2Outline: Overview Development of Diencephalon Basic Organization Dorsal Thalamus(Thalamus)HypothalamusVentral Thalamus( Subthalamus)EpithalamusVasculature of the Diencephalon
3Diencephalon The diencephalon includes - Dorsal thalamus - Hypothalamus- Ventral thalamus- EpithalamusSituated between telencephalon & brainstem.Main processing center for information
4Rt & Lt halves of the diencephalon,contain symmetrically distributed cell groups separated by the space of the 3rd ventricle
5Development of the Diencephalon The cell groups that give rise to the diencephalon form in the caudomedial portion of the prosencephalon, bordering on the space that will become the 3rd ventricle.The developing brain at this level consists initially of a roof plate and the two alar plates; it lacks a well-defined floor plate and basal plates.
7The hypothalamic sulcus A shallow groove appears in the wall of the 3rd ventricle & extends rostrally from the developing cerebral aqueduct to the ventral edge of the interventricular foramenDivides the alar plate into :Superior (dorsal) area : future dorsal thalamusInferior (ventral) portion : future hypothalamusThe dorsal thalamus- On each side of the 3rd ventricle increases rapidly in size & will partially fuse across the space of the 3rd ventricle to form :- massa intermedia, or interthalamic adhesion .(present in about 80% of the general population)
9The epithalamus- Develops from the caudal portion of the roof plate.- By 7th week, a small thickening of the roof plate forms. It gradually increases in size & evaginates to form the epiphysis, which develops into the pineal gland of the adult .- The portion of the roof plate immediately rostral to the epiphysis gives rise to the habenula, a small thickening in which the habenular nuclei will developJust anterior to the habenular region, the roof plate epithelium & adjacent pia mater give rise to the choroid plexus of the third ventricle, This choroid plexus is continuous through the interventricular foramina with that of the lateral ventricles.
11In locations around the perimeter of the 3rd ventricle, specialized patches of ependyma lie on the midline & form unpaired structures called the circumventricular organsThese structures include :Subfornical organOrganum vasculosum of the lamina terminalisSubcommissural organ,Pineal gland.
13The development of the pituitary gland during the 3rd week is linked to that of the diencephalon . A downward extension of the floor of the 3rd ventricle, the infundibulum, meets the Rathke pouch, an upward outpocketing of the stomodeum, the primitive oral cavity.
14By the end of the 2nd month, the Rathke pouch loses its connection with the developing oral cavity but maintains its attachment to the infundibulum.As development continues, the Rathke pouch gives rise to the anterior lobe (adenohypophysis) and pars intermedia of the pituitary glandInfundibulum differentiates into the posterior lobe of the pituitary gland, or neurohypophysis
15A craniopharyngioma (Rathke pouch tumor) can arise from a portion of the Rathke pouch that fails to undergo proper migration & apposition to the infundibulum.These tumors mimic lesions of the pituitary & may cause visual problems, diabetes insipidus, & ↑ ICP
16Basic OrganizationThe junction between the diencephalon & midbrain lies along a line extending from the posterior commissure to the caudal edge of the mammillary body on the medial aspect of the hemisphere .On the surface of the hemisphere, this interface is represented by a line starting at the caudal aspect of the mammillary body, extending anterolaterally over the edge of the crus cerebri & following the caudal edge of the optic tract .The boundary between the diencephalon & surrounding telencephalon is less distinct & represented :- laterally by the internal capsule- rostrally by the interventricular foramen, lamina terminalis & optic chiasm .
18The 3rd third ventricle has small evaginations or recesses associated with Optic chiasm (supraoptic recess)Infundibulum (infundibular recess)Pineal gland (pineal& suprapineal recesses)
19The dorsal thalamusLocated superior to the hypothalamic sulcusExtends from the interventricular foramen caudally to the level of the splenium of the corpus callosum.The hypothalamusLocated inferior to the hypothalamic sulcusBordered :- Rostrally by the lamina terminalis- Caudally by a line that extends from the posterior aspect of the mammillary body- Superiorly to intersect with the hypothalamic sulcus.
