Brainstem Important Question And Answers

Pyramidal Decussation

Pyramidal Decussation Introduction: It is a crossing of corticospinal fibres at the level of medulla oblongata.

1. Pyramidal Decussation Pyramidal Decussation Origin: Corticospinal fibres arise from the area no. 4, 6, 8 of precentral gyrus and from area no. 3, 1, 2 of postcentral gyrus.
2. Location: On the anterior surface of medulla oblongata.
3. Pyramidal Decussation Features
   • The corticospinal fibres cross at medulla oblongata. It forms a bulging which resembles pyramid.
   • The decussation starts at upper part of medulla and completed at lower part of medulla.

Pyramidal Tracts

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• • The 80% fibres cross at medulla oblongata and 20% fibres cross at the respective region of spinal cord.
  • The crossed fibres course through lateral column and is called lateral corticospinal tract.
  • The uncrossed fibres traverse through the anterior column and are called anterior corticospinal tract.
  • Pyramidal tract fibres are composed of 55%-upper
    • 55% upper limb
    • 25%-lower limb
    • 20%-trunk

1. Pyramidal Decussation Termination: The crossed fibres continue in the lateral column of the opposite side as lateral corticospinal tract. These fibres terminate in the laminae IV to VII of spinal grey matter. They are connected to the alpha and gamma motor neurons of the lamina IX through the interneurons.
2. Effect of decussation
   • It displaces the central grey matter and central canal dorsally.
   • The continuity between the ventral grey column and central grey matter of the spinal cord is lost.
   • The column subdivides into the supraspinal nucleus, and the nucleus of the accessory nerve.
   • The anterior median fissure is interrupted by the pyramidal decussation.
3. Pyramidal Decussation Applied anatomy: Lesions of the corticospinal tract is called the upper motor neuron lesion.
   • The lesion above the level of pyramidal decussation results in paralysis of the muscles of the upper limb and lower limb on opposite side of the body.
   • The lesion below the level of pyramidal decussation results in the paralysis of the muscles on the same side of body.

Blood Supply Of Medulla Oblongata

1. Arterial supply
   • Branches of vertebral artery
     1. Anterior spinal artery
     2. Posterior spinal artery
     3. Posterior inferior cerebellar artery ( PICA)
   • Branches from the basilar artery
     1. Medullary branches, and
     2. Anterior inferior cerebellar artery.
2. Venous drainage: It is divided into lower and upper parts of medulla oblongata.
   1. Veins of lower part of medulla penetrate deeper regions. They open into anterior and posterior spinal veins.
   2. Veins of the upper part of medulla drain into sigmoid or petrosal sinus.

Draw and label a transverse section of medulla oblongata at the level of pyramidal decussation.

Medulla Oblongata Anatomy

Arcuate Fibres

Arcuate Fibres Introduction: The axons of various nuclei are arranged like the arc.

1. Arcuate Fibres Types: These are
   • Internal arcuate fibres:
     1. These are 2nd order neurons arising from gracilis and cuneatus nuclei. The nuclei situated on dorsal side of medulla.
     2. These fibres cross to the opposite side and form medial lemniscus.
     3. Somatotopic arrangement:
        1. Before decussation
           • The fibres of lower limb are placed medially.
           • The fibres of upper limb are placed laterally.
        2. After decussation
           • The fibres of the lower limb are placed anteriorly.
           • The fibres of the upper limb are placed posteriorly.

1. • External arcuate fibres: These are of following types
     1. Anterior external arcuate fibres:
        • These fibres arise from arcuate nuclei present on the anterior surface of medulla oblongata. They reach the nuclei of the same side of cerebellum.
        • Function: They convey information from cerebrum to cerebellum. The cortico-arcuato-cerebellar pathway is functionally equivalent to cortico- ponto-cerebellar pathway.
     2. Posterior external arcuate fibres (cuneocerebellar)
        • These fibres arise from accessory cuneate nucleus and reach the nuclei of cerebellum on the same side.
        • Function: These are homologous of posterior spinocerebellar tract in upper limb and carries unconscious proprioceptive sensations from upper limb.
2. Arcuate Fibres Applied anatomy: Following are the effects of lesion of internal arcuate fibres.
   1. Before decussation: Loss of sensation of touch and pain on the same side.
   2. After decussation: Loss of touch and pain on the opposite side.

