Limbic System

What are the 4 primary functions of the limbic system?

1. Homeostasis

2. Olfaction

3. Memory

4. Emotion

1. What structures in the brain are for memory and learning, and are attached to an event?

2. The emotional response which causes attachment is mediated by what?

1. Hippocampus & amygdala

2. Hypothalamus

Where is the amygdala located?

Deep in the temporal lobe.

What is the function of the amygdala?

• Involved in emotions and motivations. Allows us to associate an action with a consequence.

• Involved in processing fear, anger, pleasure, and determining what memories are stored and where those memories are stored.

• Constantly evaluates and integrates a variety of sensory information from surroundings.

  • Assigns this information the appropriate values of emotional dimensions.

• Has connections with the medial dorsal thalamic nucleus: smell

• Connections with the frontal cortex and develops as we do; shapes judgement.

What are 2 general things that happen when the amygdala is injured?

• Mood disorder

• No fear when lesioned bilaterally

What is valence in terms of the amygdala?

Amygdala determines how much someone likes, fears, and love/hates something.

What can happen when someone has an overactive amygdala?

Trembling

What is the Papez circuit?

Circuit of communication from hippocampus to the cingulate gyrus and back.

What is the function of the Papez Circuit?

Allows memory recall. You can access memories because of this circuit.

What effect does the pre-frontal cortex have on the amygdala?

• Emotional evaluation

• Decision making

• Behavior

What affect does the amygdala have on the striatum (basal ganglia)

• Motor responses

• Reward or repulsion processing

What affect does the amygdala have on the basal nucleus of Meynert?

Learning

What affect does the thalamus have on the amygdala?

• Exteroception (sight, sound, tactile stimuli)

• Interception (heart beat, BP)

What affect does the amygdala have on the hypothalamus?

• Hypothalamus affects the:

  • Periaqueductal gray (PAG): modulation of pain

  • Anterior pituitary: hormone (cortisol)

What affect does the amygdala have on the hippocampus?

• Emotion modulation

• Change to memory

What structure of the brain is involved with working memory*?

*Ability to hold a limited amount of information that is immediately available for a variety of cognitive functions.

Prefrontal cortex

What is another function of the prefrontal cortex that is discussed this lecture?

• Self-awareness and self-recognition

  • Cognitive ability to differentiate between self and environmental cues; understand behaviors or emotion of others; insight.

  • Preferentially involves right prefrontal cortex

    • Perceptual deficits (run into things and wonder why their arm hurts)

What are possible symptoms of a prefrontal injury/disorder depending where the lesion is?

• Apathetic, lifeless, abulic (unable to make decisions) state.

  • Dorsolateral lesion

• Impulsive, disinhibition, poor judgement, emotional lability (constantly changing).

  • Orbitofrontal lesion

• Depression

  • Left prefrontal lesion

• Behavioral disturbances resembling mania, indifference, or euphoria (bipolar)

  • Right prefrontal lesion

AN overactive amygdala likely needs more what?

Needs more pre-frontal cortex activation.

This is why therapy works.

What is an example of a reduction in pre-frontal modulation of the amygdala?

• ETOH intake.

  • ETOH disrupts the connections between pre-frontal cortex and amygdala.

    • This impairs the processing of emotions (why a drunk person may want to fight someone)

What happens when there is an abnormal increase in amygdala activity?

Anxiety; activates the hypothalamic-pituitary-adrenal (HPA) axis

What hormones are increased as a result to increased amygdala activity (HPA axis activation)?

• ACTH

• Adrenaline

• Cortisol

What brain structure is perhaps the most strongly implicated in the pathophysiology of PTSD? Why does this cause PTSD?

Amygdala; it is so active that the prefrontal cortex is under active.

The prefrontal cortex allows us to dampen anxiety and anger.

What is the relationship between the amygdala size and PTSD?

Association with smaller amygdala volumes with increased levels of fear conditioning.

What is emotion’s link with the immune system?

It treats the immune system as optional when stressed. This is okay in the short term, but if the stress response does not go down and the cortisol does not go down, bad things occur. This can cause:

• Colitis (inflamed colon)

• CV disorders

• Adult-onset diabetes

What is the link between stress and neuronal growth rate?

increased stress = increased cortisol = decreased neuronal growth rate = may lead to decreased cognitive ability.

What is the Engram theory?

Theory of learning: a group of neurons that were active under specific circumstances and that group’s information was retained.

What are the 4 steps to making a memory?

1. Encoding: a perception is gained by the brain (info about the body or world)

2. Consolidation: information is made stable

3. Storage: permanent changes made to retain the information

4. Recall: reactivation of the information in storage under precise cues

What is the hippocampal formation? (Hippocampus as a whole)

A group of structures involved in declarative memory formation. Performs the chemical activity needed to convert short term memory to long term memory (hippocampus)

What does the hippocampal formation include?

• Dentate gyrus (injured in Alzheimers dementia)

• Entorhinal cortex

  • Relay center between cingulate gyrus and hippocampus

  • Participates in memory consolidation

• Parahippocampus

What is long-term potentiation (LTP)?

Chemical process that forms memories. It permits implicit and explicit (facts) learning.

What are the 3 stages for LTP?

1. Immediate (1-2 secs)

2. Short-term

   1. For recognizable stimuli

   2. Loss within 1 min unless info is rehearsed

3. Long-term

   1. Relatively permanent storage (can lose it)

   2. Consolidation

What is ultimately the biggest part of successful LTP?

Synapse strength

What happens if there is unilateral damage to the hippocampus?

cannot form new long-term memories, lose much of the short-term memory

What does the Alzheimer’s disease have to do with the hippocampus?

It is associated with loss of neurons and synapses in the hippocampus.

What is Korsakoff Syndrome?

Characterized by loss of recent memory and tendency to fabricate accounts of recent events.

Changes to hippocampus, mammillary bodies, and thalamic nuclei.