Open transmission channels

Certain G-proteins can induce opening of channel proteins. In this way, potassium channels can be activated (e. g., acetylcholine effect on sinus node, p.104 opioid effect on neural impulse transmission, p.208). 

Neuronal production of NO is triggered when an activated neuron releases a chemical messenger (glutamate) from the presynaptic neuron to bind to receptor sites (A-methyl-D-aspartate) on the postsynaptic neuron. The transmission of this nerve impulse opens a channel in the receptor allowing the influx of calcium ions. These ions can then bind to calmodulin already present in the cell, forming a Ca / CaM complex that binds to and activates nNOS [90]. The NO radical that is produced in turn activates soluble guanylate cyclase in the postsynaptic neuron and potentially in the presynaptic neuron. 

In previous sections, we have, for simplicity, confined our attention to situations involving one open channel. For isolated Fano profiles, this leads to an exact zero in the cross section at the transmission window, and to a series of exact zeros in the cross section associated with each resonance for a complete Rydberg series. Obviously, if one wishes to study profile shapes in detail and the connection between the bound states and autoionising resonances implied by Seaton s theorem (see previous section) it is desirable to work just above the first ionisation potential, where the number of open continuum channels is at a minimum. In general, this is not possible, and the number of open channels increases rapidly with increasing energy. 

In 1988, such a scenario was played out for the first time on a worldwide scale. Since then, numerous incidents have reinforced the public s sense of vulnerability to attacks by insidious code fragments on software and data stored in all kinds of computers. While earlier virus attacks spread via diskettes and later via electronic bulletin boards (in ways that required some user participation through loading infected programs), in recent years, the World Wide Web and more sophisticated e-mail systems have provided transmission channels that facilitated the worldwide spread of the attackers at a unprecedented speed. Moreover, infection which earlier required some explicit action by the victim has become much more stealthy, with the advent of viruses that become activated through the opening (or even previewing) of an apparently innocent attachment to an e-mail document. 

All these postsynaptic events last only for a few milliseconds synaptic transmission through LGICs is fast. When the postsynaptic cell membrane is sufficiently depolarized, voltage-dependent Na+ channels open and an action potential is generated. 

In cerebellar Purkinje cells, a TTX-sensitive inward current is elicited, when the membrane was partially repolarized after strong depolarization. This resurgent current contributes to high-frequency repetitive firing of Purkinje neurons. The resurgent current results from open channel block by the cytoplasmic tail of the (34 subunit. The med Nav 1.6 mutant mice show defective synaptic transmission in the neuromuscular junction and degeneration of cerebellar Purkinje cells. 

Veratridine is a complex lipophilic alkaloid that also binds to sodium channels, causing them to stay open and thereby disrupting the transmission of nerve action potential. It is found in the seeds of a member of the Liliaceae, Schoenocaulon... 

Unwin, N (1995) Acetylcholine receptor channel imaged in the open state. Nature 373 37-43. Unwin, N (2000) Nicotinic acetylcholine receptor and the structural basis of fast synaptic transmission. Phil. Trans. Roy. Soc. Lond. Ser. B 355 1813-1829. 

Presynaptic events during synaptic transmission are rapid, dynamic and interconnected. The time between Ca2+ influx and exocytosis in the nerve terminal is very short. At the frog NMJ at room temperature, 0.5-1 ms elapses between the depolarization of the nerve terminal and the beginning of the postsynaptic response. In the squid giant synapse, recordings can be made simultaneously in the presynaptic nerve terminal and in the postsynaptic cell. Voltage-sensitive Ca2+ channels open toward the end of the action potential. The time between Ca2+ influx and the postsynaptic response as measured by the postsynaptic membrane potential is 200 ps (Fig. 10-7). However, measurements made with optical methods to record presynaptic events indicate a delay of only 60 ps between Ca2+ influx and the postsynaptic response at 38°C [21]. 

---