh. In arthropods and vertebrates, axonal growth is guided by proteins encoded by the same set of genes, Netrins and Slits. Thus, although interesting the nervous system of an arthropod ends up being ventral and that of a vertebrate ends up being dorsal, the genetic regulation of their development shows fundamental similarities. Neuroanatomy The blueprint of the arthropod CNS consists of a dorsal cephalic ganglion, the ‘brain’, followed by a chain of ventral ganglia, the ventral cord (Figure 1). In each metameric segment, bilateral ganglia are fused at the midline and their lateral nerves extend into the body segment and appendages (Figure 1A). The segmental ganglia are connected to each other through axon bundles, or connectives. During larval development in crustaceans, the bilateral symmetric ganglia are not completely fused at the midline and a ladder-like structure, as in annelids, is visible. In each segment, axons cross the midline in commissures to connect the hemiganglia. In all other arthropods, the ganglia are tightly fused at the midline, while commissures are still present but hidden in the layer of the ganglion (Figure 2). One characteristic which is clearly visible in most arthropods is the heavily segmented body. In evolutionarily older phylae, like myriapods, each segment is similar (metameric) to the others and carries pairs of appendages (one or two on each side); only head and tail segments are specialized. The body segmentation in other arthropods is more diverse. There is a clear distinction between thorax and abdomen. The appendages at the thorax level are the walking legs, or periopods, and in insects even wings. The abdomen’s morphology can be even more distinct: it can be either equipped with specialized appendages, known as pleopods, as seen in long tailed crustaceans, or without appendages (insects), or even compressed (crabs, spiders). The segmentation is preserved internally. The brain of an arthropod lies in the head (dorsally) and consists of three pairs of ganglia: the protocerebrum
Ci
Bi
A
b
b
ch
Di
pp
b
b
wl1 SOG wl2 TG
wl3 wl4
SOG
TG
AG
AG
Cii
b AG
Bii b
Dii wl1
chn ppn
wl1
b
wl1 wl2
wl2
wl3
wl3
wl2 wl3
wl4 wl4 Current Biology
Figure 1. Morphology of the arthropod CNS. Schematic of arthropod CNS: (A) Myriapoda; (B) Insecta; (C) Crustacea; (D) Chelicerata. The brain (b) is located anteriorly in the head. The ventral nerve cord is a chain of segmental ganglia, some compressed. (A) In a centipede, ganglia are metameric and connected via the paired connectives. The ventral nerve cord of some insects, such as dragonflies (Bi), and crustaceans, such as crayfish (Ci), are spatially distributed; only the subesophageal ganglion (SOG) and the terminal ganglion are compressed. The CNS of evolutionarily younger insects, such as fruit flies (Bii), and of brachyurian crabs (Cii) is compressed. In insects the brain is fused to the SOG; and the thoracic ganglia form with the abdominal ganglia a thoracic mass. In crabs the subesophageal, the thoracic and the abdominal ganglia are compressed into a subesophageal mass, the thoracic ganglion. (D) Chelicerate CNS is strongly compressed. The first three neuromers form the brain, or syncerebrum. It lies dorsal to the fused subesophageal and thoracic segments and form together the synganglion. (Di) The abdominal ganglia of a scorpion are spatially distributed. (Dii) In arachnids, the abdominal segments are compressed into the synganglion. Abbreviations: b, brain, when fused with the following segments and positioned above of others shown in grey; SOG, subesophageal ganglion; TG, thoracic ganglion; AG, abdominal ganglion; ch, cheliceral; pp, pedipalp; wl 1–4, walking leg nerves 1–4. Figures adapted from: (Bi) Wendler (1999) with permission of Springer, (Ci) Storch and Welsch (2009), with permission of Springer; (Cii) Thomson (1916); (Di) Preprinted by permission from Macmillan Publishers Ltd: Nature Tanaka et al. (2013); (Dii) Adapted from Lehmann et al. (2016) by permission of Oxford University Press.
