Limbic System Anatomy

Question 1

What are the major limbic cortical areas of the limbic system?

Answer 1

Parahippocampus

Cingulate gyrus

Prefrontal cortex

Temporal pole

• other structures include*
• hippocampus
• amygdala
• septal area

Limbic system structures are the structures that directly or indirectly communicate with the hypothalamus and/or the PAG

Question 2

What are the 4 main functions of the limbic system and the structures associated?

Answer 2

“HOME”

Homeostatic functions
- key structure = hypothalamus

Olfaction
- key structure = olfactory cortex

Memory
- key structure = hippocampal formation

Emotions and drives
- key structure = amygdala

Question 3

Hippocampal formation

Answer 3

Contains 3 structures lumped together

Hippocampus

• contains CA1-4 fields of pyramidal cells
• CA1 aka = “sommers sector”

Dentate gyrus
- connects with CA3 pyramidal cells in hippocampus via mossy fibers and granular cells

Subiculum

• transitional region between hippocampus and entorhinal cortex
• regulates the HPA axis and endocrine functions assocaited with the hippocampal formation

Question 4

What portion for the hippocampus is the most susceptible to hypoxia?

Answer 4

CA1 = sommers sector

- this is why when people, have epilepsy or seizures or clots/infarcts, memory issues tend to occur.

Question 5

Afferent pathways in the hippocampal formation

Answer 5

2 pathways

Perforant pathways

• projects from the entorhinal region -> hippocampus
• CA1 -> CA2 -> CA3 -> CA4 -> entorhinal cortex

Alvear pathway
- projects from entorhinal cortex -> CA1/3

Question 6

Efferent pathways of the hippocampal formation

Answer 6

Travels along the fornix to various structures

Precommissural fornix fibers -> septal area of the cerebrum

Post commissural fornix -> diencephalon

Commissual component -> contralateral hippocampus
* believed to be the reason why primary seizures on one hippocampus side can cause contralateral temporal damage/secondary seizures

Question 7

What are the two primary thalamic nuclei associated with the hippocampus and limbic system

Answer 7

Medial dorsal and the anterior thalamic nuclei

Anterior = propagate head movements, alertness levels and learning/episodic memory formation

Medialdorsal = emotional responses to pain, movement of the eyes and learning/episodic memory formation

Question 8

Functions of the hippocampal formation broadly

Answer 8

1) Spatial learning
- Short term memory and consolidation of memory to long term
- Receives and categorizes sensory information during learning

2) Modulates aggressive behavior
- activation of the portion closest to the AMYGDALA = Facilitates predatory attack behavior
- activation of the portion closest to the SEPTAL POLE = antagonizes this action

3) modulates endocrine functions
- specifically uses the ventral subiculum to regulate the hypothalamic-pituitary-adrenal axis via the medial corticohypothalamic tract

Question 9

What are the parts of the parahippocampal gyrus?

Answer 9

Entorhinal cortex (anterior portion)

Parahippocampal cortex (posterior portion)

Perirhinal cortex (lateral portion)

• involved in the formation of new declarative memories (anterograde amnesia) and in spatial processing*
• damage leads to loss in the ability to form new memories and in ability to recognize scenes
• however, you can still recognize objects and shapes, just cant put it all together.

Question 10

Papez circuit

Answer 10

Hippocampus -> fornix -> mammillary body -> mammillothalamic tract -> anterior thalamic nuclei -> thalamocingular tract -> cingulate gyrus -> cingulohippocampal fibers -> hippocampus

neuronal circuit used to establish connections between information stored in the unconscious and conscious behavior

Question 11

Septal area function

Answer 11

Serves as the relay between hippocampal formation and the hypothalamus. Also is a feedback center for hypothalamus functions. Specifically

• aggression
• rage
• autonomic functions
• drinking/drug use
• suppression of ACTH and adrenal activity
• lesions here leads to Cushing’s syndrome as part of the sequela*

There are a medial and lateral septal areas and includes the bed of nucleus of atria and nucleus accumbens neuronal bodies.

