The diencephalon is the most rostral portion of the brain stem. It is located between the cerebral hemispheres, and is divided dorsoventrally into epithalamus, thalamus, metathalamus[1], subthalamus[2] and hypothalamus[3].

 

The epithalamus is formed by: the habenula[4], the pineal[5] gland, the caudal commissure and the subcommissural organ. The habenula is found in the dorsocaudal portion of the roof of the third ventricle. It forms part of a limbic circuit. The pineal gland is an unpaired structure found in front of the caudal commissure. It forms part of the circuits that regulate the circadian rhythms by means of secreting the hormone melatonin (N-acetyl-5-metoxytrytamine) into the general circulation. It induces sleepness restrains sexual activity and has an antigonadotophic activity. The melatonin is synthesized from the serotonin. Light inhibits the synthesis of melatonin.

 

For centuries, the pineal gland has been a subject of philosophical and religious theories. It was valued as the organ of clairvoyance and meditation, allowing man to remember his previous lives. In oriental cultures, it has been considered the “mystic eye” which supported the idea of clairvoyance or the ability to see the invisible. In occidental cultures, the first person to define the pineal gland was Herophilos of Chalcedon[6]. He assumed that the gland worked like a sphincter to control the “current of thoughts”. Due its location, the gland could regulate the quantity of “substance” allowed to pass between the third and fourth ventricles. Galen of Pergamon (130-200) used a Greek term to describe the pineal gland, konarion, because of the pineapple shape it assumes in some mammals. For him, the pineal gland was simply a lymphatic gland. Descartes[7] was the last to assign a possible function to the pineal gland, saying that the gland acted as a receptor and integrator of external stimuli, with the theory that the eyes perceive the happenings of the surrounding environment and transmit these perceptions to the pineal gland via “cerebral threads”. In his book, “De homine,” he argued that human beings are different from animals because they have an intellect and soul coming from divine origins. He thought that movements were ordered by the soul residing in the brain and that it was in communication with the body by way of the pineal gland, which functioned as a regulatory valve.

 

The thalamus[8] is the biggest part of the diencephalon. It is formed by two big masses of nuclei that join at the midline forming the interthalamic adhesion. It functions as a relay station for the sensory pathways that reach the cerebral cortex and is included in neuronal circuits involving the basal nuclei.

 

With the exception of the olfactory sense, that reaches the piriform lobe without making any relay, all the sensations reach the thalamus before they can project to the sensory cerebral cortex. The thalamus also receives information from cortical cerebral areas, from the cerebellum and extrapyramidal nuclei.

 

The thalamus can be divided in three areas separated by two laminae of white mater: the internal medullary lamina and the external medullary lamina. The internal medullary lamina separates the thalamic nuclei in a rostro-dorso-medial group and a lateral-ventro-caudal group. The nuclei located inside the internal medullary lamina are the intralaminar nuclei. The external medullary lamina separates the latest group from the reticular nucleus. Externally to the reticular nucleus is the external capsule.

 

The rostro-dorso-medial group is formed by the rostral nucleus, the dorsomedial nucleus and the paraventricular nucleus. The lateral-ventro-caudal group is formed by the lateral dorsal nucleus, the lateral caudal nucleus, the ventral rostral nucleus, the ventral lateral nucleus, the ventral caudal nucleus, and the pulvinar[9]. Some authors include in this group, the lateral geniculate body or nucleus and the ventral geniculate body or nucleus. In this text we will consider these nuclei as part of the metathalamus.