(which innervates structures such as the eyes, controlling vision); the
deutocerebrum (which innervates structures such as the antennae,
Current Biology 26, R937–R980, October 24, 2016
R961
Current Biology
Magazine Insect ganglia
Crustacean ganglia MDT LDT
Ai
Aii MG
Transversal
DMT DIT VLT mcN VIT hN d
LG
VMT MVT
IVAC
VLT
HN
B
B
vcN
MDT LDT DMT DIT DLT VIT VMT MVT
A A
LVT
v
DC1 DC3 DC4 DC6 DC2 DC5
Bi
DC1 DC4 DC2 DC3 DC5 DC6 DC7
Bii MDT DMT
MDT
C
C
Sagittal
DMT VMT MVT
dC7
VIT
d IVAC
p
a
PVC SVC
HN
AVC
PVC
DMT
p
LDT
Frontal
a
DC1 DC2
Cii
DC3
LDT
DIT
Ci
DMT DIT
v
DC1 DC2
DC4
DC3
DIT
DC4 DC5
DC6
Current Biology
Figure 2. Ganglia in insects and crustaceans. Schematics of three pairs of orthogonal sections through: (i) a thoracic ganglion of an insect; and (ii) an abdominal ganglion of a crustacean. The basic structure is strongly preserved across species and across homologous segments. Tracts of axons run rostro-caudally through the ganglia, while commissures connect the bilateral ganglia. Neuropils are the center for synaptic interaction. The color coding is preserved across the figure. (A) Transversal section at the height of the first nerve root. The tracts are layered and separated by commissures. (B) Sagittal section through the ganglia at the height of the medial dorsal tract. The numbering of the dorsal commissures is different in the two species, but they are layered in a similar fashion. (C) Frontal section in the dorsal part of the ganglia showing the tracts, and commissures in the two species. Abbreviations: MDT, medial dorsal tract; LDT, lateral dorsal tract; DMT, dorsal medial tract; DIT, dorsal intermediate tract; DLT, dorsal lateral tract; VIT, ventral intermediate tract; VMT, ventral medial tract; VLT, ventral lateral tract; MVT, medial ventral tract; LVT, lateral ventral tract. Commissures: DC 1–7, dorsal commissure 1–7; AVC, anterior ventral commissure; PVC, posterior ventral commissure. Neuropils: hN/HN, horseshoe neuropil; lVAC, lateral part of ventral association center; mcN, medial coarse neuropil; vcN, ventral coarse neuropil; LN, lateral neuropil. Giant axons: MG, medial giant; LG, lateral giant. Figures adapted from: (Ai) Goldammer et al. (2012); (Bi) after Kittman et al. (1991); (Ci) after Gregory (1974); (Aii,Bii,Cii) Elson (1996) and Skinner (1985).
controlling chemosensation and tactile sensation); and the tritocerebrum (which, for example, integrates sensory information from protocerebrum and deutocerebrum). The brain’s main function is to assimilate sensory information, process it, select the appropriate behavioral program, and send it downstream. Homologies between the brains of different arthropod classes can provide R962
interesting insights informative for multiple types of investigation, and comparative studies are looking at structural, developmental and physiological aspects of brains of representative species in different arthropod subclasses (see the papers mentioned in the Further reading list for more details). The first of the ventrally located ganglia, the subesophageal ganglion,
Current Biology 26, R937–R980, October 24, 2016
also integrates sensory information and innervates the neck and mouthparts. In many species, this ganglion is built by a chain of compressed ganglia. Following the subesophageal ganglion are the thoracic ganglia, the abdominal ganglia, and the specialized terminal ganglion (Figure 1). Thoracic ganglia are morphologically very much alike, as are the abdominal ganglia; both arise from homologous neuromers and they are also very similar to each other. Ventral ganglia contain the neurons necessary for the movement of appendages. This includes pattern generating interneurons and motor neurons which innervate the muscles. The thoracic ganglia innervate legs and wings (walking and flying, respectively), while the abdominal ganglia innervate the pleopods (swimming, ventilation), the reproductive organs, and the anus. Sensory information from the body appendages, like proprioception and hearing, is processed in its own segment and sent to neighboring ganglia. There are some arthropods with longitudinal compression of the chain of ganglia (Figure 1Bii,Cii,D). This goes along with the body shape of the animal. The CNS of evolutionarily older insects is spatially segregated (Figure 1Bi). In flies (Figure 1Bii), the thorax and abdomen are compressed into a thoracic mass. The crustacean CNS exhibits similar diversifications: in lobster and crayfish, the CNS is a well segmented ganglia chain (Figure 1Ci); in crabs, the chain is shortened, fused, and compressed into a thoracic mass (Figure 1Cii); in chelicarates another form of compression can be found. Scorpions possess, like locusts and lobster, a chain of separated abdominal ganglia, but the anterior segments (brain and thorax) are strongly compressed and are referred to as ‘synganglion’. The protocerebrum is the most dorsal and anterior segment, immediately followed by the deutocerebrum. The CNS experiences a dorsal bend at the tritocerebrum near the esophagus. These three parts form the syncerebrum (brain). Because of the bend, it is located immediately dorsal of the fused subesophageal section of the synganglion, which is formed by the neuromers for the pedipalps and four walking legs. The CNS of spiders is strongly compressed; brain, thorax
