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Central Nervous System of Insects





Body plan and nervous system of insects


The insect body consists of a number of consecutive segments and is differentiated into three parts: the cephalon (head), the thorax, and the abdomen . The cephalic part is fused from the anteriormost six segments. The compound eyes belong to the first segment, the antennae to the second segment. The third segment bears the labrum as an appendage. The other three segments are associated with the mandibles, the maxillae, and the labium. The next three segments form the thoracic part. A pair of limbs is attached to each of the segments. The first thoracic segment is the prothorax. In many insect species, a pair of anterior and posterior wings is attached to the second thoracic segment (mesothorax) and third segment (metathorax) respectively. The abdominal part consists of eleven or twelve segments but in many cases they are fused into eight or nine segments.

The insect nervous system comprises the central nervous system and the peripheral nervous system. The central nervous system consists of a chain of ganglia. The peripheral nervous system includes a stomatogastric ganglion and sensory and motor nerves. In addition, insect nervous system includes pars intercerebralis-corpus cardiacum-corpus allatum and neuroendcrine system that consists of store and release organ.

The structure of the insect central nervous system resembles that of a rope ladder, therefore, it is also called ladder-like nervous system. The stiles of a ladder correspond to the connectives, running aligned to the body axis (except in the head, where the neuraxis is often bent). A ladder rung corresponds to a ganglion with commissures. (The CNS can be considered to be an ectodermal rudiment)

The most anterior ganglion is the cerebral ganglion (also supraoesophageal ganglion or brain), a structure that evolved from the fusion of the three preoral neuromeres (anterior with respect to the oesophagus). The suboesophageal ganglion consists of the fusion of three postoral neuromeres. The oesophagus is surrounded by the circumoesophageal connectives that link supra- and suboesophaegeal ganglia. More posterior to them are thoracic ganglia and abdominal ganglia. Ganglia posterior to the supraoesophagial ganglion lie ventral with respect to the digestive tract and are, with the exlusion of the suboesophageal ganglion, also called ventral nerve cord. In comparison, the vertebrate spinal cord lies dorsal with respect to the digestive tract. This difference is derived from their skeleton, each of which is external skeleton and internal skeleton. Vertebrate has a body plan that vertebral column puts up visceral organ whereas invertebrate has a body plan that ventral plate puts up nerve cord and visceral organ.

The variety of fusion patterns of ganglia reflects the phylogeny of taxonomic groups at the level of the CNS. For example, honey bee, fly, and moth have a fused form of brain and suboesophageal ganglion with an oesophageal foramen, through which the oesophagus extends towards the mountparts. The locust embryo has eleven pairs of ganglion rudiments. During ontogeny, the first to third abdominal ganglia fuse with the metathoracic ganglion and the eighth to eleventh ganglia fuse to and form a large terminal abdominal ganglion. In Diptera, fusion of thoracic and abdominal ganglia occurs resulting in a large segmented agglomerate ganglion

       

Figure: Silkmoth brain (male). The silkmoth brain consists of protocerebrum, deutocerebrum, and tritocerebrum. The brain has many identifiable regions consisting of densely packed neurites of the constituent neurons: the neuropils. Information processing occurs mostly in the neuropil. . Representative neuropils are the optic lobes (in principle, this would have to be divided), mushroom bodies, the lateral protocerebrum, and the central body in the protocerebrum, and the antennal lobe in the deutocerebrum. Ventrally, the brain is fused with the suboesophgeal ganglion. Scale bar: 0.5 mm.




Basic structure of ganglia


Ganglia are externally delimited by an insulating cell layer, the neurolemma, forming a chemical and mechanical barrier. Outer layer of the neurolemma is called neural lamella (or sheath) that is composed of extracellular matrix produced by an inner layer, the perineurial sheath cell layer consisting of glia cells. The lamella protects the neuronal tissue mostly mechanically while the sheath cell layer acts as a blood-brain barrier, being relatively impermeant as demonstrated by lanthanum ion exposure.

Neuronal tissue within a ganglion is grouped into two layers, an outer soma layer and an inner neuropil core. Insect central neurons are usually monopolar cells. A single neurite extends from the soma and branches into several neurites, forming synapses with adjacent neurites of other neurons. Generally, the majority of synapses are localised in the neuropil.

