Cultured neuronal network

Advantages The use of cultured neuronal networks as a model for their in vivo counterparts has been an indispensable resource for decades. When the neurons are suspended in solution and subsequently dispensed, the connections previously made are destroyed and new ones formed. Ultimately, the connectivity (and consequently the functionality) of the tissue is changed from what the original template suggested. Another disadvantage lies in the fact that the cultured neurons lack a body and are thus severed from sensory input as well as the ability to express behavior – a crucial characteristic in learning and memory experiments. It is believed that such sensory deprivation has adverse effects on the development of these cultures and may result in abnormal patterns of behavior throughout the network. Cultured networks on traditional MEAs are flat, single-layer sheets of cells with connectivity only two dimensions. Most in vivo neuronal systems, to the contrary, are large three-dimensional structures with much greater interconnectivity. This remains one of the most striking differences between the model and the reality, and this fact probably plays a large role in skewing some of the conclusions derived from experiments based on this model. ==Growing a neuronal network==

Neurons used Because of their wide availability, neuronal networks are typically cultured from dissociated rat neurons. Studies commonly employ rat cortical, hippocampal, and spinal neurons, although lab mouse neurons have also been used. Currently, relatively little research has been conducted on growing primate or other animal neuronal networks. Harvesting neural stem cells requires sacrificing the developing fetus, a process considered too costly to perform on many mammals that are valuable in other studies. One study, however, did make use of human neural stem cells grown into a network to control a robotic actuator. These cells were acquired from a fetus that spontaneously aborted after ten weeks in gestation. Long-term culture One of the most formidable problems associated with cultured neuronal networks is their lack of longevity. Like most cell cultures, neuron cultures are highly susceptible to infection. They are also susceptible to hyperosmolality from medium evaporation. Another study investigates establishing a stable one-to-one connection between neurons and electrodes. The goal was to meet the ideal interface situation by establishing a correspondence with every neuron in the network. They do so by caging individual neurons while still allowing the axons and dendrites to extend and make connections. Neurons are contained within neurocages or other sorts of containers, and the device itself could be referred to as the caged neuron MEA or neurochip. Other research suggests alternative techniques to stimulating neurons in vitro. One study investigates the use of a laser beam to free caged compounds such as neurotransmitters and neuromodulators. A laser beam with wavelength in the UV spectrum would have extremely high spatial accuracy and, by releasing the caged compounds, could be used to influence a very select set of neurons. ==Network behavior==

Spontaneous network activity Spontaneous network bursts are a commonplace feature of neuronal networks both in vitro and in vivo. To eliminate aberrant activity, researchers commonly use magnesium or synaptic blockers to quiet the network. However, this approach has great costs; quieted networks have little capacity for plasticity Array-wide burst stability Stegenga et al. set out to establish the stability of spontaneous network bursts as a function of time. They saw bursts throughout the lifetime of the cell cultures, beginning at 4–7 days in vitro (DIV) and continuing until culture death. They gathered network burst profiles (BPs) through a mathematical observation of array-wide spiking rate (AWSR), which is the summation of action potentials over all electrodes in an MEA. This analysis yielded the conclusion that, in their culture of Wistar rat neocortical cells, the AWSR has long rise and fall times during early development and sharper, more intense profiles after approximately 25 DIV. However, the use of BPs has an inherent shortcoming; BPs are an average of all network activity over time, and therefore only contain temporal information. In order to attain data about the spatial pattern of network activity they developed what they call phase profiles (PPs), which contain electrode specific data. Corollary to this argument is the necessity for interaction with the environment around it, something that cultured neurons are virtually incapable of without sensory systems. Plasticity, on the other hand, is simply the reshaping of an existing network by changing connections between neurons: formation and elimination of synapses or extension and retraction of neurites and dendritic spines. But these two definitions are not mutually exclusive; in order for learning to take place, plasticity must also take place. In order to establish learning in a cultured network, researchers have attempted to re-embody the dissociated neuronal networks in either simulated or real environments (see MEART and animat). Through this method the networks are able to interact with their environment and, therefore, have the opportunity to learn in a more realistic setting. Other studies have attempted to imprint signal patterns onto the networks via artificial stimulation. This can be done by inducing network bursts or by inputting specific patterns to the neurons, from which the network is expected to derive some meaning (as in experiments with animats, where an arbitrary signal to the network indicates that the simulated animal has run into a wall or is moving in a direction, etc.). The latter technique attempts to take advantage of the inherent ability of neuronal networks to make sense of patterns. However, experiments have had limited success in demonstrating a definition of learning that is widely agreed upon. Nevertheless, plasticity in neuronal networks is a phenomenon that is well-established in the neuroscience community, and one that is thought to play a very large role in learning. ==See also==