Current Biology
Morphology of arthropod ganglia Arthropod CNSs are very precisely determined during development. Each ganglion has a predefined structure that is genetically specified. Neurons follow the paths laid out by the pioneer neurons, the neuroblasts, which are guided by the Netrin and Slit systems. Neuroblasts agglomerate at certain points along the connectives and therefore build the ganglia. In all ganglia, the cell bodies of the neurons are mostly located in the outer layer; the ganglia are connected to each other by axons which run in the connectives. The core of each ganglion consists of several tracts, commissures and neuropils (Figure 2). The tracts are axon bundles which cross each ganglion from anterior to posterior. They are the extension of the connectives, which come from neighboring segments, or the axons of neurons which enter the connective from the home ganglion. The tracts lie in specific dorsal or ventral layers and differ in their morphological structure, so that they are identifiable. The commissures are the paths through which axons and primary neurites cross the ganglion from one side to the other; they are topologically and morphologically specific regions and can be identified. The neuropil regions in each ganglion are the place where neurons connect synaptically to each other. Sensory neuropils are mostly in the ventral medial region; neuropils for neurons responsible for locomotion are located laterally. The pattern of longitudinal tracts and commissures within a ventral ganglion is especially conserved in insects and crustaceans, and between thoracic and abdominal segments in a given species (Figure 2). Neuronal anatomy At first glance, there are striking differences in the morphology of neurons in vertebrates and invertebrates. Arthropod neurons are mostly monopolar (Figure 3A), not multipolar like vertebrate neurons (Figure 3B). Their soma is rarely involved in the integration of synaptic processes — if
Ai
Aii
Aiii
Ci
Cii
na nl 2-4
1
2
ncr 100µm
Insect
Bi
Bii
Biii
Motor neuron pools
and abdominal segments fuse to the synganglion (Figure 1D). The CNS of vertebrates is also segmented, even when this is not clearly visible.
Identified interneuron
Magazine
Di
Curstacean
Dii
Current Biology
Figure 3. Morphology of neuron types in invertebrate and vertebrate CNSs. Schematics of invertebrate (A) and vertebrate (B) neurons. The color code is preserved across the figure: red, dendrites; yellow, soma; cyan, axon; blue, primary neurite; orange, spike initiation zone/axon hillock; green synaptic boutons. (Ai) Bipolar sensory neuron of invertebrate. (Aii) Monopolar invertebrate interneurons. The dendritic tree is the point of integration and transfer of information. (Aiii) Monopolar invertebrate motor neurons. (Bi) Unipolar or pseudounipolar sensory neurons of vertebrates. (Bii) Two types of multipolar vertebrate interneurons. (Biii) Multipolar motor neuron of vertebrates. (C) Example of an identified interneuron in (i) stick insects, E4, and (ii) crayfish, Commissural Interneuron 1. Both project through a homolog commissure to the contralateral side. (D) Invertebrate motor neuron pools in (i) stick insects and in (ii) crayfish. Figures adapted from: (Ci) Büschges and Wolf (1995); (Di) Goldammer et al. (2012); (Dii) Mulloney and Smarandache-Wellmann (2013).
activity is recorded from them it is mainly due to retrograde propagation from the dendritic branches and axons, although there are exceptions. From each soma, a thin primary neurite (Figure 3Aii, Aiii, D) projects towards a neuropil. In the neuropil, the neurite increases in diameter, branches out and forms many dendritic arborizations; these arborizations are also referred to as integrative segments and correspond to the soma membrane of a typical vertebrate neuron. On the dendrites, the neurons can receive synaptic inputs or make synaptic contacts to neighboring cells. These contacts are presumably not randomly spread over the whole dendritic tree but cluster in specific regions. Synaptic information is integrated in the dendrites and adds up to generate an action potential at one of the spike initiation zones (somewhere on the primary neurite, usually near the zone where the integrative segment becomes the axon). Only when an action potential is generated is the information actively propagated along the axon towards the periphery, the muscles or other segments. Electrotonic propagation of neuronal activity — the passive spread of charge within a neuron — is restricted to the segmental ganglion.
