Gap junction connections

Gaussian white noise is implemented according to the Box-Mueller algorithm [101]. Ic is the coupling current in network simulations which accounts for electrotonic gap-junction connections between the neurons. For bidirectional coupling of two neurons the coupling current Ic are of the form [94] ... 

The concentric arrangement of some smooth muscle fibers enables them to control dilation and constriction in the intestines, blood vessels, and other areas. While innervation of these cells is not individual, excitation from one cell can spread to adjacent cells through nexuses which join neighbor cells. Smooth muscle cells have a small diameter of about 5-15 micrometers and are long, typically 15-500 micrometers. They are also wider in the center than at their ends. Gap junctions connect small bundles of cells which are, in turn, arranged in sheets. 

In addition to the improved structure and cell connectivity, three-dimensional cultures have shown increased survival and enhanced neuronal differentiation compared to traditional mono-layer cultures [36, 37], The increased survival could be related to the improved cell-cell and cell-matrix contact as it leads to improved cell signaling and gap junction connections. Also cellcell signaling can promote proliferation of glial and NSCs that can contribute to increased culture survival. Moreover, the improved cellcell interaction in the 3D structure, especially between astrocytes and neurons, enhances neurogenesis, synapse formation, and axon myelination. 

Gheorghiu, E., Balut, C., Gheorghiu, M., 2002. Dielectric behaviour of gap junction connected cells a microscopic approach. Phys. Med. Biol. 47, 341—348. 

By exchange of small signaling molecules through particular channels ( gap junctions ) connecting neighboring cells this type of Interaction requires direct cell-cell contacts [11]. 

When cells lie adjacent to each other in animal tissues, they are often connected by gap junction structures, which permit the passive flow of small molecules from one cell to the other. Such junctions essentially connect the cells metabolically, providing a means of chemical transfer and communication. In certain tissues, such as heart muscle that is not innervated, gap junctions permit very large numbers of cells to act synchronously. Gap junctions also provide a means for transport of nutrients to cells disconnected from the circulatory system, such as the lens cells of the eye. 

AQPO, formerly known as the Major Intrinsic Protein of 26 kDa (MDP26), is specifically expressed in the plasma membrane of eye lens fiber cells. It transports water to a low degree, but has also been implicated in cell adhesion and gap junction formation. Its main role is to maintain the transparency of the lens by maintaining a tight cellular connection to neighboring cells and/or by controlling the fluid circulation. 

An internodal conduction pathway also extends from the SA node and transmits the impulse directly to the atrioventricular (AV) node. This node is located at the base of the right atrium near the interventricular septum, which is the wall of myocardium separating the two ventricles. Because the atria and ventricles are separated from each other by fibrous connective tissue, the electrical impulse cannot spread directly to the ventricles. Instead, the AV node serves as the only pathway through which the impulse can be transmitted to the ventricles. The speed of conduction through the AV node is slowed, resulting in a slight delay (0.1 sec). The cause of this AV nodal delay is partly due to the smaller fibers of the AV node. More importantly, however, fewer gap junctions exist between the cells of the node, which... 

In certain neurons, a different type of synapse, called a gap junction, may be formed. Gap junction transmission occurs through membrane channels made of six subunits, which directly connect with other postsynaptic gap junction channels. When the channels open, there is a continuity of cytoplasm and exchange of ions between the two neurons. This mode of transmission is faster because it does not involve the time-consuming processes of neurotransmitter release, diffusion across the synapse, and receptor binding. 

Fig. 3.1. Principal mechanisms of intercellular communiciation. a) communication via extracellular messengers and receptor systems, b) communication via gap junctions. Gap junctions are direct connections between cells. They are coated by proteins (drawn as circles) that can have a regulatory influence on the transport, c) communication via cell surface proteins.

At least three types of disturbance in the intercellular communication have to be distinguished (1) separation of the cardiac muscle fibers by strands of connective tissue as occurring in microfibrosis (2) changes in the distribution of gap junction channels, and (3) changes in the conductance of gap junctional channels either by alteration of the open probability or of the single channel conductance. 

The next issue to discuss is the diversity of connexins, i.e. the various isoforms, and species variability. Gap junctional channels exist in a broad variety of tissues including the heart, vascular system, brain, epithelial tissues, uterus, lens cells, pancreas and kidney. However, these tissues are connected by different isoforms of gap junctional connexins which can be distinguished with regard to their molecular weight. These differences are mainly due to various lengths of the C-terminal loop. 

Heart muscle fibers are coupled by gap junctions. These intercellular channels provide the exchange of small molecules (<1,000 D), like second messengers, between the cells and they allow electrical coupling. Thus, these cells connected to each other form a syncytium. However, from mapping studies it became evident that under certain conditions, e.g. regional ischemia, the ischemic region uncouples. In addition, mapping studies demonstrated that there is a special activation pattern which accounts for a directed activation of the whole heart. This activation pattern exhibits a considerable similarity from beat to beat. It is well known that the conduction velocity varies between... 

However, it is well known that with increasing age microfibrosis is observed which in turn will seperate the fibers from each other and thereby enhance the degree of nonuniformity as discussed in the first chapter of this book. This is accompanied by a reduction in side-to-side connections [Spach and Dolber, 1986]. Thus, with increasing age the intercellular communication can be expected to be reduced probably due to structural changes in the tissue with deposition of collagenous fibers. Concomitant changes in the gap junction distribution are probably secondary to cardiac diseases, although at present an effect of age per se cannot be excluded. 

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