Limbic system Flashcards
List the limbic system functions
Refers to the structures in the medial cerebral hemispheres that border or wrap around the corpus callosum (limbus is Latin for rim or border).
It is not a single structure, but a system of highly interconnected structures that participate in a wide variety of behaviors
In broad terms, the function of the limbic system is to interface our internal drives and emotional states with our decision-making cognitive functions.
It acts to match the appropriate visceral (meaning autonomic) responses with a given behavioral and conscious emotional response.
For example, when you encounter a fearful stimulus it is your cognitive processes that recognize that stimulus as fearful, but the limbic system matches the appropriate visceral response to that fear (increased heart & respiratory rates, pupil dilatation, sweating, etc.).
“Oh a bear.” vs. “OH SHIT A BEAR”
How did Papez circuit come about? (history)
The concept of the limbic system being the anatomical substrate for the generation and expression of emotions was proposed first by James Papez in 1937.
He hypothesized that since emotions have both unconscious (autonomic) and conscious (somatic) components, that there must be an interaction between the regions of the brain that control the autonomic and somatic components of an emotional response.
The two “ends” of this circuit were the hypothalamus, which integrates a number of autonomic functions and is required for expression of emotions, and the cingulate gyrus, a region of the cortex that is also necessary for emotional experience.
Papez circuit (diagram)
Write out Papez circuit
The circuit he proposed was
Cingulate gyrus → hippocampus → mammillary body of the hypothalamus → anterior thalamic nuclei → cingulate gyrus
This circuit was subsequently expanded to include the amygdala and inputs from the prefrontal cortex (but you will not be responsible for knowing those additional components). Notice that the path from the anterior thalamic nucleus to the cingulated gyrus is not labeled. This is a diffuse pathway that runs through the internal capsule (sometimes referred to as the thalamocingulate radiation) and will not be on the exam.
Limbic system anatomy
Location/brief function of the amygdala, and Kluver bucy syndrome
The amygdala located at the tip of the temporal lobe just in anterior to the hippocampus.
Klüver-Bucy syndrome
In monkeys, bilateral lesion/removal amygdala and hippocampus, produced animals that showed no fear or anger and became completely docile. Other behavioral features included increased appetite, hypersexuality, oral behavior and visual agnosia.
Similar, but not identical, changes in behavior are seen in humans who have bilateral damage to the amygdala; became emotionally “flat”.
What is the conditioned fear response?
Further evidence of the amygdala’s role in emotion were studies of conditioned fear response.
A type of learning; take a neutral stimulus (e.g. an auditory tone) and pair that with an aversive stimulus (mild shock).
The aversive stimulus normally elicits a fearful response. In rodents this is a freezing behavior. As the animal learns that the tone predicts the arrival of the shock they start freezing in response to the tone by itself (also increases in blood pressure to the tone).
Lesions of the auditory projections to the amygdala or the amygdala itself interrupt this associative learning process.
The amygdala is receiving converging sensory input from various sensory cortical areas (in this case auditory and somatosensory/pain) and is having output to cortical regions that elicit purposeful movement (the freezing) and regions of the CNS that elicit visceral responses (increase in blood pressure).
Amygdala output neurons do undergo LTP
What role does the amygdala have to do with the conditioned fear response?
Lesions of the auditory projections to the amygdala or the amygdala itself interrupt this associative learning process.
The amygdala is receiving converging sensory input from various sensory cortical areas (in this case auditory and somatosensory/pain) and is having output to cortical regions that elicit purposeful movement (the freezing) and regions of the CNS that elicit visceral responses (increase in blood pressure).
Amygdala output neurons do undergo LTP
What is emotional memory? What role does the amygdala play?
A form of memory that stores information on preferences and aversions to events.
We are unconscious of its’ contribution. It can interact with other forms of memory, e.g. declarative memory (facts and events), to strengthen a particular memory.
Emotional stress and content (both tragic or happy events) can facilitate memory formation, a process that is mediated by the amygdala.
There is an interaction between amygdala and the hippocampus that produces this effect and this interaction is bidirectional.
Amygdala activity can enhance memory storage by the hippocampus
The recollection of an event that has strong emotional content can increase activity in the amygdala
Describe S.M.
suffers from a rare genetic condition called Urbach0 Wiethe disease that causes bilateral calficiation and atrophy of the anterior-medial temporal lobes.
Extensive damage to the amygdala, not to hippocampus and remporal neocortex
S.M. has no deficits in motor function, sensory function, learning and memory, or language.
However, when shown pictures of faces whose expressions convey a specific emotion (happy, angry, surprised, etc.) her ability to recognize these emotions was reduced compared to people with an intact amygdala.
When asked to draw various facial expressions that conveyed emotions, she could produce drawings of every emotion except fear.
SM is relatively unresponsive to stimuli that elicit fear in most people (both self-reported and startle response). She does seem able to experience other emotions.
Affective/mood disorders
abnormal regulation of feelings of sadness or happiness; e.g. depression or bipolar disorder.
Depression
abnormal sense of sadness (despair); can be accompanied by disorders of sleep and eating, diminished sex drive, diminished capacity for pleasure (anhedonia)
Depression exhibits significant heritability (an indication of its’ biological basis)
How does the amygdala activity change with depression?
Depressed individuals exhibit increased activation of the amygdala (based on increase in blood flow) and the severity of the depression seems to correlate with the increase in amygdala activity.
Accompanied by increases in prefrontal cortex and thalamus
How does anterior cingulate cortex activity change in patients with depression?
Anterior Cingulate Cortex
The role of the anterior cingulate during depression is complex with some regions exhibiting an increase in activity and other showing a decrease.
However, successful anti-depressant therapy has been associated with a normalization of anterior cingulate activity.
Non-responders to anti-depressant treatment continue to exhibit areas of hyper- and hypofunction.
How does the hippocampus activity change with depression?
Hippocampus
Appear to be decreases in hippocampal volume that are correlated with onset of depression.
