Notes

Overview of the Limbic System

The term limbic system means the entire neuronal circuitry that controls emotional behavior and motivational drives.

What is the Limbic System? Anatomically speaking:

• Hypothalamus
• Limbic lobe
• Hippocampus
• Amygdala

Limbic Lobe: C-Shape border of hemisphere

Hypothalamus

A major part of the limbic system is the hypothalamus with its related structures. They control:

• Emotional behaviour
• Internal conditions of the body such as temperature, osmolality of the body fluids, and drives to eat and drink, and to control body weight. These are collectively called Vegetative Functions of the Brain.

Functional Anatomy of the Limbic System: Key Position of the Hypothalamus

Hypothalamus, a Major Control Headquarters for the Limbic System

From this view of a cut of the Hypothalamus, the functions of specific regions can be appreciated.

Hypothalamus represents less than 1% of the brain mass. It is one of the most important of the control pathways of the limbic system. It controls most of the Vegetative and Endocrine Functions of the body as well as many aspects of Emotional Behaviour.

Vegetative and Endocrine Control Functions of the Hypothalamus

• Cardiovascular Regulation
• Regulation of Body Temperature
• Regulation of Body water
• Regulation of Uterine Contractility and of Milk Ejection from the Breasts
• Gastrointestinal and Feeding Regulation
  • Stimulation of the lateral hypothalamic area results to extreme hunger, voracious appetite and intense desire for food.
  • Damage this area causes lose of desire for food, causing lethal starvation.

Hypothalamic Control of Endocrine Hormone Secretion by the Anterior Pituitary Gland.

Behavioral Functions of the Hypothalamus and Associated Limbic Structures

Effects Caused by Stimulation

• Stimulation of the lateral hypothalamus causes thirst and eating, increased general level of activity, leading to overt rage and fighting (strict connection with ANS).
• Stimulation of the ventromedial nucleus causes sense of satiety, decreased eating and tranquility.
• Stimulation of a thin zone of periventricular nuclei, leads to fear and punishment reactions.
• Sexual drive can be stimulated from several areas of the hypothalamus especially the anterior and most of the posterior portions of the hypothalamus.

Effects Caused by Hypothalamic Lesions Cause effects opposite to those caused by stimulation.

"Reward" and "Punishment" Function of the Limbic System

The limbic structures are concerned with the affective nature of sensory sensations - that is, whether the sensations are pleasant or unpleasant or also called reward or punishment or satisfaction or aversion. (It is an important aspect, the hippocampus is associated with memory, indeed a strong way of remembering is by associating emotions to memories).

Reward Centers

• The major reward centers are located along the course of the media forebrain bundle (series of fibers that run along the C-Shape and connect the areas together), especially in the lateral and ventromedial nuclei of the hypothalamus.
• Weaker stimuli give a sense of reward, and stronger ones a sense of punishment.
• Less potent reward centers -- septum, amygdala, certain areas of the thalamus and basal ganglia.
• Stimulation of these areas gives a sense of reward. When offered the choice of eating some delectable food, the animal often chooses the electrical stimulation.

Punishment Centers

• Most potent areas have been found in the central gray area surrounding the Aqueduct of Sylvius in the mesencephalon.
• Less potent punishment areas are found in the amygdala and hippocampus.
• Stimulation in these areas causes the animal to show all sign of displeasure, fear, terror, pain and even sickness.

Rage - Its Association with Punishment Centers Strong stimulation of the punishment centers, especially in the periventricular zone of the hypothalamus and the lateral hypothalamus causes the animal to:

• Develop a defense posture
• Extend its claws
• Lifts its tail
• Hiss
• Spit
• Growl
• Develop piloerection, wide-open eyes and dilated pupils

(All these body manifestations are carried out by the Sympathetic Nervous System). Placidity and Tameness, exactly the opposite emotional behaviour patterns occur when the reward centers are stimulated.

Importance of Reward or Punishment in Behaviour Almost everything that we do is related to reward and punishment. If we are doing something rewarding, we continue to do it; if it is punishing we cease to do it. Reward and punishment centers constitute one of the most important of all the controllers of our bodily activities, our drives, our aversions, our motivations.

