ANATOMY&PHYSIOLOGY FOR NURSES

NUCLEUS.   The nucleus is separated from the cytoplasm by the nuclear membrane which is also selectively porous , allowing substances to escape from the nucleus into the cytoplasm or substances to pass into it. The nucleus contains many protein-rich threads lying in a nuclear sap. In the "resting cell" the threads are collectively spoken of as chromatin, These threads or chromosomes are vital to the everyday activities of the cell and are responsible for determining the hereditary characteristics of the human body.  On the chromosomes in linear arrangement sit the genetic or hereditary determinants, the genes. The number of chromosomes in a body cell is constant for a particular forty-six chromosomes. Irritability and Conductivity. Mention has been made of some of these cells' characteristic functional properties, their metabolic activities and powers of growth. By these two properties the cell is active.  When a cell is stimulated either by chemical, physical, mechanical, or nervous means, the cell responds: it may contract as does a muscle cell; it may produce a secreation, as do the cells of the stomach, pancreas, and other organs and glands; or it may conduct an impulse, as in the case of the nerve cell.  This last is the best example of cell conductivity as a nerve impulse generated by the stimulation of a nerve cell may be conducted for a considerable distance, a yard or more, according to the length of the nerve fibre.  But in all cases, a stimulus which excites a cell to action is conducted along the entire length from end to end of the cell. .....