1

Dr. Mohanad R. Alwan

The Diencephalon

Forms the central core of the forebrain and is surrounded by the cerebral hemispheres

The Diencephalon• The diencephalon

consists of three structures– Thalamus– Hypothalamus– Epithalamus

• These structures effectively enclose the third ventricle

Hypothalamus

Thalamus

Epithalamus

Thalamus

The egg shaped thalamus makes up 80% of the diencephalon and forms the superolateral walls of the third ventricle

Thalamus• The thalamus is

composed of bilateral masses of gray matter held together by a mid- line commissure called the intermediate mass

Thalamus

ThirdVentricle

Intermediatemass

ThalamusThe thalamus has

many different nuclei, most named for their location

Each of these nuclei has a functional specialization

Each projects fibers receives fibers from a specific region of the cerebral cortex

The Thalamus

Sensory inputs are not the only type of information relayed( communicate) through the thalamus

Every part of the brain that communicates with the cerebral cortex must relay signals through the nucleus of the thalamus

The thalamus can therefore be thought of as the gateway to the cerebral cortex

Thalamus Afferent impulses

from all senses and all parts of the body converge( meet) on the thalamus and synapse with at least one of its nuclei

Within the thalamus, a sorting-out and information “editing” process occurs

ThalamusImpulses having to

do with similar functions are grouped together and then

relayed via the internal capsule to the appropriate area of the sensory cortex as well as specific cortical association areas

Thalamuso In addition to

sensory inputs, virtually all inputs ascending to the cerebral cortex funnel through thalamic nuclei

VA

VPL VPM

LGNMGN

Pulvinar

Ant

VL

MD

Int medullary laminaInt medullary lamina

Ext medullary laminaExt medullary lamina

THALAMIC NUCLEITHALAMIC NUCLEI

= ANT-MED (limbic)= ANT-MED (limbic)= ANT-MED (limbic)= ANT-MED (limbic)

= ANT-LAT (EPS)ANT-LAT (EPS)= ANT-LAT (EPS)ANT-LAT (EPS)

= POST (sensory)= POST (sensory)= POST (sensory)= POST (sensory)

= NON-SPECIFIC (relay)= NON-SPECIFIC (relay)

Thalamus• Ventral posterior lateral nucleus

– General somatic sensory receptors (touch, pain pressure)

• Lateral geniculate body– Visual relay from retina

• Medial geniculate body– Auditory inputs

• Anterior nuclear group– Regulation of emotion and

visceral function• Ventral lateral nuclei

– Direct motor activity of cerebellum

• Ventral anterior nuclei– Direct motor activity of basal

nuclei

ThalamusPulvinar, medial

dorsal and lateral nuclei are involved in the integration of sensory information and projection to specific association cortices

= ANT-MED (limbic= ANT-MED (limbic)= ANT-MED (limbic= ANT-MED (limbic)

= ANT-LAT (EPS)= ANT-LAT (EPS)= ANT-LAT (EPS)= ANT-LAT (EPS)

= POST (sensory)= POST (sensory)= POST (sensory)= POST (sensory)

= NON-SPECIFIC (relay)= NON-SPECIFIC (relay)= NON-SPECIFIC (relay)= NON-SPECIFIC (relay)

VA/VL (GP+SN)

Ant+MD (Papez)

VPL (sensory--body)

VPM (sensory--head)

LGN (vision)

MGN (hearing)

Pulvinar (visual sensory association)

Intralaminar

CM (very diffuse to cerebral ctx, ends in layer I for cortical excitability)

Reticular (GABA- to thal)

VA/VL (GP+SN)

Ant+MD (Papez)

VPL (sensory--body)

VPM (sensory--head)

LGN (vision)

MGN (hearing)

Pulvinar (visual sensory association)

Intralaminar

CM (very diffuse to cerebral ctx, ends in layer I for cortical excitability)

Reticular (GABA- to thal)

VA

VPL VPM

LGNMGN

Pulvinar

Ret

icul

ar

Ret

icul

arAnt

VL

MDIntralaminar

Intralaminar

CMCM

THALAMIC NUCLEITHALAMIC NUCLEI

VA

VPL VPM

LGNMGN

Pulvinar

Ant

VL

MD

Mamillary bodies

Cingulate gyrus

Prefrontal ctx

VA

VPL VPM

LGNMGN

Pulvinar

Ant

VL

MD

Area 6

Area 4

GP, SN (EPS)