The neuropil is classified into glomerular neuropil, non-glomerular neuropil, and neuronal tracts. Glomerular neuropil is a region where finely branched neurites form dense tangles and numerous synapses. The border of dense neuropil is clearly discernable and often delineated by a glia cell layer. non-glomerular neuropil has a low synapse density and poorly defined borders. Its internal structure can not be revealed with general staining methods. A neuronal tract is a bundle of axons that connects neuropil regions. Tracts formed by a large number of axons, in particular large diameter ones, are easily delineated, but this may not be so when the number of axons is small.

Dense neuropils are further grouped into small modular structures of different shapes such as spherical, columnnar, layered. The word, module reminds a column in cerebral cortex of vertebrate. Although the vertebrate column can be distinguished functionally they are not compartmented morphologically (not true because there are the LAYERS! And for the horizontal coordinates, there is barrel cortex and the like, and binocular dominance columns can also be morphologically demonstrated!). On the other hand the insect module is functionally and morphologically compartmented so that it can be used in the study easily.

Modules in the insect nervous system are local microcircuits with similar properties supposedly arranged to process information in a parallel and distributed manner. For example a single lamina cartridge in the peripheral optic lobe of a fly processes information received by 6 photoreceptors looking at the same point in space whereas a glomerulus in the antennal lobe generally processes information from a population of olfactory receptor cells that express the same odorant receptor protein. Neuropil regions specifically concerned with a sensory modality (i.e. lamina of the optic lobe or antennal lobe) are built from such modules that are concerned with more fine-grained aspects within this modality (spatial distribution of light for the lamina, specific odorants for the antennal lobe) In other word one densed neuropile takes charge of one modality and each modules in the neuropile takes charge of a certain quality of modality



Brain structure and function


Insect brains consist of 105 to 106 neurons and brain neurons outnumber those composing the ventral nerve cord . A house fly brain includes about 350,000 neurons, that of a honeybee (worker) about 850,000. On the other hand each thoracic ganglion in any insect consists of 3000 to 5000 neurons and each abdominal ganglion of only 400 to 850 neurons .

The insect brain can be divided into three large regions, protocerebrum deutocerebrum, and tritocerebrum. As mentioned above each of them is associated with one of the three anteriormost ancestral body segments . [the use of “frontal” is always a bit dubious, but especially in insects.

The protocerebrum bears to lateral protrusions, the optic lobes. . Depending on the insect species, the optic lobe can be further subdivided into three or four neuropils. Medially,inbetween the optic lobes are located another conspicuous pair of bilaterally symmetrical neuropils, the mushroom bodies. . The median protocerebrum also contains unpaired neuropils, in particular the protocerebral bridge and the central body, which together form the central complex. The shape, size, and disposition of these dense neuropils differs somewhat between insect groups but the basic morphology is essentially the same. The optic lobes process visual information, the mushroom bodies are important for learning and memory related in particular to olfactory stimuli, and the central complex is thought to play a crucial role in generating behavioral output. The dense, clearly delineated neuropils are surrounded by neuropil areas without conspicuous structural characteristics.

The deutocerebrum is a bilaterally symmetrical brain region that consists of the antennal lobe and the dorsal lobe. The antennal lobe receives the axons of olfactory receptor neurons from the antenna . In at least some insects such as flies and moths, the antennal lobe also receives olfactory receptor axons from the labium. The dorsal lobe receives mechanosensory and gustatory receptor neurons from the antenna and also contains the motor neurons controlling the antennal muscles. The dorsal lobe is called antennal mechanosensory and motor center (AMMC) in some insects. For flies, this name may be retained because they differ from almost all other insects in not having an antennal gustatory system. The anntennal nerve is carrying all sensory axons from the antenna to the deutocerebrum and also contains efferent neurons, in particular the motor neurons of the pedicellus muscles. Generally, a second nerve emanates from beneath the antennal lobe that contains the axons of the motor neurons of the scapus muscles, which are located in the head capsule.

