Chapter 12 - The Nutrition of the Brain
Circulation of the brain
There are four arteries supplying the brain (figure 35). The two carotid arteries and their branches comprise the anterior circulation and the two vertebral arteries comprise the posterior circulation. There are small penetrating blood vessels that come off the larger vessels. These nourish deep areas of brain while the larger branches remain on the surface. Figure 36 shows the distribution of superficial vessels on the top and deeper vessels in the lower figure. The internal carotid arteries pass through the cavernous sinus (carotid siphon) and give rise to ophthalmic arteries immediately after exiting from the anterosupeiror part of the sinus. They give rise to anterior choroidal branches that follow the optic tract posteriorly and then divide into anterior cerebral and middle cerebral branches. The anterior cerebral arteries enter the fissure separating the left and right hemispheres, supplying the medial and superior surface of the cortex. The middle cerebral artery traverses the Sylvian fissure, sending branches onto the lateral part of the cerebral cortex.
The posterior circulation begins with the vertebral arteries that typically join one another just ventral to the rostral medulla. There they form the basilar artery. There are several cerebellar branches off the vertebrobasilar system as well as penetrating branches into the brain stem and thalamus. The basilar artery ends as the posterior cerebral arteries that supply the medial temporal lobes and most of the occipital lobe.
Neurons obtain virtually all of their nutrition via aerobic metabolism of glucose. Since this process requires large amounts of oxygen, the nervous system requires a high blood flow, typically around 50cc/100grams of tissue/minute. If this value falls below around 15cc/100grams of tissue/minute, neurons will begin to malfunction and lower levels result in death quite rapidly.
Most of the venous drainage of the brain is into "sinuses" that are tunnels in the dural coverings of the brain (figure 37, figure 38). The largest, the superior sagittal sinus, is located where the falx cerebri attaches to the dura over the skull. There are bridging veins that cross from the brain to the sinuses and there is one particularly important vein that drains the deep structures of the brain (the great cerebral vein of Galen).
The blood-brain barrier is represented by the specialized endothelium that is present in brain capillaries. These specializations (by comparison with most somatic capillaries) include: tight junctions between endothelial cells; few pinocytotic vesicles; no fenestra; and high amounts of metabolic activity involved in active transport. These specializations develop under the influence of footplates of astrocytes, which basically cover the abluminal aspect of the endothelium.
The blood brain barrier permits selective entry of substances into the brain. Highly lipophilic substances enter the brain by directly crossing the membrane. Water will also cross by simple diffusion. Very small molecules are also somewhat more likely to cross, although even these are usually tightly regulated and transport of ions is via active pumps. Most nutrients cross the barrier via facilitated diffusion, usually by mechanisms that couple the movement of the nutrient with movement of an ion that is moving down its concentration gradient.
There are regions of the brain that lack a blood-brain barrier. These are in brain regions that are responsible for sensing the internal melieu of the body (such as serum osmolarity) or in areas that are involved in either sensing hormone levels or releasing hormonal factors into the blood stream. In the former category are areas such as the subfornical organ or the area postrema of the brain stem. The infundibulum of the hypothalamus and the pituitary gland are examples of the latter category. Releasing factors enter the capillaries of the infundibulum and travel to the anterior pituitary gland where they influence the release of the trophic hormones. This circulation from the hypothalamus to the pituitary gland is called the hypothalmo-hypophyseal portal system and contains fenestrated capillaries.
There are specialized glial cells called tanicytes that separate those areas of the brain with and without blood-brain barriers. This is necessary in order to prevent substances from moving between these brain regions, circumventing the blood-brain barrier.
There is no barrier between the cerebrospinal fluid and the brain. Therefore, there must be a barrier between the blood and CSF. This barrier is formed by the epithelium of the choroids plexus, which has very well defined tight junctions between cells as well as a high metabolic rate, and specific transport mechanisms for nutrient passage and electrolyte regulation.
Control of regional blood flow
Cerebral blood flow is highly regulated and is activity-dependent. At baseline, the flow in gray matter is about 50-70cc of blood per 100 grams of tissue per minute. White matter has approximately half of the flow of gray matter. When flow goes below approximately 15cc per 100 grams per minute, neurons begin to suffer and any prolonged period below 10cc per 100 grams per minute results in ischemic injury to neurons with accumulation of free radicals, release of intracellular enzymes and entry of calcium into the neuron. Neurons do not normally have the capacity for anaerobic metabolism, so hypoxia is a primary cause of damage and cell death.
Cerebral blood flow is regulated by several mechanisms. There are myoepithelial cells in brain precapillaries that contract when stretched. This is an intrinsic property of these cells and does not require innervation. Therefore, when blood pressure rises, the precapillaries constrict, preventing much rise in blood flow. Through all normal ranges of mean arterial blood pressure, blood flow will remain constant. This autoregulation can be exceeded by malignant hypertension and can result in damage to the endothelium and the blood-brain barrier. Longstanding hypertension results in a shifting of this autoregulatory curve such that low-normal pressures can result in tissue ischemia.
There are a variety of substances present in the circulation or released from brain cells that dilate cerebral blood vessels. These include CO2, low pH (which can occur because of increased CO2), adenosine, and nitric oxide. These latter compounds are often released in regions of increased neuronal activity and result in increased blood flow (beyond that needed for the increased metabolism) in active areas of the brain.
Cerebrospinal fluid (CSF) is created in the choroid plexi via an active process. These highly vascular structures are located within the ventricles (figure 38). This fluid, amounting to about 75cc, has few cells and a low concentration of protein. It does contain 1/2 to 3/4 the concentration of blood glucose. CSF circulates, and turns over several times per day (depending on the level of hydration). The fluid in the lateral ventricles enters the third ventricle via the foramen of Monroe. The third ventricle connects to the fourth ventricle via the narrow cerebral aqueduct, which is located in the core of the midbrain. The fourth ventricle, which lies dorsal to the tegmentum of the pons and rostral medulla, connects with the tiny central canal of the caudal medulla at the obex. This canal continues through the caudal medulla and spinal cord to end blindly in the sacral spinal cord. The fourth ventricle also connects with the subarachnoid space via the midline foramen of Magendi and the two lateral foramina of Lushka.
The subarachnoid space surrounds the brain and spinal cord and provides buoyancy for the brain. It also provides some nourishment for the brain. There are several subarachnoid collections of CSF called cisterns. The largest is the lumbar cistern located between the end of the spinal cord (around the L1 disc) and the end of the thecal sac, around S2 (figure 38). Other cisterns include the cisterna magna located between the dorsal medulla and the posterior part of the cerebellum, the quadrageminal cistern located dorsal to the midbrain and containing the pineal gland, and the basal cistern, located ventral to the hypothalamus and containing the beginning of the great vessels.
Cerebrospinal spinal fluid is resorbed into the venous system at the arachnoid granulations that are associated with the venous dural sinuses, especially the superior sagittal sinus. Any condition that impairs this circulation of CSF is likely to result in increase in pressure inside the head and may result in herniation or other neurologic catastrophies.