Diencephalon: Thalamus, Hypothalamus & Function

Diencephalon Components

• Dorsal thalamus
• Hypothalamus
• Epithalamus
• Subthalamus

Diencephalon Location

• Deep in cerebral hemispheres
• Bilateral structure that caps the brainstem
• Lies medial to the lentiform nucleus and caudal to the head of the caudate nucleus in horizontal section
• Medial to the posterior limb of the internal capsule
• Bordered superiorly by the fornix and corpus callosum
• Lies along a line from the posterior commissure to the caudal edge of the mammillary body

Dorsal Thalamus Function

• Gateway to the cerebral cortex
• Relays sensory and motor information to the cerebral cortex
• Associates different thalamic inputs with widespread areas of the cortex
• Integrates sensory, motor, and limbic information
• Extends anterior-posteriorly from the interventricular foramen to the posterior commissure
• Extends transversely from the internal capsule to the 3rd ventricle
• Consists of a large mass of gray matter
• Has extensive reciprocal input to the cortex
• The superior aspect is grooved by the fornix

Thalamic Nuclei

• Divided into medial, lateral, and central regions by the internal medullary lamina
• The internal medullary lamina splits to enclose the anterior nucleus and central intralaminar nuclei
• Names of nuclei reflect their anatomical locations within the thalamus

Specific Relay Nuclei of the Thalamus

• Ventral posterior nuclei receive ascending somatic sensory fibers
• Ventral Posterolateral (VPL) nucleus receives somatosensory information from the contralateral body
• Ventral Posteromedial (VPM) nucleus receives trigeminalthalamic fibers from the spinal trigeminal tract, Chief sensory nucleus of V, and gustatory fibers from the nucleus solitarius
• Both VPL and VPM project to the primary sensory area in the postcentral gyrus and paracentral lobule
• Ventral anterior nuclei receive input from the corpus striatum
• Ventral lateral nuclei receive input from the cerebellum via the dentato-rubro-thalamic tract
• Both the VA and VL project to the motor cortex (precentral gyrus and premotor area) and contribute to initiation, organization, and control of movement
• Lateral geniculate nucleus receives optic tract from retina and projects via optic radiations to the primary visual area
• Medial geniculate nucleus receives auditory fibers from the inferior colliculus projects to primary auditory cortex
• Anterior nuclei are involved in emotional drives, instinctive behavior, and memorization, and is part of the limbic system which is involved in processing basic emotions related to survival

Associational Nuclei of the Thalamus

• Receive input from numerous areas and project to many cortical areas
• Dorsomedial nucleus is associated with mood and emotional balance
• Feelings of euphoria or depression may result from stimulation of the dorsomedial nucleus
• Damage to the dorsomedial nucleus results in personality change, affects intelligence, emotional drive, memory, and psychological reactions to pain
• Lateral dorsal nucleus projects to the cingulate gyrus and acts in concert with the anterior nucleus to regulate emotions, memory, and behavioral functions
• Pulvinar receives input from the lateral geniculate nucleus and superior colliculus, projects to visual association cortex, temporal, parietal, and frontal lobes for visual function and eye movements, and is important for visual discrimination and ability to interpret written symbols
• Lateral posterior nucleus projects to the parietal, occipital, and temporal cortex, and is important for visual discrimination and ability to interpret written symbols

Non-Specific Nuclei and Thalamic Reticular Nucleus

• Evoke widespread activity in both hemispheres
• Project to widespread cortical areas in both hemispheres
• Concerned with correlation and interpretation of information
• Intralaminar nuclei are associated with the ARAS
• The internal medullary lamina encloses several intralaminar nuclei, the largest of which is the centromedian
• Intralaminar nuclei receive afferents from ARAS and ascending pain pathways, and project to other thalamic nuclei and corpus striatum
• Intralaminar nuclei evoke arousal response, consciousness, alertness, and awareness of pain
• Midline nuclei are small in humans, located on the wall of the 3rd ventricle, and several are involved in arousal and awareness
• Thalamic reticular nucleus is a thin sheet between the external medullary lamina and the internal capsule that gates responses of thalamic neurons to cortical input and modulates thalamic input

Hypothalamus Anatomy

• Regulates the autonomic nervous system and affective/emotional behavior
• Forms the lower/lateral walls and floor of the 3rd ventricle
• Located posterior to the lamina terminalis and extends posterior to the mammillary bodies
• Influenced by osmotic pressure, temperature and hormone concentrations
• Receives sensory input regarding the internal environment and regulates visceral motor systems
• Receives afferents from the prefrontal cortex (emotions, mood, personality) and limbic system (fear, anger, pleasure)

Hypothalamus Functions

• Synthesizes hormones
• Regulates body temperature
• Regulates the biological clock (circadian rhythms)
• Controls electrolyte balance
• Controls emotional behavior (anger, fear, euphoria)
• Controls motivational arousal (hunger, thirst, aggression, sexual arousal)
• Regulates anterior pituitary by production of releasing factors
• Has an endocrine function by releasing Oxytocin and Vasopressin to general circulation via the posterior pituitary
• Median eminence attaches the infundibulum to the pituitary gland