The axons of motor neurons leave each ganglion through lateral nerve roots, which project to muscles (Figure 3D). In invertebrates, innervation of muscle fibers by motor neurons is polyneuronal and multiterminal, whereas in vertebrates it is mononeuronal and monoterminal. Furthermore, in invertebrates, one motor neuron is not restricted to one muscle fiber, but rather innervates multiple muscle fibers. There is a significant difference between vertebrates and invertebrates in the synaptic information the muscle fibers receive: The primary excitatory neurotransmitter of motor neurons in vertebrates is acetylcholine (ACh); in contrast, invertebrates use glutamate. Invertebrate muscle fibers also receive inhibitory synaptic input, mediated by the transmitter -amino-butyric acid (GABA) at their neuromuscular junction which modifies muscle contraction in a very precise manner. The soma of sensory neurons in arthropods is mostly located in the periphery. The bipolar morphology (Figure 3Ai) enables them to obtain sensory information with their dendrites. They send the information via the axon to terminals in the ganglion’s sensory neuropils. Interestingly, these sensory neurons use acetylcholine as excitatory neurotransmitters. Somototopy is
Current Biology 26, R937–R980, October 24, 2016
R963
Current Biology
Magazine also preserved in arthropods; sense organs along appendages project to different targets in the neuropils. The primary neurons in the visual system of arthropods use serotonin as transmitter; in contrast the primary neurons in the visual system of vertebrates use glutamate. Rhythmic neural activity in all animals is driven by neuronal networks, mostly central pattern generators. In arthropods, each segmental ganglion is formed by a small number of cells, where local neurons play an important role. In many systems, these neurons have been identified physiologically and morphologically. They are mostly unique in each hemiganglion, but are repeated in the CNS segments of a given species. Connections between these identified local neurons have been characterized and studied in some detail. Usually they are contained to one hemisegment, but there are some examples of interneurons that project to the contralateral hemiganglion through the commissures (Figure 3C). Interneurons have distinct properties; some of them are even non-spiking. They lack the ability to produce action potentials and drive the postsynaptic neurons via graded transmitter release. This mode of information transfer is extremely fine-tuned and can relay more information than the all-ornothing principle of action potentials. The potential attenuation due to electrotonic propagation is their only downside; presumably it is due to the short distance between neurons in an invertebrate ganglion that non-spiking neurons even exist. In vertebrates, pattern generating neurons or premotor interneurons are always spiking neurons because they send their axons over longer distances. Examples of nonspiking cells in vertebrates are bipolar and amacrine cells in the retina, which transmit information over short distances. Voltage-gated ion channels The processing and propagation of information by neurons is made possible by the cells’ electrical properties. Voltage-gated ion channels are one of the key components of the molecular machinery underlying the biophysical properties of neurons. They belong to a large superfamily of proteins which have a similar architecture, R964
with four subunits per channel, each subunit having two transmembrane helices. Ion channels of this superfamily are found in both invertebrates and vertebrates. Despite their structural similarities, these ion channels are highly specialized. Fast transient sodium channels (NaV) are ubiquitous in the animal kingdom, with a key role in the generation of action potentials; no significant variation in the functioning of these channels has been found to date. Two NaV channel genes are expressed in arthropods, NaV1 and NaV2, while vertebrates produce several copies of the NaV1 type. Voltage-dependent potassium channels are necessary for the repolarization of membrane potentials in all phyla and are the largest and most diverse class of channel proteins. They all let only K+ ions cross through the pore, but their voltage sensitivity and opening duration varies between the different types. Chloride channels are also ubiquitous in all neuron types, their function is not completely understood, but it seems that they complement the potassium leak current in helping to maintain the resting potential, and play a major role at the inhibitory synapses. Calcium channels show as well a great diversity and are encoded by several different genes. Calcium currents depolarize the neuron. The inflow of calcium through the voltagegated ion channels also has indirect hyperpolarizing effects by opening calcium-dependent potassium channels or triggering a whole set of second messenger processes. Synaptic connections All neurons are polarized cells: they gather information via their dendrites and project it to other cells where they make synaptic connections. In both arthropods and vertebrates, the synapses can be either chemical or electrical. Chemical synapses have in all systems a similar morphology and function. Vesicles filled with the neuronal transmitter are located in the synaptic bouton and fuse with the membrane to release the transmitter. These molecules bind to specific receptors in the postsynaptic membrane in order to open ionic channels. The same types of low molecular weight transmitters are found in vertebrates and
