A control system must be used to govern this action.
Neural impulses are created by light reaching the retina and are proportional to the light intensity.
At a place along the nervous system of vision, this information is compared to a preset reference value stored in the brain.
The reference can be altered by hormones.
Nerve impulses are sent to the muscles of the iris to adjust the size of the opening in response to this signal.
The general characteristics of control systems in biological organisms have been studied by engineers for many years.
Features in common with biological control systems include voltage regulators, speed controls, and thermostatic heat regulators.
Engineers are able to analyze and predict the behavior of control systems.
The study of biological systems has been aided by these techniques.
In terms of input and output, an engineering analysis of such systems is done.
In the light-intensity control example, the input is the light reaching the retina, and the output is the response of the retina to light.
The system produces an output in response to the input.
This is the nerve circuits in our case.
The iris control system wants to keep the output constant.
Sexual arousal is an example of positive feedback, whereas sweating or shivering is an example of negative feedback.
Negative feedback keeps the system response constant.
Most biological feedback systems are negative.
An example from electrical engineering will be used to illustrate the method of system analysis.
We will look at a voltage amplifier that has part of its output fed back to the input.
Let us introduce feedback.
Negative feedback is what this is.
Without a detailed knowledge of the individual system components, this type of analysis gives us an advantage.
We can measure the output and input voltages with different frequencies, magnitudes, and durations.
We can get some information about the amplifier and the feedback component without knowing anything about the components that make up the device.
A detailed analysis of the device in terms of its basic components would involve a lot more work.
The systems approach is useful in the study of complex biological functions because the details of the various component processes are unknown.
In the iris control system, very little is known about the processing of the visual signals, the mechanism of comparing these signals to the reference, or the nature of the reference itself.
By shining light at various intensities, wavelength, and durations into the eye and measuring the corresponding changes in the iris opening, we can get significant information about the system as a whole.
The techniques developed by the engineers are useful in analyzing the system.
Many biological systems are so complex that even the simplified systems approach cannot be tractable.