The Worm Brain

Introduction

Caenorhabditis Elegans is a microscopic soil-dwelling worm. This worm is one of those creatures which biologists have adopted as a "model organism". Other model organisms include the simple bacterium "E. Coli", the fruit fly, and the common laboratory rat. Despite the huge diversity of life on Earth, all organisms share the same basic biology at the molecular and cellular level. Thus by studying the details of one of these model organisms, it is possible to infer knowledge about all organisms, or at least all organisms of a certain class.

C.Elegans has been specifically used as a model for our knowledge of genetics, developmental biology, and most interestingly for us, simple nervous systems. Indeed, of all organisms C.Elegans has the smallest nervous system to have been studied in great detail. The location and characteristics of every single nerve cell and their interconnections has been mapped. The availability of this knowledge makes C.Elegans a good target for reverse engineering and recreation of a complete artificial nervous system.

General Biology

C.Elegans belongs to the taxonomic group called "nematoda", or nematode worms. The adult has a long cylindrical body of 1.2mm in length and 80 microns in diameter. Although usually overlooked due to its small size, this worm is very common and is found in soil all over the world. A square meter of moist vegetated soil will often contain several million individuals. The worms inhabit the thin film of water on and between soil particles and rotting vegetation. Their diet consists almost exclusively of bacteria.

The worm was first chosen as a model organism in 1965. It was chosen because of its structural and behavioural simplicity, its rapid life cycle, and its ease of cultivation. It will grow readily in a petri dish on a bed of agar and bacteria (E.Coli). The normal lifespan is around 2 weeks. After hatching from an egg, the worm grows through 4 larval stages before emerging as an adult. These larval stages, termed L1 to L4, normally last about 36 hours in total. The third larval stage, L3, is actually optional. In the event of harsh conditions, i.e. a lack of moisture or food, the L2 stage will become what is known as a "Dauer Lava". In this non-feeding state the larva can survive many months waiting until conditions improve before skipping L3 and continuing its life-cyle from L4.

There are two sexes of worm, the male and the self-fertilising hermaphrodite. The hermaphrodite, although structurally a female, produces both sperm and eggs. The majority of offspring produced are from self-fertilised hermaphrodites. Only sometimes do males and hermaphrodites mate and cross-fertilise. Each hermaphrodite adult will produce 300 to 350 progeny, about one in a hundred of these will be male. The nervous system of the male is slightly more complex than the hermaphrodite. Whilst the hermaphrodite is passive during mating, the male has an extra ganglion in its tail for the control of copulatory behaviour. This chapter focuses on the nervous system of the simpler hermaphrodite.

The sequencing of the C.Elegans genome was completed in 1998. The DNA consists of 97 million base pairs, this is approximately 1/30th of the size of the human genome. This DNA contains the code for an estimated the 17,800 genes and is distributed across 6 chromosomes.

Body Structure

The body of the worm is transparent when viewed under a microscope. The body plan is essentially two concentric tubes which are separated by a fluid filled space. It is the hydrostatic pressure of this body space which gives the animal its rigidity. The outer tube comprises the musculature and the secretory, reproductive, and nervous systems. The inner tube forms the digestive tract. The entire animal is covered with a collagenous extracellular cuticle. This outer coating is impermeable, tough, and elastic.

[diagram of body structure]

The musculature is made up of 4 strips of muscle which are attached to the cuticle and run the entire length of the body. Towards the anterior of the body, on the ventral mid-line, there is a small secretory pore. The function of this pore is still unknown. The reproductive system consists of a pair of ovaries, uteri, oviducts, and sperm producing spermathecae. The vulva is located on the ventral mid-line about halfway down the body.

The digestive system runs from the mouth at the front of the the head, through the pharynx and intestine, to the anus near the tail. The pharynx, where food is chewed, has two bulbs which are joined by an isthmus. The intestine is where the food is digested. Nutrients are absorbed through the intestinal walls which are lined with microvilli. The mouth is surrounded by 6 lips, or flaps, called labia. These labia contain the primary sense organs.

All adult worms contain exactly 959 cells. The lineage of each of these cells as they develop from a single egg has been completely determined. It is this constant number of cells, and their entirely predictable lineage, that makes C.Elegans so popular as a study for developmental biologists.