Current Biology 26, R937–R980, October 24, 2016
invertebrates: glutamate, GABA, ACh, serotonin, dopamine and octopamine (in arthropods)/noradrenaline (in vertebrates), all of which are released into the synaptic cleft. Two types of postsynaptic receptors open ionic channels in the postsynaptic membrane. The ionotropic receptors are also ion channels, so that binding of the transmitter changes the conformation and ions pass through the membrane. The metabotropic receptors activate G-proteins to directly or indirectly open ion channels in the membrane for ion channels to pass, as well as influencing the electrical and biochemical properties of the postsynaptic cell. The same variety of ionotropic and metabotropic receptors can be found in both arthropods and vertebrates. Even if the receptors function in the same way, they do not necessarily open the same type of ion channels; the particular effect of the transmitter can therefore vary between different systems. Gap junctions are channels between cells that promote the conductance of ions and small molecules between neurons and form the electrical synapses between cells. Electrical synapses can be found in vertebrate and invertebrate nervous systems. They can be located anywhere along the neuron. Their proteins form a hexameric hemichannel and establish a channel only when the membranes of two neurons are in very close proximity. In invertebrates, gap junctions are made by members of the innexin protein family; in vertebrates, by contrast, the hemichannels are formed by the evolutionarily unrelated connexins. The innexins in invertebrates are homologous to a third gap junction protein family, the pannexins, which are also found in vertebrates. Some electric synapses are simple channels through which small molecules (>1 kDa) are exchanged and cross in both directions. Other electrical synapses are more complex: in the rectifying type, for example, ions can only flow in one direction. Electrical synapses play an important role in synchronizing or desynchronizing neuronal networks. Rectifying synapses can even change the synaptic strength between neuronal networks and can themselves be modulated by changes in the postsynaptic membrane potential.
Current Biology
Magazine Conclusions The fact that the CNSs of diverse members of the arthropod phyla are essentially similar and show clear differences from vertebrate CNSs has never been disputed. Nevertheless, it is interesting to see how many features are shared between the nervous systems of arthropods and vertebrates. Their development is driven by genes and transcription factors that are similar, indeed homologous. Neurons build a nervous system and have similar ionic channel compositions in arthropods and vertebrates. In both cases, neurons communicate with each other through synapses, chemical or electrical, and these are even built by the products of homologous genes. They use the same transmitters. And even the segmentation of the CNS (brain and nerve cord) has a similar overall organization. The differences are in the details, particularly in features that are important for nervous system specializations. In vertebrates, the need for fast propagation of action potentials is supported by the myelination of axons. The vertebrate brain is generally larger and more complex (with exceptions such as the very large brains of some cephalopods), and responsible for higher cognitive functions. Motor neurons and sensory neurons can put a similar set of neurotransmitters to different uses. And gastrulation generates a different position for the nerve cord, ventral versus dorsal, though this may not be a fundamental difference. The overwhelming similarities between the nervous system, and especially the neuronal network structure, argue that work on invertebrates will continue to provide general insights, and the case for research on invertebrate species is strengthened by their experimental tractability and the powerful tools for genetic analysis available, particularly for the fruit fly Drosophila. And of course, invertebrates are fascinating in their own right. FURTHER READING Arendt, D., and Nübler-Jung, K. (1999). Comparison of early nerve cord development in insects and vertebrates. Development 126, 2309–2325. Burrows, M. (1996). The Neurobiology of an Insect Brain (Oxford; New York: Oxford University Press)
