At the level of the caudal pons, the anterior inferior cerebellar artery (AICA) branches off of the basilar artery and perfuses the lateral portion of the caudal pons (eFig. 9.43A). Rostral levels of the lateral pons are perfused by circumferential branches of the basilar artery (eFig. 9.43A and B). Medial portions of the pons are perfused by paramedian branches off of the basilar artery as it continues rostrally toward the midbrain.
Just before the midbrain, the superior cerebellar arteries branch off of the basilar artery and supply the superior cerebellar peduncles and caudal aspect of the dorsal midbrain before reaching the superior portion of the cerebellar hemispheres. Paramedian branches from the basilar artery supply the medial aspect of the midbrain (eFig. 9.43C). The final branches emerging from the top of the basilar artery are the PCAs, which perfuse the lateral aspect of the midbrain before reaching the thalamus, medial occipital, and inferior temporal lobes (eFig. 9.43C and D).
Part V: Spinal cord
The spinal cord is continuous with the medulla oblongata near the foramen magnum at the base of the skull. Cylindrical in shape, it occupies the vertebral canal of the vertebral column to the LI and LII vertebral level in an adult (eFig. 9.44). Numerous ascending and descending axonal tracts course through the spinal cord and connect with the brain to convey sensory (afferent) and motor (efferent) information for facilitation of movement, reflexes, sensory input, and feedback mechanisms.
Like the brain, the spinal cord is surrounded by three concentric meninges: the dura mater, arachnoid mater, and pia mater. The spinal dura mater is continuous with the inner meningeal layer of the cranial dura mater and extends inferiorly to the posterior surface of the vertebral body of S2 (eFig. 9.45). It is separated from the bony vertebral canal by the epidural/extradural space (eFig. 9.45). In addition, unlike in the cranial cavity, the underlying arachnoid mater is not tightly adherent to the dura mater, and instead has a theoretical plane or potential space called the subdural space.
Although the arachnoid mater of the spinal cord has a less adherent relationship with the dura mater than in the cranial cavity, the overall structure of the arachnoid mater is the same (eFig. 9.46A). The subarachnoid space created by the loose relationship of the arachnoid and underlying pia mater extends inferiorly to the level of the SII vertebra (eFig. 9.45). Given that the spinal cord terminates near the LI–LII vertebrae, this lower termination point of the subarachnoid space creates a safe and enlarged space for accessing CSF in the clinical setting (eFig. 9.45).
The innermost pia mater is a highly vascular layer that is adherent to the surface of the spinal cord. Midway between the anterior and posterior roots the pia mater forms a flat continuous sheet, the denticulate ligament (eFig. 9.46B). At the posterior and anterior rootlets, sleevelike projections from the denticulate ligament extend out through the arachnoid mater to attach onto the dura mater. These delicate attachments anchor and position the spinal cord within the central area of the subarachnoid space.