Diencephalon

The diencephalon is the caudal part of the forebrain (prosencephalon), located between the midbrain (mesencephalon) and the cerebrum (telencephalon). It consists of the thalamus, hypothalamus, epithalamus, and subthalamus. The thalamus is the relay center for sensory information. The hypothalamus regulates autonomic function and the endocrine system. The epithalamus, which comprises the pineal gland, habenula, habenular commissure, and stria medullaris, regulates the sleep-wake cycle. The subthalamus, which contains the subthalamic nucleus, is part of the indirect basal ganglia circuit and is involved in the inhibition of involuntary movements. The limbic system consists of the amygdala, hypothalamus, anterior thalamus, hippocampus, mammillary bodies, cingulate cortex, and entorhinal cortex. These structures are involved in memory formation, regulation of appetite and satiety, attention, emotional responses, and sexual arousal.

• Location: caudal portion of the forebrain, situated between the cerebrum and midbrain
• Embryogenesis: arises from prosencephalon
• Composition and function
  • Thalamus: relay center of sensory information between subcortical and cortical structures
  • Hypothalamus: main center of autonomic function and endocrine system function
  • Epithalamus: involved in the regulation of the sleep-wake cycle
  • Subthalamus: contains the subthalamic nucleus, a part of the basal ganglia

Characteristics

• Location: dorsal part of diencephalon, located above the midbrain
• Function: a group of nuclei constituting the main relay center of ascending sensory information between subcortical and cortical (except for the sense of smell)
• Composition
  • Divided into lateral, anterior, medial, and intralaminar nuclear groups and the metathalamic region
  • Lateral, anterior, and medial nuclear groups are separated by the Y-shaped internal medullary lamina
  • The lateral group can be subdivided into posterior, dorsal, and ventral subgroups

Thalamic nuclei

Thalamic stroke causes contralateral hemiparesis and hemisensory loss, miotic and unreactive pupils, and upgaze palsy with gaze deviation away from the side of the lesion (a phenomenon known as wrong-way eyes).

Olfaction is the only sensory modality that is not relayed through the thalamus.

VPL: receives sensory information from the Limbs. VPM: receives sensory information from your Mug (face).

The Lateral geniculate body processes Light (part of the visual pathway); the Medial geniculate body processes Music (part of the hearing pathway).

Blood supply ^[1]

• Mainly derived from branches of the posterior cerebral artery (P1 and P2 segments):
  • Posterior thalamosubthalamic paramedian arteries
    • Area of supply: ventral, intralaminar, and medial nuclear groups
    • Infarction symptoms: impaired learning, memory loss, visual disturbance, and confabulation
  • Thalamogeniculate arteries
    • Area of supply: ventral nuclear group of the thalamus, medial geniculate thalamic nucleus
    • Infarction symptoms: impaired movement, contralateral hemisensory loss, thalamic pain syndrome
  • Posterior choroidal arteries
    • Area of supply: intralaminar nuclei, pulvinar, medial, and lateral geniculate nuclei
    • Infarction symptoms: impaired movement, contralateral hemisensory loss, thalamic pain syndrome, visual field loss
• Branches of the posterior communicating artery:
  • Anterior thalamosubthalamic paramedian arteries
    • Area of supply: anterior, ventral, and intralaminar nuclear groups of the thalamus
    • Infarction symptoms: facial asymmetry during emotional displays, memory loss, hemiparesis, apraxia, hemispatial neglect
• For more information on the clinical significance of infarction of the thalamus, see “Thalamic stroke” in “Overview of stroke.”

Characteristics

• Location: anterior part of the diencephalon, ventral and inferior to the thalamus
• Function
  • Control center for the autonomic nervous system
  • Regulates hormonal pathways and autonomic functions
  • Production of hypothalamic hormones that (e.g., GHRH, somatostatin, dopamine) mostly affect the hormonal secretion of the anterior pituitary gland
  • For more information on the function of the hypothalamus, see “Hypothalamus and pituitary gland” in “General endocrinology”
• Composition: four nuclear groups
  • Supraoptic (anterior)
  • Tuberal (middle)
  • Mammillary (posterior)
  • Preoptic (anterior-posterior)
• Inputs
  • Area postrema in dorsal medulla oblongata
  • Vascular organ of lamina terminalis: detects changes in osmolarity with osmoreceptors

Hypothalamic nuclei

Hypothalamic functions (Thirst and water balance; Adenohypophysis produces releasing hormones that act on the anterior pituitary, Neurohypophysis produces ADH and oxytocin for the posterior pituitary; Hunger; Anger; Temperature; Sexual functions): TAN HATS

Injury to the Lateral nucleus makes you Lean (due to the absence of hunger), and injury to the VentroMedial nucleus makes you Very Massive (due to the absence of satiety).

