Transmission of the Nervous Impulse

Both the intracellular and the plasma membranes are actively involved in the cell s vital functions. In the surface membranes of axons, processes of information transfer in the form of electrical signals (nerve impulses) lake place. Bioenergy conversion processes occur at the intracellular membranes of the mitochondria and chloroplasts. 

The intracellular and plasma membranes have a complex structure. The main components of a membrane are lipids (or phospholipids) and different proteins. Lipids are fatlike substances representing the esters of one di- or trivalent alcohol and two aliphatic fatty acid molecules (with 14 to 24 carbon atoms). In phospholipids, phosphoric acid residues, -0-P0(0 )-O-, are located close to the ester links, -C0-0-. The lipid or phospholipid molecules have the form of a compact polar head (the ester and phosphate groups) and two parallel, long nonpolar tails (the hydrocarbon chains of the fatty acids). The polar head is hydrophihc and readily interacts with water the hydrocarbon tails to the 

FIGURE 30.1 Lipid bilayer membrane (a) and the location of protein molecules in it (b). 

Cell membranes consist of two layers of oriented lipid molecules (lipid bilayer membranes). The molecules of these two layers have their hydrocarbon tails toward each other, while the hydrophilic heads are outside (Fig. 30.1a). The mean distance between lipid heads is 5 to 6mn. Various protein molecules having a size commensurate with layer thickness float in the lipid layer. Part of the protein molecules are located on the surface of the lipid layer others thread through the layer (Fig. 30.1fc). Thus, the membrane as a whole is heterogeneous and has a mosaic structure. 

All biological systems contain aqueous electrolyte solutions. These solutions consist of strong electrolytes (inorganic salts) as well as various organic substances with acidic or basic functional groups which usually behave as weak electrolytes. The solutions are often gel-like in their consistency because of the polyelectrolytes, proteins, and other macromolecules contained in them. The pH values of biological solutions as a rule are between 6.7 and 7.6. 

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In the present chapter a brief outline of two major lines of modem bioelectrochemistry is given studies of transmission of the nervous impulse (Section 30.1) and... 

The Glvcosamlnoglvoans. An Important category of biological polyelectrolytes, the glycosamlnoglycans. Includes biopolymers (formerly named mucopolysaccharides) which occur at neuronal synapses. In the extracellular volume of connective tissues, etc. Accumulation of these substances In the nervous tissues. Including the central nervous system, of young animals, prevents normal transmission of the nervous Impulse, and Is termed mucopolysaccharidosis. This disease Is very severe and rather widespread, which underlines the Importance of a better... 

That transmission of the nervous impulse across synapses was chemical occurred to T.R. Elliott while still an undergraduate at Cambridge. Published work suggested to him that sympathetic nerve impulses released minute amounts of an agent that diffused across the synapse and was taken up on the far side (Elliott, 1905). Experimental confirmation was slow to arrive. Henry Dale (1914) supposed that acetylcholine could be the parasympathetic transmitter and, because it could not be detected, he further assumed that an enzyme must be nearby to hydrolyse it. Not until 1921 was there a convincing experimental proof of chemical transmission across this synapse (Loewi, 1921), and in 1926 the first neurotransmitter was finally identified and found to be acetylcholine (Loewi and Navratil, 1926). Dale was shown to be right about acetylcholinesterase, too. 

A chemoreceptor can be attached in its natural state to a potentiometric electrode to form biosensors which are called receptrodes . One such receptor is the antennule of the blue crab Callinectes sapidus, whose nerve fibers are used as a transducer for the transmission of the nervous impulsions caused by olfaction or gustation [91]. The sensitive nerves are found near the crab endopod and are dissected before being fixed directly to the platinum electrode. The measurement... 

Rgure 22-2. Neurotransmission in the central nervous system. Neurotransmitter molecules (eg, norepinephrine), released by the presynaptic nerve, cross the synapse and bind with receptors in the cell membrane of the postsynaptic nerve, resulting in the transmission of the nerve impulse. 

Acetylcholine is a neurotransmitter that functions in conveying nerve impulses across synaptic clefts within the central and autonomic nervous systems and at junctures of nerves and muscles. Following transmission of an impulse across the synapse by the release of acetylcholine, acetylcholinesterase is released into the synaptic cleft. This enzyme hydrolyzes acetylcholine to choline and acetate and transmission of the nerve impulse is terminated. The inhibition of acetylcholineasterase results in prolonged, uncoordinated nerve or muscle stimulation. Organophosphorus and carbamate pesticides (Chapter 5) along with some nerve gases (i.e., sarin) elicit toxicity via this mechanism. 

