The Midbrain

The central aqueduct is distinctive. Dorsal to this is the tectum with four colliculi.

  • Superior colliculi process visual sensations.
  • Inferior colliculi process auditory sensations.

Red nucleus is most distinctive in the midbrain tegmentum. It integrates information from cerebrum and cerebellum (involuntary motor control)

Its fibers to the spinal cord (rubrospinal) transmit impulses that facilitate flexor muscle tone.

Structures dorsal and lateral to the red nucleus form the midbrain reticular formation.

What do you consider to be the main functions of the reticular formation?

 

The mibrain consists of four main parts:

  • Tectum(1)
  • Red nucleus(2)
  • Substantia nigra (3)
  • Crus cerebri (4)

Around the aqueduct is the periaqueductal gray. It contains nuclei of oculomotor and trochlear nerves.

The most prominent structure in the tegmentum is the red nucleus but the tegmentum, also contains ascending tracts.

The substantia nigra consits of melanin containing neurons tha regulate motor output of cerebral cortex

Crus cerebri represents a bundle of projection fibers.

 

 

 

 

The following structures can be identified In a cross section of the midbrain.

  • Mesencephalic nucleus.
  • Medial lemniscus.
  • Decussation of superior cerebellar peduncle.
  • Inferior/superior Colliculi.
  • Descending fibers from motor cortex.
  • Nuclei of cranial nerve and gray matter.
  • Periaqueductal gray
  • Oculomotor/trochlear nucleus
  • Edinger Westphal
  • Red nucleus
  • Substantia nigra
  • Mesencephalic reticular formation

   

Cranial nerve nuclei include

  1. Oculomotor nuclei and accessory oculomotor nuclei (Edinger Westphal).
  2. Trochlea nuclei at the level of the inferior colliculi.
  3. Mesencephalic tract and nucleus of CN V.

b)  Other nuclei include

Nucleus

Function

Main connections

Red nucleus

-  Control of pattern of movement
- Postural reflexes

-  Cerebellum, reticular formation,      spinal cord
-  Sensori motor corlex

Substantia Nigra

Control of movement

-  Sensorimotor cortex,
-  Corpus striatum
-  Reticular formation

Superior colliculus

Visual reflexes

-  Retina
-  Spinal cord

Pre-tectal nuclei

Pupillary reflex

-  Retina
-  Accessory CN III

Inferior colliculus

Auditory reflex

-  Cochlear nuclei
-  Spinal cord

Reticular formation

-  Motor control
-  Cortical activation

-  Spinal cord
-  Cerebral cortex
-  Thalamus

c)   Fibre tracts

Crus cerebri corticospinal, corticnuclear and corticopontine fibres
Medial lemniscus from nucleus Gracilis and cuneatus (dorsal columns)
Spinal lemniscus spinothalamic and spinotectal
Lateral lemniscus auditory from cochlear nuclei
   


             
d) Mid brain decussations

Decussation of superior cerebellar peducles: Fibres arise form cerebellar dentale nucleus and end in the contalateral red nucleus    (Dentantorubral)
Tectospinal decussation (posterior tegmental) Contralateral fibres from superior colliculus to cervical cord.
Rubrospinal decussation (Anterior tegmental) Fibres from red nucleus to contralateral anterior horn cells.  These fibres relay also in the    reticular formation.
   


               


Blood supply to the midbrain
Most of the blood supply is derived from branches of the basilar artery. 

  1. Posterior cerebral
  2. Superior cerebellar
  3. Posterior communicating
  4. Anterior choroidal

Vascular lesions of the midbrain include:
Weber‘s syndrome, which involves the basal portion of the midbrain due to occlusion of a mesencephalic branch of the posterior cerebral artery.


Area of infarction involves:

  1. Pyramidal fibres- contralateral hemiparesis. 
  2. Oculomotor nerve - ipslateral paralysis of the ocular muscles except the lateral rectus and superior oblique.

Red area: shows area of vascular lesion involvement

 

 

 

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