Color Atlas of Neurology

Posterior Cerebral Artery (PCA)

The precommunicating segment of the PCA (P1) extends from the basilar bifurcation to the origin of the posterior communicating artery

(PCommA). Its course lies within the interpeduncular cistern, which is demarcated by the clivus and the two cerebral peduncles. The oculomotor nerve, after its emergence from the brain stem, runs between the PCA and the superior cerebellar artery. The postcommunicating segment (P2) curves laterally and backward around the crus cerebri and reaches the posterior surface of the midbrain at an intercollicular level.

The precommunicating and postcommunicating segments are together referred to as the pars circularis of the PCA. (Alternatively, the pars circularis may be divided into three segments— interpeduncular, ambient, and quadrigeminal— named after the cisterns they traverse.)

Distal to the pars circularis of the PCA is the pars terminalis, which divides above the tentorium and caudal to the lateral geniculate body to form its terminal branches, the medial and lateral occipital arteries.

Pars circularis. The precommunicating segment gives off fine branches (posteromedial central arteries) that pierce the interpeduncular perforated substance to supply the anterior thalamus, the wall of the third ventricle, and the globus pallidus. The postcommunicating segment gives off fine branches (posterolateral central arteries) to the cerebral peduncles, the posterior portion of the thalamus, the colliculi of the mid brain, the medial geniculate body, and the pineal body. Further branches supply the posterior portion of the thalamus (thalamic branches), the cerebral peduncle (peduncular branches), and the lateral geniculate body and choroid plexus of the third and lateral ventricles (posterior choroidal branches).

Pars terminalis. Of the two terminal branches of this terminal portion of the PCA, the lateral occipital artery (together with its temporal branches) supplies the uncus, the hippocampal gyrus, and the undersurface of the occipital lobe. The medial occipital artery passes under the splenium of the corpus callosum, giving off branches that supply it (dorsal branch to the corpus callosum) as well as the cuneus and pre-

cuneus (parieto-occipital branch), the striate cortex (calcarine branch), and the medial surfaces of the occipital and temporal lobes (occipitotemporal and temporal banches), including the parasagittal portion of the occipital lobe.

Superior cerebral veins, bridging veins

Cavernous sinus

Inferior petrosal

sinus

Transverse sinus

Sigmoid sinus

Superior sagittal sinus

Inferior sagittal sinus Venous angle Internal cerebral v.

Great cerebral v. (Galen) Basal v. (Rosenthal) Straight sinus Confluence of sinuses

Internal jugular v.

Cerebral veins

Emissary v.

Superior sagittal sinus

Cerebral vein

Superior cerebral veins, bridging veins

Inferior sagittal sinus

Venous angle

Cavernous sinus

Ophthalmic v.

Sphenoparietal sinus

Basilar plexus

Middle meningeal v.

Petrosal v.

Scalp vein

Diploic veins

Superior cerebral v., bridging vein

Extracerebral veins

Superior sagittal sinus

Basal v. (Rosenthal)

Great cerebral v.

Straight sinus

Confluence of sinuses

Sigmoid sinus

Transverse sinus

Superior petrosal sinus

Cerebral veins and sinuses

Craniocervical Veins

Anastomotic channels connect the cutaneous veins of the two sides of the head. Venous blood from the facial, temporal, and frontal regions drains into the facial and retromandibular veins and thence into the internal jugular vein. Some blood from the forehead drains via the nasofrontal, angular, and superior ophthalmic veins into the cavernous sinus. The occipital vein carries blood from the posterior portion of the scalp into the deep cervical vein and thence into the external jugular vein. Blood from the jugular veins continues to the brachiocephalic vein, superior vena cava, and right atrium. The venous channels in the spinal canal and the transcranial emissary veins play no more than a minor role in venous drainage. The pterygoid plexus links the cavernous sinus, the facial vein, and the internal jugular vein.

The numerous anastomoses between the extracranial and intracranial venous systems provide a pathway for the spread of infection from the scalp or face to the intracranial compartment. For example, periorbital infection may extend inward and produce septic thrombosis of the cavernous sinus.

which anastomoses with the occipital venous plexus and finally drains into the external jugular vein.

