Physiological Signals

Cells modify their activity in response to information received from their immediate environment, including such physical signals as light, heat, distortion of their surfaces by stretching or compression, and a vast array of chemical signals that may originate in the external environment or in other cells. Chemical signals are the most common form of information transfer that cells use to communicate with each other. Chemical signals may be simple substances such as ions, metabolites, or derivatives of amino and fatty acids, or they may be complex molecules such as peptides, proteins, glycoproteins, and steroids (Table 1). Signal molecules may be fixed on the surfaces of cells or the supporting extracellular matrix, or they may be dissolved in extracellular fluids. In general, signals confined to surfaces of cells or to extracellular matrix direct cellular behavior during development, wound healing, blood clotting, immunosurveillance, and other processes that involve cell migration. Soluble messages govern the various activities associated with maintenance of the internal environment, coping with the external environment, and reproduction. Mechanisms for information transfer, however, follow a similar pattern, regardless of whether the cell moves to contact a fixed signal or a mobile signal reaches a fixed cell. These activities are the substance of physiology; hence, the molecular bases for cellular communication are considered in detail below and in later chapters.

Secretory Products

Soluble signals have been categorized according to the cell of origin or the mode of delivery to the recipient, or target, cells. Generic terms for signal molecules are agonists, because they produce an effect, or ligands, because, in producing their effect, they bind to target cells. Secretions that are released from one cell and reach nearby target cells by diffusion through the extracellular space are called paracrine. Some secretory products may also act on the cells that produced them, in which case they are called autocrine secretions. Communication that depends purely on diffusion is highly efficient when the distances between communicating cells are short, but it becomes increasingly inefficient when distances exceed a few cell diameters. Nerve cells are specialized to conduct electrical impulses with great speed (as high as 120 m/sec) over the long distances they span and to release neurotransmitters from their terminals. Neuro-transmitters diffuse across the synaptic spaces in a paracrine manner to reach adjacent target cells. Alternatively, some secretory products enter the circulation and are delivered to distant target cells by the blood stream. This mode is called endocrine secretion, and the secretory products are called hormones. Secretions of nerve cells that are carried by the blood to distant target cells are called neurohormones, and this category of communication is referred to as neuroendocrine. Secretions of cells of the immune system are called cytokines or lymphokines and may behave in a paracrine,

TABLE 1 Some Examples of Chemical Signals

Signal type

Signal molecule

Cell or tissue of origin

Target cell

Gas

Nitrous oxide

Vascular endothelium

Smooth muscle

Simple ions

Sodium

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