Nerve cell connections

The mode of action of ivermectin has been reviewed (2). It has tentatively been identified as agonism at GABA receptors, with inhibition of ion channels that control specific nerve cell connections. The functioning of chloride channels should thus be altered in most organisms, leading to paralysis and death of parasites. Several sites of action have been proposed ... 

Let s consider just a few of those effects in brief detail here. In the brain, lead probably corrupts more than one biochemical system. It interferes with events at nerve cell connections called glutamate synapses, which means at approximately half the synapses (connections) in the cerebral cortex. These synapses are apparently critical for learning during development. The important glutamate receptor that s involved is the so-called NMDA (iV-methyl-D-aspartate) receptor. This receptor is selectively blocked by lead, and the behavioral consequence is a deficit in learning ability. 

The important point here is that the production and migration of nerve cells are provoked by local biochemical events easily affected by the local prenatal environment, and the same is true for axon guidance processes that connect nerve cells to other nerve cells—connections of critical importance for later cognitive and emotional behavior. 

A point about the life and death of nerve cell connections in recent years, it has become apparent that during brain development the formation of many neural circuits involves selective elimination of many synapses—selective pruning. This is an old idea reinvigorated in a theory of neural Darwinism, 15 which in general proposes that groups of synapses are selected for survival or extinction according to experience—a model... 

Neuroanatomists have taken advantage of the phenomenon of fast retrograde transport to locate remote nerve cell bodies in the CNS of an experimental animal that are connected to an identified axonal fiber tract whose origin is uncertain. The tracer material [purified horseradish peroxidase (HRP) enzyme] is injected in the region of the axon terminals, where it is taken up by endocytosis and then is carried by retrograde axonal transport over a period of several hours to days back to the nerve cell body. The animal is sacrificed, and the enzyme tracer is localized by staining thin sections of the brain for peroxidase activity. 

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. 

Fig. 9.2 A general impression of the construction of nerve cells (a) cross-section of a nerve trunk, (b) a neuron, (c) sensory and motor nerve connection via a synapse.

Chemical transmission between nerve cells involves multiple steps. Until the late 19th century, many physiologists believed that there were direct physical connections between nerves and that an impulse from one nerve... 

These 49 words have formed what now is known as Hebb s Learning Rule. Although some minor modifications have been required over the years, the essence of Hebb s rule remains unchanged a memory is produced by coincident neural activity when two connected nerve cells are active simultaneously, the strength of their synaptic connection increases this confers a basis for the persistence of memory. 

What is a synapse In the brain, the nerve cells or neurons are connected at special functional junctions called synapses, which depend on many proteins, including large complexes. They participate in basic functions with important roles in coordinating every characteristic of the nervous system, including physiology, emotions, learning, sleep, memory, and pain signal transmission. 

Negative entropy is also expressed in the ordered three-dimensional structures of complex proteins, protein-nucleic acid complexes, molecular machines, biological membranes, and so on. Metabolism and chemical signaling too are highly and meticulously ordered processes. Use your brain to think about the brain the degree of order in the creation of trillions of connections among nerve cells that is required to allow us have that most amazing facility, consciousness. 

Similarly, the structure of the nervous system is interwoven with its function. At all levels, from the microscopic highly branched nerve cell to the multiple connections between large brain regions that are visible to the naked eye, the structure of the nervous system is obviously designed to serve its chief purpose communication. As a result, it is difficult to talk about structure separately from function. Nevertheless, a divided, stepwise approach may help make these complicated matters easier for you to understand. 

The brain three pounds of soft matter that can take a split second of experience and freeze it forever in its cellular connections. A hundred billion nerve cells are the architecture of our experience. Is Proust still here in the wet organ balanced in my palm Could we reconstruct his memories Would Proust approve such a breach of privacy ... 

Synapse. The chemical connection for communication between two nerve cells or between a nerve cell and a target cell such as a muscle cell. 

Briefly, the flatworm brain is most commonly a ring-like or semicircular structure formed by the connection of two concentrations of nerve axons and nerve cell bodies by a well-developed commissure or commissures. These bilobed accumulations of nerves form the cerebral ganglia that occupy the lateral margins of the brain and comprise a core of nerve axons that are enveloped by a loose gathering of the associated cell bodies at least some flat-worm brains appear to be further surrounded by multilayer sheaths of mesenchyme that represent primitive glia-like cells (Sukhdeo and Sukhdeo, 1994). 

Volatile solvents are useful in industry and in homes because of their ability to dissolve fat. When inhaled, however, this property poses problems to the brain and the network of nerves that connect the brain and spinal cord to the rest of the body, ...thus, because the brain is a lipid-rich organ, chronic solvent abuse dissolves brain cells, the American Academy of Pediatrics wrote in a 1996 policy paper about inhalants. The chemical vapors also damage the myelin sheath, the fatty wrapper that insulates the fibers of many nerve cells that carry signals. 

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