Limbic System

Question 1

The limbic system referred to the curved rim of cortical tissue that borders the cerebral hilum (limbic means bordering or edge). Functionally, this definition also includes the subcortical nuclei associated with these cortices, and have special functions, and the axonal fiber tracts that link various nuclei and cortices. Currently, what does the limbic system refer to?

Answer 1

The limbic system is a collection of cortical and subcortical brain components that are structurally interconnected, and functionally integrated, with each other, and with other parts of the nervous system; that are concerned with self perpuation and self propagation. These are a variety of diverse functions from behavior and emotions to memory.
OR
The entire neuronal circuitry that controls emotional behaviour and motivational drives.

Question 2

State the key characteristics of the limbic system.

Answer 2

1. Presence of allocortices (archicortex and paleocortex) on some structures. [Allocortices do not have the 6 histological layers unlike the neocortex. Archicortex has 3-4 layers, paleocortex has 3 layers. The mesocortex (cortex of the cingulate gyrus) is a transitional cortex.]
2. Neurons display prolonged afterdischarge within some circuits (this could explain why there’s long term memory or why some emotional experiences are felt for so long).
3. Although present, there is paucity of neocortical input.

Question 3

State the functions of the limbic system.

Answer 3

1. It contributes to the processes of learning and memory: important in processing new information, causing reinforcement or habituation (ignoring the info).
2. Emotional control: creates emotional responses (fear, rage and placidity), as well as visceromotor (endocrine and autonomic) responses to certain sensory inputs (e.g. blood pressure rising due to fear).
3. Addiction and motivation: has reward circuitry that causes excitations and cue-induced relapses that lead to addictive behaviours.
4. Sexual behaviour: receives genitosensory signals and provides responses that are consistent with sexual excitement/it facilitates behaviours integral to sexual reproduction
5. Food intake: control appetite and eating behaviours e.g. food choice and satiety.
6. Social cognition: help develop thought processes involved in understanding and dealing with other people.
7. Sleep and dreams: interweaves unconscious emotions with conscious cognitive thoughts and perceptions, linking emotions and memory during sleep.
8. Vegetative functions: responses influence temperature, osmolality, appetite e.t.c.
9. Olfaction: processing of olfactory sensation, emotional response to smell and creating olfactory memory

Here’s something easier to remember: The 5 Fs Mnemonic
Key functions of the limbic system:
1. Feeding (satiety and hunger)
2. Forgetting (memory)
3. Fighting (emotional response)
4. Family (sexual reproduction and maternal instincts/social cognition)
5. Fornication (sexual arousal)

Question 4

The limbic system has subcortical nuclei associated with the limbic lobe, the limbic lobe itself and fiber tracts connecting the various nuclei and cortices. List the cortical and subcortical structures making up the limbic system.

Answer 4

Cortical structures:
- limbic lobe
- hippocampal formation
- septal area
- olfactory areas

Subcortical structures:
- amygdaloid nuclear complex
- reticular formation
- hypothalamus
- anterior and medial nuclei of thalamus
- habenular nucleus

Question 5

List the components of the limbic lobe (4).

Answer 5

• cingulate gyrus
• isthmus
• parahippocampal gyrus
• uncus
• [Diagram 1] [Diagram 2]

Question 6

What are the parts of the hippocampal formation (these are some structures within the mesial/medial temporal lobe)? (7)

Answer 6

• hippocampus
• dentate gyrus
• gyrus fasciolaris
• indusium griseum
• medial and lateral longitudinal striae
• entorhinal cortex
• subiculum (subicular complex, consisting of subiculum, presubiculum and parasubiculum) [gives multiple subcortical efferents, some going to fornix]
• [Diagram 1] [Diagram 2]

Question 7

The limbic cortex largely functions as a transitional zone through which signals are transmitted from the neocortex. TRUE or FALSE?

Answer 7

TRUE

Question 8

What is the role of the orbitofrontal cortex?

Answer 8

• actively involved in the cognitive process of decision making
• important in processing reward behaviours

Question 9

State the function of the subcallosal (paraolfactory) area/Zuckerkandl’s gyrus (note that this is different from the subcallosal gyrus/paraterminal gyrus).

Answer 9

Involved in development of negative mood, and regulation of some autonomic responses.

Question 10

What is worth noting about cingulate gyrus? (Afferents and efferents, role, implicated in disorders like)

Answer 10

• receives inputs from neocortex, and projects the signals to the hippocampal formation
• has a role in emotion formation and processing, learning and memory, autonomic functions and pain processing
• implicated in disorders such as depression and schizophrenia

Question 11

State the role of the parahippocampal gyrus.

Answer 11

• provides a major path by which sensations converge on the hippocampus
• involved in encoding and recognising social and environmental scenes

Question 12

State the functions of the insular cortex.

Answer 12

• processes convergent information to produce a subjective emotion that is relevant for the context
• taste, hearing, autonomic and salience

Question 13

Click on Answer to view images of the hippocampal formation. Try to identify all the features of the hippocampal formation.

