Nervous system concepts

Environment Sensors monitoring air and water quality will be able to provide early warning of pollution events arising at industrial plants, landfill sites, reservoirs, and water distribution systems at remote locations. The environmental nervous system concept likens the rapid access and response capabilities of widely distributed sensor networks to the human nervous system that is, it is able to detect and categorize events as they happen, and organize an appropriate response. 

Because of its motor, i.e., activating effect on vascular smooth muscle and its inhibitory effect on intestinal smooth muscle, the sympathetic nervous system has been cast into the role of the component of the nervous system that executes control of visceral function in times of physical emergency for the organism. The phrase fight or flight has been often used to describe the circumstances in which the adrenergic transmitters of the sympathetic system are dominant over the cholinergic parasympathetic system. This concept is perhaps oversimplified but it has the utility of a first approximation of how the two components of the ANS interact in the periphery. Sensory inputs which lead to increased blood pressure, for example, activate the sympathetic pathways. 

In the central nervous system (CNS) the concept that localized microdomains of calcium are responsible for triggering vesicle fusion generally refers to neurons (Rizzuto and Pozzan 2006). Concerning glial cells although the existence of structural... 

Neuropathic pain is defined as spontaneous pain and hypersensitivity to pain associated with damage to or pathologic changes in the peripheral nervous system as in painful diabetic peripheral neuropathy (DPN), acquired immunodeficiency syndrome (AIDS), polyneuropathy, post-herpetic neuralgia (PHN) or pain originating in the central nervous system (CNS), that which occurs with spinal cord injury, multiple sclerosis, and stroke. Functional pain, a relatively newer concept, is pain sensitivity due to an abnormal processing or function of the central nervous system in response to normal stimuli. Several conditions considered to have this abnormal sensitivity or hyperresponsiveness include fibromyalgia and irritable bowel syndrome. 

The concept of chemical neurotransmission originated in the 1920s with the classic experiments of Otto Loewi (which were themselves inspired by a dream), who demonstrated that by transferring the ventricular fluid of a stimulated frog heart onto an unstimulated frog heart he could reproduce the effects of a (parasympathetic) nerve stimulus on the unstimulated heart (Loewi Navratil, 1926). Subsequently, it was found that acetylcholine was the neurotransmitter released from these parasympathetic nerve fibers. As well as playing a critical role in synaptic transmission in the autonomic nervous system and at vertebrate neuromuscular junctions (Dale, 1935), acetylcholine plays a central role in the control of wakefulness and REM sleep. Some have even gone as far as to call acetylcholine a neurotransmitter correlate of consciousness (Perry et al., 1999). 

NGF also has actions within the CNS, although it is not particularly abundant in the CNS. Its synthesis appears to be largely restricted to the hippocampus and neocortex, and even in these regions it is present at relatively low concentrations relative to the other neurotrophins. The most prominent population of NGF-responsive neurons expressing TrkA are the basal forebrain cholinergic neurons. The principal projections of these neurons are to the hippocampus and cortex, which conforms with the concept that NGF acts as a target-derived trophic factor in the CNS, just as it does in the peripheral nervous system (PNS). NGF also acts on a subpopulation of cholinergic neurons within the striatum. These interneurons express the NGF receptor, TrkA, and respond to NGF. However, they do not appear to rely entirely on NGF for their survival, and the specific actions of NGF on this neuronal population have not been clearly defined. NGF may also have autocrine actions in the CNS, as some neuronal populations have been identified that express both TrkA and NGF. 

Triploidy refers to cells that contain three copies of each chromosome (69 total). THploidy, which usually occurs as a result of the fertilization of an ovum by two sperm cells, is common at conception, but the vast majority of these conceptions are lost prenatally. However, about 1 in 10,000 live births is a triploid. These babies have multiple defects of the heart and central nervous system, and they do not survive. 

Trisomy 13 (Patau syndrome, 47,XY,+13 47,XX,H 13) is seen in approximately 1 in 10,000 live births. More than 90% of conceptions are lost prenatally, and more than 90% of those who survive to term do not survive to 1 year of age. Common disease features include oral-facial clefts, microphthalmia (small eyes), renal defects, and polydactyly (extra fingers). Central nervous system malformations and heart defects are common and contribute to mortality. 

The gap across the synapse is so small that the chemical messenger (neurotransmitter) crosses the cleft in less than a millisecond. Within the brain there are more than 50 neurotransmitters, which include amino acids, amines, purines, peptides and some gases. In contrast, in the peripheral nervous system there are only two, acetylcholine and noradrenaline. One of several questions concerning the concept of neurotransmitters is whether they differ, in principle, from local hormones (See below and Chapter 12). 

The concept of chemical transmission in the nervous system arose in the early years of the century when it was discovered that the functioning of the autonomic nervous system was largely dependent on the secretion of acetylcholine and noradrenaline from the parasympathetic and sympathetic nerves respectively. The physiologist Sherrington proposed that nerve cells communicated with one another, and with any other type of adjacent cell, by liberating the neurotransmitter into the space, or synapse, in the immediate vicinity of the nerve ending. He believed that transmission across the synaptic cleft was unidirectional and, unlike conduction down the nerve fibre, was delayed by some milliseconds because of the time it took the transmitter to diffuse across the synapse and activate a specific neurotransmitter receptor on the cell membrane. 

A crucial concept in this area is that of novelty seeking. The mammalian nervous system appears to be biologically programmed to attend to novel information more readily than to familiar information. For example, human subjects respond more rapidly to novel auditory stimuli than to familiar repetitive stimuli (Tiifinen et al. 1994). In the... 

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