Nerve cell properties

Other ion channels are closed at rest, but may be opened by a change in membrane potential, by intracellular messengers such as Ca + ions, or by neurotransmitters. These are responsible for the active signalling properties of nerve cells and are discussed below (see Hille 1992, for a comprehensive account). A large number of ion channels have now been cloned. This chapter concerns function, rather than structure, and hence does not systematically follow the structural classification. 

Nerve cells have two distinct properties that distinguish them from all other types of cells in the body. First, they conduct bioelectrical signals for relatively long distances without any loss of signal strength. Second, they... 

Phenytoin, introduced as an anticonvulsant drug in 1938, remains one of the drugs most frequently prescribed for convulsive disorders. The precise mode of action is unknown, but it appears to inhibit the accumulation of sodium in nerve cells, thus stabilizing hyperexcitable cell membranes (A13)—a property also utilized in the treatment of cardiac arrhythmias. 

It is interesting that a nnmber of antihistamine, anticholinergic, and adrenergenic drngs having analogous chemical structures, also exhibit local anesthetic properties. It is possible that by interacting with internal axoplasmic membranes, they rednce the ion flow in particnlar, the flow of sodinm ions inside nerve cells. 

The nervous system has several properties in common with the endocrine system, which is the other major system for control of body function. These include high-level integration in the brain, the ability to influence processes in distant regions of the body, and extensive use of negative feedback. Both systems use chemicals for the transmission of information. In the nervous system, chemical transmission occurs between nerve cells and between nerve cells and their effector cells. Chemical transmission takes place through the release of small amounts of transmitter substances from the nerve terminals into the synaptic cleft. The transmitter crosses the cleft by diffusion and activates or inhibits the postsynaptic cell by binding to a specialized receptor molecule. In a few cases, retrograde transmission may occur from the postsynaptic cell to the presynaptic neuron terminal. 

Flavonoids are a family of antioxidants found in fruits and vegetables as well as in popular beverages such as red wine and tea. Although the physiological benefits of flavonoids have been largely attributed to their antioxidant properties in plasma, flavonoids may also protect cells from various insults. Nerve cell death from oxidative stress has been implicated in a variety of pathologies, including stroke, trauma, and diseases such as Alzheimer s disease and Parkinson s disease. 

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. 

The structure of a nerve is not simple. In the following account, the stress is upon a single aspect of the mechanism of the action of a nerve, the origin of the spike potential in sections of the nerve called nodes in which the axon is in contact on the outside with the extracellular fluids. The relevant properties of a nerve cell free of a myelin sheath can be seen in Table 14.1. 

In this example, a scientist at a Big Pharma noticed that a new sodium chaimel was upregulated in spinal cord samples in an animal model of pain. She isolated the ion charmel and made, by a process called transfection, a cell-line that expressed this sodium ion chaimel. She could then examine the characteristics of the channel more closely and found that it was blocked by one of the drugs already made by the company. Perhaps not surprisingly, since most sodium channels are associated with excitability in nerve cells, the drug was an antiepileptic—a drug that blocks excitability in nerve cells. The same sodium charmel blocker also turned out to have excellent antidepressant properties in bipolar (manic-depressive) patients. 

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