Regional+Anatomy+of+the+CNS+and+Brain+Vasculature

=**Regional Anatomy of the CNS and Brain Vasculature**=


 * 1. Spinal Cord:**

__Anterior and Dorsal Cell Columns__ These are the unmyelinated grey matter ventral and dorsal horns of the spinal cord. They are surrounded by myelinated white matter though preserved specimens often cause the white matter to appear darker.

Lower cervical and lumbosacral regions have a larger anterior and dorsal cell columns because the innervation to the upper and lower limbs requires more neurons and interneurons. The thoracic and cervical regions contain lots of white matter because they are made up of much of the ascending and descending sensory and motor nerves. Portions of the lower spinal cord will have lots of ascending neurons because most of the descending neurons have already branched off at their appropriate levels. Alternatively, anything going to the lower parts of the spinal cord must pass through the upper cervical region; thus, it is not surprising that the upper cervical regions contain lots of white matter and relatively less grey matter.

The most medial part of the anterior cell column supplies motor innervation to the axial skeleton while the more lateral parts supply the muscles of the limbs.

__Dorsal, Lateral, and Anterior Funiculi__ The white matter of the spinal cord can be divided into the dorsal, lateral and anterior funiculi. Essentially the dorsal funiculus is between the midline and the dorsal horn, the lateral feniculus is between the lateral and ventral horn, and the anterior feniculus is between the midline and the ventral horn.

The dorsal horn contains mostly cell bodies receiving sensory information from the dorsal root ganglion, either ascending information or local reflex information. Ventral horn contains mostly motor neuron cell bodies and interneurons.


 * 2. Brainstem:**

The brainstem is composed of the midbrain, the pons, and the medulla.

__Medulla Oblongata__ The medulla oblongata is the most caudal subdivision of the brainstem that is continuous with the spinal cord. It lies in the posterior cranial fossa; the cavity of the medulla forms the inferior part of the 4th ventricle.

__Pons__ The pons is the part of the brainstem between the midbrain rostrally and the medulla oblongata caudally. It lies in the anterior part of the posterior cranial fossa. The cavity in the pons forms the superior part of the 4th ventricle.

__Midbrain__ The midbrain or mesencephalon is the rostral part of the brainstem at the junction of the middle and posterior cranial fossae. The cavity of the midbrain forms a narrow canal called the cerebral aqueduct that conducts CSF from the lateral and 3rd venticles to the 4th ventricle.

__Cerebellum__ The cerebellum is a large brain mass lying dorsal to the pons and medulla and ventral to the posterior part of the cerebrum. It lies beneath the cerebellar tentorum in the posterior cranial fossa and consists of two lateral hemispheres united by a narrow middle part called the vermis.

The cerebellar tentorum holds up the posterior portion of the cerebrum. Cranial pressure in the supratentoral region can cause the cerebrum to try to go through the tentoral notch or incisures, putting pressure on the midbrain and resulting in unconsciousness. Infratentoral cranial pressure can cause the cerebellum to be pushed on to foramen magnum, pressing on the medulla, causing uncontrolled heart and respiration rate.

__Cortex and Deep Nuclei__ Basal ganglia are technically deep cerebral nuclei. They are found posterior to the thalamus region and largely control motor behavior.

__Cranial Nerves__ Eleven of the twelve cranial nerves arise from the brain; they all exit the cranial cavity. Generally these nerves are surrounded by the dural sheath as they leave the cranium. The olfactory tract are extensions of the cerebrum going out to meet CN I. Extending out to meet CN II is the optic chasm, an extension of the retina coming into the thalamus; the cell bodies of CN II are found in the ganglion cells of the retina. CN III and CN IV have cell bodies in the midbrain; CN IV actually is the only cranial nerve that exists from the dorsal aspect. CN V has its motor component in the pons and sensory component mostly in the pons but dispersed. CN VI, CN VII, and CN VIII originate at the pons-medullary junction, where their cell bodies are. CN IX, CN X, CN XI, and CN XII originate at the medulla. The olive is a nucleus which bulges out at the medulla. CN IX, CN X, and CN XI emerge from the pre-olivary sulcus while the CN XII emerges from the post-olivary suclus.


 * 3. Cerebrum:**

The cerebrum is divided into the frontal, parietal, occipital and temporal lobes based on which ever cranial bone it is closest to. The gyri are raised ridges of the brain while sulci are grooves.