Answer 13

[Diagram 1] [Diagram 2] [Diagram 3]

Question 14

The parahippocampal gyrus is the entorhinal area, Brodmann Area number _____. Which functional cortex was that?

Answer 14

28. Primary olfactory cortex

Question 15

State the connections of the hippocampus.

Answer 15

Afferent: Hippocampus receives fibres mainly from entorhinal area (area 28), olfactory cortex, amygdala, opposite hippocampus, parahippocampal gyrus.

Efferent: The fornix is the main efferent tract of the hippocampus. The fibres leaving the hippocampus pass:
*To the opposite hippocampus through the commissure of fornix/hippocampal commissure
* To the septal and anterior hypothalamic regions
* To the mamillary body, which sends impulses to cingulate gyrus through anterior nucleus of thalamus, through Papez circuit

Question 16

State the key functions of the hippocampus.

Answer 16

a) consolidate information from short-term memory to long-term memory
b) spatial navigation

NOTE: Some of the disorders associated with the hippocampus include amnesia, Alzheimer’s, epilepsy and schizophrenia.

Question 17

The dentate gyrus is a crenated strip of archicortex located between the hippocampus and subiculum. It is one of the most plastic regions, exemplified by its ability for postnatal neurogenesis. State its role.

Answer 17

It contributes to the formation of new episodic memories, and spontaneous exploration of novel environment.

Question 18

What are hippocampal rudiments?

Answer 18

These are vestigial extensions of the components of the hippocampal formation outside the temporal lobe.

Question 19

List the hippocampal rudiments

Answer 19

1. Indisium griseum.
2. Subcallosal (paraterminal) gyrus (continuous with indisium griseum).
3. Medial and lateral longitudinal stria (two fiber bands on either side of indisium griseum, on the dorsal surface of the corpus callosum).

Question 20

State the functions of the following nuclei related to the limbic system:
1. Amygdala [Diagram: Connections (1)] [Diagram: Connections (2)]
2. Mammillary nuclei
3. Anterior thalamic nucleus
4. Nucleus accumbens
5. Habenular nucleus

Answer 20

1. • On the basis of all its connections and experimental studies, amygdala plays an important role in the control of emotional behaviour, especially fear conditioning and aggression response.
   • Since it receives olfactory inputs, it is believed that the amygdaloid body plays an important role in smell-mediated sexual behaviour.
2. Recollective memory
3. Spatial learning and memory.
4. • acquisition and development of reward behaviours
   • involved in drug addiction and drug seeking habits
5. • Regulation of monoamine neurotransmitters notably dopamine and serotonin.
   • may be involved in control of sleep and temperature regulation.

Question 21

Name the major fiber tracts of the limbic system and state the origin and termination of each

Answer 21

1. Fornix: connects hypothalamus and mammillary bodies
2. Cingulum fibres: projects from the cingulate gyrus to the entorhinal cortex
3. Stria terminalis: from amygdala to septal nuclei and thalamus [Diagram: septal nucei] [Diagram: septal nuclei (2)]
4. Stria medullaris thalami: from habenular nucleus to hypothalamus
5. Mamillothalamic tract

Further notes:
~ Some of the fibres of the stria medullaris thalami connect to the pinealocytes in the pineal gland as they cross the superior lamina of the pineal stalk to reach the habenular nuclei on the opposite site.
~ The pineal body is involved in secreting melatonin, and a tumour of the pineal body can produce precocious puberty since melatonin is believed to regulate the onset of puberty.
~ The septum pellucidum is a thin, triangular double membrane separating the frontal horns of the right and left lateral ventricles of the brain. [Diagram]

Question 22

(a) Briefly describe the Papez circuit.
(b) What is the function of the Papez circuit?

Answer 22

(a) The Papez circuit begins at the hippocampus and projects to the mammillary bodies via the fornix. From the mammillary bodies, the mammillothalamic tract emerges and goes to the anterior nucleus of the thalamus. The anterior nucleus of the thalamus projects to the cingulate gyrus via the internal capsule, and the cingulate gyrus projects via the cingulum to the parahippocampal gyrus (specifically the entorhinal cortex) and finally back to the hippocampus.
[Diagram 1] [Diagram 2]
(b) The Papez circuit plays a crucial role in the control of emotional expression and is also involved in short-term memory.

Question 23

Briefly discuss Kluver-Bucy syndrome. (cause and symptoms)

Answer 23

Cause: bilateral destruction of the amygdala
Symptoms:
☑ Hyperphagia [compulsive eating]
☑ Hypersexuality
☑ Hyperorality [a compulsion to examine objects by mouth]
☑ Visual agnosia: an inability to recognize familiar objects or people
☑ Docility: exhibiting diminished fear responses or reacting with unusually low agression

Question 24

State two efferents of the mammillary bodies.

Answer 24

☯︎ mammillothalamic tract
☯︎ mamillotegmental tract