In normal transmission of a nervous impulse from nerve to nerve, acetylcholine is released into the synapse in order to excite the receiving neuron (Figure 5.10). Unless acetylcholine is rapidly broken down, the receiving nerve is constantly fired, resulting in uncoordinated muscle movement, nausea, dizziness, and eventually seizures and unconsciousness. The serine enzyme acetylcholinesterase is responsible for the expedient breakdown of the neurotransmitter acetylcholinesterase. 

Dopamine is a chemical messenger produced within the nerve cells that is essential for the transmission of the nerve impulse and hence involved in a wide range of important functions, including movement, cognition and behaviour. Dysfunctions in the central nervous dopamine system can lead to diseases such as Parkinson s and schizophrenia. Alterations in the levels of this neurotransmitter have also been implicated in a variety of behavioural problems such as attention deficit and hyperactivity. 

In the neuromuscular junction, one nervous impulse results in the liberation of about 5 X 10 ACh molecules, about 10" M The interaction of ACh molecules with a small part of all the receptors (about 20%) is probably sufficient for the transmission of the nerve impulse. Thus a high reliability of transmission is provided. 

In nervous tissue it provides a reserve supply of choline, which in tmn gives rise to the neurocrine, acetyl choline, necessary for the transmission of the nerve impulse. 

Historically, it was T. R. Elliot who, while still a student at Cambridge, concluded that sympathetic nerve impulses must release minute amounts of an agent that was taken up by a receptor on the other side of the synapse. Hence he was the first to postulate that transmission across synapses was chemical (Elliott, 1905). Henry Dale (1914) proposed acetylcholine as the parasympathetic neurotransmitter and, because of its short life, he added that an enzyme must be on the spot to hydrolyse it. Otto Loewi (1921) achieved the first convincing proof of chemical transmission of a nervous impulse across a synapse, and in 1926 (with Navratil) identified the commonest neurotransmitter as acetylcholine. Dale (1914) noted that some of the actions of acetylcholine can be imitated by nicotine (7.1 ) and others by muscarine (j 39 and the terms nicotinic and muscarinic have turned out very useful in classifying the sites at which this neurotransmitter acts. 

Figure 1.1 Functional components of the nervous system. The sensory division of the peripheral nervous system is sensitive to changes in the internal and external environment. The information gathered by this component is transmitted to the CNS where it is processed, integrated, and interpreted. The CNS then determines the appropriate response to this input. This response is carried out by the transmission of nerve impulses in the motor division of the peripheral nervous system to the effector tissues.

Yovo et al. stated that these alkaloids act via inhibition of ganglionic impulse transmissions of the sympathetic nervous system. It is evident that each alkaloid has its own effect. Anagyrine caused skeletal deformity in foetuses when pregnant cows consumed toxic lupines . On the other hand, some quinolizidine alkaloids are used as a drug in folk medicine". They probably have chronic toxicity. However, adequate knowledge about the chronic toxicity of these alkaloids and especially of chronic toxication across generations is not available. The premise that quinolizidine alkaloids have not produced hereditary symptoms has not been checked with total reliability. 

The nervous system consists of sensory and motor compo-nenLs. The. sensory compiment responds to various external stimulations, which it transmits in the form of a nerve impulse to the CNS for interpretation. The motor component of the nervous. system carries a signal from the CNS to the appropriate part of the bixly to elicit the rasponse to the stimulation. One of thc.se rcspoascs is the sensation known as pain. Nerve impulses arc now known to take the form of an electrical impulse. Experimental evidence suggests that both stimulation and the transmission of a nerve impulse may be bUx ked by the action of local anesthetic agents. Consequently, understanding this action requires a knowledge of the structure and action of the nervous system. 

The central nervous system is made up of billions of individual nerve cells (neurons). Transmission of information within a neuron is by electrical means and the transmission of such nerve impulses can be recorded with suitable equipment. However, most transmission of information between neurons or between neurons and effector systems (muscles, for example) is by chemical means. The first chemical... 

In nervous tissue, some neurons are connected by gap junctions through which Ions pass rapidly, thereby allowing very rapid transmission of electrical signals. Impulse transmission through these connections, called electrical synapses. Is almost a thousandfold as rapid as at chemical synapses (Chapter 7). Gap junctions are also present in many non-neuronal tissues where they help to Integrate the... 

We have already noted above that in analyzing excitable systems one has, more often than not, to deal with a parabolic equation with a nonlinear source. In this section we will concern ourselves with an excitable medium of a different type, where the signals are transmitted in the neuron network not by the local currents but by the nervous impulses traveling along the axons. The propagation speed of the activity wave will, if this transmission mode is possible at all, depend not only on the signal transmission speed but also on the other characteristics of nerve cells such as cell body capacitance, conductance, etc. 

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