20The ventral thalamus (subthalamus) Located :Caudal to the hypothalamusRostral to the diencephalon-midbrain junctionLateral to the midlineEpithalamic structuresLocated posteriorly & caudally in close apposition to the posterior commissureInclude : - pineal gland- habenular nuclei- main afferent bundle of these nuclei- stria medullaris thalami .
22Dorsal Thalamus (Thalamus) The dorsal thalamus (or thalamus) is a massive collection of neuronal cell groups that participate in a widely diverse array of functions involving motor, sensory & limbic systems.Typically, thalamic output neurons project to the cerebral cortex → !! very little information reaches the cerebral cortex without first being processed by thalamic neurons→ the thalamus is functional "gateway" to the cerebral cortex
23In turn, nearly all regions of the cerebral cortex give rise to reciprocal projections that return to the thalamic region from which they originally received input.
24The thalamus is covered on its lateral aspect by a layer of myelinated axons, the external medullary lamina ( includes fibers that enter or leave the subcortical white matter)Within the external medullary lamina are clusters of neurons that form the thalamic reticular nucleus.
26An internal medullary lamina: - Consisting of myelinated fibers- Extends into the substance of the thalamus, where it forms partitions or boundaries that divide the thalamus into its principal cell groups :- anterior, medial, lateral & intraluminar nuclear groups.There are midline thalamic nuclei located just superior to the hypothalamic sulcus.Finally, attached to the caudolateral portion of the thalamus are the medial and lateral geniculate bodies (and their nuclei) .
28Anterior Thalamic Nuclei This group of cells consists of a large principal nucleus & two smaller nuclei → form the anterior nucleus of the thalamusThe anterior nucleus forms a prominent wedge on the rostral aspect of the dorsal thalamus just caudolateral to the interventricular foramen → this wedge is the anterior thalamic tubercle.Rostrally, the internal medullary lamina divides to partially encapsulate the anterior nucleus.
29Anterior Thalamic Nuclei The cells of this nucleus receive dense limbic-related projections from (1) the mammillary nuclei via the mammillothalamic tract and (2) the medial temporal lobe (hippocampus) via the fornix.The output of this nucleus is primarily directed to the cingulate gyrus through the anterior limb of the internal capsule.
30Medial Thalamic Nuclei Comprises the dorsomedial nucleusComposed of :-Large parvicellular (located caudally)-Magnocellular (located rostrally)-Small paralaminar adjacent to the internal medullary lamina2 larger portions are linked to parts of the frontal & temporal lobes & to the amygdaloid complex .Cells of the paralaminar subdivision receive input from the frontal lobe & substantia nigra(may play a role in the control of eye movement)
31Lateral Thalamic Nuclei 2 subdivisionsDorsal subdivisionLateral dorsal- Functionally part of anterior group (limbic system)Lateral posterior
32Pulvinar Pretectal area Border with lateral Superior colliculus Inputs Pretectal area Superior colliculusRoles: Visual relay center Selective attention SpeechPulvinarBorder with lateralposterior is vagueReciprocal connections: Lateral geniculate nucleus Parietal lobe Temporal lobe Occipital lobe
33Ventral subdivision - Ventral anterior - Ventral lateral - Ventral posteriorReceive direct input from long ascending tractsReprocal connections with cortexRetrograde degeneration on cortical lesions
37The lateral (LGB) and medial (MGB) geniculate nuclei are considered parts of the lateral thalamic nuclear group .MGB receives ascending auditory input via the brachium of the inferior colliculus → projects to the primary auditory cortex in the temporal lobe.LGB receives visual input from the retina via the optic tract → projects to the primary visual cortex on the medial surface of the occipital lobe .
38Located in the posterior thalamus at about the level of the pulvinar and geniculate nuclei is a cluster of cell groups collectively called the posterior nuclear complex.This complex consists of :- Suprageniculate nucleusNucleus limitansPosterior nucleusThese nuclei are positioned superior to the medial geniculate and medial to the rostral pulvinar.The posterior nuclear complex receives& sends to the cortex nociceptive cutaneous input that is transmitted over somatosensory pathways
39Intralaminar NucleiEmbedded within the internal medullary lamina are the discontinuous groups of neurons that form the intralaminar nuclei.Projections to the neostriatum & to other thalamic nuclei, along with diffuse projections to the cerebral cortex.2 of the most prominent cell groups are :Centromedian : projects to the neostriatum & to motor areas of the cerebral cortexParafascicular nuclei : projects to rostral & lateral areas of the frontal lobe.Other intralaminar nuclei receive input from ascending pain pathways and project to somatosensory and parietal cortex.
