5/28- Limbic System Flashcards
What is the most common type of epilepsy?
Medial temporal lobe seizure
Flowchart outlining fit of limbic system into the rest of the brain?
Input from:
- Cerebral cortex (sensory)
- Reticular formation
Outputs to:
- Cerebral cortex (association and motor)
- Hypothalamus
Basically tells you what’s most important with everything coming into the brain
Where is the limbic system (broadly)?
Top/middle of prefrontal brain
Major limbic system components and their functions (one word/phrase)?
Prefrontal cortex: conscience
Basal forebrain: relay system (like thalamus for rest of nervous system)
Hippocampus: memory center
Amygdala: emotions/fear
Large connections between limbic system components (what they are and what they connect)?
Uncinate fasciculus: medial and temporal lobes
Cingulate fasiculus: temporal and frontal lobes (via cingulate gyrus)
Medial forebrain bundle: many connections to hypothalamus
What is the prefrontal cortex (PFC)- characteristics/job?
Connects to what via what?
- Large relative size in humans compared to other primates/mammals
- Emotional parts: orbitofrontal and ventromedial cortex (dorsolateral is more rational thought)
- “Conscience”- powerful inhibition of lower hypothalamic-mediated aggression (superego)
- Connected via the uncinate fasciculus to medial temporal structures
Ex) Prefrontal lobotomy effects
Ex) Phineas Gage
Why was the prefrontal lobotomy performed? Effects?
- Wide variety of conditions from schizophrenia to chronic pain syndromes
(“Ice-pick lobotomy” also became popular through orbitofrontal cortex)
- Resulted in emotionally-stunted individuals; couldn’t express emotion
(Replaced by anti-psychotic medications)
What happened to Phineas Gage?
- Knocked out his orbitofrontal cortex with a tamping rod through his cheek/eye
- Remained conscious, but significant change in personality
- Became rude, very disinhibited, couldn’t hold a job (not inhibiting lower functions of hypothalamus)
Nuclei in basal forebrain?
- Nucleus accumbens
- Substantia innominata (involves basal nucleus of Meynert)
- Septal area (near septum pallucidum)
- Diagonal band of Broca
More posterior nuclei in basal forebrain?
- Septal area continued
- Stria terminalis and bed nuclei
- Diagonal band of Broca (now separated by anterior commissure)
Septal area connects most to what?
Septal area has many hippocampal connections
Afferent fibers: from hippocampus to medial/lateral septal areas
Efferent fibers: to diagonal band of Broca to hippocampus and back
Other septal area connections?
- To prefrontal cortex
Via medial forebrain bundle:
- To hypothalamus
- To mammillary bodies
- To medial thalamus
Function of septal area?
- Relay station between hippocampus and hypothalamus
- Modulates function of both of these
- Electrical stimulation modulates aggressive behavior and elicits drinking behavior
- Lesions cause “septal rage” (uninhibited lower structures/hypothalamus)
Nucleus accumbens connections?
Afferent: receives large dopaminergic projection from ventrotegmental area (VTA) in the mesolimbic pathway
Efferent:
- Projections back to VTA
- Substantia innominata
- Substantia nigra
Functions of nucleus accumbens?
- Integrates sequencing of motor responses associated with emotion and rewards
- Implicated in addiction (e.g. cocaine induces release of dopamine into this region)
– e.g. mice would hit reward level over food, reproduction, etc…
What is the substantia innominata? Connections?
Aka “perforated substance” b/c many blood vessels
- Relay nucleus form amygdala to lateral hypothalamus
- Also contains Nucleus Basalis of Meynert
- Cholinergic nucleus that projects to widespread areas of cortex and limbic system
Functions of substantia innominata?
- Arousal and memory
(Loss in Alzheimer’s disease and some in Parkinson’s)
Where is the hippocampal formation located?
Temporal lobe
Three divisions of the hippocampal formation?
- Subicular cortex/subiculum
- Hippocampus (CA regions, esp CA3 and CA1)- Archicortex
- Dentate gyrus (mossy fiber cells as opposed to pyramidal)
Two basic pathways in hippocampus
Both enter via entorrhinal cortex:
- Perforant pathway- through subiculum to dentate gyrus (projections back to CA 3 and 1 and back out entorrhinal path)
- Alvear pathway- forms fornix going to other regions of limbic system and cortex
What are the Schaffer collaterals?