Pyramidal Tracts

Draw and label a transverse section of medulla oblongata at the level of sensory decussation.

Draw and label a transverse section of medulla Oblongatio of the lev of inferior olivary nucleus or pontomedullary junction

Name The Cranial Nerve Nuclei In Medulla Oblongata.

Name the cranial nerve nuclei in midbrain?

1. The central (periaqueductal) grey matter contains
   • Lower part of midbrain-nucleus of the trochlear nerve in the ventromedial part, and
   • Upper part of midbrain-nucleus of oculomotor nerve with Edinger-Westphal nucleus in the ventromedial part.
2. The lateral part contains mesencephalic nucleus of the trigeminal nerve. The mesencephalic nucleus is made up of unipolar cells (first neuron) and receives proprioceptive impulses from the muscles of mastication, the facial and ocular muscles, the teeth and temporomandibular joint.

Name the cranial nerve and nuclei at pontomedullary junction.

1. Vestibular nuclei
   • Medial,
   • Lateral,
   • Superior, and
   • Inferior.
2. Cochlear nuclei
   • Dorsal, and Ventral.

Name the cranial nerve nuclei in pons.

1. Abducent (6th) nerve nucleus lies deep to facial colliculus.
2. Facial nerve (7th) nucleus lies in the reticular formation of the pons.
3. Vestibular and cochlear (VIIIth) nuclei lie in relation to the inferior cerebellar peduncle.
   1. Vestibular
      1. Situation: The vestibular nuclei lie deep to the vestibular area in the floor of the 4th ventricle, partly in the medulla and partly in the pons.
      2. Divisions: They are divisible into four parts
         1. Superior,
         2. Inferior,
         3. Medial, and
         4. Lateral.
      3. Connections of vestibular nuclei
         1. Afferents: They receive the fibres of the vestibular nerve.
         2. Efferents
         3. Cerebellum (vestibulocerebellar),
         4. Medial longitudinal bundle,
         5. Spinal cord (vestibulospinal tract arising in the lateral vestibular nucleus), and
         6. Lateral lemniscus.
   2. Cochlear
      1. Situation: The dorsal and ventral cochlear nuclei are situated dorsal and ventral to the inferior cerebellar peduncle.
      2. Connections
         1. Afferent: Cochlear nerve
         2. Efferents
            1. Superior olivary nucleus, and
            2. Nuclei of the
               1. Corpus trapezoideum,
               2. Lateral lemniscus.
         3. These fibres form the trapezoid body.
4. Spinal nucleus of the trigeminal nerve lies in the lateral part.

Draw and label a transverse section of pons at the level of facial colliculus.

Medulla Oblongata Anatomy

Draw And Label Ts Of Pons At Lower Level Showing Exit Of Cranial Nerves.

Draw and label TS of pons at lower level showing blood supply

Draw and label TS of pons at lower level showing features.

Stria medullaris

Stria medullaris Introduction: These are transversely running glistening white fibres present in the floor of 4th ventricle.

1. Stria medullaris Origin: They arise from arcuate nuclei present on the ventral surface of medulla oblongata.
2. Stria medullaris Course: They run medial to medial lemniscus, traverse through the substance of medulla oblongata.
   • They emerge from the medial sulcus and run transversely into the inferior peduncle.
   • They terminate into dentate nucleus of cerebellum.
   • They divide the floor of 4th ventricle into upper and lower regions.

Draw and label TS of pons at upper level.

Draw And Label Ts Of Pons At Upper Level Showing Blood Supply.

Draw and label a TS of midbrain at the level of inferior colliculus

Draw And Label A Ts Of Midbrain At The Level Of Inferior Colliculus Showing Features.

Draw and label a TS of midbrain at the level of inferior colliculus showing blood supply.

 Trapezoid body
Trapezoid Body Introduction: It is a transverse band of fibres lying just behind the ventral part of pons.

