Internal Anatomy of the Brainstem
Figure 1.5. (a) Superolateral, (b) superior, (c) inferior, and (d) medial views showing the relationship of the thalamus, the lateral ventricle, and the third ventricle relative to adjacent structures.
Figure 1.10. (a) Superior view of the venous relationship of the lateral and third ventricles. (b) Superior view showing the separation of the body of the fornix to expose the third ventricle and vascular structures.
Figure 1.13. Ventral surface of the brainstem. The midbrain is located between the level of the optic tract above and the pontomesencephalic sulcus below. The pons is bordered by pontomesencephalic and pontomedullary sulci. The medulla is situated below the level of the pontomedullary sulcus and transitions into the spinal cord at the level of the first cervical nerve rootlets.
Figure 1.15. Dorsal column–medial lemniscus pathway. The dorsal column–medial lemniscus pathway is the major sensory pathway relaying vibration, proprioception, fine touch, and two-point discrimination from the skin and joints. First-order neurons of this pathway that are located in the dorsal root ganglion send their axons via the gracile fasciculus (information from the lower half of the body) and the cuneate fasciculus (information from the upper half of the body) to the level of the second-order neurons, which are located in the gracile nucleus and cuneate nucleus in the medulla. A subset of these second-order neurons decussates in the medulla, and hence these fibers, which form the medial lemniscus, are named the internal arcuate fibers. Second-order neurons send axons to the ventral posterolateral nucleus of the thalamus, where they synapse with third-order neurons, which then relay information to the postcentral gyrus (not shown).
Figure 1.16. Anterolateral system. The anterolateral system, also known as the spinothalamic system, is one of the major sensory pathways. This pathway consists of two adjacent pathways: the anterior spinothalamic tract, which is responsible for carrying crude touch, and the lateral tract, which carries pain and temperature sensation. Unlike the corticospinal and medial lemniscus pathways, this tract decussates in the spinal cord instead of the brainstem. Neurons in the dorsal root ganglion send ascending fibers, or descending fibers migrate caudally for one or two levels via Lissauer’s tract and then synapse with second-order neurons, called tract cells, in the substantia gelatinosa or nucleus proprius. The axons of the second-order neurons decussate via the anterior white commissure, usually one or two levels above the point of entry, and migrate via the spinal cord to the rostral ventromedial medulla. These second-order neurons then connect with third-order neurons in the medial dorsal, ventral posterolateral, and ventral medial posterior nuclei of thalamus. From this point, the information is passed to the primary somatosensory, insular, and cingulate cortices.
Figure 1.17. Spinocerebellar tract. The spinocerebellar tract is a proprioception tract that transmits signals from the spinal cord to the cerebellum ipsilaterally. The tract is divided into a ventral spinocerebellar tract, a rostral spinocerebellar tract, a dorsal spinocerebellar tract, and a cuneocerebellar tract. Proprioceptive stimuli are received from the Golgi tendon organs and muscle spindles, which have their cell bodies located in the dorsal root ganglion and constitute the first-order neurons in the circuit. These fibers then pass through Rexed laminae I–VI to synapse with second-order neurons in Rexed lamina VII. For the dorsal and cuneocerebellar tracts, the sensory neurons synapse in Clarke’s nucleus in layer VII at L3–T1 and then send axons via the spinal cord to the medial zones in the cerebellum using the inferior cerebellar peduncle. For the ventral and rostral spinocerebellar tracts, the sensory neurons synapse in layer VII of S3–L4 and most of these neurons cross to the contralateral lateral funiculus through the anterior white commissure and the superior cerebellar peduncle. In the cerebellum, the bulk of these fibers ultimately cross over again to the ipsilateral side.
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.
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.
Figure 1.20. Tectospinal and reticulospinal tracts. The tectospinal tract connects the midbrain tectum to the spinal cord and is responsible for mediating reflex postural movements of the head to auditory and visual input. This tract originates in the superior colliculus, which has afferents from the oculomotor nuclei and projects to the contralateral and ipsilateral domains of the cervical neuromeres and the oculomotor, trochlear, and abducens nuclei. This tract descends in the spinal cord to terminate in Rexed laminae VI, VII, and VIII. The reticulospinal tract consists of the medial and lateral parts. The medial reticulospinal tract provides excitatory input to the antigravity extensor muscles. The fibers of this tract originate in the caudal pontine reticular nucleus and the oral pontine reticular nucleus, and they project to Rexed laminae VII and VIII of the spinal cord. The lateral reticulospinal tract exerts inhibitory control over excitatory axial extensor muscles. The fibers of this tract arise from the medullary reticular formation and descend in the anterior spinal cord in the lateral column to terminate in Rexed laminae VII and IX of the spinal cord.
Figure 1.21. Dorsal view of a cadaveric dissection showing the surface and internal structures of the brainstem. The midbrain has superior and inferior colliculi. The brachium of the inferior colliculus runs from the inferior colliculus to the medial geniculate body. The floor of the fourth ventricle has a rhomboid shape. The upper two-thirds of the floor of the fourth ventricle are on the dorsal surface of the pons, and the lower one-third is on the dorsal surface of the medulla. The floor of the fourth ventricle can be divided into a superior or a pontine part, an intermediate or a junctional part, and an inferior or a medullary part. The superior part of the floor of the fourth ventricle has a triangular shape. Its apex is at the cerebral aqueduct, its base is formed by an imaginary line connecting the lower margin of the cerebellar peduncles, and its lateral margins are the medial edge of the superior cerebellar peduncles. The intermediate or junctional part is at the same level as the lateral recesses. The medullary part has a triangular shape limited laterally by the inferolateral margin of the floor, along which the tela choroidea is attached, and its apex is at the obex. The medullary part contains the hypoglossal and vagal trigones and the area postrema, which is shaped like a pen nib and is thus called the calamus scriptorius.
Figure 1.22. Cadaveric dissections showing (a) the lateral and (b) the ventral views of the long tracts of the brainstem. The colored areas denote the various tracts within the bilateral cerebral peduncles.
Vascular Anatomy of the Brainstem
Figure 1.24. Ventral view of a cadaveric dissection of the vascular anatomy of the brainstem. At the level of the pontomedullary sulcus, the vertebral arteries come together to form the basilar artery, which ascends superiorly along the ventral surface of the pons to terminate as the two posterior cerebral arteries. This termination usually occurs at the level of the pontomesencephalic junction.
Figure 1.25. Lateral view of the vascular anatomy of the brainstem. The posterior inferior cerebellar artery arises from the vertebral arteries and supplies the medulla, the glossopharyngeal nerve (CN IX), the vagus nerve (CN X), the accessory nerve (CN XI), the inferior cerebellar peduncle, and the suboccipital surface of the cerebellum. The basilar artery gives rise to the pontine-perforating arteries and the anterior inferior cerebellar artery. The anterior inferior cerebellar artery is related to the abducens nerve (CN VI), the facial nerve (CN VII), the vestibulocochlear nerve (CN VIII), the middle cerebellar peduncle, and the petrosal surface of the cerebellum. The superior cerebellar artery arises from the basilar artery at the level of the pontomesencephalic sulcus and is related to the midbrain, superior cerebellar peduncle, and tentorial surface of the cerebellum.
Axial Sections of the Midbrain
Figure 1.30. (a–d) Sequential axial slices of the midbrain.