40Midline NucleiThe midline nuclei are the least understood components of the thalamus??The largest is the paratenial nucleus, which is located just ventral to the rostral portion of the stria medullaris thalami; other cells are associated with the interthalamic adhesion (massa intermedia).Inputs are poorly definedEfferent fibers reach the amygdaloid complex &the anterior cingulate cortex, suggesting a role in the limbic system.
41Thalamic Reticular Nucleus The cells are situated within the external medullary lamina & between this lamina and the internal capsule .Axons of these cells project medially into the nuclei of the dorsal thalamus or to other parts of the reticular nucleus, but not into the cerebral cortex.Afferents are received from the cortex and from nuclei of the dorsal thalamus via collaterals of thalamocortical & corticothalamic axons.Thalamic reticular neurons modulate, or gate, the responses of thalamic neurons to incoming cerebral cortical input .
43Thalamic nucleus : efferent projections (thalamocortical axons) → corterx Cortex → reciprocal projection (corticothalamic axons) → thalamic nucleusVL/motor/precentral gyrus and anterior paracentral gyrusVPL/sensory for the body/postcentral gyrus and posterior paracentral gyrusVPM/sensory for the face/postcentral gyrusMGB/auditory/transverse temporal gyrusLGB/vision/cortex on the calcarine sulcusThe anterior nucleus projects primarily to the cingulate gyrus and functions in the broad area of behavior
44Thalamic nuclei : relay nuclei or association nuclei Thalamic nuclei : specific or nonspecific
47HypothalamusThe hypothalamus is mainly involved in visceromotor, viscerosensory & endocrine activities.The hypothalamus & related limbic structures receive sensory input regarding the internal environment & in turn, regulate through four mechanisms the motor systems that modify the internal environment.
48HypothalamusIs a principal modulator of autonomic nervous system function.Is a viscerosensory transducer, containing neurons with specialized receptors capable of responding to changes in the temperature or osmolality of blood, as well as to specific hormonal levels in the general circulation.It regulates the activity of the anterior pituitary through the production of releasing factors (hormone-releasing hormones)It performs an endocrine function by producing & releasing oxytocin &vasopressin into the general circulation within the posterior pituitary.
49The hypothalamus can be divided into lateral, medial & periventricular zones :
50Lateral Hypothalamic Zone Composed of diffuse clusters of neurons intermingled with longitudinally oriented axon bundlesCells are involved in cardiovascular function & in the regulation of food & water intake.
51Medial Hypothalamic Zone In contrast to the lateral zone, it contains discrete groups of neurons whose function & connections are established.Within the chiasmatic (anterior) region are 5 nuclei:PreopticsupraopticparaventricularAnteriorSuprachiasmatic
52(Preoptic, supraoptic, periventricular) → are generally involved in regulating hormone release Anterior → cardiovascular functionSuprachiasmatic → circadian rhythmsPreoptic → body temperature & heat loss mechanisms
53In the tuberal region :DorsomedialVentromedialArcuate nucleiThe ventromedial nucleusFood intake (satiety) center.Bilateral lesions produce hyperphagia, a greatly increased food intake with resultant obesity.Cells of the arcuate nucleus deliver peptides to the portal vessels & through these channels, to the anterior pituitary.
55At caudal levels, the mammillary region - Posterior nucleus- Mammillary nucleiThe mammillary nuclei consist of a large medial & a small lateral nucleus.Although both of these nuclei receive input via the fornix, only the medial nucleus projects to the anterior thalamic nucleus through the mammillothalamic tract.
56The neurons of the posterior nucleus are involved in : ↑ BPPupillary dilationShivering or body heat conservation.The mammillary nuclei are involved inControl of various reflexes associated with feedingMechanisms relating to memory formation.
57Afferent Fiber Systems The fornix & stria terminalis are 2 major afferent fiber bundles that reach the hypothalamusThe fornix consists of axons that largely originate in the hippocampus, and the stria terminalis arises from neurons in the amygdaloid complex .Fibers composing the ventral amygdalofugal bundle exit the amygdala and course through the substantia innominata to enter the hypothalamus and thalamus .The medial forebrain bundle passes bidirectionally through the lateral hypothalamic region.Ascending axons → forebrain & brainstem.