 

The rostral nucleus has reciprocal connections with the cingulate gyrus and the dorsomedial group with the prefrontal cortex and hypothalamus. The paraventricular nucleus receives fibers from the hypothalamus, reticular formation, nucleus accumbens and hypocampal formation, and projects to the cingulate gyrus. The lateral dorsal nucleus has reciprocal connections with the cingulate gyrus. The lateral caudal nucleus receives fibers from the rostral colliculi and pretectal area, and projects to the temporal, parietal and occipital cortices. The ventral rostral and ventral lateral nuclei receive fibers from the cerebellum and pallidum, and project to the motor cortex. The ventral lateral nucleus also receives fibers from the substantia nigra, vestibular nuclei, fastigial nucleus, and dentate nucleus. The ventral caudal nucleus receives fibers from the medial lemniscus and trigeminal lemniscus, and projects to the sensory cortex. The pulvinar nucleus receives fibers from the rostral colliculi, and projects to the temporal parietal and occipital cortices. The reticular nucleus receives collateral branches from thalamocortical and corticothalamic fibers and project to thalamic nuclei. The ventroposterior nucleus and intralaminar nuclei receive fibers from the reticular formations, pallidum and cerebral and cerebellar cortices (pág. 136 - 139 of "Tratado de Anatomía Veterinaria" Tomo IV by .Sandoval, J. 2003).

           

The metathalamus is formed by the lateral and medial geniculate[10] bodies. They are situated in the caudolateral portion of the thalamus. The lateral geniculate body receives visual information and the medial one receives auditory information. Their axons form part of the optic and acoustic radiations respectively in order to reach the visual (occipital) and auditory (temporal) cortices.

 

The hypothalamus integrates the endocrine system and the ANS by maintaining homeostasis and by regulating ingestion, behavior and circadian[11] rhythms. It is formed by the hypothalamic nuclei, the optic chiasm, the hypophysis, the optic tracts, the tuber cinereum[12] and the mamillary bodies.

 

The optic chiasm is a decussation[13] (around 75% of fibers in the dog) of fibers of the optic nerves coming from the retinal ganglion cells of each eye.

 

The optic tracts form part of the visual pathways. They extend from the optic chiasm to the right and left lateral geniculate bodies of the metathalamus.

 

The tuber cinereum is an eminence of gray matter situated between the optic chiasm and the mammillary bodies. It gives rise to the infundibulum[14] and the neural lobe of the hypophysis. The infundibulum or pituitary stalk is divided into a root or median eminence and a distal part that extends into the neural lobe (neurohypophysis). The mammillary bodies receive stimuli from the hippocampus by way of the fornix. It is thought that this connection may play an important role in memory.

 

The hypophysis is located in the sellar region (formed by the sella turcica) in the dorsal surface of the basisphenoid bone. This sella is limited rostrally by the tuberculum sellae, laterally by the rostral and caudal clinoid processes, and, caudally, by the dorsum sellae. The inner layer of the dura mater extends at the level of the pituitary stalk, forming the diaphragm sellae. On each side to the hypophysis and the sella turcica, the inner and outer layers of the dura mater separate to locate the cavernous sinus, arteries and nerves. The internal carotid artery crosses the outer layer of the dura mater and travels in relation to the cavernous sinus to cross the inner layer of the dura mater, at the level of the diaphragma sellae, in order to reach the circulus arteriosus cerebri. In its pathway through the parasellar region, the internal carotid artery gives off a communicating ramus with the external ophthalmic artery (the latter is a branch of the rostral cerebral artery that leaves the cranial cavity through the orbital fissure). The oculomotor, trochlear, abducent and ophthalmic nerves invaginate the inner layer of the dura mater on the lateral aspect, and travel in the parasellar region to exit the cranial cavity through the orbital fissure. 

 

The hypophysis[15] is composed of the neurohypophysis and the adenohypophysis. The neurohypophysis is a ventral extension of the tuber cinereum that gives rise to the infundibulum and the neural lobe. The infundibulum or pituitary stalk is an extension of the third ventricle. It is divided in a root or median eminence, and a distal part that extends into the neural lobe or neurohypophysis. The adenohypoiphysis derives from the Rathke’s bursa. Inside the adenohypophysis, close to the neurohypophysis, a cavum hypophysis can be found. This a remnat of the Rathke’s bursa.

