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One factor in schizophrenia may be a decrease in NMDA receptor mediated excitation of interneurons, both acting during brain development and also more acutely. The NMDA receptor antagonist, ketamine, given to normal volunteers, can elicit psychotic symptoms. Ketamine also affects gamma (30 – 70 Hz) and beta oscillations experimentally, in in vitro brain slices. The effects of ketamine, however – whether to enhance or diminish oscillation power, and acting on which frequency range – depend on the brain region in question. There is no pan-cortical phenomenology of oscillations.

The neocortex is built from five principal-cell types and numerous classes of interneurons. Early formulation of cortical structure emphasized the modularity of the neocortex. Its robust local tensegrity organization has allowed for continuous growth. Medium- and long-range connections that compose the white matter and interconnect nonadjacent cortical neuronal circuits are relatively sparse but sufficient to keep the synaptic path lengths constant in brains of different sizes. Such interconnectedness is a prerequisite for global operations in finite temporal windows. The small-world-like, scale-free organization of cortical architecture may provide some quantitative rules for the growth of both cell numbers and associated axonal connections while minimizing the cost of connectivity, though available anatomical data indicate that cortical areas processing similar kinds of information are more strongly connected than required. Limiting excitatory spread and segregation of computation are solved by balanced interactions between the excitatory principal cells and inhibitory interneurons.

In addition to principal cells, the cerebral cortex contains diverse classes of interneurons that selectively and discriminately innervate various parts of principal cells and each other. The hypothesized “goal” of the daunting connectionist schemes of interneurons is to provide maximum functional complexity. Without inhibition and dedicated interneurons, excitatory circuits cannot accomplish anything useful. Interneurons provide autonomy and independence to neighboring principal cells but at the same time also offer useful temporal coordination. The functional diversity of principal cells is enhanced by the domain-specific actions of GABAergic interneurons, which can dynamically alter the qualities of the principal cells. The balance between excitation and inhibition is often accomplished by oscillations. Connections among interneurons, including electrical gap junctions, are especially suitable for maintaining clocking actions. Thus, the cerebral cortex is not only a complex system with complicated interactions among identical constituents but also has developed a diverse system of components.

The varied and complex roles of interneurons in the control of spinal cord function have provoked widespread interest in understanding how these neurons operate to enable appropriate and synchronised output from the spinal cord. This chapter focuses on the role of spinal interneurons in autonomic control and examines their properties, morphology, inputs and outputs. The location, neurochemical diversity, and firing patterns of spinal presympathetic interneurons suggest diverse roles for these interneurons while changes in these neurons after spinal cord injury may indicate their importance in the pathological changes occurring in this condition. Parasympathetic interneurons play roles in the precise orchestration of specific reflexes, such as those involved in bladder and bowel control, which is reflected in the lack of overlap of interneurons sub-serving different functions. Those interneurons that do show a less specific output may only be activated in unusual circumstances or conditions.

This chapter concentrates on the mechanisms regulating the onset of ocular dominance (OD) plasticity. It describes how alteration in the timing of the critical period can be implicated in neurodevelopmental diseases. It reveals a critical role for basket cell interneuron maturation in the onset of critical period plasticity. This chapter shows that maturation of GABAergic inhibition regulates the onset of the critical period for OD plasticity in the visual cortex. It suggests that alterations in molecular pathways involved in regulating critical period plasticity are linked with neurodevelopmental diseases.

To effectively send a message, a single neuron must cooperate with its peers. Such cooperation can be achieved by synchronizing their spikes together within the time window limited by the ability of the downstream reader neuron to integrate the incoming signals. Therefore, the cell assembly, defined from the point of view of the reader neuron, can be considered as a unit of neuronal communication, a “neuronal letter.” Acting in assemblies has several advantages. A cooperative assembly partnership tolerates spike rate variation in individual cells effectively because the total excitatory effect of the assembly is what matters to the reader mechanism. Interacting assembly members can compute probabilities rather than convey deterministic information and can robustly tolerate noise even if the individual members respond probabilistically.