 Mostly hidden from view  Between cerebral hemispheres  2% of CNS by weight  Widespread and important sensory connections

 Majority of sensory, motor and limbic pathways involve one or more stops in this region  4 parts – each part includes the term ‘thalamus’ [ inner chamber]

1. Epithalamus –including pineal gland and few nearby neural structures 2. Dorsal thalamus=thalamus 3. Subthalamus 4. Hypothalamus

 Visible part of diencephalon is inferior surface of hypothalamus  Includes mammillary bodies and infundibulum  Entire medial surface is wall of 3 rd ventricle, visible in a hemisected brain

 Superiorly, it borders body of lateral ventricle  Laterally- internal capsule  Caudal boundary-plane through posterior commissure and caudal edge of mammillary bodies  Rostral boundary-plane through back of anterior commissure and front of optic chiasm

 Boundaries are approximate  Neural tissue is continuous across boundaries  Certain thalamic nuclei protrude through posterior boundary to a position alongside midbrain

 Includes pineal gland and habenular nuclei

 Midline, unpaired  Resembles a pine cone  Rostral to superior colliculi  Once considered to be the seat of the soul

 Pineal tumours compress midbrain leading to 1. Hydrocephalus 2. Deficits in eye movements and pupillary reactions 3. Altered sexual development

 Receives light – regulated input by a circuitous pathway  Retina → hypothalamus → intermediolateral cell column → postganglionic fibres of superior cervical ganglion → pineal gland  No known neural output

 Secretes a hormone- melatonin [derived from serotonin]  Secretion increases during darkness  Related in humans to sleep-wake cycles  Gland undergoes calcification after the age of 17

 Calcified gland is a useful radiologic landmark  Slight shifts in pineal position can be indicative of expanding masses of different types

 Small part of diencephalon [ 4g in weight]  Important as a nodal point in pathways concerned with autonomic, endocrine, emotional and somatic functions designed to promote homeostasis

 Widespread sets of connections 1. Various components of limbic system 2. Outputs influencing pituitary gland 3. Interconnections with various visceral and somatic nuclei[ motor and sensory,of brainstem and spinal cord]

 Optic tracts, optic chiasma, mammillary bodies  This area exclusive of mammillary bodies is called tuber cinerium [‘gray swelling’]  Medial eminence protrudes from surface of tuber cinerium, and is continuous with infundibular stalk, which in turn is continuous with posterior lobe of pituitary

 Infundibular stalk +posterior lobe of pituitary=neurohypophysis

 Anterior extent-lamina terminalis  Superiorly- hypothalamic sulcus  Posteriorly- caudal edge of diencephalon

 Neural tissue anterior to a plane passing through anterior edge of optic chiasma and posterior edge of anterior commissure is functionally continuous with hypothalamus=preoptic area  Considered a part of anterior hypothalamus

 Anterior  Tuberal  Posterior

 Anterior region- above optic chiasma  Tuberal – above and including tuber cinerium  Posterior – above and including mammillary bodies

 Periventricular- in the wall of 3 rd ventricle [rostral continuation of PAG]  Lateral –lateral to fornix  Medial zone [in between the two] –populated by series of hypothalamic nuclei  The 1 st 2 zones contain neurons and are avenues via which ascending and descending axons enter, leave or traverse hypothalamus

 Traversed by dorsal longitudinal fasciculus[bundle of hypothalamic afferents and efferents]  Contains suprachiasmatic and arcuate nuclei  Suprachiasmatic – tiny – less than 1 mm square and fewer than 10,000 neurons  ‘master clock’ for our circadian rhythms

 Receives direct retinal projections which entrain it to the actual day length  Its neurons also contain melatonin receptors  Night-time rise in pineal melatonin secretion probably helps ‘set’ the circadian rhythm

 Arcuate nucleus- critically involved in feeding behavior

 Mainly scattered cells interspersed among longitudinally running fibers of Medial forebrain bundle  Anteriorly- continuous with lateral preoptic nucleus- an important sleep-promoting area  Caudally- continuous with midbrain reticular formation

 Also has 1. Parts of supraoptic nucleus 2. Lateral tuberal nuclei 3. Tuberomammillary nucleus [source of histaminergic fibers that project widely to cerebral cortexand thalamus-participate in sleep-wake cycles]

 Anteriorly has 2 nuclei containing large neurosecretory cells- paraventricular, supraoptic

 Sits astride optic tract  Extends to lateral hypothalamic zone

 Located higher up in the wall of 3 rd ventricle  Most cells of supraoptic nucleus and many cells of paraventricular nucleus secrete hormones that travel down axons of these cells and are released in neurohypophysis

 Divided into dorsomedial and ventromedial nuclei  Also has clusters of orexin-containing neurons near fornix extending into lateral and medial hypothalamus  Source of second set of wakefulness promoting neurons

Contains  Mammillary body [complex of many nuclei]  Posterior hypothalamic nuclei continuous with PAG [periaqueductal gray matter]of midbrain