22 - Limbic System Flashcards
Describe the overall functions of the limbic system
The limbic system brings together the seemingly unrelated functions of short-term memory and learning (hippocampus), expression of primal emotions such as fear, libido (amygdala), responses to odors (olfaction) and higher order modulation of the autonomic nervous system (hypothalamus). All are related to behavioral responses, both motor and visceral, appropriate to the sensory stimulus and/or cortical thought process.
What are the components to the lymbic system?
- Cingulate gyrus
- Subcallosal gyrus
- Parahippocampal gyrus
- Hippocampus
- Amygdala
- Septal area (uncus - olfactory cortex)
- (Hypothalamus)
- (Anterior nucleus of thalamus)
What is the “limbic lobe”?
A subset of limbic structures that form a border (limbus) around the diencephalon
How does olfactory input reach the limbic system?
Via the medial and lateral stria from the olfactory tract and bulb
What is considered the primary olfactory cortex?
The piriform lobe (uncus)
What is considered the olfactory association area?
The entorhinal area
Describe direct olfactory input
This input does NOT need to go to the cortex first, but rather goes directly to the amygdala
This input is responsible for the emotions and drives that are associated with olfaction
Describe indirect olfactory input
This input must go through the cortex first and then is sent to the hippocampus
This input is responsible for memory of olfaction
What is responsible for visceral responses to odors?
The connections between the hippocampus (smell memory area) and the hypothalamus (the homeostasis center)
How does the orbitofrontal cortex contribute to olfaction?
It brings odors into conscious awareness
How does the anterior olfactory nucleus and tract contribute to olfaction?
They send axons through the anterior commissure to both sides, which “fine tunes” olfaction sensitivity
Where is the hippocampus located?
It is located under the cerebral cortex and forms the floor of the inferior horn of the lateral ventricle
What else is the hippocampus called?
“Ammon’s horn” or Latin cornu ammonis which means the rounded, circular horns of a ram, which resembles the hippocampus
Also, Greek for “seahorse”
What is the input to the hippocampus?
- Amygdala
- Cingulate gyrus
- Pre-frontal cortex via entorhinal cortex
What is the “input gate” to the hippocampal formation?
The dentate gyrus
Which pathways bring input to the dentate gyrus of the hippocampus?
There are two pathways
- Alvear pathway
- Ferforant pathway
What is the “output” gate of the hippocampus?
The subiculum
Describe the way in which output fibers leave the hippocampus
The subiculum or “output gate” of the hippocampus sends fibers through the fornix before dividing into two sets of fibers to leave the hippocampus
What are the two destinations that output fibers are sent
Septal nuclei (pleasure center) of the limbic system that are located in the subcortical region (below the cerebral cortex)
The mamillary bodies of the hypothalamus
There is also a recurrent pathway from the subiculum. What does this mean and where does it go?
This means that the “output gate” of the hippocampus sends fibers directly to the entorhinal cortex (olfactory association area) which is part of the hippocampus - so it is “recurrent”
What does repetition and reinforcement do to the formation of memories?
Leads to consolidation of memories
What are internal field cells?
There are three
CA1
CA2
CA3
CA = cornu Ammonis
Which of the three cornu Ammonis are most vulnerable to anoxia and ischemia?
CA1
Which of the three ornu Ammonis is believed to be the trigger zone for temporal epilepsy
CA1
What is the function of the hippocampus?
Temporary memory until one of two things occurs…
- there is a conscious decision to memorize the information
- a strong emotional overtone stimulates the hippocampus to fire neuron circuits repetitively. This produces a memory code which is permanently established in the association cortex of the cerebral hemispheres.
What is the process of converting memories from short to long term memory called?
Consolidation
Both of the above methods of short to long term memory are considered consolidation (conscious decision to memorize, emotional memorization)
There are two types of memory. What are they?
1 - Working memory
2 - Declarative memory
Where does working memory take place? What type of memory is the working memory responsible?
Working memory takes place in the cortex
Information like adding up a list of numbers in order to figure out your balance is processed in the working memory. This information is typically quickly forgotten unless there is repetition or a decision to memorize your balance.
Where does declarative memory take place? What type of memory is the declarative memory responsible for?
Declarative memory takes place in the hippocampus
- Semantic information like facts, figures, names and other “memorized” information that you know is part of the declarative memory.
- Episodic information like remembering personal events
What else is the hippocampus responsible for in terms of memory?
The hippocampus also converts short-term memories into long-term memories (consolidation).
These memories are stored in the cerebral association cortex. The specific location the memory is stored in is thought to be related to the original mode of sensory input.
What is an example of specific location memory?
Visual memories are stored in areas 18 and 19 within the visual association cortex
How is a thiamine deficiency related to memory loss?
Although all systems in the body are affected by a thiamine deficiency, the CNS is particularly vulnerable due to its dependence on oxidative metabolism (which requires thiamine).