Importance of Reward or Punishment in Learning and Memory - Habituation vs Reinforcement

• If the sensory experience does not elicit a sense of either reward or punishment, repetition of the stimulus over and over leads to almost complete extinction of the cerebral cortical response, thus the animal becomes habituated to that specific sensory stimulus and thereafter ignores it.
• If the stimulus does cause either reward or punishment, the cerebral cortical response becomes progressively more and more intense during repeated stimulation and the response is said to be reinforced.

Medial Temporal Lobe - Hippocampus and Amygdala

Hereafter, two important structures of the limbic systems are highlighted: amygdala and hippocampus. The latter is involved in many functions like emotional behaviour and learning.

Specific Functions of Other Parts of the Limbic System Role of the Hippocampus in Learning. Effect of bilateral removal of the Hippocampus Inability to learn. Theoretical Function of the Hippocampus in Learning Without the Hippocampus, consolidation of long-term memories of verbal or symbolic thinking type is poor or does not take place.

Hippocampus It is part of the cortex and seems to be a particularly sensitive region of the brain. Often epileptic seizures start in the Hippocampus and often represents the first region where Alzheimer's disease symptom take place.

• Vulnerable to damage from anoxia, stress and environmental toxins.
• Often is epileptogenic focus.

HM: bilateral removal of Hippocampus

• Unable to lay down new declarative memories.
• Old memories intact.
• No change in intellect.
• No problems with procedural memory.

Amygdala It is made of several nuclei and it is part of the temporal lobe of the brain rather than the cortex. It receives neuronal signals from all portions of the limbic cortex. Because of it multiple connection, it is called the "window" through which the limbic system sees the place in the world. It seems to be one of the gateways that allows sensation and perception to be associated with particular emotional connotations.

Amygdala: Function and Connections

• Highly processed sensory input.
• Widespread outputs to cortex, hippocampus hypothalamus, brainstem.
• Responsible for learning and maintenance of link between a stimulus and its emotional value.
• Stimulation - fear and anxiety, déjà vu.
• Lesion - e.g., Kluver-Bucy syndrome (Lesions have been studied also in primates and humans, which lead to particular symptoms such as the Kluver-Bucy Syndrome, they studied the effects of mescaline on the brain, resulting in lesions of the amygdala).

Effects of Bilateral Removal of the Amygdala - Kluver-Bucy Syndrome

• Placid, flat affect.
• Fearless.
• Inappropriate social and sexual behaviour.
• Hyperoral and overly curious. Amygdala: learning and memory of emotional significance of stimuli.

Overall Function of the Amygdala It is the behavioral awareness areas that operate at a semiconscious level. The amygdala is believed to make the person's behavioral response appropriate for each occasion.

Functional roles

• Hippocampus: learning and declarative memory (memory for facts, events, faces, places, etc.).
• Amygdala: learning and memory of emotional significance stimuli.

Circuits in the Limbic Cortex

Limbic Cortex It is the least understood part of the brain. It functions as a cerebral association area for control of behavior. (Very few studies of electrical stimulation in the limbic cortex that tell us what it does, it seems to be related to motivational drive, emotions, sensations, coordinating behaviour and motivational drive. But it's still an open chapter as it is very poorly understood).

Basic Limbic Circuit: Loop!

It is a series of different systems that work together in this loop, this type of interconnection can be seen in a number of parallel circuits studied in the past as shown below.

Hippocampal Circuit

Papez: french researcher that described the first emotional circuit of the brain, which was the hippocampal circuit, it was considered the whole limbic system in the beginning until the discovery of additional parallel circuits such as the amygdala circuit.

Amygdala Circuit

Summary

• Limbic structures and hypothalamus are highly interconnected with each other and with cortex and brainstem.
• Amygdala orchestrates emotional and drive-related behaviour through connections with brainstem, hypothalamus and spinal cord.
• Hippocampus is important for laying down new declarative memories.