The Nervous System

The complete circuity of the nervous system was determined in 1988 by John White of Cambridge University, England. The nervous system was reconstructed from electron micrographs of consecutive serial sections taken from one animal. Each section was just 50 nanometers thick. This investigation revealed that the hermaphrodite nervous system is bilaterally symmetric and has exactly 302 neurons. Not only is this number of neurons invariant, but the position and morphology of the cell bodies is also fairly constant across individuals. These neurons are connected by about 5,000 chemical synapses, 600 gap junctions, and 2,000 neuro-muscular junctions. The pattern of synapses also shows little variation.

The neurons of C.Elegans, unlike their mammalian counterparts, have very few if any branches. They are typically bipolar, non-branching, and highly connected to their immediate neighbours. The cell bodies are small, i.e. less than 5 microns in diameter. Unfortunately this makes them hard to impale and study using conventional micro-electrodes. ???

[diagram of nervous system]

The largest group of neuron bodies is located in the head of the worm between the two lobes of the pharynx. This group of cells could be called the brain, although the word "brain" is rather a grand term for what is a very simple nervous system. The brain consists of a bundle of nerve fibers which encircle the isthmus of the pharynx. This is called the circumpahryngeal nerve ring. Adjacent to the nerve ring, both anterior and posterior, are the cell bodies. The anterior cells form the anterior ganglion which is connected to the sensory organs in the labia. Posterior to the nerve ring are four ganglia. These are the dorsal, ventral, and two lateral ganglia. They are each separated by a thin lamina. The dorsal ganglia is connected to sensory organs in the head, whilst the ventral ganglia contains inter- and motor neurons. The lateral ganglia contain sensory, inter- and motor neurons. Adjacent to the ventral ganglion is another cluster of neurons called the retro-vesicular ganglion.

The anterior ganglia contains about 40 neurons. The dorsal, ventral and lateral ganglia together contain about 100 neurons, whilst the retro-vesicular ganglion contains about 60. This brings the total number of neurons in the head of the worm to around 200. The remaining 100 or so are located in the ventral nerve cord (~50) and lateral ganglia (~10) and the tail ganglia (~40).

The ventral nerve cord is a single row of cell bodies which runs down the ventral midline of the animal from the retro-vesicular ganglion to the tail. Along the way there is also a pair of small neuron clusters called the posterior lateral ganglia. There are also a few isolated cells along the side of the body. The tail contains four ganglia. These are the preanal ganglion, two laterally symetrical lumbar ganglia, and a single small dorso-rectal ganglia.

It seems that each particular ganglia does not have a specific function. Instead, groups of cells which are analogous in structure and function are distributed across several different ganglia.

The Sensory System

The nematode worm has three types of sensors, these are chemo, thermal, and touch sensors. The worm has no sense of vision or hearing. The primary set of sensors in the head are called the amphids. There are two of these and they are innervated by twelve neurons each. The neurons are bipolar with their cell bodies located in the ganglia posterior to the nerve ring. Their single dendrite terminates as a sensory cilia in the sense organ whilst the axon extends to the nerve ring where it synapses with other neurons, including other amphid neurons. The amphids are chemo and thermo sensors. Eight of the twelve neurons in each amphid are able to detect soluble chemicals, each detecting a different but overlapping set of several substances. Three of the neurons are sensitive to volatile chemicals, and the one remaining neuron is temperature sensitive.

Also located on the head are the labial and cephalic sensilla. These are positioned in concentric circles around the mouth. There are six inner labials, each innervated by two neurons from the anterior ganglion. The tips of the inner labial neurons are exposed to the outside of the body. This suggests that they are of chemosensory function similar to the amphids. The six outer labials, and the 4 cephalic sensilla are each innervated by a single neuron. The endings of these neurons are not exposed to the outside but embedded in the cuticle. This means that they are probably mechano-receptive rather than chemosensory.

On the sides of the body, both anterior and posterior, are two pairs of sensilla called the deirids. These have a morphology similar to the cephalic sensilla so they are probably also mechanoreceptive. Finally, located in the tail are two sensors called the phasmids. These are innervated by neurons from the lumbar ganglia. The function of the phasmids is still not known for sure, but they have a similar morphology to the amphids and so are probably chemosensory. Other neurons which are not connected with any specific sensory organ are probably also responsive to touch.

