Thalamus and Hypothalamus PDF

THALAMUS  Thalamus is an egg-shaped mass of gray matter forming the major part of the diencephalon It is formed of various groups of nuclei Each thalamus is situated on either side of the third ventricle Each thalamus is connected with the interthalamic adhesion (is not a commissural...

THALAMUS  Thalamus is an egg-shaped mass of gray matter forming the major part of the diencephalon It is formed of various groups of nuclei Each thalamus is situated on either side of the third ventricle Each thalamus is connected with the interthalamic adhesion (is not a commissural pathway)  Inferior end is bordered by the tegmentum of the midbrain Anterior end extends up to the interventricular foramen Posterior part is called the pulvinar Medial surface forms part of the lateral wall of the third ventricle Hypothalamic sulcus determines the border between the thalamus and hypothalamus Subdivisions of the thalamus The superior part of the thalamus is covered by a thin layer of white matter known as the stratum zonale and its lateral part also has a layer of white matter known as the external medullary lamina A “Y” shaped vertical layer of white matter extends within the thalamus that is known as the internal medullary lamina – Internal medullary lamina divides the thalamus into three parts Anterior part Medial part Lateral part Anterior part Contains the anterior thalamic nuclei Mamillothalamic tract terminates here (refer to the limbic system) Has reciprocal connections with the cingulate gyrus and hypothalamus Functions of these nuclei are closely related with the limbic system and deal with emotions and recent memory Medial part Contains the large dorsomedial nucleus – Has reciprocal connections with the prefrontal cortex and hypothalamic nuclei – Has connections with all other thalamic nuclei – Responsible for the integration of sensory information Lateral part – Is subdivided into a dorsal tier and a ventral tier Dorsal tier – Comprise the pulvinar – The connections and functions are not well identified Ventral tier Ventral anterior nucleus – Influences the activity of the motor cortex Ventral lateral nucleus – Influences the activity of the motor cortex Ventral posterior nucleus (subdivided into VPL and VPM nuclei) – VPM receive the trigeminal and gustatory fibers – VPL receive the medial and spinal lemnisci Efferents of these nuclei form the thalamocortical projections Pass through the posterior limb of the internal capsule and corona radiata to reach to the primary somatic sensory areas Other thalamic nuclei Intralaminar nuclei Small collections of nuclei within the internal medullary lamina Probably influence the levels of consciousness and alertness Midline nuclei Consist of groups of neurons adjacent to the third ventricle and interthalamic adhesion Function is unknown Other thalamic nuclei (continued) Reticular nucleus Lie between the external medullary lamina and the posterior limb of internal capsule Probably deals with the mechanisms through which the cerebral cortex regulates thalamic activity Other thalamic nuclei (continued) Medial geniculate body Related with the auditory pathways Afferent fibers to this nucleus from the inferior colliculus form the brachium of inferior colliculus Efferent fibers form the auditory radiation Information from both ear is represented here, the contralateral being predominant Other thalamic nuclei (continued) Lateral geniculate body Related with the visual pathway All fibers of the optic tract terminates here, except those passing to the pretectal nuclei – Afferent fibers to this nucleus starts from the ganglionic cell layer of the retina – Contralateral parts of each visual field are represented here Efferent fibers form the optic radiation  Functions of the Thalamus Thalamus has been thought of as a "relay" that simply forwards signals from various sensory pathways to the cerebral cortex However, newer research suggests that thalamic function may be more complicated It is currently believed that the thalamus also modulates sensory signals to and from cortex The thalamus plays a major role in regulating arousal, the level of awareness, motor activity, and memory Clinical note Lesions of the thalamus (Thalamic sydrome) There are two considerations