Brain Anatomy: Cerebral Crura and Surrounding Structures

Brainstem Surface Features

• The ventral surface of the brainstem converges towards the upper border of the pons, with a posterior perforated substance between the crura cerebri.
• The III and IV nerves emerge at the medial margin of each crus cerebri.
• The optic tract curls around each crus cerebri.

Midbrain (Mesencephalon)

• The midbrain has a dorsal surface featuring two pairs of small elevations called the superior and inferior colliculi.
• The superior colliculi are connected to the lateral geniculate body by the superior brachium (visual).
• The inferior colliculi are connected to the medial geniculate body by the inferior brachium (auditory).
• The IV nerve leaves the dorsal surface just below the inferior colliculus.
• The superior cerebellar peduncles converge from the cerebellum into the dorsal surface of the midbrain.

Parts of the Midbrain

• Tectum: a small part posterior to the aqueduct, consisting of four colliculi.
• Cerebral peduncles: one on each side, anterior to the cerebral aqueduct, subdivided into:
  • Crus cerebri (anterior)
  • Substantia nigra (middle)
  • Tegmentum (posterior)

Superior Colliculus Section

Grey Matter

• Nucleus of superior colliculus: involved in general visual reflexes, located in the tectum (posterior lateral).
• Pretectal nucleus: involved in pupillary light reflexes, located just cranial to the superior colliculus at the junction of the midbrain and diencephalon.
• Motor nucleus of CN III: motor to extraocular muscles, located ventromedial to the aqueduct.
• Edinger-Westphal nucleus: involved in pupillary light reflexes, located posterior lateral to the CN III motor nucleus.
• Mesencephalic nucleus of CN V: receives proprioception from the masticatory, facial, ocular, and lingual muscles, located lateral to the aqueduct.
• Reticular nuclei: control alertness, circadian rhythm, etc., located lateral to the central grey matter.
• Red nucleus: part of the extrapyramidal system, located paramedian behind the substantia nigra.
• Substantia nigra: gives Negrostriate fibers, secretes dopamine, located between the tegmentum and crus cerebri.

White Matter

• Crus cerebri: contains descending tracts (frontopontine, corticospinal, corticonuclear, temporopontine), located ventral to the substantia nigra.
• Medial lemniscus: ascending for proprioception and most of touch, located posterior lateral to the red nucleus.
• Trigeminal lemniscus: carries opposite fibers for facial pain and temperature, located posterior lateral to the medial lemniscus.
• Spinal lemniscus: ascending for pain, temperature, and some touch, located posterior lateral to the trigeminal lemniscus.
• Medial longitudinal bundle: connects the vestibular nuclei with the III, IV, VI, and spinal XI nuclei, and reticular nuclei to coordinate eye movement and audiovisual reflexes of the head and neck.

Decussations

• Tectospinal decussation: decussating descending fibers from the colliculi (tectum) to the spinal cord and medulla, located at the midline ventral to the medial longitudinal bundle.
• Rubrospinal decussation: decussating descending fibers from the red nucleus to the spinal cord, located at the midline between the two red nuclei.

Inferior Colliculus Section

Grey Matter

• Nucleus of inferior colliculus: involved in general auditory reflexes, located in the tectum (posterior lateral).
• Mesencephalic nucleus of CN V: receives proprioception from the masticatory, facial, ocular, and lingual muscles, located lateral to the aqueduct.
• Trochlear nerve nucleus: motor neurons to the superior oblique muscle, located ventromedial to the aqueduct.

White Matter

• Lateral lemniscus: ascending auditory fibers from the cochlear nuclei to the inferior colliculus and thalamus, located dorsal to the spinal lemniscus.
• Tectospinal tract: descending fibers from the colliculi (tectum) to the spinal cord, located ventral to the medial longitudinal bundle.

Functions

• The superior colliculus receives visual fibers from the optic tract and visual cortex, and gives efferent to the motor nuclei of the cranial and spinal nerves for reflex movements of the eyes, head, neck, and limbs away from or towards light stimuli.
• The inferior colliculus receives auditory fibers from the lateral lemniscus and sends efferent to the reticular formation and the medial geniculate body.
• The pretectal nucleus is involved in pupillary light reflexes, receiving visual afferent from the optic tract and occipital cortex, and sending efferent to the Edinger-Westphal nuclei of both sides. Damage to the pretectal nucleus causes Argyll-Robertson pupil, where the light reflex is lost but the accommodation reflex is present.