Tritocerebrum is the smallest region and little studied. In the primitive insect ground plan, the tritocerebrum is connected to the suboesophageal ganglian by a pair of circumoesophageal connectives (for example in locusts, crickets, cockroaches), in more derived groups, the tritocerebrum, the posterior part of the deutocerebrum, and the suboesophageal ganglion are fused such that it can be difficult to determine the borders between them. The tritocerebrum contains at least two commissures, one of which is running above the oesophagus in the primitive ground plan. The tritocerebrum receives sensory inputs from the labrum through the labral nerve, from the tegument through the tegumentary nerve, and at least in some insects also direct sensory input from the mouthparts (mandible, maxilla, and labium). The frontal nerves of the tritocerebra on each side run to the frontal ganglion that connects the brain and the stomatogastic system, lying dorsal with respect to the oesophagus and innervating cardiac and gastrointestinal muscles. The frontal ganglion is connected to the hypocerebral ganglion thourgh the recurrent nerve.In some insects (honeybee), the frontal and labral nerves are fused at the root to form a labro-frontal nerve. Tritocerebrum is connected to suboesophageal ganglion through a pair of connective. Frontal ganglion is center of stomatogastric ganglion innervating cardiac and gastrointestinal muscle and is connected to suboesophageal ganglion via recurrent nerve.

The suboesophageal ganglion is largely concerned with the processing of sensory information from and the motor control of the mouth parts. The mouth parts consist of three appendages, the mandible, the maxilla, and the labium. These appendages can be modified in various ways in different insect groups. For instance, many insects have paired maxillary and labial palps. In the bee, a part of the labium forms the unpaired glossae Axons of the receptor cells of the sensilla distributed on the mouth-parts project to the suboesophageal ganglion and in at least some insects also to higher areas. The suboesophageal ganglion also receives direct sensory input from the labrum and through non-olfactory sensory cells of the antenna. Generally, there are separate mandibular, maxillary, and labial nerves innervating the appendages (for example in the bee), but they can also be fused (such as in Dipera, in which there is a fused maxillary-labial nerve) At least in the bee, a little-known fourth nerve emanates from the labial neuromere: the labial gland nerve.

The thoracic ganglia possess motor centers that control important behaviors, such as flight, walking, and vocalization.

The abdominal ganglia possess motor centers related to posture control, rhythmic behavior, such as respiration, and circulation and the control of copulation and egg laying.

The motor centers residing in thoracic or abdominal ganglia autonomously express and correct motor patterns in response to their own local sensory inputs (trigger inputs) largely without depending on inputs from the brain. However, the initiation and maintenance of their motor patterns are generally controlled by descending interneurons from the brain.

The higher order neuropils in protocerebrum, such as mushroom body, central complex, and lateral protocerebrum, receive sensory information from optic lobe, deutocerebrum, tritocerebrum, suboesophageal ganglion, and ventral nerve cord through several layers of filters in a preprocessed form. The higher neuropils integrate information from various modalities and relay outputs that can eventually results in activating output neurons such as motor neurons of the brain or descending interneurons transmitting command information to the ventral nerve cord, which is important in controlling a wide range of behaviors. Thus, overall control mechanisms such as for triggering particular behaviors are localized in the brain and are separated from local control mechanisms that are responsible for aspects like the maintenance of motor patters for wing flapping or walking are localized in the thoracic ganglia. This is not to say that the ventral nerve cord is a fully hardwired system: In seminal experiments, Horridge could demonstate that cockroaches and locusts are capable of leg position learning, which is even improved by removal of the head. Thus, plasticity is possible at short time scales even in thoracic circuits controlling very basic behaviors.




Neuronal composition of the insect brain


Insect brain consists of neurons and glia cells. Both neuron and glia cell are differentiated from the same stem cells. Glia cells have traditionally been mostly seen as structural components, but there is no doubt that they also have important functional roles, which are still poorly understood in the insect brain.

Neurons (nerve cells) play the major role in information processing in the brain. Ethymologically, “neuron” is Greek for “string”, “tendon”, “nerve”, i. e. a term for elongated objects.

In the mammalian brain, glia cells are ten times more numerous than neurons whereas in the insect brain, the ratio is approximatedly reversed (Reference 1).