Nerve Processes

Most of the nerve processes are to be found in the nerve ring surrounding the pharynx, as well as in the ventral and dorsal nerve cords. The processes in the nerve ring extend mostly to sensors in the head, as well as to some in the body. The ring is estimated to contain about 175 nerve processes. Their function is probably to integrate sensory information and then to connect motor neurons in the head and nerve cord. The ventral nerve cord and the smaller dorsal nerve cord are longitudinal bundles of processes which connect sensors in the head with somatomotor neurons and tail ganglia. Circumferential bundles around the body of the worm connect these two nerve cords. In addition to the processes mentioned above, there may also be sets of neurons which communicate without direct synaptic connections. This communication is probably mediated through the long-range transmission of neuropeptides.

Each neuron in the C.Elegans nervous system has been assigned a unique name. These names consist of 3 upper case letters followed by a number of up to 2 digits. An additional symmetry descriptor can also be added (D - dorsal, V - ventral, L - left, R - right). An example neuron is IL1DL. This name refers to the dorsal lateral neuron of class IL1. A complete table of all neuron labels is available on the book website.

Behaviour and Neural Function

The behavioural repertoire of the nematode worm is not particularly complex. The worm spends its time swimming around searching for food. The search process is based on the principle of chemotaxis whereby the worm swims towards attractive stimulants, i.e. food, and away from noxious chemicals. The worm also uses thermotaxis to move towards, or stay within, an area of preferred temperature. When food is found the worm will stop swimming and start feeding, foraging, and defecating. If food is plentiful then egg laying will be triggered. In the text below we will describe each of these behaviours in more detail together with how they are controlled by the neural circuitry.

Locomotion

The worm is able to swim both forwards and backwards as well as turning left and right. This swimming movement is mediated by the four longitudinal muscles extending down the length of the body. When cells in the two ventral muscles are contracted, and cells in the 2 opposing dorsal muscles are relaxed, the body will bend downwards. Conversely, when the ventral muscles are relaxed and the dorsal muscles contracted, the body will bend upwards. By propagating these out-of-phase waves of contraction and relaxation down the length of its body the worm is able to produce the undulating swimming movements which propel it forward. Anterior to posterior waves of contraction cause forward movement, posterior to anterior waves cause backward movement. Because the worm is only able to bend up and down, as opposed to side-to-side, this means that it actually swims on its side. This is of course irrelevant to the worm herself as she doesn't have much of a comprehension of up, down, and sideways anyway.

Swimming is controlled by five classes of neurons in the ventral cord. These classes are labeled A,B,D,AS, and VC - neurons within each class have similar morphologies and connectivities. Five neurons, one from each class, are connected together in a group. This grouping is then repeated about 11 times along the ventral cord. The circuitry of each group is shown in the diagram below.

[diagram - swimming control circuit]

The neurons VA,VB,VC, and VD innervate the ventral muscles, whilst neurons AS, DA, DB, and DD innervate dorsal muscles. Neurons A,B, and AS synapse onto the muscle cells and trigger contractions, they also synapse onto the adjacent D neuron. D neurons synapse muscle cells on the opposite side of the body and trigger relaxation, i.e. they act as cross inhibitors. This pattern of muscle activation and opposing muscle inhibition causes the basic bending movement required for swimming. The method by which the oscillatory repetition of activation and inhibition is generated has not yet been elucidated. However, many such oscillatory pattern generators have been studied in other organisms and it is likely that similar systems are at work in the worm.

The decision making circuit which control whether the worm is to swim forwards or backwards is located in the posterior ganglia in the head. This circuit is made from four bilaterally symmetric interneuron pairs - AVA, AVB, AVD, and PVC. Each of these neurons has large diameter axons which extend down the entire length of the ventral cord. The neurons AVA and AVD synapse onto the A-type motor neurons and drive backward swimming. Neurons AVB and PVC synapse onto the B-type motor neurons and drive forward swimming. Interaction between these opposing circuits may be mediated by connections between ventral cord neurons. ??? (more info on this circuit)

Relatively little is known about how the wave of activation and inhibition is propagated. Neighbouring muscle and neural cells all share gap junctions which could act to relay the activation signals. Alternatively, or even additionally, the A- and B-type motor neurons may also act as stretch receptors. A-type cells are known to have anteriorly directed processes, whilst B-type cells have posteriorly directed processes. Thus activity in once cell could be detected by, and trigger activity in, an adjacent cell.

