mesencephalon

Diencephalon

by Sharadsaini on

DiencephalonThe diencephalon extends from the region of the mamillary bodies and the posterior commissure at its caudal end to the interventricular foramen at its most rostral end. It forms the lateral wall of the third ventricle and is made up  principally of the hypothalamus, epithalamus,  thalamus, and subthalamus (also termed ventral thalamus). The thalamus lies above the hypothalamic sulcus, and the hypothalamus below it. The thalamus makes up the dorsal wall and the hypothalamus the ventral wall of the ventricle. Little can be seen of the diencephalon, since most of it is surrounded by the cerebral hemispheres, and it is best seen in sagittal section. The only part that is visible on the brain surface is in the ventral view, when the infundibulum, bilateral mamillary bodies and the tuber cinereum can be seen, as well as a surface rostral boundary, the optic chiasm. The mamillary body holds the mamillary nuclei of the hypothalamus.

                 In sagittal section, the hypothalamus is seen from the mamillary body at its caudal end to the interventricular foramen rostrally.  Functionally, the hypothalamus is critical for normal life, since it controls body temperature, fluid and water balance, and neuroendocrine function, and has an important role in the control of the autonomic nervous system and  emotional and sexual behavior. At the base of the hypothalamus is the infundibulum or pituitary stalk, which connects the hypothalamus to the pituitary gland through blood portal and nervous links. Several small but important nuclei  have been identified in the hypothalamus.

                The thalamus is the largest member of the diencephalon, and if it were dissected free might resemble a hen’s egg in shape. It is separated from the hypothalamus by a groove, the hypothalamic sulcus. There are two thalami, joined by amassa intermedia or interthalamic adhesion. The thalamus is a huge relay station, and has massive reciprocal connections with the cerebral cortex. The thalamus extends forward to the interventricular foramen, and is bounded laterally by the posterior limb of the internal capsule, and the head of the caudate nucleus. Internally, the thalamus consists of several nuclei, which project to the ipsilateral cerebral cortex, and the cortex in turn sends reciprocal fibers back to the areas from which it received them. Functionally, this relationship serves to control the organism’s response to inputs from the special and the general senses, and to ensure a proper motor response to them.

                Immediately below the thalamus lies the subthalamus, which is situated dorsolaterally to the hypothalamus. The epithalamus consists of the habenular nucleus and the pineal gland. The pineal gland synthesizes the hormone melatonin, which may modulate sleep waking rhythms, and in recent years melatonin  has been advocated to alleviate the condition known as jet lag.Diencephalon

Pons

by Sharadsaini on

Ponspons

The pons (metencephalon) lies beneath (anterior to) the cerebellum and is around 2.6 cm in length. The pons has been arbitrarily divided into the dorsal or posterior tegmentum, and a basal or anterior part, sometimes referred to as the pons proper.

Transection of the caudal pons at the level of the facial colliculi shows the fourth ventricle prominently, as well as  the middle cerebellar peduncles. (The term colliculus refers to the visible swellings caused by the mass of the nucleus.) The superior and inferior cerebellar peduncles and the nuclei and spinal tracts of several cranial nerves are also visible. The medial lemniscus runs at the base of the tegmentum, and above it the area occupied by the reticular formation is now much larger than that of the medulla. The trapezoid body consists of fibers from the cochlear nuclei and the nuclei of the trapezoid nucleus in the pons; these convey, for example, auditory information arriving in the pons. Ascending and descending fiber tracts, such as the corticospinal tractcourse through the pons.

The basal (anterior or ventral) portion of the pons consists of transverse and longitudinal bundles of fibers. The fibers constitute, mainly, a massive relay system from the cerebral cortex to the contralateral cerebellar cortex.

Dorsolateral to the reticular formation, lying in the floor of the fourth ventricle are the vestibular nuclei, which receive afferent inputs concerning equilibrium and balance and which are then well placed to be relayed to the cerebellum. The cerebellum in turn sends afferents from Purkinje cells to the vestibular nucleus; these are inhibitory, and release the neurotransmitter !-aminobutyric acid (GABA). The vestibular nuclei project efferent fibers to the middle ear.

The motor nucleus of the facial nerve innervates facial muscles, and its functionis clearly manifested when the facial nerve is damaged. This results in partial paralysis of the facial muscles (Bell’s palsy), and possibly autonomic disturbances. Transverse section through the pons higher up (rostrally) reveals similar structural features, except that the motor and sensory nuclei of the trigeminal nerve are now clearly visible. The principal sensory nucleus of the trigeminal nerve lies lateral to the motor nucleus, and its sensory incoming fibers lie laterally to the efferent fibers of the trigeminal nerve, which leave the trigeminal motor nucleus. The superior cerebellar peduncle is now more prominent, as is the lateral lemniscus, which runs dorsolateral to the medial lemniscus.

Damage to the pons results, typically, in muscle paralysis or weakness of structures innervated by cranial nerves. For example, a childhood tumor of the pons called astrocytoma of the pons, is the most prevalent  brainstem tumor, and causes a number of symptoms that reflect the paralysis of the ipsilateral cranial nerve; thus there may be weakness (hemiparesis) of facial muscles due to damage to the facial nucleus. The pons may be damaged by hemorrhage of the cerebellar arteries or of the basilar artery, and, depending on whether the damage is unilateral or bilateral, will result in facial paralysis and contralateral paralysis of lower limbs, through damage to corticospinal fibers which traverse the ventral pons.mid pons