Types of response

There are broadly three kinds of response to stimuli: cell membrane changes, alterations in internal state, and alterations in the gene expression pattern.

Two kinds of change might be induced at the membrane level. First, the speed with which something moves into or out of the cell can be altered. In this case the "contact molecule", M, is part of the membrane. When M is activated a specific "gate" is opened or closed, or a "pump" is switched on or off. For example, the activated insulin receptor increases the rate at which glucose enters the cell. Neurotransmitters, the chemicals released from nerve cell termini, alter the rates at which sodium and potassium ions pass through the membrane of the "receiving" cell; this changes the likelihood of electrical activity in that cell. Without the effect of insulin on glucose permeation, the cells of your body would be starved of nutrient. Without the effects of neurotransmitters on sodium and potassium permeation, your nervous system would not work.

Second, another signalling pathway might be modified. The response to one signal might be to inhibit, or to activate, the receptors for another signal. This is another example of cross-talk among signalling pathways.

A subtler "membrane response" is the formation of cell-cell junctions. We mentioned this earlier. The junction might be a tight seal, or it might afford direct communication between the cells (a "zero-resistance junction"). The stimulus for junction formation is direct physical contact between two cells, inducing internal state changes in both. These changes include rearrangements of the cytoskeleton and changes in the patterns of gene expression; so although the final effect is seen at the cell membranes, the response is mediated by changes within the cell.

Many stimuli can affect the internal state without any changes in the membrane other than those directly involved with receptor activation. The "RESP1" and "RESP2" of Fig. 9-2 might, for instance, be the activities of two metabolic enzymes. By making these two enzymes more or less active, the stimuli can dramatically change the relative rates of two or more metabolic pathways. For example, adrenaline makes your heart beat faster and more strongly, while acetylcholine slows it down. Many mechanisms are involved here, but they include alterations in glucose metabolism. The faster your heart beats, the more energy it consumes, so the more fuel it needs. One effect of adrenaline is to activate enzymes of glucose catabolism. One of the effects of acetylcholine is to deactivate them.

A stimulus might also bring about a change in compartmentalisation within the cell. Releasing a substance from a store (or sequestering it in a store) can drastically alter metabolism. When adrenaline binds to heart muscle cells, it causes calcium to be released from internal stores; calcium is not only essential for muscle contraction, it also has further effects on glucose metabolism. The calcium is rapidly returned to storage when the adrenaline stimulus terminates. Alternatively, the stimulus might elicit a change in the cytoskeleton; so the cell might alter its shape or move to a new location, or its internal transport processes might be modified.

In short: via the various branches of its signalling pathway, a stimulus can affect virtually any aspect of the cell's internal state. Indeed, several aspects (metabolism, structure and transport) can be affected simultaneously by the same stimulus.

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