THALAMUS& LIMBIC SYSTEM THALAMUS&

Lecture 27Functions of thalamus and limbic system At the end of the session the students should be able to: a Classify the nuclei of thalamus b Describe the connections of thalamus and relate them to the thalamic functions. c Discuss the applied part including thalamic syndrome. d Discuss briefly the structures and functions of limbic system.  Guyton and hall textbook of medical physiology 12 th edition, chapter 60 page , 986.

THE THALAMUS  It is an ovoid gray mass located in the diencephalon. The two thalami of both sides are separated by the 3 rd ventricle with interconnection by a narrow band called interthalamic adhesion.  The gray matter of the thalamus is divided internally by laminae of white matter into many separate nuclei. The thalamic nuclei are functionally divided into two groups:

Specific i.e. receive well defined afferents and project to specific areas in the cerebral cortex and they include: - Anterior nuclei. - Medial nuclei. - Lateral nuclei (dorsal lateral + ventral lateral). Non-specific i.e. receive input mainly from R.A.S, and project diffusely to all parts of the neocortex and they include: - Intralaminar nuclei. - Midline nuclei. - Reticular nuclei.

The afferent (input) and efferent (projections) of the thalamic nuclei are illustrated in the following figure:

THALAMIC FUNCTIONS I- Sensory functions. II- Motor functions. III- Integration functions. IV- Arousal function.

All ascending sensory pathways before reaching, the cerebral cortex relay in the thalamus specially those carrying fine epicritic sensations from opposite side of the body. i)Somatic sensory: Pathways relay in V.P.L and VP.M nuclei of the opposite side, where those from the body relay in V.P.L and those from the head and the face plus taste relay in V.P.M. I) Sensory functions a- It is a sensory relay station:

ii) Special sensations: -Vision: the optic fibers from both retinae reach L.G.B. and projected to the visual cortex. - Hearing: auditory fibers from both ears relay in M.G.B. before reaching the auditory cortex. - Olfaction: recently, olfactory pathway has found to pass through the dorsomedial nucleus to the orbitofrontal cortex.

 Coticofugal impulses are transmitted back from the cerebral cortical sensory areas to the thalamic relay nuclei which already project to these areas.  This cortical-feedback is inhibitory, and it decreases transmission through these nuclei particularly when the sensory input is very high and the sensory system is overloaded with sensory input b- Gating of the ascending sensory information:  The discrimination of many sensory impulses occurs in the thalamus but the sensations felt are of crude protopathic nature e.g: diffuse pain, crude touch & extremes of temperature change. c- Thalamus as a sensory center:

 The V.A. and V.L. nuclei relay motor signals from the B.G. and cerebellum to the motor and premotor areas of the frontal lobe to control their functions.  The V.P.L nucleus relays tactile and proprioceptive signals to the motor cortex. This provides sensory information about position and movements of the different parts of the body.  The non-specific nuclei adjust the general level of excitability of the motor cortex. II) Motor functions

 The anterior and medial nuclei together with hypothalamus and limbic system play a role in integrating the visceral and somatic motor responses evoked during emotional reactions with incoming sensory signals.  The reciprocal connection between the D.M. nucleus of the thalamus and prefrontal areas may play a role in the coding, storing and recalling of memory. III Association and integration function

 The non-specific thalamic nuclei receive excitatory signals from the R.A.S of the brain stem, and project it to almost all areas of the cerebral cortex, producing arousal and wakefulness IV) Arousal function

 The limbic system consist of those parts of the brain that are of great importance in: 1. Initiation of emotions. 2. Regulation of emotional behavior  The limbic system is interconnected group of cortical and subcortical nuclei within the brain. It includes both; Limbic System

1- Limbic cortex: A ring of old cortex: 1. Beginning in orbitofrontal area on the ventral surface of the frontal lobe. 2. Extending upward infront and over the corpus callosum on the medial aspect of cerebral hemispheres to cingulate gyrus. 3. Passing posteriorly to the corpus callosum and downward on the ventromedial surface of the temporal lobe, to the para- hippocampal gyrus, pyriform area and uncus. 2- Subcortical limbic structures: It includes the following structures:  Hypothalamus  Thalamus  Paraolfactory areas  Amygdala  Hippocampus  Septal nucleus  Parts of basal ganglia.

Connections of limbic system: A- With cerebral cortex: Through connection with cerebral cortex, the limbic system receives processed sensory information from the cortical association area about various stimuli in the internal and external environments, for deciding the nature of appropriate internal and external response initiated by the limbic system. B- With reticular formation and autonomic centers in the brian stem: The hypothalamus as well as the amygdala which is the major output pathway for the limbic system, have numerous connections with the R.F and the autonomic centers of the brain stem, then through,  Descending projections to motor neurons of cranial and spinal nerves  control of skeletal muscle contraction and movements.  Descending projections to autonomic neurons in brian stem and spinal cord  activation of smooth muscles and exocrine glands.  In addition to the controlling effect of the hypothalamus on the endocrine system.