Büschges, A., and Wolf, H. (1995). Nonspiking local interneurons in insect leg motor control.1. Common layout and species-specific response properties of femur-tibia joint control pathways in stick insect and locust. J. Neurophysiol. 73, 1843–1860. Büschges, A., Scholz, H., and El Manira, A. (2011). New moves in motor control. Curr. Biol. 21, R513–524. Elson, R.C. (1996). Neuroanatomy of a crayfish thoracic ganglion: sensory and motor roots of the walking-leg nerves and possible homologies with insects. J. Comp. Neurol. 365, 1–17. Goldammer, J., Büschges, A., and Schmidt, J. (2012). Motoneurons, DUM cells, and sensory neurons in an insect thoracic ganglion: a tracing study in the stick insect Carausius morosus. J. Comp. Neurol. 520, 230–257. Gregory, G.E. (1974). Neuroanatomy of mesothoracic ganglion of cockroach Periplaneta americana (L). 1. Roots of peripheral nerves. Phil. Trans. R. Soc. Lond. B. 267, 421–465. Kittmann, R., Dean, J., and Schmitz, J. (1991). An atlas of the thoracic ganglia in the stick insect, Carausius morosus. Phil. Trans. R. Soc. Lond. B 331, 101–121. Lehmann, D., Melzer, R.R., Hörnig, M.K., Michalik, P., Sombke, A., and Harzsch, S. (2016) Arachnida - excl. Scorpiones. In Structure and Evolution of Invertebrate Nervous Systems, Schmidt-Rhaesa, A., Harzsch, S., and Purschke, G. ed. (Oxford: Oxford University Press), pp. 453-477 Loesel, R., Wolf, H., Kenning, M., Harzsch, S., and Sombke, A. (2013). Architectural principles and evolution of the arthropod central nervous system. In Arthropod Biology and Evolution, A. Minelli, G. Boxshall, and F.G., eds. (SpringerVerlag Berlin Heidelberg), pp. 299–342. Mulloney, B., and Smarandache-Wellmann, C. (2012). Neurobiology of the crustacean swimmeret system. Prog. Neurobiol. 96, 242–267. Mulloney, B., Tschuluun, N., and Hall, W.M. (2003). Architectonics of crayfish ganglia. Microsc. Res. Tech. 60, 253–265. Orlovsky, G.N., Deliagina, T.G., and Grillner, S. (1999). Neuronal Control of Locomotion: From Mollusc to Man (Oxford: Oxford University Press). Schmidt-Rhaesa, A., Harzsch, S., and Purschke, G. (2016). Structure and Evolution of Invertebrate Nervous Systems (Oxford: Oxford University Press). Skinner, K. (1985). The structure of the fourth abdominal ganglion of the crayfish, Procambarus clarki (girard). I. Tracts in the ganglionic core. J. Comp. Neurol. 234, 168–181. Smarandache-Wellmann, C., Weller, C., Wright, T.M., Jr., and Mulloney, B. (2013). Five types of nonspiking interneurons in local patterngenerating circuits of the crayfish swimmeret system. J. Neurophysiol. 110, 344–357. Storch, V., and Welsch, U. (2009). Crustacea, Krebse. In Kükenthal - Zoologisches Praktikum. (Hamburg: Spektrum Akademischer Verlag). Strausfeld, N. J. (2012). Arthropod Brains: Evolution, Functional Elegance, and Historical Significance (Cambridge, MA: Harvard University Press). Tanaka, G., Hou, X., Ma, X., Edgecombe, G.D., and Strausfeld, N.J. (2013). Chelicerate neural ground pattern in a Cambrian great appendage arthropod. Nature 502, 364-367. Wendler G. (1999). Fortbewegung und sensomotorische Integration. In Lehrbuch der Entomologie, K. Dettmer and W. Peters, eds. (Stuttgart, Jena, Lübeck, Ulm: Gustav Fischer), pp. 229-272.
Emmy Noether Group / Institute of Zoology, University of Cologne, D50674 Cologne Germany. E-mail: [email protected]
Primer
Evolution of highly diverse forms of behavior in molluscs Binyamin Hochner1,* and David L. Glanzman2,3,* Members of the phylum Mollusca demonstrate the animal kingdom’s tremendous diversity of body morphology, size and complexity of the nervous system, as well as diversity of behavioral repertoires, ranging from very simple to highly flexible. Molluscs include Solenogastres, with their worm-like bodies and behavior (see phylogenetic tree; Figure 1); Bivalvia (mussels and clams), protected by shells and practically immobile; and the cephalopods, such as the octopus, cuttlefish and squid. The latter are strange-looking animals with nervous systems comprising up to half a billion neurons, which mediate the complex behaviors that characterize these freely moving, highly visual predators. Molluscs are undoubtedly special — their extraordinary evolutionary advance somehow managed to sidestep the acquisition of the rigid skeleton that appears essential to the evolution of other ‘successful’ phyla: the exoskeleton in ecdysozoan invertebrates and the internal skeleton in Deuterostomia, including vertebrates. A skeletal body provides stability and enables, through lever action, efficient exploitation of muscle forces for the generation of rapid movements. By contrast, molluscs must use their muscles for both body support and movement. Having a skeleton also simplifies motor control, because motor commands are limited to a rather restricted number of control variables (degrees of freedom) dictated by the number of joints. Indeed, except for cephalopods, molluscs are not renowned for their motor capabilities. The flexibility and speed of cephalopod motor behaviors have been achieved through radical changes in morphology, and the
Current Biology 26, R937–R980, October 24, 2016 © 2016 Elsevier Ltd.
R965