58Efferent FibersSeveral nuclei give rise to descending fibers that contribute to the dorsal longitudinal fasciculus and the medial forebrain bundle and to diffuse projections that pass into the tegmentum.These fiber systems project directly to numerous brainstem nuclei, as well as to preganglionic sympathetic and parasympathetic neurons in the spinal cord.Other projections reach the thalamus and frontal cortex, and still others extend to the posterior pituitary or to the tuberohypophysial portal system for delivery of substances to the anterior pituitary.
60Ventral Thalamus (Subthalamus) 3 main structures Fields of ForelH field of Forel PrerubralH1 field of Forel Thalamic fasciculusH2 field of Forel Lenticular fasciculus
61Ventral Thalamus (Subthalamus) 3 main structures Zona incerta Implicated in many functions :Locomotion,Oculomotor, arousal,Attention, feeding,Sociosexual,Somatosensory
62Epithalamus Principal components : Pineal gland Habenular nuclei Stria medullaris thalamiThe pineal gland consists of richly vascularized connective tissue containing glial cells & pinealocytes but no true neurons.Pinealocytes are cells that synthesize melatonin from serotonin via enzymes that are sensitive to diurnal fluctuations in light.Production of melatonin by pinealocytes is rhythmic & calibrated to the 24-hour cycle of photic input to the retina = circadian rhythm.
63EpithalamusThe habenular nuclei consist of a large lateral nucleus and a small medial nucleus .Both nuclei contribute axons to the habenulointerpeduncular tract (fasciculus retroflexus), which terminates in the midbrain interpeduncular nucleus.The stria medullaris thalami, conveys input to both habenular nuclei.The habenular commissure, a small bundle of fibers riding on the upper edge of the posterior commissure, connects the habenular regions of the two sides.
65Vasculature of the Diencephalon The diencephalon is supplied bySmaller vessels that branch from the various arteries making up the cerebral arterial circle (circle of Willis)Larger arteries that originate from the proximal parts of the posterior cerebral artery
66The hypothalamus & subthalamus are supplied by central branches of the circle (perforating or ganglionic)
67Anterior parts of the hypothalamus are served by central branches (anteromedial group) arising from the anterior communicating artery and the A1 segment of the anterior cerebral artery and from branches of the proximal part of the posterior communicating artery.Caudal hypothalamic regions and the ventral thalamus are supplied by branches of the (posteromedial group)arising from the posterior communicating artery and the P1 segment of the posterior cerebral artery.
68Some of the branches of the posteromedial group that arise from the P1 segment near the basilar bifurcation are called the thalamoperforating arteries.Supply → rostral areas of the thalamus
69Slightly more distal branches, which usually arise from the P2 segment, are the posterior choroidal and thalamogeniculate arteries.Supply → portions of the diencephalonA narrow portion of the caudal and medial thalamus bordering on the 3rd ventricle is supplied by the medial posterior choroidal artery, whereas the thalamogeniculate branches irrigate the caudal thalamus, including the pulvinar & geniculate nucleiBranches of the medial posterior choroidal artery also serve the choroid plexus of the 3rd ventricle.
72The anterior choroidal artery originates from Cerebral portion of the internal carotid arterySends penetrating branches into the genu & posterior limb of the internal capsuleServes the optic tract, inferior portions of the lenticular nucleus, the choroid plexus of the inferior horn of the lateral ventricle, and large parts of the hippocampal formation.
73Although the thalamus receives a blood supply largely separate from that of the internal capsule , vascular lesions in the thalamus may extend into the internal capsule or vice versa.
74Ischemic or hemorrhagic strokes in the hemisphere may result in contralateral hemiparesis in combination with hemianesthesia.These losses correlate with damage to corticospinal and thalamocortical fibers in the internal capsule.
75Strokes involving the larger thalamic arteries, such as the thalamogeniculate artery, may result in total or dissociated sensory losses.These patients may subsequently experience persistent, intense pain (thalamic pain, Dejerine-Roussy syndrome).