LIMBIC SYSTEM

LEARNIBG OBJECTIVES

• Brief physiological anatomy of limbic system.• Describe the roles of limbic system.• Discuss especially the roles of – hippocampus– amygdale– various reward and punishment centers.

• Discuss the lesions of various parts of limbic system,with neurophysiologic basis and effects.

LIMBIC SYSTEM

• Limbic:border• Refers to a ring of gray matter on the medial

aspect of the cerebral hemispheres.

LIMBIC SYSTEM

• the entire neuronal circuitry that controls emotional behavior and motivational drives.

• Network of structures is associated with emotions, basic survival and sociosexual behavioral patterns, motivation, and learning.

Motivation

• is the ability to direct behavior toward specific goals.

• Some goal-directed behaviors are aimed at satisfying specific physical needs related to homeostasis:Homeostatic drives

• As an example, the sensation of thirst,hunger.

Components

• Amygdaloid body• Hippocampus (“seahorse”)• Cingulate gyus• Parahippocampal gyrus• Hypothalamus• Mamillary bodies• Anterior nucleus of thalamus• Medial forebrain bundle

Hypothalamus:

• Major part• Vegetative roles• Behavioral control• governs the involuntary internal responses of various body

systems in preparation for appropriate action to accompany a particular emotional state. For example, the hypothalamus controls the increase of heart rate and respiratory rate, elevation of blood pressure, and diversion of blood to skeletal muscles that occur in anticipation of attack or when angered.

• these preparatory changes in the internal state require no conscious control

• Stimulation in the lateral hypothalamus : thirst and eating, overt rage and fighting.

• Stimulation in the ventromedial nucleus and surrounding areas mainly causes effects opposite to those caused by lateral hypothalamic stimulation—that is, a sense of satiety, decreased eating, and tranquility.

• Stimulation of a thin zone of periventricular nuclei, located immediately adjacent to the third ventricle (or also stimulation of the central gray area of the mesencephalon that is continuous with, this portion of the hypothalamus), usually leads to fear and punishment reactions.

• Sexual drive can be stimulated from several areas of the hypothalamus, especially the most anterior and most posterior portions of the hypothalamus

Effects Caused by Hypothalamic Lesions.

• Bilateral lesions in the lateral hypothalamus :decrease drinking and eating almost to zero, lethal starvation,extreme passivity of the animal as well, with loss of most of its overt drives that are observable emotional expressions (for example, laughing, crying, or blushing).

• Bilateral lesions of the ventromedial areas of thehypothalamus : excessive drinking and eating ,hyperactivity and often

continuous savagery (cruelty) along with frequent bouts of extreme rage on the slightest provocation.

• Stimulation or lesions in other regions of the limbicsystem, especially in the amygdala, the septal area,and areas in the mesencephalon, often cause effectssimilar to those elicited from the hypothalamus.

Anterior nucleus of thalamus

• Afferent connections:– Mammillothalamic tract,cingulate gyrus and

hypothalamus• Efferent connections:– Cingulate gyrus,hypothalamus

• Functions:– Emotional tone i-e attitude,mechanism of recent

memory.

Mamillary bodies

• They are connected to other parts of the brain• act as a relay for impulses coming from the

amygdalae and hippocampi, via the mamillo-thalamic tract to the thalamus.

• This circuit, from amygdalae to mammillary bodies, and then on to the thalamus, is part of the larger 'Papez circuit'.

• Mammillotegmental tract: terminates in reticular formation of mid brain.

• They, along with the anterior and dorsomedial nuclei in the thalamus, are involved with the processing of memory.

• They are believed to add the element of smell to memories.

Dorsomedial nucleus of thalamus

• Prefrontal cortex,hypothalamus,other thalamic

• Integration of somatic,visceral,olfactory information and relation to subjective feelings and emotional states

Damage to the mammillary bodies

• due to thiamine deficiency is implied in pathogenesis ofWernicke-Korsakoff syndrome. Symptoms include impaired memory, also called anterograde amnesia.

• Lesions of the medial dorsal and anterior nuclei of the thalami and lesions of the mammillary bodies are commonly involved in amnesic syndromes in humans.

Hippocampus

• The hippocampus and its adjacent temporal and parietal lobe structures, all together called the hipocampal formation.

• has numerous but mainly indirect connections with many portions of the cerebral cortex ,the amygdala, the hypothalamus, the septum,and the mamillary bodies

• Hyperexcitable:Prolonged discharges with slight stimulus

• 3 layered structure• Involved in sensations– Almost any type of sensory experience causes

activation of at least some part of the hippocampus– incoming sensory signals– can initiate behavioral reactions for different

purposes

• Seizures and hallucinations:focal epileptic seizures with psychomotor effects (olfactory,visual, auditory, tactile, and other types of hallucinations that cannot be suppressed as long as the seizure persists)

• pleasure,rage, passivity, or excess sex drive.

• Roles– Learning(bilateral lesion,cant learn even names)– Memory(working memory,consolidation of memory, declarative memory

functions)– Evolutionary role (smell things to eat)– Decision making for life and death– It helps control corticosteroid production.– It also has significant contribution to understanding spatial relations within the

environment.