that must be taken into account when attempting to diagnose lesions of the thalamus – Thalamic nuclei are small so that lesions producing highly specific effects are uncommon (although they do occur) – The thalamus is immediately bounded by the internal capsule on its lateral side and is in close proximity to the deep motor nuclei (basal ganglia) of the cerebral hemisphere (putamen, caudate and globus pallidus) so that thalamic lesions frequently are accompanied by symptoms from damage to these other structures (most commonly from hemorrhage from the striate arteries) Clinical note Lesions of the thalamus (continued) If the damage includes VPL and VPM a contralateral hemianesthesia usually results – Typically, all somatic sensory modalities are affected: light touch, conscious proprioception, 2-point discrimination & vibration, and pain & temperature – This loss of all somatic sensory modalities is an important diagnostic sign for thalamic damage (lesions of the internal capsule or cortex that impair somatic sensory function typically affect different modalities to different extents, often leaving pain sensation unchanged) Clinical note Lesions of the thalamus (continued) Sometimes seen after a period of recovery from damage to VPL and VPM (days to months) is hyperalgesia (an exaggerated unpleasant or painful sensation resulting from mild cutaneous stimulation) or in some cases spontaneous pain with no apparent stimulation (causalgia) Such pain can be severe and intractable Hyperalgesia and spontaneous pain do not occur with lesions confined to the cerebral hemispheres (cortex, internal capsule, or deep nuclei) Clinical note Lesions of the thalamus (continued) If the damage extends into the VA/VL nuclei complex movement disorders can result The movement disorders can be reminiscent of cerebellar damage (ataxia and intention tremor) and/or basal ganglia damage (choreoathetoid movements) This reflects, in part, the fact that both the cerebellum and basal ganglia project to VA and VL Clinical note Lesions of the thalamus (continued) All such problems occur contralateral to the side of the lesion The above signs are termed as thalamic syndrome and usually results from a lesion that is vascular in origin as can be seen in occlusion of the posterior cerebral or its thalamic branches HYPOTHALAMUS  Lies below the thalamus and forms the floor and the inferior part of the lateral walls of the third ventricle Main function is to control the autonomic nervous system and the endocrine system  The part of hypothalamus at base of the 3rd ventricle, between the optic chiasm and mamillary bodies is called the tuber cinerum The median eminence is the inferior protrusion of the tuber cinerum Median eminence inferiorly continues with the infundibulum which is continuous with the neurohypophysis  HYPOTHALAMIC NUCLEI The hypothalamic nuclei are divided into medial and lateral zones Mamillary bodies are part of hypothalamus and are functionally related with limbic system Medial Zone Part of the preoptic nucleus Part of the suprachiasmatic nucleus Anterior nucleus Paraventricular nucleus Dorsomedial nucleus Ventromedial nucleus Infundibular (arcuate) nucleus Posterior nucleus Lateral Zone Part of the preoptic nucleus Part of the suprachiasmatic nucleus Supraoptic nucleus Lateral nucleus Tuberomamillary nucleus Lateral tuberal nuclei  Functions of the Hypothalamus Autonomic control Anterior and preoptic nuclei are involved in parasympathetic function Posterior and lateral nuclei are involved in sympathetic function The autonomic impulses from the hypothalamus is distributed to the brainstem and the spinal cord through the descending autonomic fibers (their course is not defined yet) Endocrine control Certain hypothalamic nuclei control the pituitary gland by secreting the releasing-inhibiting hormones – Growth hormone releasing hormone and inhibiting hormone – Prolactin releasing hormone and inhibiting hormone – Corticotropin releasing hormone – Thyrotropin releasing hormone – Luteinizing hormone releasing hormone, follicule-stimulating releasing hormone Neurosecretion Supraoptic nucleus synthesize antidiuretic hormone - ADH (vasopressin) Paraventricular nucleus synthesize oxytocin Temparature regulation Anterior hypothalamic