MESENCEPHALON

The mesencephalon (midbrain) is situated caudally to the diencephalon. It connects the prosencephalon (forebrain) with the rhombencephalon (hindbrain). It is divided into a dorsal portion or tectum, and a ventral portion. 

 

The tectum is characterized by four elevations called colliculi[1]: two rostral and two caudal. Both are involded in reflexes allowing the head to turn towards a visual or auditory stimulus. The rostral colliculus, along with the pretectal[2] region, receive visual information through fibers that originate directly from the retina. The caudal colliculus receives acoustic information (through the lateral lemniscus) and project to the medial geniculate body by the brachium of the lateral geniculate body.

 

The ventral portion of the mesencephalon forms the cerebral peduncles. Each cerebral peduncle is formed, dorsally to ventrally, by the tegmentum, the substantia nigra, and the pes pedunculi or crus cerebri.

 

            The tegmentum is formed by the periaqueductal gray matter and the following nuclei and tracts:

 

  • Motor nucleus of the oculomotor nerve

  • Parasympathetic nucleus of the oculomotor nerve

  • Motor nucleus of the trochlear nerve[3]

  • Red nucleus

  • Interpeduncular nucleus[4]

  • The mesencephalic tract and nucleus of the trigeminal nerve[5]

  • Decussation of the rostral cerebellar peduncles[6]

  • Dorsal longitudinal fasciculus[7]

  • Tectospinal tract[8]

 

The red nucleus[9] consists of a round nuclear mass that receives impulses from the cerebrum and cerebellum and ascending pathways. Its efferent axons cross at the ventral portion of the tegmental decussation, and project to the cranial nerves nuclei and the spinal cord, forming the rubrospinal tract[10].

 

The substantia nigra, named as such because it contains neuromelanin, is formed by a pars compacta and a pars reticulata. The pars compacta is formed by dopaminergic cells that receive axons from the cerebral cortex, the thalamus, and corpus striatum. It projects itself to the basal nuclei regulating their action. The pars reticulata receives fibers from the subthalamus and projects to midbrain nuclei to control the saccadic eye movements.

 

The crus cerebri are formed by the pyramidal tract and the corticopontine tract[11].

 

 

[1] From the Latin colliculus, “small elevation.”

[2] The pretectal region is located between the caudal commissure and the colliculi. It is integrated by three nuclei, all of them related to visual pathways.

[3] From the Latin, trochlearis, “that which resembles a pulley”.

[4] Found dorsal to the interpeduncular fossa, it receives axons from the habenula via the fasciculus retroflexus.

[5] The nucleus of the mesencephalic tract of the trigeminal nerve is formed by the bodies of the proprioceptive sensory neurons. The mesencephalic tract of the trigeminal nerve is formed by the proprioceptive axons of the trigeminal nerve that reach the nucleus of the mesencephalic tract of the trigeminal nerve.

[6] The decussation of the rostral cerebellar peduncles is found ventral to the medial longitudinal fasciculus.

[7] The dorsal longitudinal fasciculus is formed by ascending and descending myelinated and unmyelinated fibers. It is found in the ventrolateral central gray matter of the mesencephalon, in the floor of the fourth ventricle, and in the dorsal gray commissure of the spinal cord. It connects the hypothalamus with general visceral nuclei of the cranial nerves and the spinal cord.

[8] The tectospinal tract is formed by fibers that descend from the colliculi, ventrally to the medial longitudinal fasciculus, reaching the spinal cord. They decussate at the level of the dorsal portion of the tegmental decussation in the mesencephalon, dorsally to the interpeduncular nucleus and ventrally to the medial longitudinal fasciculus. They connect brain stem nuclei with the cervical segments of the spinal cord.

[9] The red nucleus is also known as the red nucleus of Stilling, even though the name was given to it 20 years before by Burdach.

[10] The rubrospinal tract was described by Von Monakow. It is formed by fibers that originate in the red nucleus, cross the midline in the ventral portion of the tegmental decussation, and finish in the ventral horn of the spinal cord.

[11] The corticopontine tract is formed by fibers originating in the cerebral cortex and ending in the pontine nuclei.

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