 

The neurohypophysis (also called the posterior hypophysis in human anatomy textbooks) serves as a support for axon terminals from nuclei responsible for secreting the following hormones: ADH (antidiuretic), oxytocin and vasopressin. The adenohypophysis, also named pituitary[16] gland by the Belgian anatomist Andreas Vesalius[17]  because he thought that it secreted nasal mucus into the nose, is the anterior lobe of the hypophysis in human anatomy. It releases the following hormones into the circulatory system (Page 22 of “Neuroscience Dictionary” by F. Mora and A.M. Sanguinetti): FSH (follicular stimulating hormone), LH (leutenizing hormone), TSH (thyroid stimulating hormone), prolactin, GH (growth hormone), ACTH (adrenocorticotrophic hormone), β-LPH (beta lipotropin), α-MSH (alpha melanocyte stimulating hormone), and opiate peptides (β-endorphin).because he thought that it secreted nasal mucus into the nose, is the anterior lobe of the hypophysis in human anatomy. It releases the hormones, FSH (follicular stimulating hormone), LH (leutenizing hormone), TSH (thyroid stimulating hormone), prolactin, GH (growth hormone), ACTH (adrenocorticotrophic hormone), β-LPH (beta lipotropin), α-MSH (alpha melanocyte stimulating hormone), and opiate peptides (β-endorphin), into the circulatory system (Page 22 of “Neuroscience Dictionary” by F. Mora and A.M. Sanguinetti).

 

The subthalamus is composed of gray matter, limited medially by the hypothalamus, laterally by the internal capsule and ventrally by the crus cerebri. It is formed by the subthalamic body and the zona incerta[18]. The subthalamic body is formed by the subthalamic nucleus and nuclei located lateral to it. It is dorsolateral to the mamillary body, and ventral to the mamillo-tegmental tract and cerebello-rubro-thalamic tract. The zona incerta is an extensión of the thalamic reticulate nucleus (pp. 676 from "Miller's Anatomy of the dog" by Evans and de Lahunta. Elsevier. 2013) that extends dorsolaterally to the subthalamic body, and dorsal to the crus cerebri.

 

[1] From the Latin meta, “after”; from the Greek thalamos, “internal chamber”.

[2] From the Latin, sub, “below”; from the Greek tlamos, “internal chamber”.

[3] From the Greek hypo, “below” ; from the Greek thalamos, “internal chamber”.

[4] From the Latin diminutive of habena, “a small strap or rein”.

[5] From the Latin pinea, “like a pine”.

[6] Herophilos of Chalcedon (335–280 B.C.) was a Greek physician from the medical school of Alexandria. He was the first, along with Erastistratus of Ceos, to perform anatomic dissections publicly.

[7] Rene Descartes was born in La Haye in 1596 and died in Stockholm in 1650. He was a French philosopher and mathematician, and Jesuit alumnus. His work influences philosophy, medicine, mathematics and physics alike. His thoughts were centered on the inherently analytic character of experimentation.

[8] From the Greek thalamos, “internal chamber”.

[9] From the Latin pulvinus, “pillow, cushion”.

[10] From the Latin geniculatus, “bent as a knee”.

[11] Pertaining or related to a 24-hour period.

[12] From the Latin cinereum, “of ashes”.

[13] From the Latin decussate, “to cross in the form of an X”.

[14] From the Latin infundibulum, "funnel".

[15] From the Greek hypo, “below”; phyein, “grow”.

[16] From the Latin pituita, "mucus".

[17] Flemish anatomist, born in Brussels in 1514 and died on the island of Zakynthos in 1564. His test, De corporis humani libri septem (1543), caused a revolution in the scientific community of his time since it contradicted the opinions of Galenus and others. He was judged by the Inquisition and condemned to death (1561) for accusations of having started an anatomical investigation on a living man. His penalty was amended to a pilgrimage to Jerusalem. He died in a shipwreck while trying to return.

[18] From the Latin zona, “zone”, incierta, “between”.