Area 4 + EPS

GP, SN, cerebellum

(EPS + dentatothalamic

tract)

VA

VPL VPM

LGNMGN

Pulvinar

Ret

icul

ar

Ret

icul

ar

Ant

VL

MDIntralaminar

IntralaminarCMCM

Areas 3,1,2

Sensory--body

Sensory--face

Vision

Hearing

Area 17

Areas 41,42

Integration of somesthetic, visual, & auditory

Areas 18,19

Functions

1. It is a great sensory relay station and integrating centre for most inputs before relaying to the cerebral cortex

2. It is the subcortical perception for some sensations3. It is an integrating centre for sleep4. It is concerned with recent memory and emotion 5. It is concerned with language function 6. It plays an imp. role in genesis of synchronization of

EEG7.Being incorporated in the motor loops it serves very imp.

role in motor activity 8. It plays important roles by acting as a link between basal

ganglia and cerebellum with the cortex

Thalamic syndrome

• Typically results due to damage of the posterior thalamic nuclei due to blockage of thalamogeniculate artery

1. Sensations from the opp. side of the body are diminished

2. Sensory ataxia 3. Thalamic phantom limb4. Overreaction to pain 5. visual field defect 6. motor disturbances : abnormal movements,

tremor, hemiparesis etc

The Hypothalamus

The hypothalamus is located below the thalamus, capping the brain stem

Hypothalamus

Hypothalamus

Merging into the midbrain inferiorly, it extends from the optic chiasma to the posterior margin of the mammillary bodies

Mammillarybodies

Opticchiasma

Mammillary Bodies

The mammillary bodies are paired nuclei that bulge anteriorly from the hypothalamus which receive and relay olfactory impulses

Hypothalamus

• Between the optic chiasma and the mammillary bodies is the infundibulum as a stalk of the hypothalamic tissue connects the pituitary gland to the base of hypothalamus

Hypothalamus

• The hypothalamus contains about a dozen functionally important nuclei

• Despite its small size, the hypothalamus is the main vital and visceral control center of the body and is vitally important to overall body homeostasis

Connections- Afferent I. Limbic system - Medial forebrain bundle - Stria terminalis - Fornix - pre and post commissural fibers - Medial corticohypothalmic tract II Midbrain tegmentum ( It forms the floor of the midbrain which

surrounds the cerebral aqueduct) - adrenergic fibers - serotonergic fibers - nor adrenergic fibers III Retina, thalamus , basal ganglia - Retinohypothalamic tract - Thalamohypothalamic tract - Pallidohypothalamic tract

Connections- Efferent

Ventral pathwaysStria terminals amygdalae (are

considered part of the limbic system)

Medial forebrain bundle Mammillo-thalamic tract Mammillotegmental tract Hypothalamo-hypophyseal tract (pertaining to

the hypothalamus and pituitary complex).

Neurons having hypophysiotropic hormones

Suprachiasmatic Suprachiasmatic nuc nuc (direct retinal (direct retinal

input, circadian input, circadian rhythmsrhythms))

Suprachiasmatic Suprachiasmatic nuc nuc (direct retinal (direct retinal

input, circadian input, circadian rhythmsrhythms))

Preoptic area (Medial Preoptic Nuc) (sexually dimorphic,

regulates Gonadotropic hormones)

Preoptic area (Medial Preoptic Nuc) (sexually dimorphic,

regulates Gonadotropic hormones)

Ant Hypothal Nuc (dissipates heat,

parasympathetic)

Ant Hypothal Nuc (dissipates heat,

parasympathetic)

Lat Nuc (appetite center,

stim. induces eating, destruction causes

starvation)

Lat Nuc (appetite center,

stim. induces eating, destruction causes

starvation)

Ventromedial Nuc (satiety center, stops eating)

Ventromedial Nuc (satiety center, stops eating)

Dorsomedial Nuc (stim causes

obesity & savage behavior)

Dorsomedial Nuc (stim causes

obesity & savage behavior)

PVN & SON (produce oxytocin &

ADH/vasopression H,) contain

SUPRAOPTICO-HYPOPHYSIAL

TRACT –and-

Magnocellular Neuron

Post Hypothal Nuc (conserves heat,

sympathetic)

Post Hypothal Nuc (conserves heat,

sympathetic)

Parvocellular PVN

supraoptic nucleus SON

Autonomic Control CenterThe Hypothalamus regulates involuntary

nervous activity by controlling the activity of autonomic centers in the brain stem and spinal cord