The lateral fissure is a large, deep sulcus between the fronto-parietal lobes and the temporal lobes, underneath which is the insula. The insula is covered over by the other parts of the cerebrum but covered over during development. The central gyrus of the frontal lobe divides areas of the frontal lobe were the gyri are predominately anterior-posterior with the areas where it is predominately medial-lateral. The central gyrus and the lateral fissure can be used as landmarks to orient the lobes of the cerebrum.

The precentral gyrus is important for motor control while the postcentral gyrus is important for somatic sensation. The medial occipital lobe is the primary visual cortex of the brain. The medial temporal gyrus is an important area for auditory function.

The limbic lobe is a functional lobe containing parts of the frontal, parietal, medial temporal, and some occipital anatomical lobes. It resides at the junction of the brainstem and the cerebrum and is important for emotion and memory.


 * 4. Other Structures:**

The corpus callosum is a large commissure composed of interneuronal fibers that join equivalent areas of the left and right hemispheres of the cerebrum. This allows the right hand to know what the left hand is doing and vice versa. Commissural fibers join equivalent portions on the same level of two sides of the brain. Association fibers join portions of the same level of the same side but dissimilar regions. Projection fibers join different levels of the nervous system.

The diencephalon is formed by the thalamus and hypothalamus and is located above the midbrain. The diencephalon surrounds the 3rd ventricle, forming its walls. The white matter adjacent to the thalamus contains fibers going to the cortex, fibers going from the cortex to the thalamus, and from the cortex to the brainstem. These projection fibers are called the internal capsule.

__Caudate Nucleus__ The caudate nucleus is a telencephalic nucleus located within the basal ganglia in the brain. The caudate known to be an important part of the brain's learning and memory system.

__Putamen__ The putamen is a structure in the middle of the brain, which, together with the caudate nucleus forms the corpus striatum of the basal ganglion system. It forms the outermost part of the lenticular nucleus and appears to play a role in reinforcement learning.

__Globus Pallidus__ The pallidum or globus pallidus, is a sub-cortical structure of the brain. It is a major element of the basal ganglia system.


 * 5. Ventricular System:**

The lateral ventricles (1st and 2nd ventricles) are the largest cavieties of the ventricular system and occupy the larges areas of the cerebral hemispheres. Each lateral ventricle opens through the interventricular forament into the 3rd ventricle. The 3rd ventricle is a slit light cavity between the two halves of the diencephalon and is continuous posterioinferiorly with the cerebral aqueduct which connects to the 4th ventricle. The 4th ventricle is in the posterior part of the pons and medulla, extending inferoposteriorly. It is continuous through the central canal in the inferior part of the medulla with the central canal in the spinal cord. CSF drains from the 4th ventricle through a single median aperture and paired lateral apertures in the subarachnoid space. These apertures are the only means by which the CSF enters the subarachnoid space. If blocked, CSF accumulates, distending the ventricles and compressing the cerebral hemispheres.


 * 6. Describe:**

__Composition of the Cranial Vault__ The cranial vault is divided into 3 fossae that are like steps leading down from anterior to posterior. The anterior cranial fossa is the shallowest, Formed by the frontal bone, cribriform plate of the ethmoid bone, and the lesser wing of the sphenoid. It forms the frontal lobe of the brain and the roof of the orbit.

The middle cranial fossae is formed by the greater wing of the sphenoid and the temporal bone. It contains the sella turcica that is a fossa for the pituitary gland and supports the temporal lobe of the brain. It contains the foramen for five or 6 cranial nerves (depending on if you count branches).

The posterior cranial fossa is the deepest and most posterior. It is made up of primarily the occipital bone. It contains the foramen magnum which transmit the spinal cord and the jugular foramen where the internal jugular vein and nerves pass at the lateral border between the posterior and middle cranial fossae. The posterior cranial fossa support the cerebellum, the pons, and the medulla.

See Skull and Cranial Vault

__Dura mater and dural reflections,arachnoid membrane, pia mater__ The cranial meninges are three layers of connective tissue that serve as protection for the brain. These layers are continuous with those surrounding the spinal cord. Going deep to superficial, the meninges are pia mater, arachnoid mater, and dura mater. The pia mater is intimate with the brain. The arachnoid mater has web-like processes that attach to the dura and makes a subarachnoid space between the arachnoid mater and pia mater and contains cerebral spinal fluid.