Receives sensory afferents that modulate activity from

• medial olfactory stria
• hippocampal formation
• amygdala
• lateral hypothalamus (feedback only)

Sends efferent connections to the hypothalamus
- also can use nuclei of the diagonal band of Broca’s to widespread connections to other parts of the limbic system if need.

Question 12

Function of the bed nucleus of the stria terminalis

Answer 12

Serves as the connection between the amygdala -> hypothalamus

Believed to regulate autonomic and endocrine processes

Question 13

Function of the nucleus accumbens

Answer 13

Receives dopaminergic projections from the brainstem and other inputs from the amygdala and hippocampal formation
- projects to the substantia Nigra and innominate as well as the ventral tegmental area

Believed to integrate motor responses and associated them with specific afferent processes (usually extreme afferent process)
- i.e “unconsciously balling up your fist when very emotional”

Question 14

Function of the substantia innominate

Answer 14

Contains the basal nucleus of Meynart
- projects to cerebral cortex and to other limbic systems as well as shares reciprocal connections with the amygdala.

Also believed to serve as the relay between amygdala -> lateral hypothalamus

is believed to play a part in Alzheimer’s disease when damaged

Question 15

Nuclei of the amygdala

Answer 15

Contains 2 nuclei groups

Corticomeidal group

• cortical nuclei
• basal nucleus (medial part)

Basolateral group

• central nuceli
• lateral nuclei
• basal nucleus (lateral part)

Question 16

Amygdala lesions cause what

Answer 16

Kluver-Bucy syndrome

• hypersexuality
• excessive eating
• decreases anxiety/fear
• hyperorality
• visual agnosia (can see but cant recognize what your seeing)
• hypermetamorphosis (desire to touch everything you see)

Question 17

Amygdala connections

Answer 17

Afferent inputs

• olfactory and taste nuceli
• temporal neocortex
• prefrontal cortex
• ventromedial hypothalamus
• substantia innominate
• nuceli of the diagonal band of Broca’s
• medial thalamus

Efferent outputs

• to hypothalamus via stria terminalis
• to PAG via stria terminalis
• to prefrontal cortex via ventral amgyfugal pathway

Question 18

Functions of the amygdala

Answer 18

Mediates conditioned fear responses to auditory and visual stimuli
- both corticomedial and basolateral nuclei contribute equally.

aggressive behavior

• stimulation of the corticomedial nuclei = increased rage
• stimulation of the basolateral nuclei = decreased rage

feeding and drinking behavior

• stimulation of the corticomedial nuclei = decreases drinking and hunger
• stimulation of the basolateral nuclei = increases drinking and hunger

Induces ovulation (corticomedial nuceli stimulation only) 
* transaction of the stria terminalis removes this completely*

Endocrine functions

• stimulation of corticomedial nuclei = inhibits ACTH and GH inhibts
• stimulation of the basolateral nuclei = increases ACTH and GH release

Question 19

Limbic connections to basal nuclei

Answer 19

Caudate nucleus and ventral striatum fibers project to:

• internal segment of globus pallidus
• substantia Nigra pars reticulata
• ventral tegmental area
• lateral preoptic area
• caudal mesencephalon

*All of the above then project to the mediodosal and ventral anterior thalamic nuclei *

Question 20

What psychiatric disorder is well connected to abnormalities in basal nuclei, anterior cingulate gyrus and serotonin function?

Answer 20

Obsessive-compulsive disorder
- abnormally increased activity in the basal nuclei especially in the head of the caudate and the anterior cingulate gyrus.

Question 21

What psychiatric disorder is well connected to abnormalities throughout the limbic system, frontal lobe and basal nuceli and dopamine production?

Answer 21

Schizophrenia

- decreases in volume of the amygdala, HF and parahippocampal gyrus

Question 22

What psychiatric disorder is well connected to abnormalities in the cerebral cortex, hippocampus and both BDNF and VEGF production?

Answer 22

Depression

• global decrease in activity of the cerebral cortex, especially the frontal lobes.
• decreased hippocampal volume, especially left hippocampus