Which syndrome caused by thiamine deficiency encompasses memory loss?
Wernicke–Korsakoff syndrome (WKS)
WKS is usually secondary to alcohol abuse, which causes a thiamine deficiency and the pathology of this syndrome.
It mainly causes vision changes, ataxia and impaired memory.
Is thiamine deficiency related to Alzheimer’s?
We know that treatment of Alzheimer’s patients does NOT improve the condition or lessen the memory impairment.
Where is the amygdala of the limbic system located?
Deep to the uncus, which is located in the anterior pole of the temporal lobe
The amygdala has three major nuclei. What are they?
1 - Basolateral nuclei
2 - Corticomedial nuclei
3 - Central nuclei
Which nuclei make up the “input gateway” of the amygdala?
The basolateral and corticomedial nuclei
What nuclei makes up the “output gateway” of the amygdala?
The central nuclei
What fibers are input fibers coming to the amygdala via the basolateral and corticomedial nuclei?
Input fibers come from…
- General sensory cortex
- Special sensory cortex
- Olfactory input (DIRECT)
Where exactly does the direct olfactory input enter into the amygdala?
The corticomedial nuclei
Remember this means that it does NOT get processed by the cortex before being sent to the amygdala
There are two major outputs of the amygdala. What are they?
1 - Stria terminalis
2 - Ventroamygdalofugal pathway
Describe the stria terminalis pathway
Provides reciprocal connections with the septal area and the hypothalamus, modulating visceral responses
Describe the ventroamygdalofugal pathway
This pathway to the ventral striatum (nucleus accumbens) and brainstem allows emotions to influence movements and brainstem nuclei
These brainstem nuclei include CN VII (facial), CN X (vagus, dorsal motor nucleus that provides parasympathetics)
What part of the prefrontal cortex integrates emotions with decision making?
The ventromedial prefrontal cortex
It is also responsible for conscious control of emotions
What is the function of the amygdala?
Attaching emotional significance to sensory stimuli and organizing behavioral responses.
Generally, it is responsible for providing appropriate behavioral responses and reactions to sensory information, especially, information that is threatening or potentially dangerous.
Why would you say the amygdala has a role of “assessing”?
Because it is responsible for assessing sensory input in order to determine if it is non-threatening, frightening, enraging, sexually arousing, etc.
Where are conscious perceptions of this sensory input formulated?
In the cingulate gyrus and prefrontal cortex
In these areas you will bring the sensory input into conscious awareness and process “how you feel about it”
This is where we get our sense of physical and emotional “comfort level”
Where are visceral responses to this sensory input formulated?
Hypothalamus (autonomic reactions) and septal areas
Where are motor responses to this sensory input formulated?
The motor cortex
What type of clinical presentation will we see with amygdala damage?
Amygdala damage results in the inability to…
- Express appropriate behavioral and visceral responses to known stimuli
- Learn appropriate responses to novel stimuli
You may lose the ability to “read” facial expressions - this means you would not have the ability to look at someone’s face and interpret their emotional state and what that means
Where is the septal area of the limbic system located?
Below the genu of the corpus callosum
What is the function of the septal area of the limbic system?
The septal area relates emotion to respiratory and cardiovascular centers, salivatory nuclei and autonomic centers
There is some connection with the septal area being a pleasure/reward center
What is the major input to the septal area?
The hippocampus
Fibers from the hippocampus reach the septal area via the fornix
From the septal area, these fibers continue to the amygdala via the stria terminalis
What reciprocal connections does the septal area have?
The septal area has reciprocal connections with…
- hypothalamus (homeostasis)
- amygdala (emotional significance to sensory stimuli)
- cingulate gyrus (conscious awareness of feelings)
What is the major output from the septal area?
The medial forebrain bundle to the hypothalamus and brainstem nuclei
What example of amygdala function did Dr. Spocter give during lecture?
Mice experiment
- Researchers trained a mouse to push a button
- When the button was pushed, an electrical shock was sent to the septal area of the brain to stimulate this area
- The mouse gave up eating, drinking, sex, sleep and socialization to constantly push this button
- Maxed out at 6000 times per minute pressing this button
- The septal area is associated with being a reward/pleasure center
What is the Papez Circuit?
An interconnection of components of the limbic system. The following components are included in the circuit:
- hippocampus
- fornix
- septal area
- hypothalamus (mammillary body)
- mammillothalamic tract
- thalamocingulate tract
- cingulate gyrus (which allows for interconnections with prefrontal, parietal and temporal association areas)
- cingulum
- parahippocampal gyrus
- back to hippocampus
NOTE: although the amygdala is part of the limbic system, it is NOT part of the Papez Circuit
Diagram on slide 21
What type of drug is the limbic system a target organ for?
Anxiolytic drugs
What are anxiolytic drugs?
Anti-anxiety medications
What is an example of a class of drugs that are anxiolytic drugs?
Benzodiaxepines
Example:
- Diazepam (Valium)
