Limbic System. 88m Robert Sapolsky video.
The Limbic system is the emotional decision-making system of the brain. Sapolsky doesn't define "emotional" and it is not fully clear to me what the word means.
Unifying theory of the limbic system: every circuit in the limbic system tries to influence the hypothalamus. Since the hypothalamus is the central hub of the neuroendocrine system, it is the main control system for the body: "Emotions change how your body works". That is, the limbic system tells the hypothalamus what to do. Each part of the limbic system has mechanisms to inform the hypothalamus about what to do and mechanisms to inhibit the signaling of other parts of the brain from sending their signals to the hypothalamus. The brain is a complex drama, a cooperative fight of stimulus & inhibition. Those brain regions with fewer synapses between them and the hypothalamus have a greater "say" in what the hypothalamus should do. So it is surprising that every form of human sensory input has to go through a minimum of 3 or 4 synapses to get to the hypothalamus except olfaction which is 1 synapse from the limbic system! Is smell our most importance sense or is it a spandrel?
Most of the lecture is a survey of the neuroanatomy of the limbic system. But toward the end it gets interesting. First he examines the nature of brain research, how we know what we know, and the limitations of such research. Finally, Sapolsky provides six strong pieces of evidence for the James-Lang theory of emotion which proposes that the physiological changes happen in the body first and the brain senses that and decides what emotion it is feeling. More and more evidence suggests that some of the basis of emotion is indeed physiological (= your body's state). Detailed notes below.
14. Limbic System
The limbic system was known as the rhinencephalon, or the nose brain, due to rat studies which found the olfactory bulb constituted some 40% of the brain of a rodent. The ethological perspective clarifies this connection: rat emotions are associated with smell.
The limbic system is not the nose brain, rather it is the part of the brain that processes the information most pertinent to emotional life.
Paul D. MacLean developed the model of the triune brain with three layers of brain function: reptilian (automatic / regulatory which includes the hypothalamus & pituitary), the paleomammalian or limbic system, and the neocortex or neomammalian (the analytical brain; present in all vertebrates but more developed in some).
Contrary to popular conception, the neocortex is deeply influenced by the limbic system which is why all of us have made terrible emotional decisions under duress as our neocortexes are subject to hormonal influence from the limbic system. Moreover, the neocortex affects the limbic system so that if you think of your own mortality, your limbic system will start pumping out CRH (the hypothalamic hormone involved in the hypothalamic-pituitary-adrenal (HPA) axis regulating the fight-flight response behavior). The brain is interconnected and the parts of the triune brain are in constant communication with no strict separation of functions.
Ondine's curse (affects about 1/200,000 children): damage to the reptilian brain prevents automatic breathing so you die of sleep deprivation as you keep waking up to breathe.
Antonio Damasio wrote "Descartes' Error": Sapolsky:"it is complete nonsense" that there is a strong separation between thought and emotion.
James Papez sorted out the anatomy of the limbic system in the 1930s. He noted the interconnectedness of the parts (the Papez circuit) that are now identified as the emotional brain.
Cell bodies tend to be grey in color: grey matter are nuclei: cell bodies
packed into a region of space
myelination tends to be white in color: white matter is "cables" of axons
wrapped in myelin
Anatomy of the limbic system
PFC: Prefrontal cortex: larger in humans than other species: most recently evolved: the last part of the brain to fully mature (isn't fully myelinated until the mid-20s): provides impulse control. Ian Dunbar determined that the size of the PFC is correlated in ∼150 primate species to the size of their average social group. Dunbar's works suggests that the PFC evolved for gossip, social relations, appropriate behavior, and social intelligence. It is involved in learning when to be aggressive and appropriate sexual behaviors.
AC: Anterior cingulate: part of the PFC which is intimately connected to all parts of the limbic system: the cortical component of the limbic system. It is affected in people with clinical depression ("hypersensitivity to pains of life and the world"). It is involved in empathy and feeling the pain of others.
Hipp: hippocampus (Latin for seahorse, but Sapolsky claims "it actually looks like a jelly-roll"). Sapolsky spent 30 years obsessing over it, but now thinks the PFC is the most interesting part of the brain. The hippocampus is involved in learning & memory and turning off the stress response (it measures the levels of glucocorticoids). Which makes sense because survival may depend on remembering how you escaped last time.
hippocampal-amygdala pathway: bidirectional projections between Hipp and Amyg: learning to be afraid
fimbria/fornix: bidirectional projections between Hipp and SEPT (fornix means arch and the pathway is arch-shaped)
Amyg: Amygdala (means almond): center of fear & anxiety & aggression. It also plays a role in male sexual motivation which may explain various pathologies confusing sexual & aggressive behaviors. It receives olfactory data directly: only 1 synapse. Sapolsky: "You cannot understand the neurobiology of being violent without understanding the neurobiology of being afraid and being anxious".
stria terminalis: from Amyg to Hipp by arching the long way around the fimbria/fornix. Such "nutty wiring" tells us that during embryonic development the structures were not close to each other resulting in the long pathway or it might tell us something about the evolutionary history ("evolution is not an inventor, it is a tinkerer").