Cranial dura mater is composed of an endocranium or periosteal layer and a meningeal layer. The periosteal layer is attached to the inside of the cranial cavity. The meningeal layer is in contact with the arachnoid mater. These two layers are typically not separatable except at four specific places where they form dural folds.

Dural folds are locations where the meningeal layer of the dura mater separates from the periosteal layer and folds in onto itself to form a dual layer reflection. These reflections form compartments in the brain which help support the brain.

The dural folds are the falx cerebi, falx cerebelli, tentorium cerebelli, and diaphragmn sella. The two biggest folds are the falx cerebri and diaphragm sella. The falx cerebri is a sickle-shaped fold that is suspended vertically on the inside of the sagittal suture and separates the right and left cerebral hemispheres. The falx cerebelli is a small, sickle-shaped fold separating the right and left cerebellar hemispheres in the posterior cranial fossa. The tentorium cerebelli is a tent-like projection that is perpendicular to the falx cerebri from the falx cerebelli, separating the cerebral hemispheres from the cerebellar hemispheres. The diaphragm stella is a circular fold covering the pituitary gland to attach it to the pituitary fossa.

See Skull and Cranial Vault

__Distribution of major branches of the internal carotid artery, vertebral artery__ The internal carotid artery arises from the common carotid arteries in the neck and ascends vertically, entering the cranial cavity through the carotid canals of the temporal bones. Within the carotid canals, it passes anteriorly and medially, and then through the cavernous sinus on the carotid groove on the body of the sphenoid. The terminal branches of the internal carotid arteries are the anterior and middle cerebral arteries. The anterior cerebral arteries are connected by the anterior communicating artery. Near their termination, the internal carotid arteries are joined to the posterior cerebral arteries by the posterior communicating arteries, completing the cerebral arterial circle around the interpeduncular fossa, a deep depression on the inferior surface of the midbrain between the cerebral peduncles.

The vertebral artery passes through the transverse foramen of the C1 through C6 cervical vertebrae as it ascends the neck. The vertebral artery supplies blood to radicular arteries and segmental medullary arteries. Segmental medullary arteries also supply the spinal cord. It also supplies the dorsal root ganglion and spinal nerves. Eventually the vertebral artery ends up in the brain, entering the cranial vault via foramen magnum and piercing the dura and arachnoid mater. The vertebral arteries unite at the caudal border of the bons, forming the basilar artery.

__Major patterns of venous drainage__ The dural sinuses are produced when the periosteal dura separates from the meningeal dura and form a cavity or space. These spaces are lined with endothelium and form the dural venous sinuses. They drain blood from the brain and all eventually empty into the internal jugular vein.

Blood generally drains posteriorly, with the inferior sagittal sinus draining into the straight sinus and the superior sagittal sinus, straight sinus, and occipital sinus draining towards the confluence of sinuses. At that point, the blood drains into the left and right transverse sinuses and then goes into the sigmoid sinuses and out the jugular foramen into the internal jugular veins. Blood from the cavernous sinuses, drain into the superior and inferior petrosal sinuses. The superior petrosal sinuses drains into the sigmoid sinuses with the transverse sinuses while the inferior petrosal sinuses drains directly into the origin of the internal jugular veins.

See Skull and Cranial Vault

__The formation, circulation, and resorption of cerebrospinal fluid__ CSF is secreted by the choroidal epithelial cells of the choroid process in the lateral 3rd and 4th ventricles. CSF leaves the lateral ventricles through the interventricular foramina and enters the 3rd ventricle. From it, it passes through the cerebral aqueduct into the 4th ventricle. It leaves the 4th ventricle through its median and lateral apertures and enters the subarachnoid space. Most of the CSF flows to the interpeduncular and quadrigeminal cisterns. CSF from the various subarachnoid cisterns flows superiorly through the sulci and fissures on the medial and superolateral surfaces of the cerebral hemispheres. CSF also passes into the extensions of the subarachnoid space around the cranial nerves, especially CN II. The main site of CSF absorption into the venous system is the arachnoid granulations, tiny protrusions of arachnoid villi into the walls of the dural venus sinuses, especially the superior sagittal sinus and its lateral lacunae. The subarachnoid space containing CSF extends into the cores of arachnoid granulations. CSF enters the venous system either by transport through the cells of the arachnoid granulations into the dural venous sinuses or by moving between cells making up the arachnoid granulations.

Enlargements of the subarachnoid space form cisterns where CSF is found. Cisternae magnum is found between the medulla and the brainstem. The ventriclar system is the lumen maintained from the tube the CNS developed from.