1. Trapezoid Body Formation: It is formed by the fibres that arise in the cochlear nuclei of both the sides. Most of the axons arising in these nuclei cross to the opposite side (in the trapezoid body) and terminate in the superior olivary nucleus. Many fibres end in the nucleus of trapezoid body
2. Trapezoid Body Situated in midline of the tegmental part of pons. Most of the fibres of the trapezoid ascend upward as the lateral lemniscus.

Crus Cerebri (crus-leg, cerebri-cerebrum)

Crus Cerebri Introduction: It is a white matter present on ventral part of cerebral peduncle. It contains all descending tracts.

1. Crus Cerebri Relations
   1. Medially it is related to interpeduncular fossa which contains
      1. Oculomotor nerve at the level of superior colliculus.
      2. Posterior cerebral artery (terminal branch of basilar artery).
   2. Laterally it is related to trochlear nerve.
   3. Posteriorly it is related to substantia nigra.
2. Crus Cerebri Divisions: It is divided into three parts.
   1. Medial 1/6th: It contains frontopontine fibres.
   2. Middle 2/3rd: It contains
      1. Corticonuclear fibres going to different motor nuclei of cranial nerves.
      2. Corticospinal fibres: The fibres of the leg are placed laterally and I for the fibres of the upper limb are placed medially.
   3. Lateral 1/6th: It contains PTO
      1. Parietopontine,
      2. Temporopontine, and
      3. Occipitopontine.
3. Crus Cerebri Blood supply: Short circumferential artery, a branch of posterior cerebral artery.
4. Crus Cerebri Applied anatomy
   1. Weber’s syndrome: A lesion in the midbrain causes contralateral hemiplegia and ipsilateral paralysis of the muscles of the eyeball. This condition is known as Weber’s syndrome. It includes
      • Upper motor neuron lesion of corticospinal tract. It results into contralateral hemiplegia.
      • Lower motor neuron lesion of 3rd and 4th nerves. It results into ipsilateral paralysis of muscles of eyeball.
      • Lesion of oculomotor nerve produces
        1. Ptosis (pseudoptosis of eyelid).
        2. Strabismus: Squint due to unopposed action of lateral rectus.
        3. Loss of light and accommodation reflex due to involvement of ciliary muscle and sphincter pupillae.
           • Lesion of corticospinal tract produces increased deep reflex and loss of superficial reflex.

Draw and label a transverse section of midbrain at the level of superior colliculus.

Lateral medullary syndrome (Wallenberg’s syndrome)

Lateral medullary syndrome: The lateral part of the medulla oblongata receives blood from posterior inferior cerebellar artery (PICA) and occlusion of the vessel produces damage of the posterolateral part of the medulla.

Medial Medullary Syndrome

Medial medullary syndrome: The medial part of the medulla oblongata receives blood from anterior spinal artery and its paramedian branches. The occlusion of the artery leads to medial medullary syndrome.

Dentate Nucleus

Dentate Nucleus Introduction

• It is the chief and largest nucleus of cerebellum.
• It is situated in deep part of cerebellar hemisphere.
• It is present in white matter of cerebellum.
• It belongs to neocerebellum.

Dentate Nucleus Function

1. Dentate Nucleus Gross anatomy
   1. Dentate Nucleus Location: It is located in the roof of 4th ventricle, just lateral to emboliform nucleus.
   2. Dentate Nucleus Size: 3-4 cm.
   3. Dentate Nucleus Shape: It gives appearance of a crumpled purse, the hilum directed medially.
   4. It resembles inferior olivary nucleus.
2. Dentate Nucleus Connections
3. Dentate Nucleus Histology
   1. It consists of large and multipolar neurons with branching dendrites.
   2. The axons acquire myelin sheath within the nucleus.
4. Dentate Nucleus Applied anatomy: The lesion of dentate nucleus produces
   1. Hypotonia: Diminished muscle tone, which produces pendular jerk.
   2. Asynergia: Loss of muscular co-ordination.
   3. Ataxia: In coordination of muscles of trunk.