Cell Signaling

Cells communicate by signaling each other chemically. These chemical signals are regulatory molecules released by neurons and endocrine glands, and by different cells within an organ.

The membrane potential and the permeability properties of the plasma membrane to ions discussed in the previous section set the stage for the discussion of nerve impulses in chapter 7. Nerve impulses are a type of signal that is conducted along the axon of a neuron. When the impulses reach the end of the axon, however, the signal must somehow be transmitted to the next cell.

Cell signaling refers to how cells communicate with each other. In certain specialized cases, the signal can travel directly from one cell to the next because their plasma membranes are fused together and their cytoplasm is continuous through tiny gap junctions in the fused membranes (see chapter 7, fig. 7.19). In these cases, ions and regulatory molecules can travel by diffusion through the cytoplasm of adjoining cells. In most cases, however, cells signal each other by releasing chemicals into the extracellular environment. In these cases, cell signaling can be divided into three general categories: (1) paracrine signaling; (2) synaptic signaling; and (3) endocrine signaling.

In paracrine signaling (fig. 6.26), cells within an organ secrete regulatory molecules that diffuse through the extracellular matrix to nearby target cells (those that respond to the regulatory molecule). Paracrine regulation is considered to be local, because it involves the cells of a particular organ. Numerous paracrine regulators have been discovered that regulate organ growth and coordinate the activities of the different cells and tissues within an organ.

Synaptic signaling refers to the means by which neurons regulate their target cells. The axon of a neuron (see chapter 1, fig. 1.10) is said to innervate its target organ through a functional

Paracrine regulator

Neuron -

-Neurotransmitter

Endocrine gland

-Hormone

-Hormone

Endocrine gland

Target organ

■ Figure 6.26 Chemical signaling between cells. (a) In paracrine signaling, regulatory molecules are released by the cells of an organ and target other cells in the same organ. (b) In synaptic signaling, the axon of a neuron releases a chemical neurotransmitter, which regulates a target cell. (c) In endocrine signaling, an endocrine gland secretes hormones into the blood, which carries the hormones to the target organs.

Target organ

■ Figure 6.26 Chemical signaling between cells. (a) In paracrine signaling, regulatory molecules are released by the cells of an organ and target other cells in the same organ. (b) In synaptic signaling, the axon of a neuron releases a chemical neurotransmitter, which regulates a target cell. (c) In endocrine signaling, an endocrine gland secretes hormones into the blood, which carries the hormones to the target organs.

connection, or synapse, between the axon ending and the target cell. There is a small synaptic gap, or cleft, between the two cells, and chemical regulators called neurotransmitters are released by the axon endings (fig. 6.26).

In endocrine signaling, the cells of endocrine glands secrete chemical regulators called hormones into the extracellular fluid. The hormones enter the blood and are carried by the blood to all the cells in the body. Only the target cells for a particular hormone, however, can respond to the hormone.

In order for a target cell to respond to a hormone, neuro-transmitter, or paracrine regulator, it must have specific receptor proteins for these molecules. These receptor proteins may be located on the outer surface of the plasma membrane of the target cells, or they may be located intracellularly, in either the cytoplasm or nucleus. The location of the receptor proteins depends on whether the regulatory molecule can penetrate the plasma membrane of the target cell.

If the regulatory molecule is nonpolar, it can diffuse through the cell membrane and enter the target cell. Such non-

polar regulatory molecules include steroid hormones, thyroid hormones, and nitric oxide gas (a paracrine regulator). In these cases, the receptor proteins are intracellular in location. Regulatory molecules that are large or polar—such as epinephrine (an amine hormone), acetylcholine (an amine neurotransmitter), and insulin (a polypeptide hormone)—cannot enter their target cells. In these cases, the receptor proteins are located on the outer surface of the plasma membrane. The details of how these signals influence their target cells are described in conjunction with neural and endocrine regulation in the next several chapters.

Test Yourself Before You Continue

1. Distinguish between synaptic, endocrine, and paracrine regulation.

2. Identify the location of the receptor proteins for different regulatory molecules.

Fox: Human Physiology, 6. Interactions Between Text © The McGraw-Hill

Eighth Edition Cells and the Extracellular Companies, 2003

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