nuclei response to heat – stimulation induces mechanism to dissipate heat (vasodilatation of the skin vessels and sweating) Posterior hypothalamic nuclei response to cold – stimulation induces mechanism to preserve heat Regulation of food and water intake Lateral hypothalamic nuclei initiate eating and increase food intake (hunger center) – Bilateral damage to this center will cause anorexia with consequent loss of body weight Lateral hypothalamic nuclei also increase the water intake (thirst center) Medial nuclei inhibit eating and reduces food intake (satiety center) – Bilateral damage to this center will cause insatiable appetite causing obesity Supraoptic nucleus synthesize vasopressin (antidiuretic hormone-ADH) – ADH causes increase in the reabsorption of water from kidneys – Lesions of the suproptic nucleus cause diabetes insipidus Patient passes large volumes of urine, and therefore drinks large quantities of water Emotion and behavior Certain aspects of emotions are expressed by the hypothalamus Control of circadian (biological) rhythms – Certain rhythms are daily fluctuations of certain bodily functions such as sleeping, wakefulness, body temperature, hunger etc. – Suprachiasmatic nucleus most probably plays an important role in the regulation of the biological rhythms Connections of the hypothalamus with the hypophysis The hypothalamus is connected to the hypophysis by two pathways – Hypothalamohypohysial tract – Hypophysial portal system Hypothalamohypohysial tract Supraoptic and paraventricular nuclei synthesize vasopressin and oxytocin respectively – These hormones are carried to the posterior lobe of hypophysis with carrier proteins called the neurophysins along the axons forming the supraopticohypophyseal tract – The hormones are absorbed into the bloodstream through the fenestrated capillaries of the posterior lobe of hypophysis Vasopressin causes increased absorption of water from the distal convoluted tubules and collecting tubules of the kidneys Oxytocin – Stimulates the contraction of the smooth muscle of the uterus during labor – Causes contraction of the smooth muscles around the ducts of the mammary glands – When baby suckles at the breast, a nervous reflex stimulates hypothalamus to produce more oxytocin Hypophysial portal system Neurosecretory cells situated mainly in the medial zone (especially the infundibular - arcuate - nucleus) produce releasing hormones and release-inhibiting hormones These hormone are carried by their axons to the median eminence and infundibulum of hypophysis Hypophysial portal system (continued) In the hypophysis, the hormones are released onto the the fenestrated capillaries of the hypophysial portal system – Superior and inferior hypophyseal arteries supply the hypophysis – Branches of the superior hypophyseal artery terminates as capillaries, which drain into the vascular sinusoids within the median eminence and infundibulum – These sinusoids are drained by the hypophysial portal veins which in turn drains into the sinusoids of the adenohypophysis – Vascular sinusoids within the median eminence and infundibulum, the hypophysial portal veins and the sinusoids of the adenohypophysis form the hypophyseal portal system Hypophysial portal system (continued) Through this system the releasing hormones and the release-inhibiting hormones are carried to the adenohypophysis where they either stimulate or inhibit the secretion of the adenohypophysis hormones  Hypothalamus nucleus’larının fonksiyonu Nucleus Fonksiyon Nucleus supraopticus Vasopressin (ADH) sekresyonu Nucleus paraventricularis Oksitosin sekresyonu Nucleus arcuatus Releasing hormon veya inhibiting faktör salgısı Nucleus preopticus (ön) Adenohypophysis'ten gonadotropinlerin salgılanmasını sağlar Nucleus anterior ve nucleus preopticus (ön) Parasempatik kontrol Nucleus posterior ve nuclei tuberales Sempatik kontrol laterales (arka-lateral) Hypothalamus’un ön kısmı Vücuttan ısının atılmasını sağlar Hypothalamus’un arka kısmı Vücut ısısını artırır Hypothalamus'un lateral kısmı Açlık merkezi ve susuzluk merkezi (gıda ve su alımını artırır) Nucleus ventromedialis Tokluk merkezi (gıda alımını azaltır) Nucleus suprachiasmaticus Biyolojik ritmlerin merkezi Nuclei mammillares Hafıza (limbik sistem ile ilgili)