1 Anatomy

1 Anatomy


Internal Anatomy of the Brainstem














Surface Anatomy of the Brainstem




Fiber Tracts of the Brainstem






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Figure 1.18. Corticospinal tracts. The corticospinal tracts constitute the major motor pathways. Nerves of this tract originate in the pyramidal cells in layer V of the cerebral cortex. The axons of these nerves travel from the cortex via the posterior limb of the internal capsule, into the cerebral peduncle and the anterior medulla. At the pyramids of the medulla, most of the fibers (80%) cross over to the opposite side from which they originated and form the lateral corticospinal tract. Some axons remain ipsilateral and form the anterior corticospinal tract. The descending fibers constitute upper motor neurons that migrate caudally in the spinal cord to the level of the organs they innervate. Once there, they form synapses with lower motor neurons or interneurons that ultimately form synapses with lower motor neurons in the anterior horn of the spinal cord. The corticobulbar tract is also known as the corticonuclear tract, and it is responsible for relaying motor information of the non-oculomotor cranial nerves between the cerebral cortex and the brainstem. The cranial nerves innervated by this tract include the trigeminal nerve (CN V), the facial nerve (CN VII), the vagus nerve (CN X), the accessory nerve (CN XI), and the hypoglossal nerve (CN XII). This tract originates in the primary motor cortex in Brodmann area 4. The tract descends through the corona radiata and the genu of the internal capsule to the midbrain. The internal capsule transitions to become the cerebral peduncles in the brainstem. White matter tracts migrate in the ventral portion of the peduncles in the crus cerebri. The corticospinal and corticobulbar fibers travel within the middle three-fifths of the crus cerebri. The fibers of the corticobulbar tract migrate in the crus cerebri and synapse at the level of the appropriate lower motor neurons controlling the cranial nerve of interest. This tract innervates cranial motor nuclei bilaterally, except for the lower facial nuclei and the hypoglossal nerve (CN XII), both of which are innervated unilaterally.



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Figure 1.19. Rubrospinal, vestibulospinal, and central tegmental tracts. The rubrospinal tract constitutes one of the major motor pathways of voluntary movement. This pathway terminates in the upper cervical spinal cord and mediates flexion in the upper extremities. The fibers of this pathway originate in the midbrain at the magnocellular red nucleus and cross in the midbrain and descend in the lateral brainstem tegmentum. Once in the spinal cord, the rubrospinal tract travels through the lateral funiculus along with the lateral corticospinal tract. The vestibulospinal tract is a motor pathway responsible for maintaining posture and balance with head movement. It consists of a lateral tract and a medial tract. The lateral vestibulospinal tract originates in the lateral vestibular nucleus in the pons. These fibers descend ipsilaterally in the anterior lateral funiculus, ultimately terminating at the interneurons (or, in rare cases, at the alpha motor neurons) of Rexed laminae VII and VIII. The medial vestibulospinal tract originates in the medial vestibular nucleus. These fibers unite with ipsilateral and contralateral medial longitudinal fasciculi and descend in the anterior funiculus into the spinal cord, where they terminate on neurons in Rexed laminae VII and VIII. The medial vestibulospinal tract predominantly innervates muscles of the head and terminates in the cervical spinal cord. The central tegmental tract is an axon tract that connects the subthalamus and the reticular formation with the inferior olivary nucleus. The central tegmental tract contains ascending and descending fibers. The ascending fibers arise from the rostral solitary nucleus and terminate in the ventral posteromedial nucleus of the thalamus. Information about taste is relayed from the thalamus to the insular cortex. The descending fibers arise from the red nucleus and project to the inferior olivary nucleus. The rubro-olivary tract connects to the contralateral cerebellum.






Vascular Anatomy of the Brainstem








Midbrain


Axial Sections of the Midbrain


Figure 1.30. (a–d) Sequential axial slices of the midbrain.


Apr 23, 2018 | Posted by in NEUROSURGERY | Comments Off on 1 Anatomy
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