Transverse Section of the Midbrain - NEET PG Anatomy

The transverse section of the midbrain is considered an important topic for the NEET PG exam because of its anatomical significance. A thorough understanding of its transverse section helps in visualizing the complex arrangement of various structures and their relationships.

Furthermore, the NEET PG exam assesses the candidate's understanding of human anatomy, including the central nervous system. The transverse section of the midbrain is one of the important topics covered in the syllabus, and a sound understanding of this topic can contribute significantly to your overall performance.

In this blog, we have covered this high-yield anatomy topic briefly from the NEET PG/NExT exam preparation point of view. Keep reading.

Transverse section of the midbrain is taken at the level of the superior colliculus. C.N. III nucleus is seen anterior to the cerebral aqueduct of Sylvius in the periaqueductal grey matter. C.N. III (LMN) exits from the anterior midbrain. If the anterior midbrain is damaged, it damages the C.N. III → Causing ipsilateral down & out eye with wide fixed pupils.

• Crus cerebri has:
  • Cortico-nuclear tract (CNT) passing more medially
  • Corticospinal tract (CST) passing more laterally

• CNT & CST cross the midline and control the muscles on the other side. Therefore, when this crus cerebri is compromised: CNT & CST are compromised (UMN lesion).

• C.N. V, VII, XII; radial nerve & ulnar nerve are compromised on the contralateral side. Injury to the substantia nigra is also seen. This causes contralateral parkinsonism symptoms.  

Also Read: Development of the Nervous System - NEET PG Anatomy         

Cortico-Nuclear Tract

• A.k.a. corticobulbar tract. It crosses the midline at the level of motor nuclei in the brainstem.
• The cranial nerves go beyond the brainstem and control the skeletal muscles. Cerebrum controls the contralateral side of the body.

Corticospinal Tract

• It crosses the midline at the level of lower medulla. It controls the LMN on the contralateral side.

Important Information: Muscles with only contralateral innervation: Lateral pterygoid muscle (Supplied by contralateral C.N. V)Lower face muscles (supplied by contralateral C.N. VII)Genioglossus muscle (supplied by contralateral C.N. XII)

• In case of a right-sided Weber’s Syndrome (with injury of the crus cerebri):
  • Left facial & hypoglossal paralysis
  • Left spastic hemiplegia
  • Right oculomotor paralysis (eye is down and out) with wide fixed pupils
  • Therefore, we see alternating hemiplegia.

• If there is a lesion in the pons (Medial pons syndrome)
  • Ipsilateral abducens nerve (C.N. VI) palsy → Causes medial squint
  • Contralateral spastic paralysis
• If there is a lesion in the medulla (Medial medullary syndrome)
  • Ipsilateral hypoglossal nerve (C.N. XII) palsy → Causes ipsilateral tongue palsy
  • Contralateral spastic paralysis

Also read: How to Prepare Anatomy for PG Entrace Exams ?

Lateral Pontine Syndrome

• At the level of trigeminal nerve → Ipsilateral trigeminal muscles are compromised. 
• At the level of facial nerve → Ipsilateral facial muscles compromised.
• Contralateral loss of pain & temperature, because of involvement of the spinothalamic tract. No spastic paresis, because the pyramidal tract is not involved. No dorsal column (DC) involvement

Important Information: In lateral brainstem lesions, the pyramidal tract is not affected. Therefore, there is no contralateral spastic paralysis. Rather, these patients have contralateral loss of pain & temperature sensations, due to involvement of the spinothalamic tract.

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WALLENBERG SYNDROME

• It is also called the lateral medullary syndrome.

• Contralateral loss of pain & temperature, because of involvement of the spinothalamic tract. Loss of pain & temperature of the ipsilateral face ; this is because of injury to the C.N. V  (trigeminal nerve).

• Alternating hemianesthesia
• Ipsilateral Horner’s syndrome – Due damage to the horner’s pathway (hypothalamospinal pathway compromised)
  • Paralysis of ipsilateral dilator pupillae & superior tarsal muscle
  • ∴ ipsilateral miosis and ipsilateral ptosis.
• Ipsilateral cerebellar ataxia – Due to damage to the dorsal spinocerebellar tract. Difficulty in speech and swallowing on the ipsilateral side – This is due to ischemia of nucleus ambiguus.
• Ipsilateral loss of taste sensation on the tongue – Due to ischemia of the nucleus tractus solitarius. Vertigo – Because of involvement of the vestibular nucleus. The Dorsal column and pyramidal tract are spared. No problem in tongue muscle.

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