Much progress in our
understanding of animal behaviour has been made since Descartes� postulation of
water-powered statues following prescribed patterns of action. We now see
behaviour and the conditions of the environment as being linked by the
intermediary of the brain, responding to various internal and external
receptors. Motivational mechanisms are vital to our survival, because they tune
our perceptions and increase the probability of an act (potentiation) in a
given situation, e.g. hunger. They are used in self-regulation,
self-preservation and sleep. Through them, actions are directed towards
objects, goals and expectations.
Claude Bernard first
realised that many of the mechanisms involved in responding to changes in the
external environment could also be used by the organism in the regulation of
the internal environment, comprised of a complex set of bodily fluids which
stay remarkably constant despite what is going on outside the animal, e.g. the
concentration of various salts, level of dissolved O2, nutrients
(e.g. glucose), acidity and temperature. In this way, the motivational
mechanisms in animals drive feedback and control systems, similar to those
found in central heaters in homes.
The body�s water
supply is crucial to every aspect of normal functioning. We continually lose
water, through urination, respiration, sweating, defecation and occasionally
vomiting and bleeding. This water loss has to be monitored, with action taken
when it drops below a certain level.
Blass and Hall�s
study on rats with tubes implanted, which drained water out of their stomach as
fast as they could drink it, showed that a dry mouth is just one indicator of
thrist. In fact, there are two separate aspects of our internal water balance:
the water volume existing inside our cells, and the volume of the fluids that
circulate outside our cells (e.g. saliva, blood, lymph, cerebrospinal fluid
etc.). Both intracellular and extracellular aspects of our water balance are
controlled by separate receptors, making their own set of internal homeostatic
adjustments.
Extracellular
receptors are located throughout the body, especially the heart and surrounding
blood vessels, which detect the drops in blood pressure that occur whenever
there is a drop in the total amount of bodily fluid. Signals from these
pressure receptors can lead to a various attempts to restore normal blood
pressure, such as through the use of a hormone called vasopressin (or
anti-diuretic hormone), manufactured by the hypothalamus and secreted by the
piuitary gland into the bloodstream. This hormone causes the blood vessels to
constrict, driving up blood pressure as well as making the kidneys retain
rather than excrete water.
Studies on dogs with
a small baloon inserted into the large vein leading to the heart showed that,
when inflated, the dogs drank copiously. The balloon impeded the blood flow
into the heart, causing a decrease in fluid pressure, causing the pressure
receptors to signal the brain to initiate drinking (Fitzsimons and
Moore-Gillow, Rolls and Rolls 1982).
The amount of
another hormone, angiotensin II, floating in the bloodstream, is modulated by
receptors in the kidneys, which also detect the volume of extracellular fluids.
This hormone seems to act on receptors just in front of the hypothalamas
amongst other areas (Epstein, Fitzsimons and Rolls, 1970), again an extremely
powerful and immediate motivator of drinking if injected into the bloodstream
or directly into the brain.
The osmoreceptors
depend on the chemical process of osmosis, where a sem-permeable membrane
allows the free flow of water but impedes the flow of substances dissolved in
it, leading to a flow of water from the less concentrated region into the more
concentrated region.
Studies where tiny
drops of solt water injected into regions in or around the hypothalamas of a
rat led immediately to drinking, showing the role of the osmoreceptors. When
the concentration of sodium ions in the fluid surrounding the receptor was
increased, water leaked out by osmosis to equalise ion concentrations in the
two areas, causing the cell to deflate somewhat, causing the receptor to fire
(Rolls and Rolls, 1982).
The need for so many
different receptor systems monitoring the levels of bodily fluids stems from an
evolved advantage in having multiple defenses, with backups if one system were
to fail. As with any internally regulating system, there is only a certain
amount of adjustment that the body is able to achieve to restore the bodily
balance, after which the corrective measures must involve some externally
directed behaviour, obviously drinking in the case of thirst. Yet the levels of
redundancy in thirst regulation are even less than that of maintaining the
body�s nutrient levels by feeding.
Simple drives such
as eating, sleeping and drinking can explain some of the most basic elements of
animal behaviour, but composite models which take into account threats and
being faced with novel situations are necessary to understand behaviour at a
higher level. The drive-reduction theory posits that all built-in motives act
to reduce stimulation and arousal, so that all organisms strive for an optimum
level of arousal, below which they try to incresae arousal by various means.
Drugs provide one such way of modulating the arousal level, with some drugs
acting as depressants (e.g. alcohol and the opiates) and others (such as
amphetamines and cocaine) acting as stimulants. Repeated drug use leads to
addiction, increased tolerance and withdrawal if the drug is withheld.
The opponent process
theory of motivation explains drug tolerance and withdrawal, amongst other
things, with the argument that all shifts of arousal level product a
counteracting process which moderates the modulation, i.e. flattens the peaks
and troughs. When the original instigator of the shift is removed, the opponent
process is revelaed more clearly, as in withdrawal.
Speculation about
possible �pleasure centres� in the brain, which when stimulated leads to
specific rewards as reinforcement for the behaviour. The dopamine hypothesis of
reward, the it is the activation of fibres originating the nucleus accumbens
triggered by dopamine which provides neural stimulation for repetition of the
action.
Thirst is an example
of a vital bodily need, which can be moderated to a small extent by internal
adjustments such as instructing the kidneys to retain rather than excrete
water. However, beyond a certain point, there can only be one remedy, that of
consuming more water to address the need.