Nerve impulses, generation

Because the chemical signals (semiochemicals) are normally produced in minute amounts and diluted in the environment with a complex mixture of chemical compounds derived from a myriad of sources, the olfactory system in insects evolved as a remarkably selective and sensitive system, which approaches the theoretical limit for a detector. For example, it has been estimated that the male silkworm moth is able to distinguish within 1 s 170 nerve impulses generated by the female silkworm moth s sex pheromone from 1700 spontaneous nervous impulses [ 1 ], thus, operating on a remarkably low S/N ratio ... 

Some homeopathic remedies like polycrests produce a wide variety of symptoms common to many diseases. They may act through multifunctional neural networks. For this, treatment of a chronic disease with a high potency of a polycrest sometimes results in brief recapitulation of past diseases, particularly their pain components which are connected with the same multifunctional neural network. According to the neuromatrix theory pain is a multidimensional experience produced by characteristic neurosignature patterns of nerve impulses generated by a widely distributed neural network. The neuromatrix concept fits in with the holistic approach of homeopathy. 

Figure 32.2. The Main Nasal Epithelium. This region of the nose, which lies at the top of the nasal cavity, contains approximately 1 million sensory neurons. Nerve impulses generated by odorant molecules binding to receptors on the cilia travel from the sensory neurons to the olfactory bulb.

Sodium also plays an important role in nerve impulse generation and transmission. As a part of the sodium-potassium pump, the difference between the potassium and sodium concentrations is maintained through active transport across the cell membrane as needed with the help of adenosine triphosphate (ATP) as an energy source. The flow of sodium and potassium across the cell membrane of electrically charged cells results in depolarization. Thus sodium is important for nerve and muscle function. As such, sodium imbalances can affect cardiac and respiratory muscle function as well as mobility. 

As an activator of the phosphokinases, magnesium is essential in energy-requiring biological processes, such as activation of amino acids, acetate, and succinate synthesis of proteins, fats, coen2ymes, and nucleic acids generation and transmission of nerve impulses and muscle contraction (67). 

When large areas of the membrane are depolarized in this manner, the electrochemical disturbance propagates in wave-like form down the membrane, generating a nerve impulse. Myelin sheets, formed by Schwann cells, wrap around nerve fibers and provide an electrical insulator that surrounds most of the nerve and greatly speeds up the propagation of the wave (signal) by allowing ions to flow in and out of the membrane... 

Table 8 5 shows that each of the four common s-block ions is abundant not only in seawater but also in body fluids, where these ions play essential biochemical roles. Sodium is the most abundant cation in fluids that are outside of cells, and proper functioning of body cells requires that sodium concentrations be maintained within a narrow range. One of the main functions of the kidneys is to control the excretion of sodium. Whereas sodium cations are abundant in the fluids outside of cells, potassium cations are the most abundant ions in the fluids inside cells. The difference in ion concentration across cell walls is responsible for the generation of nerve impulses that drive muscle contraction. If the difference in potassium ion concentration across cell walls deteriorates, muscular activity, including the regular muscle contractions of the heart, can be seriously disrupted. 

Fig. 4.1 Hypothetical model of pathogenesis of pain in DSP. (1) Injury of peripheral nerve fibers due to multifocal inflammation and secreted macrophage activation products results in abnormal spontaneous activity of neighboring uninjured nociceptive fibers ( peripheral sensitization ). (2) Furthermore, the aberrant inflammatory response in DRG leads to alterations in neuronal sodium and calcium channel expression and ectopic impulse generation. (3) This results in central remodeling within the dorsal horn due to A-fiber sprouting and synaptic formation with pain fibers in lamina 11, and maintenance of neuropathic pain ( central sensitization ). Reproduced with permission from (Keswani et al. 2002)...

As mentioned previously, a single action potential lasting only 2 msec causes a muscle twitch that lasts approximately 100 msec. If the muscle fiber has adequate time to completely relax before it is stimulated by another action potential, the subsequent muscle twitch will be of the same magnitude as the first. However, if the muscle fiber is restimulated before it has completely relaxed, then the tension generated during the second muscle twitch is added to that of the first (see Figure 11.3). In fact, the frequency of nerve impulses to a muscle fiber may be so rapid that there is no time for relaxation in between stimuli. In this case, the muscle fiber attains a state of smooth, sustained maximal contraction referred to as tetanus. 

Local anesthetics reversibly inhibit impulse generation and propagation in nerves. In sensory nerves, such an effect is desired when painful procedures must be performed, e.g surgical or dental operations. 

Most cells normally receive a large excitatory input with a more or less constant generation of action potentials. The net result of generated IPSPs will be to decrease the number of nerve impulses per unit of time. By these mechanisms, neurotransmitters producing ei-... 

Procaine and the other local anaesthetic drugs prevent the generation and the conduction of the nerve impulses. Their main site of action is the cell membrane, since conduction block can be demonstrated in giant axons from which the axoplasm has been removed [25]. 

Local anaesthetics block both the generation and conduction of the nerve impulse. 

When light strikes the rod cells, isomerization of the C-ll/C-12 double bond takes place, and tra 5 -rhodopsin (metarhodopsin II) is formed. This cis-trans isomerization is accompanied by an alteration in molecular geometry, which generates a nerve impulse to be sent to the brain, resulting in the perception of vision. Metarhodopsin II is recycled back to rhodopsin by a multi-step sequence that involves the cleavage to all-trani-retinal and cis-trans isomerization back to 11-cA-retinal. 

---