Feeding and Defecation

The worm feeds by moving her head around and sucking bacteria into her mouth. This sucking action is mediated by the pumping action of muscles surrounding the pharynx. When these muscles contract they open the first bulb of the pharynx. This causes liquid and bacteria to be sucked in through the mouth. When the muscles relax the liquid is expelled again whilst the bacteria remain trapped. The bacteria are then transported to the rear bulb by the action of peristalsis. At the rear of the rear bulb the bacterial cells are broken up by a grinder and pumped into the intestine. The neural circuitry for the control of this pharyngeal pumping has only been partially characterized. What is known is that the circuit contains 20 neurons which are independent from the rest of the nervous system. It seems that the digestive system is in possession of its own little autonomous nervous system. The only connection to the main nervous system is through a bilateral pair of gap junctions to neurons in the anterior ganglion. These gap junctions have little effect other than to briefly inhibit pumping when touch sensors detect a light touch to the head or body.

Under laboratory conditions it seems that only four classes of neuron are required for normal feeding. These classes are labeled M3, M4, MC and SNM. The SNM neurons detect the presence of food. It is thought that they may secret a diffuse messenger chemical, namely serotonin, which has the effect of depressing locomotion and stimulating pharyngeal pumping and egg laying. The MC neurons are sensitive to serotonin. They are believed to be the pacemakers for the pumping action. When the presence of food is detected they trigger muscle contraction and pumping. The M4 neurons are located in the posterior isthmus whilst the M3 neurons are inhibitory. ???

The function of the remaining neurons in the digestive system is unknown. Although their function is required in other species of nematode, it seems that they are either unnecessary or redundant in C.Elegans. They probably play some kind of regulatory role, or they could even be detritus left over from evolution. The intestine itself has no associated muscles, and thus no requirement for neural control either. Food moves down the intestine towards the anus under pressure from the pharyngeal pump and with help from body movements.

When food is plentiful the worm defecates every 45 seconds. This frequency is reduced to every 80 seconds when food is scarce. Three anal muscles control the opening of the anus and the expulsion of waste and the motor action occurs in three steps. The first step is the contraction of the posterior body muscles, this is followed by contraction of the anterior body muscles. Finally the expulsion muscles contract and the waste is expelled. Two classes of neuron are control the expulsion muscle contraction. These are DVB, located in the dorsorectal ganglion, and AVL which has output to both the anal muscles and the head. These two neuron classes display redundancy. Either can be removed and defecation will continue normally. Remove both, however, and defecation will cease. This redundancy is a common theme throughout the C.Elegans nervous system.

Chemotaxis and Thermotaxis

Chemotaxis is the term used to describe the movement of an organism towards, or away from, chemical attractants and repellants. C.Elegans will tend to swim towards a source of bacterial metabolites. This is sensible because the presence of baterial metabolites, or their by-products, signals the availability of bacteria themselves, i.e. worm food. The attractant chemicals are various salts, amino acids, and some vitamins. Repellants are such things as acids, copper ions, and general toxins.

It seems that the worm first recognizes a chemical gradient, orients itself with this gradient, and then swims towards the area of peak concentration. The detection of a chemical gradient is not made by sensors in the head measuring a different concentration to sensors in the tail. Nor is it detected by differences in sensor readings on either side of the body. Instead all sensors are able to detect changes in concentration over time. The normal swimming pattern of the worm is to generally move forward with occasional and spontaneous backward movement. If an increase in concentration over time is detected this spontaneous backward movement is suppressed, i.e. forward movement is encouraged. The converse is also true. If the concentration is found to decrease then forward movement is suppressed and backward movement encouraged. The steepest gradient can also be detected and the worm will tend to move along this line. When a high concentration of a repellant is detected, worms will rapidly reverse and then change direction. They tend to follow the steepest gradient to a lower concentration.

The neural mechanism of chemotaxis in the worm is only partially understood. Neurons of class ASH and ADL are able to detect volatile and water soluble repellants respectively. These cells then synapse onto AVA and AVD interneurons which in turn synapse onto motor neurons which trigger backward movement.

Thermotaxis is a similar concept to chemotaxis. Worms prefer to live within certain temperatures and will swim along thermal gradients so as to maintain these temperatures. Worms are cold blooded animals and are able to survive and be fertile within the temperature range 12 to 26'C. They are able to detect thermal gradients of less than 0.1'C. Their preferred temperature is one which corresponds to that in which they were cultivated. The neural mechanism for thermotaxis is thought to be very similar to that for chemotaxis.