The pterygoid plexus lies between the temporalis, medial pterygoid, and lateral pterygoid muscles and receives blood from deep portions of the face, the external ear, the parotid gland, and the cavernous sinus, which it carries by way of the maxillary and retromandibular veins to the internal jugular vein.

Cervical Veins

The deep cervical vein originates from the occipital vein and suboccipital plexus. It follows the course of the deep cervical artery and vertebral artery to arrive at the brachiocephalic vein, which it joins.

The vertebral vein, which also originates from the occipital vein and suboccipital plexus, envelops the vertebral artery like a net and accompanies it through the foramina transversaria of the cervical vertebrae, collecting blood along the way from the cervical spinal cord, meninges, and deep neck muscles through the vertebral venous plexus, and finally joining the brachiocephalic vein.

Superficial temporal veins

Supratrochlear v.

Nasofrontal v.

Angular v.

Infraorbital v.

Facial v.

Submental v.

Anterior jugular v.

Left brachiocephalic v.

Lymph vessels joining to form thoracic duct

Extracranial veins

Occipital v.

Suboccipital venous plexus

Pterygoid plexus

Retromandibular v.

Deep cervical v.

External jugular v.

Internal jugular v.

Transverse

cervical v.

Suprascapular v.

Subclavian v.

Arteries

Most of the blood supply of the spinal cord is supplied by the segmental spinal arteries, while relatively little comes from the vertebral arteries via the anterior and posterior spinal arteries. The segmental and spinal arteries are linked by numerous anastomoses.

Segmental arteries. The vertebral, ascending cervical, and deep cervical arteries give off cervical segmental branches; the thoracic and abdominal aorta give off thoracolumbar segmental branches via the posterior intercostal and lumbar arteries.

The segmental arteries give off radicular branches that enter the intervertebral foramen and supply the anterior and posterior roots and spinal ganglion of the corresponding level. The spinal cord itself is supplied by unpaired medullary arteries that originate from segmental arteries. The anatomy of these medullary arteries is variable; they usually have 5 to 8 larger ventral and dorsal branches that join up with the anterior and posterior spinal arteries. Often there is a single large radicular branch on one side, the great radicular artery (of Adamkiewicz), that supplies the entire lower twothirds of the spinal cord. It usually enters the spinal canal in the lower thoracic region on the left side.

Spinal arteries. The spinal arteries run longitudinally down the spinal cord and arise from the vertebral artery (p. 14). The unpaired anterior spinal artery lies in the anterior median fissure of the spinal cord and supplies blood to the anterior two-thirds of the cord. The artery’s diameter steadily increases below the T2 level. The two posterior spinal arteries supply the dorsal columns and all but the base of the dorsal horns bilaterally. Numerous anastomoses of the spinal arteries produce a vasocorona around the spinal cord. The depth of the spinal cord is supplied by these arteries penetrating it from its outer surface and by branches of the anterior spinal artery penetrating it from the anterior median fissure (sulcocommissural arteries).

Spinal Veins

Blood from within the spinal cord travels through the intramedullary veins, whose anat-

omy is variable, to the anterior and posterior spinal veins, which form a reticulated network in the pia mater around the circumference of the cord and down its length. The anterior spinal vein drains the anterior two-thirds of the gray matter, while the posterior and lateral spinal veins drain the rest of the spinal cord. These vessels empty by way of the radicular veins into the external and internal vertebral venous plexuses, groups of valveless veins that extend from the coccyx to the base of the skull and communicate with the dural venous sinuses via the suboccipital veins. Venous blood from the cervical spine drains by way of the vertebral and deep cervical veins into the superior vena cava; from the thoracic and lumbar spine, by way of the posterior intercostal and lumbar veins into the azygos and hemiazygos veins; from the sacrum, by way of the median and lateral sacral veins into the common iliac vein.

Watershed Zones

Because blood can flow either upward or downward in the anterior and posterior spinal arteries, the tissue at greatest risk of hypoperfusion is that located at a border zone between the distributions of two adjacent supplying arteries (“watershed zone”). Such vulnerable zones are found in the cervical, upper thoracic, and lower thoracic regions (ca. C4, T3–T4, and T8–T9).