In this role the hypothalamus influences– Blood pressure– Rate and force of heart contraction– Motility of the digestive system– Respiratory rate and depth– Secretion of sweat and salivary glands

Center for Emotional Response

The hypothalamus has numerous connections with cortical association areas, lower brain stem centers, and it lies at the center of the limbic system which is the emotional part of the brain

Nuclei involved in the perception of fear, pleasure, and rage( angry) , as well as those involved in the biological rhythms and drives of sex are found in the hypothalamus

Center for Emotional Response

The hypothalamus acts through the autonomic nervous system to initiate most physical expressions of emotion– Physical manifestations of fear

• Pounding heart• Elevated blood pressure• Pallor• Sweating• Dry mouth

Body Temperature Regulation

The body’s thermostat is in the hypothalamusThe hypothalamus receives input from the

thermoreceptors located in other parts of the brain as well as in the body periphery

Homeostatic adjustments are then made to either cool or heat the body (sweating or shivering)

Body Temperature Regulation

Hypothalamic receptors in the preoptic region monitor the temperature of the blood flowing through the hypothalamus

Body Temperature Regulation

According to signals received by the preoptic nuclei the hypothalamus initiates mechanisms to maintain relatively constant body temperature Cooling / sweatingHeat generation / shiveringHypothalamic centers also induce fever

Regulation of Hunger & Thirst

In response to changing levels of glucose, amino acids, hormones, and salts in the blood, the hypothalamus regulates feelings of hunger and satiety (ventro- medial nuclei)

HUNGER

LACK OF

FOOD

REDUCEDAVAILABILITYOF GLUCOSE

CONTRACTIONS OF EMPTYSTOMACH

LOWTRIGLYCERIDELEVELSIN FAT CELLS

GLUCOSE RECEPTORSIN HYPOTHALAMUS(Satiety center)

MECHANO-RECEPTORSIN STOMACH

PANCREAS

HUNGER

Regulation of Water Balance

• When body fluids become too concentrated, hypothalamic neurons called osmoreceptors are activated

• These receptors excite hypothalamic nuclei that trigger the release of antidiuretic hormone (ADH) from the posterior pituitary

• ADH causes the kidneys to retain water

• The same conditions also stimulate hypothalamic neurons in the thirst center, causing to drink fluids

THIRST

WATERDEFICIENCY

OSMORECEPTORSIN SUPRAOPTICAND SUPRA-VENTRICULARNUCLEI OFHYPOTHALAMUS

THIRST

ADHSERETIONBY PITUITARY

WATERRETENTION BY KIDNEY

Regulation of Sleep-Wake Cycles

Acting with other brain regions, the hypothalamus helps regulate the complex phenomenon of sleep

It is responsible for the timing of the sleep wake cycle

Regulation of Sleep-Wake Cycles

• Hypothalamus through the operation of its suprachiasmatic nucleus (our biological clock) sets the timing of the sleep-wake cycle in response to day-light darkness cues from visual pathways

Control of Endocrine Functioning

The Hypothalamus acts as the helmsman (coordinator) of the endocrine system By producing releasing

hormones, it controls the secretion of hormones by the anterior pituitary gland

The supraoptic and paraventricular nuclei produce hormones (ADH and oxytocin or hormone of love)

Formation of Memory

The nucleus of the mammillary body receives many inputs from the major memory-processing structures of the cerebrum, the hippocampal formation and therefore may relate to memory formation

Control of Circadian Rhythm

• Circadian Rhythm: is a roughly 24-hour cycle in the biochemical, physiological, or behavioural processes of living entities, including plants, animals, fungi and bacteria

• Adrenocorticotropic (ACTH), GH, Melotonin, sleep waking cycle, body temperature rhythm, gonadotropin secretion and menstrual cycle

• Diurnal variation: fluctuations that occur during each day

Endogenous circadian rhythms

• rhythms that last about a day• humans’ last around 24.2 h

Examples:-activity-temperature-waking and sleeping-secretion of hormones-eating and drinking

Circadian Rhythms

Epithalamus

• The epithalamus is the posterior portion of the diencephalon

• It forms the roof of the third ventricle

Epithalamus

The Epithalamus

• The epithalmus consists of one tiny group of nuclei and a small, unpaired knob called the pineal body

• This gland, which derives from ependymal glial cells, is a hormone secreting organ

Epithalamus

• The pineal gland extends from the posterior border of the epithalamus

• The pineal gland secretes the hormone melatonin which signals the sleep- wake cycle

PinalBody