Wednesday, May 22, 2019

NeuroAnatomy: The midbrain

The midbrain, it is part brainstem (also known as the mesencephalon) is the most superior of the three regions of the brainstem. It acts as a conduit between the forebrain above and the pons and cerebellum below. the midbrain also serves as an orign to cranial CNIII and CNIV
 Note: we will discuss the anatomy of the midbrain – its external anatomy, internal anatomy, and vasculature of the midbrain.

 Midbrain as an origns of crainal nerves





At the surface of the midbrain two important nerves emerges which are cranial CNIII and CNIV

External Anatomy of the Midbrain

The midbrain is the smallest of the three regions of the brainstem, measuring around 2cm in length. As it ascends, the midbrain travels through the opening in the tentorium cerebelli.
It can be divided into two main parts:
  1. Tectum – located posterior to the cerebral aqueduct
  2. Paired cerebral peduncles – located anteriorly and laterally.
  3. Internally, the cerebral peduncles are further separated by the substania nigra into the crus cerebri (anterior) and the tegmentum (posterior).

Tectum

The tectum houses four rounded prominences named colliculi (midbrain)(collectively the corpora quadrigemina) which sit directly inferior to the pineal gland. The colliculi are separated by the cruciform sulcus; there are two superior and two inferior colliculi.
Extending laterally from each colliculi are the quadrigeminal brachium:
  1. Superior quadrigeminal brachium of the midbrain forms a pathway between the superior colliculus and the retina of the eye.
  2. Inferior quadrigeminal brachium conveys fibres from the lateral lemniscus and inferior colliculus to the medial geniculate body.
Inferior to the colliculi, the trochlear nerve (CN IV) emerges before sweeping across to the anterior surce of the midbrain.




Cerebral Peduncles

The paired cerebral peduncles extend from the cerebral hemispheres to converge as they meet the pons towards the midbrain. They are separated anteriorly in the midline by the interpeduncular fossa, the floor of which is termed the posterior perforated substance (as many perforating blood vessels can be identified).




The oculomotor nerve (CNIII) is seen exiting from between the peduncles while the optic tract runs around the superior border of the midbrain.
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Internal Anatomy of the Midbrain

Two transverse sections of the midbrain will be discussed: the level of the inferior colliculus, and the level of the superior colliculus.



Level of the Inferior Colliculus

The anteriolateral surface of the midbrain houses the paired crus cerebri. Four fibre tracts run within the crus:
  1. Frontopontine fibres – located most medially.
  2. Corticospinal fibres – motor fibres from the primary motor cortex.
  3. Corticobulbar tracts – motor fibres from the primary motor cortex.
  4. Temporopontine fibres – located posterolaterally.
Posteriorly surface of the midbrain is the substantia nigra – a pigmented nucleus that separates the two regions of the cerebral peduncles. It is further broken down into the pars reticulata (anterior) and pars compacta (posterior).




The tegmentum of the midbrain is located posterior to the substantia nigra. It is continuous with that found in the pons by the same name. It is important to note that unlike the crus cerebri, the tegmentum is continuous at the midline.

The cerebral aqueduct is a midline structure surrounded by central gray matter – the periaqueductal gray matter. Within this gray matter lies the mesencephalic nucleus of the trigeminal nerve, as well as the trochlear nucleus with its fibres continuing around the gray matter to exit the midbrain. Anterior to this, the medial longitudinal fasciculus can be seen.

The decussation of the superior cerebellar peduncles can be seen centrally at this level with some reticular formation (noted throughout the brainstem) lying lateral.

Between the central gray matter and the substantia nigra are four lemnisci. Moving anterior to posterior they are the medial, spinal, trigeminal, and lateral leminisci.




At the very posterior pole, we find the tectum which, at this level, contains the inferior colliculus.

 


Level of the Superior Colliculus

Much of the internal structure of the midbrain is unchanged at this level and should be assumed present unless mentioned below.




The central portion which previously was occupied by the decussation of the superior cerebellar peduncles now contains the large paired red nuclei with some decussation of the rubrospinal tract occuring anterior to this. The reticular formation now fans around the posterior borders of the red nuclei. The trochlear nucleus is replaced with the oculomotor nucleus while the oculomotor nerve projects anteriorly.

The medial, spinal and trigeminal lemnisci are all present in much the same location however the lateral lemnisci does not reach to this level. Needless to say, the tectum now contains the superior colliculi rather than the inferior colliculi.




Vasculature of midbrain

We will only briefly look at the vasculature of the midbrain.
The supply to the area is derived from the basilar artery and its branches. The major suppliers are: the posterior cerebral artery and its peduncular branch, the superior cerebellar artery, the posterior choroidal artery, and the interpeduncular branches of the basilar artery.

Function of midbrain

The mesencephalon is considered part of the brainstem. Its substantia nigra is closely associated with motor system pathways of the basal ganglia. The human mesencephalon is archipallian in origin, meaning that its general architecture is shared with the most ancient of vertebrates. Dopamine produced in the substantia nigra and ventral tegmental area plays a role in movement, movement planning, excitation, motivation and habituation of species from humans to the most elementary animals such as insects. Laboratory house mice from lines that have been selectively bred for high voluntary wheel running have enlarged midbrains. The midbrain helps to relay information for vision and hearing. 



References

  1. "mesencephalon". Oxford English Dictionary (3rd ed.). Oxford University Press. September 2005. (Subscription or UK public library membership required.)
  2. Mosby's Medical, Nursing & Allied Health Dictionary,≈ Fourth Edition, Mosby-Year Book 1994, p. 981
  3. Breedlove, Watson, & Rosenzweig. Biological Psychology, 6th Edition, 2010, pp. 45-46
  4. "Archived copy". Archived from the original on 2011-04-27. Retrieved 2011-03-05.
  5. Kandel, Eric (2000). Principles of Neural Science. McGraw-Hill. p. 669. ISBN 0-8385-7701-6.
  6. Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005
  7. Ferrier, David (1886). "Functions of the optic lobes or corpora quadrigemina". doi:10.1037/12789-005.
  8. Martin. Neuroanatomy Text and Atlas, Second edition. 1996, pp. 522-525.
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