CHAPTER 14LECTURE OUTLINEI. INTRODUCTIONA. The brain is the center for registering sensations, correlating them with one another and with stored information, making decisions, and taking action.1. It is also the center for intellect, emotions, behavior, and memory.2. It also directs our behavior towards others.3. In this chapter we will consider the principal parts of the brain, how the brain is protected and nourished, and how it is related to the spinal cord and to the 12 pairs of cranial nerves.II. BRAIN ORGANIZATION, PROTECTION AND BLOOD SUPPLYA. The brain and spinal cord arise from embryological tissue (ectoderm) beginning as a hollow, neural tube. B. The major parts of the brain are the brain stem, diencephalon, cerebrum, and cerebellum (Figure 14.1).C. Protective Covering of the Brain1. The brain is protected by the cranial bones (Figure 7.4) and the cranial meninges (Figure 14.2).a. The cranial meninges are continuous with the spinal meninges and are nameddura mater, arachnoid, and pia mater.1. Three extensions of the dura mater separate parts of the brain: the falx cerebri, falx cerebelli, and the tentorium cerebelli.D. Blood Flow and the Blood-Brain Barrier1. Blood flows to the brain mainly via blood vessels that branch from the cerebral arterial circle (circle of Willis) at the base of the brain (Figure 21.19); the veins that return blood from the head to the heart are seen in Figure 21.24.2. Although the brain comprises only about 2% of the total body weight, it utilizes about 20% of the oxygen used by the entire body. The brain is one of the most metabolically active organs of the body, and the amount of oxygen it uses varies with the degree of mental activity.a. Any interruption of the oxygen supply to the brain can result in weakening, permanent damage, or death of brain cells. Interruption of the mother’s bloodsupply to a child during childbirth before it can breathe may result in paralysis, mental retardation, epilepsy, or death.b. Because carbohydrate storage in the brain is limited, the supply of glucose to the brain must be continuous. Glucose deficiency may produce mental confusion, dizziness, convulsions, and unconsciousness.c. A blood-brain barrier (BBB) protects brain cells from harmful substances andpathogens by serving as a selective barrier to prevent passage of many substances from the blood to the brain.1. Clinical Connection: An injury to the brain due to trauma, inflammation, or toxins causes a breakdown of the BBB, permitting the passage of normally restricted substances into brain tissue. The BBB may also prevent entry of drugs that could be used as therapy for brain cancer or other CNS disorders, so research is exploring ways to transport drugs past the BBBIII. CEREBROSPINAL FLUID A. Cerebrospinal fluid (CSF) is a clear, colorless liquid that protects the brain and spinal cord against chemical and physical injuries and carries oxygen, glucose, and other needed chemicals from the blood to neurons and neuroglia.1. There are four CSF filled cavities within the brain called ventricles (Figure 14.3).2. CSF contributes to hemostasis by providing mechanical protection, chemical protection, and circulation.3. CSF is formed by filtration from networks of capillaries called choroid plexuses (found in the ventricles) and circulates through the subarachnoid space, ventricles, and central canal.4. Materials entering CSF from the choroid capillaries cannot leak between the surrounding ependymal cells; these constitute the blood-cerebrospinal fluid barrier, which permits certain substances to enter the fluid but excludes others and protects the brain and spinal cord from harmful elements (Figure 14.4).5. Most of the fluid is absorbed by the arachnoid villi of the superior sagittal blood sinus (Figure 14.2); this absorption normally occurs at the same rate at which CSF is produced in the choroid plexuses, thereby maintaining a relatively constant CSF volume and pressure (Figure 14.2).a. Clinical Connection: If CSF cannot circulate or drain properly due to some obstruction in the ventricles or subarachnoid space, a condition called hydrocephalus develops. The fluid buildup that occurs causes increased pressure on the brain, either internally or externally, depending on where the blockage is present. Surgically draining the ventricles and diverting the flow of CSF by an implanted shunt can positively and dramatically affect the individual’s prognosis.IV. THE BRAIN STEMA. Medulla Oblongata1. The medulla oblongata, or just medulla, is continuous with the upper part of the spinal cord and contains portions of both motor and sensory tracts (Figures 14.5, 14.1).2. It also contains the nuclei of origin for cranial nerves VIII (cochlear and vestibular branches) through XII (Table 14.4).3. Structural regions of the medulla include the pyramids (Figures 14.5, 14.6) and the inferior olivary nucleus (Figures 14.5, 14.6).a. Decussation of pyramids results in neurons in the left cerebral cortex controlling skeletal muscles on the right side of the body and neurons in the right cerebral cortex controlling skeletal muscles on the left side.b. Inferior olivary neurons relay impulses from proprioceptors to the cerebellum.4. Functional regions include nuclei that are reflex centers for regulation of heart rate, respiratory rate, vasoconstriction, swallowing, coughing, vomiting, sneezing, and hiccuping; the first three are considered vital reflexes. a. Clinical Connection: Injury to the medulla can be fatal or lead to serious problems. B. Pons1. The pons is located superior to the medulla. It connects the spinal cord with the brain and links parts of the brain with one another by way of tracts (Figures 14.1, 14.5).2. It relays nerve impulses related to voluntary skeletal movements from the cerebral cortex to the cerebellum.3. The pons also contains the pneumotaxic and apneustic areas, which help control respiration along with the respiratory center in the medulla (Figure 23.25).4. It contains nuclei for cranial nerves V through VII and the vestibular branch of VIII (Figure 14.5).C. Midbrain1. The midbrain conveys motor impulses from the cerebrum to the cerebellum and spinal cord, sends sensory impulses from the spinal cord to the thalamus, and regulates auditory and visual reflexes (Figures 14.1, 14.5, 14.7).2. Structures within the midbrain include the cerebral peduncles, the corpora quadrigemina, the left and right substantia nigra, the left and right red nucleus, and the