Intercellular Junctions

Some cells, such as blood cells, are separated from each other in fluid-filled spaces (intercellular (incter-selou-lar) spaces). Many other cell types, however, are tightly packed, with structures called intercellular junctions connecting their cell membranes.

In one type of intercellular junction, called a tight junction, the membranes of adjacent cells converge and fuse. The area of fusion surrounds the cell like a belt, and the junction closes the space between the cells. Cells that form sheetlike layers, such as those that line the inside of the digestive tract, often are joined by tight junctions. The linings of tiny blood vessels in the brain are extremely tight (Clinical Application 3.2).

Another type of intercellular junction, called a desmosome, rivets or "spot welds" adjacent skin cells, so they form a reinforced structural unit. The membranes of certain other cells, such as those in heart muscle and muscle of the digestive tract, are interconnected by tubular channels called gap junctions. These channels link the cytoplasm of adjacent cells and allow ions, nutrients intense exercise, it slightly alters the muscle cell membrane's electrical potential. Normally, this slight change would have no effect. In affected horses, however, the change causes sodium channels to open too widely, and admit too much sodium into the cell. The influx of sodium renders the muscle cell unable to respond to nervous stimulation for a short time—but long enough for the racehorse to fall.

Humans can inherit this condition too. In one affected family, several members collapsed after eating bananas! Bananas are very high in potassium, which triggered the symptoms of hyperkalemic periodic paralysis.

Long-QT Syndrome and Potassium Channels

A Norwegian family had four children, all born deaf. Three of the children died at ages four, five, and nine; the fourth so far has been lucky. All of the children inherited from their unaffected "carrier" parents a condition called "long-QT syndrome associated with deafness." They have abnormal potassium channels in the heart muscle and in the inner ear. In the heart, the malfunctioning channels cause fatal arrhythmia. In the inner ear, the abnormal channels alter the concentration of potassium ions in a fluid, impairing hearing.

The inherited form of long-QT syndrome in the Norwegian family is extremely rare, but other forms of the condition are more common, causing 50,000 sudden deaths each year, often in apparently healthy children and young adults. Several cases were attributed to an interaction between the antihistamine Seldane (terfenadine) and either an antibiotic (erythromycin) or an antifungal drug (ketoconazole), before Seldane was removed from the market in 1997.

Diagnosing long-QT syndrome early is essential because the first symptom may be fatal. It is usually diagnosed following a sudden death of a relative or detected on a routine examination of the heart's electrical activity (an electrocardiogram, see fig. 15.21). Drugs, pacemakers, and surgery to remove certain nerves can treat the condition and possibly prevent sudden death.

Cystic Fibrosis and Chloride Channels

A seventeenth-century English saying, "A child that is salty to taste will die shortly after birth," described the consequence of abnormal chloride channels in the inherited illness cystic fibrosis (CF). The disorder affects 1 in 2,500 Caucasians, 1 in 14,000 blacks, and 1 in 90,000 Asians, and is inherited from two unaffected parents who are carriers. The major symptoms of impaired breathing, respiratory infections, and a clogged pancreas result from secretion of extremely thick mucus. Affected individuals undergo twice-daily exercise sessions to shake free the sticky mucus and take supplemental digestive enzymes to aid pancreatic function. Strong antibiotics are used to combat their frequent lung infections.

In 1989, researchers identified the microscopic defect that causes CF as abnormal chloride channels in cells lining the lung passageways and ducts in the pancreas. The primary defect in the chloride channels also causes sodium channels to malfunction. The result is salt trapped inside affected cells, which draws moisture in, thickening the surrounding mucus. Several experimental gene therapies attempt to correct affected cells' instructions for building chloride channel proteins. ■

(such as sugars, amino acids, and nucleotides), and other small molecules to move between them (fig. 3.8). Table 3.1 summarizes these intercellular junctions.

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Essentials of Human Physiology

Essentials of Human Physiology

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