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 America are obese, ¼ in Western Europe.  Millions die every year of clogged arteries and other complications of obesity.  Drive to pleasure is killing us.

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.