• Lesion – Anterograde amnesia– No long term memory establishment.– Lobes are removed for treatement of epilepsy.

Regions of the Amygdala

• Large basolateral region:Provides direct input to basal ganglia and motor system.

• Small corticomedial group of nuclei:Related to olfactory cortex especially in

lower animals.• Medial and central nuclei:

Connected to hypothalamus

Amygdala

Amygdala

• Window of limbic system:wide aff connections with visual,auditory association areas.

• Wide efferent connections• Effects of stimulation

Amygdala stimulation produces emotional behaviors through subcortical pathways

• Involuntary movements – (1) tonic movements, such as raising the head or bending

the body (2) circling movements (3) occasionally clonic, rhythmical movements and (4) different types of movements associated with olfaction and eating, such as licking, chewing, and swallowing.

• fear and punishment• Sexual effects :erection(copulatory movements,

ejaculation, ovulation, uterine activity, and premature labor).

Functions of the Amygdala• behavioral awareness areas• project into the limbic system one’s current status in relation to

both surroundings and thoughts. • make the person behavioral response appropriate for each

occasion• Relate environmental stimuli to coordinated behavioral

autonomic and endocrine responses seen in species-preservation.

• Responses include:Feeding and drinkingfighting behaviorMating and maternal careResponses to physical or emotional stresses

Kluver-Bucy Syndrome• Results from bilateral destruction of temporal lobes or amygdala.• Characteristics:

– Increase in sexual activity.– Compulsive tendency to place objects in mouth.– Changes in eating behavior.– is not afraid of anything – has extreme curiosity about everything– forgets rapidly– often has a sex drive so strong that it attempts to copulate with

immature animals, animals of the wrong sex, or even animals of a different species.

• Although similar lesions in human beings are rare, afflicted people respond in a manner not too different from that of the monkey.

Limbic cortex

• Cerebral association area for control of behavior.• this ring of limbic cortex functions as a two-way

communication and association linkage between the neocortex and the lower limbic structures.

• most poorly understood portion• essentially all behavioral patterns can be elicited by

stimulation of specific portions of the limbic cortex. • ablation of some limbic cortical areas can cause

persistent changes in an animal’s behavior

Ablation of the Anterior Temporal Cortex

• When the anterior temporal cortex is ablated bilaterally, the amygdalas are almost invariably damaged.

• Klüver-Bucy syndrome occurs.

Ablation of the Posterior Orbital Frontal Cortex

• often causes an animal to develop insomnia associated with intense motor restlessness, becoming unable to sit still and moving about continuously.

• Function: The orbital frontal lobes act as the Senior Executive of the social-emotional brain and exert tremendous inhibitory as well as expressive influences on emotion and generalized arousal through its massive interconnections with various limbic nuclie, the dorsal medial nucleus of the thalamus and the reticular formation.

• When the orbital area is injured all aspects of emotional and inhibitory behaviors may be compromised, and patients may display disinhibition, manic-excitement, and internal utilization behaviors (increased sexuality, orality).With complete destruction of the orbital area, emotional and social functioning is abolished, but with less extensive damage, rather than a loss of emotion there is a loss of emotional control.

• cortical regions of the limbic system occupy intermediate associative positions between the functions of the specific areas of the cerebral cortex and functions of the subcortical limbic structures for control of behavioral patterns.

• anterior temporal cortex: gustatory and olfactory behavioral associations.

• In the parahippocampal gyri, there is a tendency for complex auditory associations as well as complex thought associations derived from Wernicke’s area of the posterior temporal lobe.

• In the middle and posterior cingulate cortex, there is reason to believe that sensorimotor behavioral associations occur.

Ablation of the Anterior Cingulate Gyri and Subcallosal Gyri

• portions of the limbic cortex that communicate between the prefrontal cerebral cortex and the subcortical limbic structures.

• Destruction of these gyri bilaterally releasesthe rage centers of the septum and hypothalamus from prefrontal inhibitory influence.

• Animal can become vicious i-e VIOLENT and much more subject to fits of rage than normally.

Roles of prefrontal cortex

• higher levels of the cortex are also crucial for conscious awareness of emotional feelings

• higher levels of the cortex, particularly the prefrontal and limbic association areas, are important in conscious learned control of innate behavioral patterns

• prefrontal cortex formulates plans and guides behavior, suppressing amygdala-induced responses that may be inappropriate for the situation at hand

Medial forebrain bundle

• extends from the septal and orbitofrontal regions of the cerebral cortex downward through the middle of the hypothalamus to the brain stem reticular formation.

• This bundle carries fibers inboth directions, forming a trunk line communication system between limbic system and brainstem.

• Roles of reticular formation:mediate the orders of hypothalamus,arousal.

• A second route of communication is throughshort pathways among the reticular formation of the brain stem, thalamus, hypothalamus, and most other contiguous areas of the basal brain.

Higer centers

• In executing complex behavioral activities such as attack or mating, the individual must interact with the external environment.

