DIENCEPHALON-HYPOTHALAMUS

Diencephalon [“ between brain”]

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=thalamus3. Subthalamus4. Hypothalamus

Visible part of diencephalon is inferior surface of hypothalamus

Includes mammillary bodies and infundibulum

Entire medial surface is wall of 3rd ventricle, visible in a hemisected brain

Medial surface of diencephalon

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

Dorsal aspect of the diencephalon

Epithalamus

Includes pineal gland and habenular nuclei

Pineal gland

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. Hydrocephalus2. Deficits in eye movements and

pupillary reactions3. 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

Hypothalamus

Basic facts

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

system2. Outputs influencing pituitary gland3. Interconnections with various

visceral and somatic nuclei[ motor and sensory,of brainstem and spinal cord]

Boundaries of inferior surface

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

Inferior surface

Infundibular stalk +posterior lobe of pituitary=neurohypophysis

Medial surface

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

Divisions [anteroposterior]

Anterior Tuberal Posterior

Anterior region- above optic chiasma Tuberal – above and including tuber

cinerium Posterior – above and including

mammillary bodies

Medial surface

Parasagittal section.Regions ;a-anterior, p-posterior,t-tuberal, po-preoptic

Longitudinal zones

Periventricular- in the wall of 3rd ventricle [rostral continuation of PAG]

Lateral –lateral to fornix Medial zone [in between the two] –

populated by series of hypothalamic nuclei

The 1st 2 zones contain neurons and are avenues via which ascending and descending axons enter, leave or traverse hypothalamus

Coronal section.medial-lateral subdivisions. L-lateral, m-medial,pe-periventricular

Periventricular zone

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

Lateral zone

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 has1. Parts of supraoptic nucleus2. Lateral tuberal nuclei3. Tuberomammillary nucleus [source

of histaminergic fibers that project widely to cerebral cortexand thalamus-participate in sleep-wake cycles]

Medial zone

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

Supraoptic nucleus

Sits astride optic tract Extends to lateral hypothalamic zone

Paraventricular nucleus

Located higher up in the wall of 3rd 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

Medial tuberal zone

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

Medial mammillary region

Contains Mammillary body [complex of many

nuclei] Posterior hypothalamic nuclei

continuous with PAG [periaqueductal gray matter]of midbrain

Inputs

Outputs