C- Internal connections: 1. A closed circuit called Papez circuit connects the thalamus and hypothalamus with the limbic system as follow: The hippocampus is connected to mamillary bodies (by means of fornix), which are connected to the anterior thalamic nuclei (by means of mamillo-thalamic tract), which projects to cingulated gyrus, and this finally discharges to the hippocampus again.

2. Stria-terminalis which connects the amygdaloid with the hypothalamus. 3. Few connections exist between limbic system and the neocortex (cortical tissues other than the limbic cortex) specially from the frontal lobe to the adjacent limbic areas.  Papez circuit have prolonged after discharge  allows the various component of the limbic sysem to co-ordinate various aspects of somatic, autonomic and hormonal responses that associate the emotional reaction.  Connection of limbic system with the neocortex enables the neocortex to modify the emotional behavior.

- Limbic system is primarily concerned with emotions, motivation, and emotional behavior and plays an essential role in learning and memory. Emotions: complex feeling states having two components: a) Sensory component: Awareness of a particular inner feeling and sense of motivation or urge to take an action in response to these feelings. b) Behavior (physical component):  Internal - neural by A.N.S.  HR, sweating - hormonal by endocrine glands.  External - motor response Functions of limbic system

* Motivation: means the desire or drive for a particular behavior to achieve certain goal. * Emotional behavior: refers to the internal and external responses which participate in the physiological functions aiming in maintaining the survival of the individual and preservation of species.

Role of limbic system in motivation and emotional behavior: The limbic system regulates patterns of behavior related to the following physiological functions: 1- Control of the feeding behaviour: This is one of the functions of the amygdaloid nuclei. The amygdaloid nuclei seems to differentiate food into edible and inedible on the basis of past experience. The stimulation of these nuclei causes chewing and licking movements, while its damage leads to hyperphagia associated with ingestion of all kinds of food and objects.

2- Fear and rage reactions:  The limbic system receives information from different association areas of the cortex about various environmental stimuli. Many limbic centers especially the amygdale, evoke protective emotional reactions (.e.g. fear or rage reactions).  Manifestation of fear reaction are: autonomic responses as pupillary dilatation, sweating and escape reaction.  The emotional behavior associated the fear or rage reactions are: 1. Feeling of general excitement and arousal. 2. Autonomic and endocrine responses  support somatic motor reactions.

3- Sexual behavior and reproduction: A)Hypothalamus plays a control role on the onset of puberty as well as it contains sensory neurons that respond to gonadal hormones  powerful control upon the sexual function and activity. B)The behavior components that accompany the sexual act are regulated by the limbic cortex and the hypothalamus: *Bilateral damage of amygdale  hypersexuality. *Removal of limbic cortex  loss of interest in other sex.

4- Punishment and reward response: A)Stimulation of certain areas in limbic system and hypothalamus e.g. lateral, ventromedial nuclei of hypothalamus, amygdale and septal nuclei  pleasure and satisfaction (reward effect). The transmitter that is responsible for this sense is dopamine. The cocaine seems to produce its euphoric effect by  dopaminergic activity in the reward system. B)Stimulation of certain areas in limbic system and hypothalamus e.g. periaquiductal area and periventricular area in hypothalamus  displeasure, rage, fear, escape (punishment effect), leading to avoidance of these punishing stimuli. So, there are 2 mechanisms in the hypothalamus and limbic systems. *One promoting pleasure, comfort and placidity called reward or approach system. *Other promoting anxiety, fear and rage called punishment or avoidance system. So, emotional state is determined by the balance between the two systems.

5- Role (importance) of reward and punishment in memory & learning Every thing we do is related to reward and punishment i.e. 1.If we do something rewarding, we continue doing it and if we do something punishing, we stop doing it. So, reward and punishment are very important in motivation for learning. 2. If the stimulus causes neither reward nor punishment experience can't be remembered easily i.e. no transformation into long-term memory and become hardly to be remembered. - If the stimulus causes either reward or punishment  leads to transformation of short-term memory into long-term memory and become hardly to be forgotten. That is to say the limbic system differentiates the sensory signals into significant ones which are stored in memory and insignificant ones which are ignored.

6- Olfaction: The limbic system is concerned with perception and discrimination of different odours. It stores the olfactory memories and controls the emotional responses to olfactory stimuli.

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