Connection between CA3 and CA1 region; underly memory
What process sets memory storage?
Long term potentiation (e.g. CA3 input to CA1 neuron)
- Brief high frequency stimulation results in strong and long term potention of an otherwise weak signal
- Input without tetanus will not receive LTP
Biochemical process at play in long term potentiation (LTP)?
- Strong depolarization removes Mg2+ ion block of the NMDA receptor
- Glutamate binds and allows Ca to enter the post-synaptic neuron
- Ca acts as a 2nd messenger to activate kinases which can potentiate existing AMPA receptors and recruit new AMPA receptors to the synapse
What is the Papez Circuit?
- Efferent projections from hippocampus through fornix to post-commissural fornix down to mammillary bodies
- Mamillary bodies send projections through mammillothalamic tract to anterior nucleus of thalamus
- Ant. nucleus of thalamus sends projections to cingulate gyrus
- Then from cingulate gyrus back through entorrhinal cortex back to hippocampus
Mnemonic: “He Man Ate a Cat”
Amygdala anterior or posterior to hippocampus?
Anterior
What nuclei are comprised within the amygdala?
- Corticomedial group
- Basolateral group
The amygdala receives information from what?
- Olfactory bulb (*directly)
- Diffuse sensory cortices
- Basal forebrain structures
- Medial thalamus and hypothalamus
Can integrate much information in emotional context before passing on to lower structures
Efferent projections of the amygdala?
- Corticomedial group to the medial hypothalamus via stria terminalis (directly or via bed nuclei)
- Basolateral group to the lateral hypothalamus (and also PAG, and prefrontal cortex) via ventral amygdalofugal path
Functions of amygdala?
- Functions via potent control over the visceral processes of the hypothalamus
- Plays a role in regulating aggression/rage, feeding, cardivoascular, and endocrine functions
What is the Kluver-Bucy Syndrome?
Often seen in lesions/strokes of amygdala, will see some (but normally not all) of the following symptoms first seen in monkeys with lesioned amygdalas:
- Hypersexuality
- Erratic feeding
- Loss of fear response
- Inappropriate oral exploratory behavior
What did the Little Albert experiment show?
Conditioned fear (how environment is more important than genetics and can change personality)
- PROVES THAT FEARS ARE LEARNED
- Not originally afraid of white rate, but becomes scared when accompanied by loud clanging (fear spreads also to rabbit and all furry things)
- Unconditioned stimulus: clanging bell
- Conditioned stimulus: white rat
(This same principle is also at play in conditioning therapy- pairing pleasant stimuli with things that people fear to get them over it)
What controls conditioned fear?
2 different pathways?
The amygdala
- Direct route: through thalamus (never appears through conscious mind)
- Indirect route: through cortex (through occipital cortex through associated cortices back to amygdala)
What is not required for conditioned fear? Exception?
The hippocampus is not required for conditioned fear, except in regard to contextual fear (e.g. the lab room in Little Albert’s experiment)
What is the role of monamine nuclei?
Collectively control conscious state and mood (among other things)
(e.g. mood and arousal may affect fear and emotional response)
Monoamine nuclei include what and project where?
Widely throughout forebrain, including the limbic forebrain:
- VTA -> dopamine (mesocortical pathway)
- Locus ceruleus -> NE (project bilaterally in brain)– attention to stimulus
- Midline raphe nuclei -> serotonin– mood
- Nucleus basalis (of Meynert) -> ACh– arousal and memory
Medial temporal lobe seizure symptoms (categorized by component of limbic system)?
Amygdala:
- Fear
- Bad smell
Hippocampus:
- Deja vu (familiarity in unfamiliar place)
- Jamais vu (unfamiliarity in familiar place)
- Amnesia
Basal forebrain nuclei:
- Increased heart rate (via hypothalamic connections)
- Increased blood pressure (via hypothalamic connections)
- Altered consciousness
- Laughing (hypothalamus)
- Spitting
Prefrontal cortex:
- Automatic behaviors (lip smacking, picking…)
- Post-ictal confusion/psychosis
What is most common cause of inappropriate/unstimulated laughing/laughing seizure?
Hamartoma in the hypothalamus (don’t feel happy while laughing)