• Higher cortical mechanisms are called into play to connect the limbic system and hypothalamus with the outer world so that appropriate overt behaviors are manifested.

• cortex provides the neural mechanisms necessary for implementing the appropriate skeletal muscle activity required to approach or avoid an adversary, participate in sexual activity, or display emotional expression.

• Higher cortical levels also can reinforce, modify, or suppress basic behavioral responses so that actions can be guided by planning, strategy, and judgment based on an understanding of the situation.

Pathologies (lesions)

• Voracious appetite:very hungery• Increased (perverse) sexual activity• Docility:

Loss of normal fear/anger response,very calm and easy to control

• Memory loss:Damage to hippocampus portion

“Reward” and “Punishment” Functionof the Limbic System

• Electrical stimulation of certain limbic areas• pleases or satisfies the animal or cause terror,

pain, fear, defense, escape reactions• greatly affect the behavior of the anima

Electrical self-stimulation and reward

Reward Centers

• along the course of the medial forebrain bundle

• lateral and ventromedial nuclei of the hypothalamus.

• Less potent reward centers:– the septum, the amygdala,certain areas of the

thalamus and basal ganglia, andextending downward into the basal tegmentum of the mesencephalon

Fear conditioning in rodents

Punishment Centers

• central gray area surrounding the aqueduct of Sylvius in the mesencephalon and extendingupward into the periventricular zones of the hypothalamus and thalamus.

• Less potent punishment areasare found in some locations in the amygdala and hippocampus.

• Stimulation in the punishment centers can frequently inhibit the reward and pleasure centers completely.

Rage—Its Association with Punishment Centers

• sudden and extreme anger • Strong stimulation of the punishment centers

of the brain, especially in the periventricular zone of the hypothalamus and in the lateral hypothalamus, causes the animal to

Rage

(1) develop a defense posture, (2) Extend its claws(3) lift its tail(4) hiss(5) spit(6) growl(7) develop piloerection, wide-open eyes, and

dilated pupils.

Rage

• the slightest provocation causes an immediate savage attack.

• expected from an animal being severely punished.

rage phenomenon is held in check mainly by

• ventromedial nuclei of the hypothalamus• portions of the hippocampi and anterior

limbic cortex, especially in the anterior cingulate gyri and subcallosal gyri, help suppress the rage phenomenon.

Placidity and Tameness

• Tranquil,calmness• reward centers are stimulated

Importance of Reward or Punishmentin Behavior

• Habituation Versus 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. ignores it.

– Reinforcement• reward and punishment centers of the limbic

system select the information that we learn.

• Effect of Tranquilizers on the Reward or Punishment Centers. – chlorpromazine

Neurotransmitter Systems and theLimbic System

• Dopamine: axons from ventral tegmental area travel through the medial forebrain bundle and connect with cingulate gyrus, hippocampus, amygdala• Serotonin: projections from the dorsal and median raphe nuclei project diffusely and synapse on limbic structures• Noradrenergic system projects diffusely and connects with limbic structures• Cholinergtic system: projects diffusely and connects with limbic system

• catecholamines are known transmitters in the regions that elicit the highest rates of self-stimulation in animals equipped with do-it-yourself devices.

• Drug of abuse: Many abused drugs act by enhancing the effectiveness of dopamine in the “pleasure” pathways– Amphetamine stimulates the release of dopamine from

dopamine-secreting neurons.– is cocaine, which blocks the reuptake of sdopamine at

synapses

• A functional deficiency of serotonin or norepinephrine or both is implicated in depression

• Serotonin and norepinephrine are synaptic messengers in the limbic regions of the brain involved in pleasure and motivation

schizophrenia

• Various addictive compounds affect activity of the dopamine transmission in the nucleus accumbens that is ventral striatum or (mesolimbic) and frontal cortical (mesocortical) systems.

• Additionally, these pathways appear to be functionally unbalanced in patients with schizophrenia. It appears that patients with schizophrenia have diminished dopamine effects through mesocortical systems to the prefrontal cortex. This could produce symptoms such as social withdrawal and diminished emotional responsiveness. Concurrently, there is a relative increase in dopamine effects via the mesolimbic system to the ventral striatal system, resulting in positive symptoms of delusions and hallucinations.

No functional signifance,just historical

Functions of limbic system

• The hypothalamus plays an important role in generating emotional behaviors…but

• The amygdala has been implicated in playing a prominent role in integrating information and coordinating emotional behaviors in response to sensory stimuli, events, and memories. These findings were demonstrated in:

– Fear conditioning studies in rodents

– Monkey studies (Kluver-Bucy)

– Human neuroimaging and lesion studies

– Studies of memory modulation by hormones in lower animals.

• Reward processing occurs in distinct brain circuits.– Stimulation of these circuits can provide powerful

reinforcement signals.– Dopaminergic neurons in the ventral tegmental area provide a

learning signal that reflects a computation comparing the reward received to the reward expected.

– Drugs of abuse act on reward circuits. • Psychiatric disorders such as depression, anxiety disorders, and

addiction, all involve limbic system neural circuitry.

JIZAKALLAH