CHAPTER 13 Brain and Cranial Nerves

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13.7 Blood Supply to the Brain

LEARNING OUTCOMES

After reading this section, you should be able to

A. Describe how the brain is supplied with blood.

B. Explain the role of the blood-brain barrier.

The brain requires a tremendous amount of blood to maintain itsnormal functions. The brain has a very high metabolic rate, andbrain cells are not capable of storing high-energy molecules forany length of time. In addition, brain cells depend almost entirelyon glucose as their energy source (see chapter 25). Thus, the brainrequires a constant blood supply to meet the demands of braincells for both glucose and oxygen. Even though the brain accountsfor only about 2% of total body weight, it receives approximately15–20% of the blood pumped by the heart. Interruption of thebrain’s blood supply for only seconds can cause unconsciousness,and interruption for minutes can cause irreversible brain damage.The brain’s blood supply is illustrated in chapter 21 (seefigures 21.10 and 21.11). Blood reaches the brain through the internal carotid arteries, which ascend to the head along theanterior-lateral part of the neck, and the vertebral arteries, whichascend along the posterior part of the neck, through the transverseforamina of the cervical vertebrae. The internal carotid arteriesenter the cranial cavity through the carotid canals, and the verte-bral arteries enter through the foramen magnum. The vertebralarteries join to form the basilar artery, which lies on the ventralsurface of the pons. The basilar artery and the internal carotidarteries contribute to the cerebral arterial circle (circle ofWillis). Branches from this circle and from the basilar artery sup-ply blood to the brain.The cerebral cortex on each side of the brain is supplied bythree branches that arise from the cerebral arterial circle: the anterior, middle, and posterior cerebral arteries. The middlecerebral artery supplies most of the lateral surface of each cere-bral hemisphere. The anterior cerebral artery supplies the medialportion of the parietal and frontal lobes. The posterior cerebralartery supplies the occipital lobe and the medial surface of thetemporal lobe.The arteries to the brain and their larger branches are locatedin the subarachnoid space. Small cortical arterial branches leavethe subarachnoid space and enter the pia mater, where they branchextensively. Precapillary branches leave the pia mater and enterthe tissue of the brain. Most of these branches are short and remainin the cortex. Fewer, longer branches extend into the medulla.Hemorrhagic brain injury is characterized by bleeding out-side the dura (extradural or epidural), between the dura and thebrain (subdural), or within the brain (intracerebral). A hemor-rhage, which is bleeding, results in a hematoma, an accumulationof blood. Extradural or epidural hematomas occur in about 1–2%of major head injuries. They usually affect the middle cranialfossa and involve a tear in the middle meningeal artery. Subduralhematomas are much more common, occurring in 10–20% ofmajor head injuries. They most commonly involve tears in the

cortical veins or dural venous sinuses in the superior portion ofthe cranial cavity. Intracerebral hematomas occur in about 2–3%of major head injuries and involve damage to small vessels withinthe brain itself.The arteries within the brain tissue quickly divide into capil-laries. These capillaries have a highly selective permeability bar-rier called the blood-brain barrier. The blood-brain barrier isformed by tight junctions between the capillary endothelial cells.The endothelial cells are surrounded by the foot processes of brainastrocytes (see figure 11.6). The astrocytes promote the formationof tight junctions between the endothelial cells. The blood-brainbarrier regulates the movement of materials from the blood intothe brain. Materials that would enter many tissues by passingbetween the endothelial cells of capillaries cannot pass throughthe blood-brain barrier because of the tight junctions. Most mate-rials that enter the brain pass through the endothelial cells. Water-soluble molecules, such as amino acids and glucose, requirespecific transporters to move across the plasma membranes bymediated transport (see chapter 3). However, gases, such as O 2 ,and lipid-soluble substances, such as nicotine and ethanol, canfreely diffuse through the plasma membranes of the endothelialcells and enter the brain.The permeability characteristics of the blood-brain barrier arean important consideration when developing drugs to affect theCNS. For example, Parkinson disease is caused by a lack of theneurotransmitter dopamine, which is normally produced by certainneurons of the brain. Lack of dopamine results in decreased musclecontrol and shaky movements. However, administering dopamine isnot helpful, because dopamine cannot cross the blood-brain barrier.Instead, physicians prescribe levodopa ( L -dopa), a precursor todopamine, because it can cross the blood-brain barrier. CNS neu-rons then convert levodopa to dopamine, which helps reduce thesymptoms of Parkinson disease (see chapter 14).

ASSESS YOUR PROGRESS

23. Describe the blood supply to the brain. List the arteriessupplying each part of the cerebral cortex.

24. Explain how the blood-brain barrier functions.

13.8 Cranial Nerves

LEARNING OUTCOMES

After reading this section, you should be able to

A. List the 12 cranial nerves and give the primary sensory,somatic motor, and/or parasympathetic functions of each.

B. Describe cranial reflexes.

Cranial nerves transmit and relay information to the brain analogousto the spinal nerves, except they do so by direct connections to thebrain instead of the spinal cord.A given cranial nerve may have one or more of three func-tions: (1) sensory, (2) somatic motor, and (3) parasympathetic(table 13.4). Sensory functions include the special senses, such as