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Senses at the periphery of the organism send afferent (input) signals along nerves to the centre (the brain) which processes them: comparing, amplifying, attenuating and channelling them. The efferent (output) signals emerge and travel to the muscles where they trigger the activity (behaviour) which keeps us alive and procreating: eating, speaking, giving, taking, exchanging, mating, sheltering: approaching goodies and avoiding harm.

There are two streams of input to the brain: signals from the outside environment which are transmitted by eyes, ears, nose, and skin; and signals from our internal environment which might indicate hunger (low blood-sugar levels) or fatigue (high lactic acid levels in muscles). These two streams of input are integrated by the brain so that the resulting muscular activity (behaviour) acts to oppose the 'drifting away from balance' of the organism: eg low blood-sugar levels are raised, while high lactic acid levels are lowered.

Because output acts to NEGATE changes in input, this is known as NEGATIVE FEEDBACK: it is the means by which a thermostat maintains a steady temperature despite fluctuations in the weather ... imagine a truck on an undulating road, governed so that when it speeds up, petrol is cut, and when it slows down, petrol flow is turned up. A steady speed will be maintained up hill or down dale. This is the simple process by which all the complexly interacting systems of the organism (respiration, digestion, circulation, perception etc) are maintained in balance, known as 'maintaining homeostasis.'

The opposite form of feedback, positive feedback, occurs when output from a system is DIRECTLY related to input: imagine a steam train regulated so that the faster it goes, the MORE steam it gets, and the more steam it gets, the faster it goes and the faster it goes ... clearly, breakdown or disaster are inevitable. This is commonly known as a 'vicious circle' and, in human affairs, positive feedback loops underlie conflict, disease, crime, addiction etc.

For example, imagine a mother who punishes her child for crying in an attempt to train it to cry less. But the child cries out of fear, and the more it cries, the more it is punished, and the more it is punished, the more it cries. Hence baby-battering. Or consider a man who over-eats to compensate for a poor self-image: the more he eats, the less satisfactory is the image in the mirror; the less satisfactory the self-image, the more he over-eats ... hence obesity. Or consider the function of newspapers, which record the most threatening statements of one section of the citizenry, convey the recording to another part of the citizenry, record their most threatening replies, and relay them back to the first, and so on. The result is war, which sells a lot of newspapers!

Life may be said to be the history of the negative feedback (balancing, maintaining) process, which lends stability to unlikely blobs of quivering jelly (life-forms), maintaining their balance in vertical and unstable configurations and in changing situations. Paradoxically, however, positive-feedback cascades -- normally associated with breakdown -- mediate the self-re-inforcing cycles of true love and are (in context!) part of the healthy interlinked mechanism by which a small input to the nervous system, even a single photon, can be propagated and amplified to trigger a large degree of muscular activity.

Parallel to (and interacting with) the nervous system, the hormonal system affects mood (or mode!) preparing the organism for fight or flight, precipitating the functional changes which permit the organism to cope with emergencies, eg adjusting heart-rate, redirecting bloodflow from the digestion to the muscles, inhibiting pain, etc.

But it is the flow of signals in the nervous system which is crucial to understanding the effects on metabolism and perception of drugs, most of which alter the conductivity of nerves, and only secondarily and indirectly affect the hormones.


Go Back	Understanding the Nervous System
	Senses at the periphery of the organism send afferent (input) signals along nerves to the centre (the brain) which processes them: comparing, amplifying, attenuating and channelling them. The efferent (output) signals emerge and travel to the muscles where they trigger the activity (behaviour) which keeps us alive and procreating: eating, speaking, giving, taking, exchanging, mating, sheltering: approaching goodies and avoiding harm. There are two streams of input to the brain: signals from the outside environment which are transmitted by eyes, ears, nose, and skin; and signals from our internal environment which might indicate hunger (low blood-sugar levels) or fatigue (high lactic acid levels in muscles). These two streams of input are integrated by the brain so that the resulting muscular activity (behaviour) acts to oppose the 'drifting away from balance' of the organism: eg low blood-sugar levels are raised, while high lactic acid levels are lowered. Because output acts to NEGATE changes in input, this is known as NEGATIVE FEEDBACK: it is the means by which a thermostat maintains a steady temperature despite fluctuations in the weather ... imagine a truck on an undulating road, governed so that when it speeds up, petrol is cut, and when it slows down, petrol flow is turned up. A steady speed will be maintained up hill or down dale. This is the simple process by which all the complexly interacting systems of the organism (respiration, digestion, circulation, perception etc) are maintained in balance, known as 'maintaining homeostasis.' The opposite form of feedback, positive feedback, occurs when output from a system is DIRECTLY related to input: imagine a steam train regulated so that the faster it goes, the MORE steam it gets, and the more steam it gets, the faster it goes and the faster it goes ... clearly, breakdown or disaster are inevitable. This is commonly known as a 'vicious circle' and, in human affairs, positive feedback loops underlie conflict, disease, crime, addiction etc. For example, imagine a mother who punishes her child for crying in an attempt to train it to cry less. But the child cries out of fear, and the more it cries, the more it is punished, and the more it is punished, the more it cries. Hence baby-battering. Or consider a man who over-eats to compensate for a poor self-image: the more he eats, the less satisfactory is the image in the mirror; the less satisfactory the self-image, the more he over-eats ... hence obesity. Or consider the function of newspapers, which record the most threatening statements of one section of the citizenry, convey the recording to another part of the citizenry, record their most threatening replies, and relay them back to the first, and so on. The result is war, which sells a lot of newspapers! Life may be said to be the history of the negative feedback (balancing, maintaining) process, which lends stability to unlikely blobs of quivering jelly (life-forms), maintaining their balance in vertical and unstable configurations and in changing situations. Paradoxically, however, positive-feedback cascades -- normally associated with breakdown -- mediate the self-re-inforcing cycles of true love and are (in context!) part of the healthy interlinked mechanism by which a small input to the nervous system, even a single photon, can be propagated and amplified to trigger a large degree of muscular activity. Parallel to (and interacting with) the nervous system, the hormonal system affects mood (or mode!) preparing the organism for fight or flight, precipitating the functional changes which permit the organism to cope with emergencies, eg adjusting heart-rate, redirecting bloodflow from the digestion to the muscles, inhibiting pain, etc. But it is the flow of signals in the nervous system which is crucial to understanding the effects on metabolism and perception of drugs, most of which alter the conductivity of nerves, and only secondarily and indirectly affect the hormones.