Meta-Biol

• Pathways

Electrical transmission across nerve cells is accomplished when the current generated in the upstream neuron spreads to the downstream neuron through a path of low electrical resistance. In neurons this is accomplished at gap junctions. Electrical synapses are found in neuronal tissue where the activity of neurons must be highly synchronized. The neurons responsible for hormone secretion from the mammalian hypothalamus are a class of highly synchronized electric neurons. Gap junctions connecting the presynaptic cell with the postsynaptic cell allow current generated in the presynaptic cell to flow directly into the postsynaptic cell. Transmission speed is dramatically increased in such a system. The junction itself is composed of two hemichannels, one each on the pre- and postsysnaptic cells. These channels are composed of members of the connexin family of proteins.

The human brain contains at least 100 billion neurons, each with the ability to influence many other cells. Clearly, highly sophisticated and efficient mechanisms are needed to enable communication among this astronomical number of elements. This communication occurs across synapses, the functional connection between neurons. Synapses can be divided into two general classes: electrical synapses and chemical synapses. Electrical synapses permit direct, passive flow of electrical current from one neuron to another. The current flows through gap junctions, specialized membrane channels that connect the two cells. Chemical synapses enable cell-to-cell communication using neurotransmitter release. Neurotransmitters are chemical agents released by presynaptic neurons that trigger a secondary current flow in postsynaptic neurons by activating specific receptor molecules. Neurotransmitter secretion is triggered by the influx of Ca2+ through voltage-gated channels, which gives rise to a transient increase in Ca2+ concentration within the presynaptic terminal. The rise in Ca2+ concentration causes synaptic vesicles (the presynaptic organelles that store neurotransmitters) to fuse with the presynaptic plasma membrane and release their contents into the space between the pre- and postsynaptic cells.