Function of suprachiasmatic nucleus (SCN): Sun Censing Nucleus

Functions of hypothalamic nuclei (Supraoptic nucleus releases, ADH release, Paraventricular nucleus release, OXytocin): SAD POX

The MaMmillary bOdY is important for MeMOrY.

Blood supply

• Anterior hypothalamus: anterior cerebral artery and anterior communicating artery
• Tuberal hypothalamus: posterior communicating artery
• Posterior hypothalamus: posterior communicating artery, posterior cerebral artery, and basilar artery

Epithalamus

• Location: dorsal diencephalon, superior and posterior to thalamus
• Function
  • Connects limbic system to other parts of the brain
  • Controls the circadian rhythm
• Composition: habenula, habenular commissure, stria medullaris, and pineal gland
• Blood supply: posterior cerebral artery → posterior choroidal arteries

Subthalamus

• Location: lateral to the hypothalamus, rostral to the thalamus, and medial to the internal capsule
• Function: part of the indirect pathway of the basal ganglia circuitry, involved in the modulation motor function
• Composition
  • The subthalamic nucleus is the most prominent structure.
  • The subthalamic nucleus has excitatory glutamatergic projections to both segments of the globus pallidus and substantia nigra
• Clinical relevance: A lesion of the subthalamic nucleus can lead to contralateral hemiballismus.
• For more information on the subthalamus, see “Basal ganglia” in “The cerebral cortex, meninges, basal ganglia, and ventricular system.”

Characteristics

• Function: involved in emotional and behavioral responses, motivation, memory, olfaction, and autonomic nervous system function
• Components
  • Areas of the cerebral cortex: hippocampal formation (hippocampus, dentate gyrus, entorhinal cortex), cingulate gyrus
  • Nuclei: mammillary bodies, amygdala, anterior thalamic nuclei
  • Nerve fiber tracts
    • Cerebral fornix: C-shaped nerve fiber bundle that represents the major output tract of the hippocampus
    • Cingulum: nerve fiber bundle connecting cingulate gyrus and entorhinal cortex
    • Mammillothalamic tract
    • Striae terminalis

Components of the limbic system

Papez circuit

• Description
  • An anatomical circuit within the limbic system that starts and ends in the hippocampus
  • Involved in the formation and storage of memories and the control of emotions (e.g., sexual desire, pleasure, fear, and anxiety)
  • Commonly affected in Alzheimer disease and Korsakoff syndrome
• Circuit: pyramidal cells of the hippocampus → fornix → mammillary bodies (hypothalamus) → mammillothalamic tract → anterior thalamic nucleus → anterior limb of the internal capsule → cingulate gyrus → cingulum → entorhinal cortex → hippocampus
  • The hippocampus receives input from cortical associations such as the cingulum, cingulate gyrus, and parahippocampal gyrus.
  • The hippocampus receives input from and sends output to the amygdala.
  • The amygdala is involved in emotional regulation and encoding of emotionally influenced memory (positive or negative).

The limbic system is responsible for the famous 5 F's: Feeding, Fleeing, Fighting, Feeling, and … Sex.

• Major neurocognitive disorders: Alzheimer disease
• Cerebrovascular diseases
  • Overview of stroke
  • Cerebral venous thrombosis
  • Vascular dementia
  • Epidural hematoma
  • Subarachnoid hemorrhage
  • Subdural hemorrhage
  • Intracerebral hemorrhage
• Brain tumors
  • Pinealoma
  • Astrocytoma
  • Meningioma
• Genetic disorders
  • Tay-Sachs disease
  • Wilson disease
  • Phenylketonuria
  • Niemann-Pick disease
• Seizure disorders: focal seizures and syndromes
• Other neurological disorders
  • Hydrocephalus
  • Neurocutaneous syndromes
  • Horner syndrome
  • Tourette syndrome
  • Cerebral palsy
  • Dystonia
  • Spinocerebellar ataxia
  • Traumatic brain injury
  • Wallenberg syndrome
  • Wernicke encephalopathy and Korsakoff syndrome
  • Anterograde amnesia
• Hypothalamic dysfunctions
  • Neurogenic hyperthermia: a noninfectious cause of fever, due to dysfunction of the anterior hypothalamus (usually after head trauma) that impairs the body's ability to dissipate heat
  • Hypothermia
  • Insomnia
  • Diabetes insipidus
  • Kallmann syndrome
  • Hyperphagia
  • Obesity
  • Wernicke encephalopathy
  • Anorexia nervosa
  • Failure to thrive
  • Starvation