Nerve, squid

In squid giant axons, PbTx causes a depolarization of the plasma membrane, repetitive discharges followed by depression of action potentials, and a complete blockade of excitability. This action is antagonized by TTX (83,84). PbTx depolarizes nerve terminals and induces neurotransmitter release (85,86) it depolarizes skeletal muscle cells (87) and increases the frequency of action potentials in crayfish nerve cord (88). PbTx also produces a contraction of the guinea pig ileum (89). All these effects are prevented by TTX. 

Suberitine, a small protein from the sponge Suberites domcuncula, has a variety of actions. It is not very toxic but causes hemolysis in human erythrocytes, flaccid paralysis in crabs and depolarization of squid axon and abdominal nerve of crayfish. A variety of extracts from Porifera have been shown to be toxic to fish and generally have cytotoxic and hemolytic actions (62,63). As discussed previously, a variety of sponges exude substances that are toxic to fish. 

The transport of information from sensors to the central nervous system and of instructions from the central nervous system to the various organs occurs through electric impulses transported by nerve cells (see Fig. 6.17). These cells consist of a body with star-like projections and a long fibrous tail called an axon. While in some molluscs the whole membrane is in contact with the intercellular liquid, in other animals it is covered with a multiple myeline layer which is interrupted in definite segments (nodes of Ranvier). The Na+,K+-ATPase located in the membrane maintains marked ionic concentration differences in the nerve cell and in the intercellular liquid. For example, the squid axon contains 0.05 MNa+, 0.4 mK+, 0.04-0.1 m Cl-, 0.27 m isethionate anion and 0.075 m aspartic acid anion, while the intercellular liquid contains 0.46 m Na+, 0.01 m K+ and 0.054 m Cl-. 

The ion-channel blocking mechanism has been widely tested and found to be important in both pharmacology and physiology. Examples are the block of nerve and cardiac sodium channels by local anesthetics, or block of NMDA receptor channels by Mg2+ and the anesthetic ketamine. The channel-block mechanism was first used quantitatively to describe block of the squid axon K+ current by tetraethylammonium (TEA) ions. The effects of channel blockers on synaptic potentials and synaptic currents were investigated, particularly at the neuromuscular junction, and the development of the single-channel recording technique allowed channel blockages to be observed directly for the first time. 

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]. 

FIGURE 10-7 The delay between Ca2+ influx into the nerve terminal and the postsynaptic response is brief. The temporal relationships between the Ca2+ current and the action potential in the nerve terminal and the postsynaptic response in the squid giant synapse are shown. The rapid depolarization (a) and repolarization (b) phases of the action potential are drawn. A major fraction of the synaptic delay results from the slow-opening, voltage-sensitive Ca2+ channels. There is a further delay of approximately 200 is between Ca2+ influx and the postsynaptic response. (With permission from reference [20].)... 

Nerve stimulation results in a net influx of sodium ions, and normal conditions are restored by the outward transport of sodium ions against an electrochemical gradient. While several earlier workers had identified ATPases in the sheath of giant squid axons, it was Skou who first connected the sodium, potassium ATPase [EC 3.6.1.37] with the ion flux of neurons. This discovery culminated... 

The sulfamate saxitoxins have very low potencies relative to their carbamate hydrolysis products. This relationship has been observed in every assay system tried, including the standard mouse bioassay (Figure 3), squid giant axon (1 ), frog sciatic nerve (16), mammalian brain (1 ), and single rat sarcolemma sodium channels incorporated into lipid bilayers (15). It seems unlikely that human oral potencies are an exception to this trend. 

Skeletal muscle Giant nerve fiber of the squid p mol cm-2 s Red blood cell 1... 