Thal: Thalamus (above hypothalamus): a bidirectional circuit with the PFC
M: mammillary bodies are tucked behind the hippocampus; it is involved in maternal behavior
mammillothalamic fasciculus: bidirectional connections between M & Thal
SEPT: septum (general term for a midline structure) at the midline of the brain; 2 synapses from Amyg; it inhibits aggression.
Medial forebrain bundle (MFB): major projection from SEPT to Hipp to M and back to SEPT: major limbic pathway
NAcc: Nucleus accumbens: involved in appetitive behavior (anything involving an appetite), releasing dopamine (involved in anticipating pleasure: not about the reward or pleasure but anticipating that a reward is available and empowering the behavior to get the reward, to pursue pleasure). It is affected by depression and drug addictions.
VTA: Ventral tegmental area is important in depression. It sends a lot of information to the NAcc which then connects bidirectionally to each of the other parts of the limbic system
hypothalamus: above pituitary, center of neuroendocrine function
centers in the hypothalamus:
ventral medial hypothalamus & medial preoptic area are involved in sexual behavior. Size differences based on sex (including transgender individuals) and sexual orientation.
SCN: suprachiasmatic nucleus: circadian rhythms
PVN: paraventricular nucleus: makes CRH (corticotropin-releasing factor: the backbone of the stress response)
arcuate nucleus: bottom of hypothalmic funnel: where the hormones come out into the bloodstream
lateral hypothalamus: related to hunger (not aggression as early studies wrongly concluded): measures blood glucose & insulin levels: also involved in broader types of hunger (hunger for information, etc.)
Walle Nauta (1916-1994) was the greatest neuroanatomist of the last half of the 20th century. He suggested that the prefrontal cortex (aka the frontal cortex) should be considered part of the limbic system based on its wiring projections.
There are several methods for studying brain functioning. Missile wounds and other lesions or brain damage allow us to learn by observing the functions that were destroyed by damage to that part of the brain. For example, Subject N.A. had his septum "taken out" by a "miniature fencing foil" at a wedding. Another approach is to stimulate a brain region with electrodes that artificially stimulate action potentials (rarely done on humans except for some rare therapeutic techniques). Instead of a stimulating electrode, you can put in a recording electrode to report on when these neurons are getting excited. Some people record or stimulate just one neuron at a time. "Patch clamping" can record the outputs of a single ion channel on an axon. Measuring the biochemistry such as neurotransmitter concentrations and molecular biology (which genes are being expressed). Finally brain imaging techniques let us study even live human brains in action (CAT or CT scans, MRI, etc.) which can show which areas are active and the metabolic rates and can see the whole brain at once.
Brain imaging studies have shown that some regions of the brain change in size over time. The Amygdala gets bigger over time in people with post-traumatic stress disorder (PTSD). Periods of severe stress cause neurons in the Amygdala to grow more dendritic processes which probably accounts for the size increase. People with long-term depression show shrinking in their hippocampus.
There are some difficulties in interpreting the data from the methods of studying brain function. Centers vs. pathways (neural bodies vs. axons): in lesion studies you may have severed communications and not the center for the given functionality. The notion of a center of neural functionality is a "pretty flimsy concept". The ethological context is important to understanding the wiring in terms of the animal's own language. You have to understand the species and their fixed action patterns as well as the individual and their social situation.
Different areas of the limbic system tend to work in opposition to each other (e.g., Amgy & SEPT stimulating and inhibiting aggression).
We tend to discuss the ways the brain influences outcomes throughout the body. In addition, body functions can affect the brain. The autonomic nervous system seems in particular to report physiology to influence the brain.
Evidence for the James-Lang theory of emotion:
- Stanley Schachter classic experiment (but unethical in modern pharmacology) showing that epinephrine enhances or modulates the emotional state of the social situation (anger & upset or joy & happiness, etc.). Note that epinephrine/adrenaline is associated with arousal of the sympathetic nervous system.
- Benzodiazepines (like Valium) are prescribed to decrease anxiety and muscle relaxation (the exact same dose): anxiety is about monitoring the level of tension in your body ... getting feedback from muscle tone; when your body signals relaxed muscles the brain realizes it doesn't need to feel so anxious: meditation & biofeedback: learning what thoughts allow the brain to regulate blood pressure, for example.
- After a fight, frequently a couple can apologize and everything is all right. But while still in the excited state, memories of prior issues can be brought out leading to another fight: the sympathetic nervous system hasn't realized that the cognitive situation is solved, so it still has hormones pushing you to the agitated state and the brain wonders why it is so upset and goes to find a good reason "from the second Roosevelt administration".
* In females, it takes longer for the sympathetic nervous system to return hormone levels back to baseline
* Similarly, after orgasm males go back to baseline faster than females
- Treat people with clinical depression to mechanically force themselves to smile, after a half hour they feel better
- Test scores received while sitting upright (vs. slouched over), result in happier assessments of the results and more pride. The posture of your spine influences your emotional response!