Response to Touch

As well as being sensitive to temperature and many chemicals, C.Elegans also responds to touch stimuli. Under laboratory conditions there are four touch response tests which can be carried out. These tests are a gentle touch to the nose, a gentle body touch, a harsh body touch, and a light tap to the entire dish of worms.

The gentle nose touch, using something like an eyelash, causes backward movement. This touch is sensed in parallel by the neuron classes ASH, FLP, and OLQ. Note that ASH is a multimodal sensory neuron as it is also part of the chemosensory system. These three classes of neuron synapse onto the AVA, AVB, and AVD interneurons which, as we saw earlier, control backward movement. When searching for food the worm will move it's head from side to side in a behaviour known as foraging. This exploratory movement is continuous and relatively complex, at least compared to the simple swimming movement. This foraging movement is effected by eight muscles in the head and is controlled by ten classes of motor neurons. When the side of the head is touched foraging stops and the head withdraws. This circuit involves the OLQ and IL1 sensory neurons which synapse onto the RMD motor neuron.

The gentle body touch can be applied anywhere along the length of the body. A touch towards the anterior causes backward movement whilst a touch to the posterior causes forward movement. The neuron classes sensitive to this kind of touch are ALM, AVM, and PLM. A harsh touch increases the frequency of the spontaneous reversal movement. This kind of touch is detected by the PVD neurons which have long processes along the length of the body wall. These processes synapse onto the AVA and PVC neurons. The tap response is seen to increase both forward and backward movements. ???

Other Behaviours

The other primary behaviours in the C.Elegans repertoire are egg-laying and Dauer lava formation. Neurons mediate the contraction of the eight muscles of the uterus and another eight muscles which open the vulva. Dauer lava formation is regulated by chemosensory neurons. These detect whether the environmental conditions are harsh enough to warrant the formation of a Dauer lava.

Large Scale Neural Circuitry

We have seen that, as is the case in mammalian nervous systems, there is much redundancy of function in the neural circuitry. Each neuron does not have an essential function on its own, but plays a part in a network of collaborating neurons. A single neuron can be killed and the worm will continue to function almost normally. Unlike mammalian systems, however, many of the neurons in C.Elegans are multi-functional. Whereas vertebrate neurons can be classified as exclusively sensory neurons, interneurons, or motor neurons, a single neuron in the worm can have both sensory and motor functions simultaneously.

Another difference between the worm and mammalian nervous systems is in the directionality of the network. The directionality refers to the direction of information flow between the sensory input and the motor output. The neural circuitry of the worm is very directional. 95% of synapses point forward, that is from sensory input towards motor output. Of the remaining 5% which point backwards, the vast majority are reciprocal connections. There is only a very limited amount of indirect feedback. This is in stark contrast to the mammalian cortex where just about every projection has a corresponding reverse projection.

The reciprocal feedback doesn't appear to interlink the whole circuitry. Instead neurons within a functional group only have reciprocal feedback with each other, not with neurons outside the group. For examble, the circuitry associated with processing information from the amphids is almost entirely isolated from motor processing. This suggests that the amphid circuitry is itself responsible for integration of information and a final decision is then transmitted to the motor neurons which carry out the chosen response.

The processing depth of a neural circuit is the number of synapses between sensory input and motor output. The processing depth between the sensors and muscles in head averages 3.5 synapses. this number displays variation between the functional groups. For example the amphid to muscle pathway contains on average 4.4 synapses. This indicates more integration than in other pathways. The other sensory to motor pathways in the head average only 2.2 synapses.

Learning and Memory

The lowly worm is not a particularly intelligent beast. It is certainly not capable of rational thought, abstract art, or understanding it's own existence. But then again, why would it need to? It is a simple organism and leads a simple life within it's own little niche in the ecosystem. It does, however, demonstrate the kind of adaptive behaviour which is so typical of living things. This adaptation is clearly a primitive form of learning and memory.

Examples of adaptation are chemo- and thermotaxic behaviours which are modified by experience. The worm is able to remember the temperature at which it was raised. In adult life it will then seek out and move towards an area of this temperature. If the worm is moved to a new temperature the preference slowly shifts, over a period of about four hours, to this new temperature. If the worm is starved in the presence of a high concentration of attractant it will slowly lose sensitivity to that attractant. This adaptation is clearly useful to the worm because it learns that the attractant doesn't mean that food is available. The neural and molecular mechanisms for these adaptive behaviour are currently unknown. It is likely, however, that changes in synaptic strength between sensory and integrating neurons are responsible. ???

