Neurology: Midbrain

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*Overall information:

-The midbrain is located at the base of the brain, serving as a connection between the forebrain and the hindbrain. 

-It is involved in the auditory and visual pathway, motor movement, alertness, eye movement,…

*Anatomy and Physiology:

On the posterior region of the midbrain, there are two colliculi hanging out (inferior and superior colliculus).

-When we take a cross section at the level of the inferior and superior colliculus, it is obvious that both of them have quite similar structures. 

-Inferior colliculus level cross section.

-Superior colliculus level cross section.

-Tectum:

+It is the storage of both inferior and superior colliculus 

-Crus cerebri:

+Descending fibers from the cerebral cortex come through the cerebri, some of them will synapse on the pontine nuclei and these nuclei will send fibers (pontocerebellar fibers) to the contralateral cerebellum (the cerebellum will modify the information and send back a perfect motor plan to the cerebral cortex). However, there will be some fibers scattering around and going to the dorsal grey horn to activate muscle nerves there (corticospinal fibers).  There are also some fibers that synapse on the nuclei located within the brainstem (corticonuclear or corticobulbar fibers).

-Tegmentum:

+It is made up of both substantia nigra and crus cerebri.

-Lemnisci: contains lateral lemniscus, spinal lemniscus, trigeminal lemniscus, and medial lemniscus. 

-Auditory pathway:

+Cochlear nerve picks up the information and sends it to the cochlear nucleus. Then, fibers will be sent off to the contralateral sides of the superior olive before ascending upwards as the lateral lemniscus and reaching the inferior colliculus. After that, auditory information will be sent to the nucleus located inside the thalamus before spreading out in the cerebral cortex. 

-Tectospinal fibers:

+Picks up information from the inferior colliculus as well as the superior colliculus to innervate the cranial nerves that supply head, neck, and eye movement. 

-Inside the cerebellum, there are some nucleus that send fibers (carrying blueprint motor movement) to the contralateral red nucleus. At this point, some of the fibers may go to the thalamus and to the motor cortex or go directly to the motor cortex (without passing through the thalamus), forming decussation of the superior cerebellar peduncles. 

-Rubrospinal fibers:

+Information from the cerebellum will be sent to the contralateral red nucleus. At this stage, rubrospinal fibers will carry the information downwards to control flexion movement. 

-Medial lemniscus: receives proprioceptive information from the dorsal grey horn, and there will be fibers ascending upwards and going to the thalamus before reaching the cerebral cortex.  

-Trigeminal lemniscus: cranial nerves will pick up information from the face and send them upwards via the structure called trigeminal lemniscus to the cerebral cortex. 

-Spinal lemniscus: cranial nerves pick up sensory information related to pain from the neck down to the lower extremities and send them to the cerebral cortex.

-Dorsal raphae nucleus and locus coeruleus send down chemicals like norepinephrine to inhibit the pain receiving area inside the dorsal grey horn. 

-Oculomotor nerve: supplies muscle movement in your eyes. 

-Medial longitudinal fasciculus: 

+Vestibular nuclear complex will stimulate cranial nerves 6, 3, and 4 to connect together and form medial longitudinal fasciculus, controlling eye movement. 

-Substantia nigra:

+This structure is divided into two areas: pars compacta (Dopamine) and pars reticularis (Beta).

+It plays a very essential role in modulating indirect and direct pathways of motor movement (basal ganglia function).