Electrical excitation, of nerve

The second level (1 mA) of 50/60 Hz is due to the direct electric excitation of nerves. The level increases as frequency is increased above 1 kHz (Figure 10.18). For example, 10 mA at 100 kHz is without perception, but temperature rise caused by the current may then be a limiting factor. Interpersonal variations... 

The second level (1 mA) is due to the direct electric excitation of nerve endings, which must be a function of the local current (density). The electric current threshold of perception with firm hand grip contact and contact area several square centimeters, is around 1 mA. Threshold current has a surprisingly small dependence on contact area. The reason for this is mentioned previously in the chapter on DC perception. With a small area contact around... 

We may extrapolate from these basic considerations in two ways. We may evaluate the extracellular electric gradients associated with intrinsic or imposed tissue fields against the magnitude of the gradients in the membrane potential, both in resting conditions and in association with modification of the membrane potential during synaptic excitation. In this way, we may appraise the probability of direct effects of extracellular tissue fields in excitation of nerve cells. A second approach will consider the observed biological sensitivities to these fields. This will lead to the crux of our current dilemma. A... 

Because the processes of excitation are largely dissipative (i.e., the flow of ions down existing electrochemical gradients and the use of chemical bond energy to restore the ionic gradients), a net evolution of heat would be expected. This evolution is observed, but measurements of the heat associated with excitation of nerve fibers (24) or electric organs (25) indicate three distinct phases in the heat flow during an action potential ... 

Colombo, J. and C.W Parkins (1987). A model of electrical excitation of the mammalian auditory-nerve neuron. Heart Res. 31, 287-312. 

Tonus is the natural and continuous slight contraction of a muscle. Electrotonus is the altered electrical state of nerve or muscle cells from the passage of a DC. Subthreshold DC currents through nerves and muscles may do the tissue more (excitatory effect) or less (inhibitory effect) excitable. Making the outer nerve cell membrane less positive lowers the threshold and has an excitatory effect (at the cathode, catelectrvtonus), the anode will have a certain inhibitory effect (anelectrotonus). This is used in muscle therapy with diadynamic currents (see the following section). 

Spatial derivative of the electrical field The first spatial derivative (or the second spatial derivative of the voltage along the nerve) is responsible for electrical excitation of the nerve. Therefore, an electrical field with a nonzero second spatial derivative is required for excitation. An axon with a linearly decreasing voltage distribution would not be excited despite the presence of a large voltage difference along the axon. 

Since the discovery of gangliosides by Klenk in the first half of the twentieth century, considerable effort has been made to understand their functional role in the central nervous system. In 1961, Mcllwain reported that the addition of gangliosides could restore the lost electrical excitability of cortical slices kept at 0 C for 5 h. Similarly, the administration of gangliosides could prevent the deleterious effects of ethanol on nerve function. In cell cultures, exogenously added gangliosides could induce axonal sprouting and neurite outgrowth. " ... 

Muscle contraction is initiated by a signal from a motor nerve. This triggers an action potential, which is propagated along the muscle plasma membrane to the T-tubule system and the sarcotubular reticulum, where a sudden large electrically excited release of Ca " into the cytosol occurs. Accessory proteins closely associated with actin (troponins T, I, and C) together with tropomyosin mediate the Ca -dependent motor command within the sarcomere. Other accessory proteins (titin, nebulin, myomesin, etc.) serve to provide the myofibril with both stability... 

Another important property of the outer membranes of nerve and muscle cells is their susceptibility to excitation under the effect of electric action. Excitation can be brought about, for example, by an external electric current pulse. Pulses can be applied to the membrane with the aid of two microelectrodes, one residing in the extracellular fluid and the other introduced through the membrane into the cytoplasm. 

The idea that signals are transmitted along the nerve channels as an electric current had arisen as early as the middle of the nineteenth century. Yet even the first measurements performed by H. Helmholtz showed that the transmission speed is about lOm/s (i.e., much slower than electric current flow in conductors). It is known today that the propagation of nerve impulses along the axons of nerve cells (which in humans are as long as 1.5m) is associated with an excitation of the axon s outer membrane. 

The electricity-producing system of electric fishes is built as follows. A large number of flat cells (about 0.1 mm thick) are stacked like the flat unit cells connected in series in a battery. Each cell has two membranes facing each other. The membrane potentials of the two membranes compensate for each other. In a state of rest, no electrostatic potential difference can be noticed between the two sides of any cell or, consequently, between the ends of the stack. The ends of nerve cells come up to one of the membranes of each cell. When a nervous impulse is applied from outside, this membrane is excited, its membrane potential changes, and its permeability for ions also changes. Thus, the electrical symmetry of the cell is perturbed and a potential difference of about 0.1 V develops between the two sides. Since nervous impulses are applied simultaneously to one of the membranes in each cell, these small potential differences add up, and an appreciable voltage arises between the ends of the stack. 

According to Fig. 6.17 the nerve cell is linked to other excitable, both nerve and muscle, cells by structures called, in the case of other nerve cells, as partners, synapses, and in the case of striated muscle cells, motor end-plates neuromuscular junctions). The impulse, which is originally electric, is transformed into a chemical stimulus and again into an electrical impulse. The opening and closing of ion-selective channels present in these junctions depend on either electric or chemical actions. The substances that are active in the latter case are called neurotransmitters. A very important member of this family is acetylcholine which is transferred to the cell that receives the signal across the postsynaptic membrane or motor endplate through a... 

This results in the extrusion of three positive charges for every two that enter the cell, resulting in a transmembrane potential of 50-70 mV, and has enormous physiological significance. More than one-third of the ATP utilized by resting mammalian cells is used to maintain the intracellular Na+-K+ gradient (in nerve cells this can rise up to 70%), which controls cell volume, allows neurons and muscle cells to be electrically excitable, and also drives the active transport of sugars and amino acids (see later). 

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