The basic structure and function of neurons is conserved across animal species. Simple nervous systems first emerged in Cnidaria, such as sea anemones and jellyfish, and evolved from loose nerve nets to compact centers, ganglia, which by further fusion and increase of complexity led to strctures one calls brains. From the evolutionary point of view insects represent a branch different from that to which humans belong. Nevertheless, their neurons have many functional properties in common. Therefore, the differences in the performance and capabilities between insect and human brain mainly arise from a difference in size and complexity. Insect brain contains 105 to 106 neurons whereas human brain harbours some 1011 neurons.

Insect neurons may be categorized into five types:

1) Sensory neuron: innervating sensory organs mostly in the cuticle, project from the periphery into the central nervous system.
2)Projection neuron: connects two or more separate regions in the central nervous system.
3)Local interneuron: confined to a certain region in the central nervous system.
4)Motor neuron: projects from central nervous system to muscle targets.
5)Neuroendocrine cell: innervates an endocrine organ.


Among these neuron types, only sensory neurons have somata localised outside the rind cortex (outer soma layer) of the central nervous system. This placement of somata is a prominent feature of the insect brain (and other arthopod brains). In the mammalian cortex for example, the gray matter contains somata, neurites, and synaptic contacts. In insects, the neuropil contains neurites and synapses and the somata are almost completely located in the rind, where synaptic contacts are exceptional.

Many mammalian neurites are myelinated, i.e. ensheathed by glia cells whereas many insect neurites are generally not myelinated. Insect glia cells often envelop bundles of neurites (such as tracts) or neuropil compartments, effectively forming borders.

Many neurons in insect brains are uniquely characterized by their morphology and function and are thus called identified neurons (Fig. 3). The fact that the insect brain possesses a comparatively small number of neurons most of which are identifiable allows us to study the insect brain at the cellular level. An extreme in terms of identified neurons is the nematode Caenorhabditis elegans, in which all 302 neurons have already been identified and even their lineage has been elucidated.

Dendrites are neurites that represent the predominant input regions of a neuron, receiving information from other neurons, mostly through synaptic contacts. In mammals, dendrites directly originate from the soma whereas in insect central neurons, only a primary neurite is sent off into the neuropil by the soma. The site of synapse can be inferred from the morphology of dendrite of a neuron. In many case dendritic spine (Insect neuron possesses spine?) is an input site whereas dendritic bouton and varicose are output sites. In addition dendrite near soma often resides input synapses whereas dendrite far from soma does output synapses. I find it a little confusing.

           

             
Figure 9. The structure of silkmoth brain (reduced silver stained horizontal section). AL, antennal lobe; Ca. calyx of the mushroom body; CB, central body; LAL, lateral accessory lobe; LALC, lateral accessory lobe commissure, LP, lateral protocerebrum; P, peduncle of the mushroom body. The LAL is an important premotor center generating command information.

             

Figure 3. Identified neurons from the silkmoth brain. (A) Group-1 descending neuron (DN). (B)Group-2 DN. Group-1, and -2 DNs are both identified neurons that transmit a premotor command, the flip-flop signal, to the thoracic ganglia. The schematic illustration indicates the location of the somata of Group-1 and-2 DNs.




Morphology of neurons in the insect brain

Figure 8 shows examples of neurons in the insect brain. The spherical structure containing the nucleus is called soma (cell body). The size of somata is about 10 μm in diameter in many neurons (but varies considerably depending on the cell type within a species and also accross species). From the soma protrusions extend: the neurites. Neurons with a single neurite are called monopolar cells (A). Those with two neurites are bipolar cells (B). Those with more than two neurites are multipolar cells. Most insect neurons are monopolar cells. Their primary neurite runs into the neuropil and forms complex branching patterns. In general, the longest neurite within a neuron is the axon and other neurites are dendrites. The axon is representing the output channel of a neuron. On the axon, the output is relayed to other neurons through synapses, the axon being the output element. On the other side of a synapse, a dendrite, the input region of another neuron, receives this output. The patterns of connectivity of synapses between neurons results in a complex network for information processing.


             





Further reading

Insect Mimetics (2009) Shimozawa and Hariyama eds. NTS Japan

With contributions from S.S. Haupt and A. Takashima


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