Worm Brain Simulation

As we have seen, the brain of the nematode worm C.Elegans is very simple. Although still incomplete, our knowledge of the structure and function of this simplest of nervous systems is very detailed. This simplicity, combined with detailed knowledge, make the worm brain suitable for computer simulation. Perhaps surprisingly, there are very few researchers working on such a simulation. A few worm brain simulation projects are discussed below.

Lego Worm-Inspired Robot

In 1998 Shawn Lockery and colleagues at Oregon University in the USA built a small lego robot whose behaviour was contolled by an artificial neural network. Both the mechanical design of the robot and its controlling artificial neural network were patterned after the body and brain of C.Elegans.

[diagram of robot]

The aim of the experiment was to simulate the chemotactic circuitry of the worm. It is difficult to create, and to detect, chemical gradients on the relatively large scale of a robot. So instead, the chemical gradient was substituted for a light gradient. The robot, four-wheeled cart of length 28cm, was fitted with a single sensor, a photocell, attached to its "head". This photocell, although detecting light intensity and not chemical concentration, was used to represent the chemosensors of the worm, the amphids. The servos used to steer the front wheels represented the neck muscles, whilst the motors powering the rear wheels represented the propulsion provided by the worm's swimming motion. Although a 4-wheeled cart robot may not seem very worm-like, this design was sufficient to test the circuit responsible for chemotaxis. It was not necessary to build a more complicated snake-like robot.

[photograph of robot]

The neural network consisted of a single sensory neuron, three interneurons, and two motor neurons. The sensory neuron recieved an input from the photocell while the two motor neurons control the steering of the robot. Each neuron is connected to all other neurons though a weighted synaptic connection. This neural network was created on a personal computer and connected to the robot via a cable. The onboard electronic circuitry for communicating with the neural net consisted of a 2MHz 68HC11 microprocessor and analog to digital converters connected to the photocell and steering servos.

[diagram of neural net]

Initially, the synapse weights were tuned in a computer simulation of the vehicle. The weights were then tested on the real robot. The light source used was a single 100W bulb suspended 120cm above the floor. It was shown that reliable phototaxis could be performed. The robot would move from various starting positions to the area of greatest light intensity, it then remained in orbit around this area. The robot was also shown to be robust to misalignment of the steering mechanism.

NemaSys

Thomas Ferree, at the University of California, has been working on a software project called Nemasys. The aim of this project is to simulate the complete organism as well stimuli from a virtual environment. It is intended that the simulation will implement everything that is known about the worm, including body mechanics as well as every neuron and synapse. The plan is to interface this simulation to either of the famous neural network simulators: GENESIS or NEURON. Unfortunately this project is currently on hold, as of late 2001, whilst further funding is sought.

Mind-Uploading

A third nematode brain simulation project is that run by the Mind Uploading Research Group (MURG). This group is a loose band of people from various backgrounds who are working together over the internet. They are developing technologies to enable the scanning of a complete worm at a near molecular level and the uploading this information into a computer simulator. The aim is to be able to scan a number of different worms and to observe how slight differences in the structure of the nervous system cause differences in behaviour. The much longer term aim of the mind uploading group is the uploading and simulation of a complete human brain. Of course this is well beyond the capability of current technologies, hence they are starting with the worm brain.

Conclusion

We have seen that the nematode worm, C.Elegans, has one of the simplest nervous systems in the animal kingdom. This nervous system comprises exactly 302 neurons. The morphology and connectivity of each neuron is known in considerable detail. Our understanding of how the neural circuitry leads to behaviour, however, is still incomplete. It is anticipated that a complete understanding will come eventually, but this will take a few years yet, perhaps even a decade or more. When a complete circuit diagram is finally drawn up, it would then be relatively easy to recreate the circuit within a computer simulation. Maybe such a simulation will be the first true artificial brain.

Of course, even if the complete nervous system of the worm was to be recreated artificially, what could it be used for? We have seen that the worm has a very limited repertoire of simple behaviours. It is probably due this lack of usefulness, or commercial application, that an artificial worm brain hasn't been built already.

References