Sympathetic vs Parasympathetic

• The hypothalamus is the highest center for coordination of visceral activity of the Autonomic Nervous System
• Parasympathetic nervous system: Anterior and Medial parts
• Sympathetic nervous system: Posterior and Lateral parts
• Parasympathetic stimulation causes reduced heart rate, peripheral vasodilation, reduced blood pressure, pupillary constriction, increased peristalsis, increased secretion of digestive enzymes and increased salivation
• Sympathetic stimulation causes increased heart rate, peripheral vasoconstriction, increased blood pressure, pupillary dilation, decreased peristalsis, decreased secretion of digestive enzymes and piloerection

Specific Hypothalamus Nuclei

• Supraoptic and Paraventricular Nuclei produce oxytocin and vasopressin
• Suprachiasmatic nucleus receives light input and regulates circadian rhythms
• Ventromedial nucleus is the satiety center
• Lateral hypothalamus is the feeding center
• Anterior hypothalamus regulates the sleep cycle
• Posterior hypothalamus and mamillary bodies promote wake (arousal) cycle and inhibit REM sleep
• Medial Preoptic nucleus releases GnRH, which causes release of LH and FSH from the anterior lobe of the pituitary gland
• Anterior hypothalamus is the heat dissipation center
• Posterior hypothalamus is the heat conservation center

Autonomic Reflexes

• The hypothalamus regulates the autonomic nervous system and functions to maintain blood pressure, body temperature and water balance
• Temperature-sensing neurons in the preoptic nucleus activate areas in the anterior hypothalamus to promote heat dissipation, and the posterior hypothalamus to promote heat conservation and heat production
• The baroreceptor reflex detects changes in blood pressure through CN IX and X

Hormone Producing Neurons in the Hypothalamus

• Magnocellular neurons are located in the paraventricular (PVN) and supraoptic (SON) nuclei
• The neurons have varicosed axons that project to the posterior pituitary gland
• The varicosities contain oxytocin or vasopressin (ADH). Oxytocin is necessary for uterine contractions, milk ejection, maternal and mating behaviors, penile erection, and orgasm. Vasopressin is necessary for water reabsorption in the kidneys.
• Parvocellular neurons project to the median eminence and release hypothalamic hypophysiotropic hormones (HHH) into the capillary bed

Vasculature Relationship

• Pituitary tumors can expand into areas near the optic chiasm, carotid arteries, cranial nerves (III, IV, and V) resulting in visual deficits, eye movement problems, and sensory loss as well as hormonal imbalances
• The triad of symptoms with a pituitary tumor includes diabetes insipidus, visual deficits, and panhypopituitarism
• The blood supply to the diencephalon comes from smaller vessels that branch from the circle of Willis and larger arteries that originate from the posterior cerebral artery
• Dorsal thalamus is supplied by branches of the posterior cerebral artery, primarily the medial and lateral posterior choroidal arteries
• Hypothalamus is supplied by branches of the circle of Willis and posterior cerebral artery
• Thalamic Syndrome is a vascular lesion in the thalamus, usually in the VPL or VPM and can involve the posterior limb of the internal capsule

Subthalamus

• Includes the subthalamic nucleus, prerubral field, and zona incerta
• Zone between the midbrain and diencephalon
• Located ventral and caudal to the dorsal thalamus
• Functionally related to the motor system
• Lesion causes contralateral hemiballism
• Subthalamic Nucleus is shaped like a biconvex lens with an excitatory effect on the globus pallidus
• Prerubral area is located rostral to the red nucleus
• Zona Incerta is located posterior to the subthalamic nucleus and receives/sends motor information cortically

Epithalamus

• Located posteriorly in the diencephalon roof
• Includes the habenular nuclei, pineal gland, and 3 fiber tracts: habenular commissure, stria medullaris thalami, and posterior commissure

Epithalamus Components

• Habenular nuclei are associated with limbic system and basal nuclei
• They play a role in regulating motivation, affective states, and decision-making
• Habenular commissure allows for crossing of fiber tracts from one habenular nucleus to another
• Stria Medullaris Thalami is a fiber tract that originates in the septal area and projects to the habenular nuclei

Pineal Gland

• Overhangs the superior colliculus, below the splenium of the corpus callosum
• Consists of glial cells and pinealocytes
• Produces melatonin from serotonin, exerting anti-gonadotrophic effects on the hypothalamus, and anterior and posterior pituitary
• Pinealomas are tumors with large numbers of pinealocytes, which decreases gonadal function and results in delayed puberty
• Tumors may compress the pretectal region and superior colliculi, producing fixed, dilated pupils and paralysis of upward gaze
• Calcareous deposits called “brain sand” are present in the pineal gland after puberty, marking it as a midline for radiologists