In contrast to the ion exchange theory, much evidence indicates that cells have an active ion pump that removes Na+ from cells and introduces K+. For example, the cytoplasm of the giant axons of nerves of squid can be squeezed out and replaced by ionic solutions. Erythrocyte ghosts can be allowed to reseal with various materials inside. Ion transport into or out of cells has been demonstrated with such preparations and also with intact cells of many types. Such transport is blocked by such inhibitors as cyanide ion, which prevents nearly all oxidative metabolism. However, the cyanide block can be relieved by introduction into the cells of ATP and other phosphate compounds of high group-transfer potential. 

Tsukada (30) have examined developmental changes of the enzyme in chick brain and spinal cord. Enzymic activity appears at about the eighteenth day of incubation and increases rapidly until 3 days after hatching in the brain and between 18 and 21 days of incubation in the spinal cord. These are precisely the periods of active myelination in the brain and spinal cord of the chick, respectively. Similarly, brain tissue of the newborn rat is devoid of cyclic phosphate diesterase activity it appears at about 8 days after birth and increases dramatically between the tenth and thirty-fifth day of life (29). This coincides precisely with the development of myelin in this species. The diesterase is essentially absent in the brain of the jimpy mouse (31), a lethal mutant devoid of myelin in the central nervous system. It is also absent from the spinal cord of this mutant. The enzyme is about 50% deficient in brain tissue of the quaking mouse (29), a mutant with partial deficiency of myelin. There is no activity in nerve fibers and ganglia from a variety of invertebrates such as squid, octopus, crab, shrimp, and starfish. Nerve tissue in these organisms is nonmyelinated. All these observations point to an intimate association of the enzyme with myelin in vivo. 

Skou had established previously that such anesthetics inhibited certain membrane-bound enzymes found in the membrane fraction isolated from nerve homogenates. He decided to study the endogenous ATPase activity as a possible target for future work, initially thinking that it might be the sodium channel. Luckily, since he lacked access to squid giant axons, he used the easily-obtained mixed nerve from the claw of the crab for this purpose. This proved to be a fortuitous choice since, in contrast to mammalian membrane fragments, these did not spontaneously reseal to form closed vesicles. This allowed substrates added to the... 

Using the information given in Table 2.1, calculate the potential difference (in mV) across a squid nerve cell. 

Giant axons from squid have a large diameter ranging from 300 to 700 ym. Because of this large size, we are able to insert metal electrodes directly into the axon and measure capacitance and conductance across the membrane. This is the most unequivocal method to measure transmembrane capacitance and is far better than the use of external electrodes as done previously by Cole and Curtis (11). In spite of the simplicity and ease of this technique, there are still a few unsolved problems which will be discussed later. Figure 3 shows one of the exemplary results of nerve membrane capacitance and conductance measurements. Comparison of this result with the one shown in Figure 2 readily demonstrates that there are considerable differences between these two sets of curves. 

A model, based on a perturbation analysis of the highly successful empirical formulation of Hodgkin and Huxley (1), has been developed which makes predictions of the effect of oscillating fields on a particular nerve membrane system (2). In the present paper, a theoretical model will be presented along with some of the predicted effects of AC electric fields on the Hodgkin-Huxley (HH) model of squid axon membranes. 

Blum, M.M., Timperley, C.M., Williams, G.R., Thiermann, H., Worek, F. (2008). Inhibitory Potency against human acetylcholinesterase and enzymatic hydrolysis of fluorogenic nerve agent mimics by human paraoxonase 1 and squid diisopropyl fluorophosphatase. Biochemistry 47(18) 5216-24. 

Sphingatrienes also occur naturally but in minor amounts. Sphingomyelin isolated from squid nerve and from starfish contains a branched, triunsaturated sphingoid base, 2-amino-9-methyl-4 , 8 , 10 -octadecatriene-l,3-diol (10), and glycosphingolipids from starfish contain the same long-chain base without the methyl branch (9). 

Ohashi Y, Tanaka T, Akashi S, Morimoto S, Kishimoto Y, Nagai Y. Squid nerve sphingomyelin containing an unusual sphingoid base. J. Lipid Res. 2000 41 1118-1124. 

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