Subcortical Powerhouse: The
Limbic System
1)
The evolutionary
hypothesis:
a)
The brain’s main
function is to keep the organism of which it is part alive and
reproducing. All its other tricks – the
ability to appreciate music, to fall in love, to create a unified theory of the
Universe – arise out of that single overriding ambition
b)
Thus a huge part
of brain structure and function given over to making sure its body parts do what is necessary to find food,
sex, security, etc.
2)
A carrot and
stick system. Three steps:
a)
brain responds to
a particular stimulus to create an urge that demands to be satisfied; this
triggers action (eating, sex, socializing)
b)
the action is
rewarded with positive feelings of pleasure.
Note that the action is the thing that gives much of the pleasure
c)
when the action
is complete, the rush of pleasure is replaced by a sense of contentment,
fulfillment
3)
When the system
breaks down:
a)
Urges cease to
prompt appropriate action, or
b)
Normal actions
cease to be enough to satisfy our urges
4)
Gilles De La
Tourette’s Syndrome
a)
Tourette’s
patients are usually painfully aware of how ridiculous or offensive they seem
to others. Causes people to shun them.
b)
Some can control
symptoms with an intense effort – emotional arousal leads to jerks and tics,
noises, curses well up and explode.
c)
Medication
preventing dopamine from activating receptor cells
d)
In physical
Tourette’s-type symptoms: bursts of activity in an area of the unconscious
brain called the putamen – part of
the basal ganglia in the center of the brain, looks after automatic movements
learned by repetition, allowing the conscious brain to get on with its
business. Some Tourette’s patients can control the urge temporarily, but it
must finally come out.
e)
In the case of
shouts and weird vocal habits Tourette’s: over activity in another part of the
dopamine pathway that links unconscious brain to conscious brain, touching on
language areas in the temporal zone.
5)
Obsessive-Compulsive
Disorder
a)
Part of same
underlying biological disturbance, but:
b)
More complicated
urges than shouting or twitching: OCD patients driven to carry out complicated
routines in order to still an ever-present feeling of doubt or unsettledness.
c)
Purely mental (obsessive)
or overtly behavioral (compulsive)
i)
Examples: counting
to seven between each mouthful of food; having to do everything in fours;
internal compulsions (going over past conversations, obsessed with idea, such
as killing somebody)
d)
These mental and
behavioral tics are, like in Tourette’s, fragments of pre-programmed
skills. A neural pathway is
overactive. But here the memories are
not personal ones but built into the species as instincts.
i)
The instinct to
keep clean, to check the environment for signs of danger, to keep order and
balance.
6)
Neural pathway
that runs between the frontal lobe and another part of the basal ganglia – the
caudate nucleus
a)
Putamen =
connected to premotor cortex
b)
Caudate nucleus =
connected to the frontal lobes (thinking, planning, assessing). It prompts you to wash when you are dirty,
alerts you to focus your attention, etc.
c) In OCD, the
error-detection mechanism is somehow stuck on alert, not matter how much the
appropriate turn-off action is carried out it continues to fire.
7)
Hunger
a)
Some people
experience an unquenchable appetite: can never get enough of what they want
(sex, food, risk) = reward deficiency syndrome, cannot get satisfaction.
b)
Hunger: 1/3 of
people in
c)
The mechanism by
which simple hunger is generated and satisfied centers on the
hypothalamus. Information constantly fed
to the hypothalamus about the state of the body. HT feeds these signals up to the cortex where
they excite areas that consciously register hunger and organize the finding and
preparing of food. When food is eaten,
the system goes into reverse: body signals satisfaction down to the HT, passes
on to the cortex, desire to stop eating is generated.
d)
Eating disorders:
one cause might lie in the HT itself
e)
Anorexia:
messages between HT and cortex not carried efficiently, do not feel hunger
f)
Most HT
dysfunctions are caused by disturbance of neurotransmitters, actual damage is
rare. Serotonin damps down activity in
the HT, reduces appetite. Anorexics tend
to show unusually raised serotonin, bulimics have low (binging and purging).
8) Sex
a)
Sexual drive
centers in the HT but radiates to encompass a wide range of other brain areas.
b)
The basic urge-reward-relief
system: orgasm is caused by a massive rush of dopamine and the relaxation that
follows is due to a hormone oxytocin.