Fig 183 cells and genes as analogue devices integrating inputs and giving rise to graded outputs

Suppose the device represents a gene. In this case, the output (O) is the transcription rate (the number of messenger RNA copies made per second). It is continuously variable from zero to a maximum. The inputs (I) are the enhancers occupied by transcription factors; most are positive, but some are negative. The integrator (C) is the initiation complex, the activity of which depends on the sum of the inputs. The effector (E) is RNA polymerase II. Thus, the diagram is an abstraction of the control of gene expression as described in chapter 8.

Now, suppose instead that the device represents a neurone. In this case, the output (O) is the rate of firing (the number of action potentials per second). It is continuously variable from zero to a maximum. The inputs (I) are the postsynaptic potentials at the dendrites and on the cell body; some of these are excitatory and some are inhibitory. The integrator (C) is the cell body, whose grand postsynaptic potential represents the sum of the inputs. The effector (E) is the axon hillock, where action potentials are initiated. Thus, the diagram is an abstraction of neurone function as described in chapter 16.

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This abstract model can be elaborated. For example, the integrator (C in the diagram) can be modulated, making it more or less sensitive to positive or negative inputs, or more or less able to activate the effector. In genes, chemical modification of the initiation complex proteins makes the initiation complex more or less inclined to launch the polymerase. In neurones, calcium channels in the cell body can make the axon hillock leak potassium ions and become less inclined to launch action potentials.

The purpose of gene transcription is to make the cell's proteins. Messenger RNAs are translated and proteins are produced. The proteins are responsible for the functional organisation of the cell as a whole. Analogously: the purpose of action potentials is to activate and inhibit synapses. Neurotransmitters are released and postsynaptic receptors are occupied. Synapses are responsible for the functional organisation of the brain as a whole.

To summarise:-

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