BSC 215 1nd EditionFinal Exam Study Guide Chapters 14-16Chapter 14The Brain and Cranial Nerves14.1 Overview of the Brain  Two directional terms of CNS anatomy are:o Rostal “towards the nose” or towards the foreheado Caudal “towards the tail” or towards the brainstem Three sections of the brain: cerebrum, cerebellum, and brainstem The cerebrum is about 83% of the brain’s volume and consists of a pair of half globes called the cerebral hemispheres Each hemisphere is marked by thick ridges called gyri separated by shallow grooves called sulci The deep longitudinal groove, the longitudinal fissure, separates the left and right hemispheres Hemispheres are connected by a thick bundle of nerve fibers called the corpus callosum The cerebellum occupies the posterior cranial fossa inferior to the cerebrumo Second largest region of the brain The brainstem is all of the brain except the cerebrum and cerebellum Major components are: the diencephalon, midbrain, pons, and medulla oblongata Brain is composed of gray and white matter Gray matter – seat of neurosomas, dendrites, and synapses – forms a surface layer called the cortex over the cerebrum and cerebellum White matter is composed of tracts, or bundles of axons, which connect one part of the brain to another to the spinal cord Mature brain anatomy is described by: forebrain, midbrain, and hindbrain In an embryo:o A neural tube develops first and becomes a fluid-filled space that later constitutes the central canal and ventricleso Neural crest gives rise to the arachnoid mater and pia mater, most of PNS, sensory and autonomic nerves, integumentary, and endocrine systems The forebrain divides into the telencephalon and diencephalon  The midbrain mesencephalon The hindbrain divides into two vesicles, the metencephalon and myelencephalon14.2 Meninges, Ventricles, Cerebrospinal Fluid, and Blood Supply The brain has four internal chambers called ventricles The largest and most rostral ones are the two lateral ventricles, which form an arc in each cerebral hemisphere Each lateral ventricle is connected to the third ventricle through a tiny pore called the interventricular forameno The third ventricle is a narrow median space inferior to the corpus callosum The cerebral aqueduct passes down the core of the midbrain and leads to the fourth ventricle (asmall triangular chamber between the pons and the cerebellum)o This space forms a central canal that goes to the spinal cord On the floor or wall of each ventricle is a spongy mass of blood capillaries called a choroid plexuso It produces cerebrospinal fluid Cerebrospinal fluid (CSF) is a clear, colorless liquid that fills the ventricles and canals of the CNS and bathes its external surface CSF has more sodium and chloride than blood plasma, but less potassium, calcium, and glucose and very little protein CSF continually flows through and around the CNS  A small amount of CSF fills the central canal of the spinal cord, but all of it escapes through threepores in the fourth ventricleo This leads into the subarachnoid space on the brain and spinal cord surfaceo From here CSF is reabsorbed by arachnoid granulations Arachnoid granulations are shaped like little sprigs and protruding through the dura mater into the superior sagittal sinus Cerebrospinal fluid serves three purposes:o Buoyancy: allows the brain to attain considerable size without being impaired by its ownweight – too much pressure would kill nervous tissueo Protection: CSF protects the brain from striking the cranium when the head is joltedo Chemical stability: CSF rinses metabolic wastes from the nervous tissue and regulates itschemical environment The brain is 2% of the adult body weight, but it receives 15% of the blood and consumes 20% of the oxygen and glucose Neurons have a high demand for ATP Blood is critical to the brain but also can carry potentially harmful agents and brain tissue is essentially irreplaceable The brain barrier system regulates what we can get from the bloodstream into the tissue fluid ofthe brain Two potential points of entry: blood capillaries throughout the brain tissue and the capillaries of the choroid plexuses At the blood capillaries in the brain tissue, the brain is well protected by the blood-brain barrier (BBB), which consists of tight junctions between the endothelial cells that form the capillary wallso Force anything leaving the blood must pass through the cells and not the gaps in between At the choroid plexuses, the brain is protected by a similar blood-CSF barrier formed by tight junctions between ependymal cellso But tight junctions do not exist because it is important to allow exchanges between the brain tissue and CSFo Therefore there is no brain-CSF barrier The brain barrier system (BBS) is highly permeable to water, glucose, and substances like oxygen,carbon dioxide, alcohol, caffeine, nicotine, and anesthetics BBS is an important protective device, it also inhibits the delivery of medications such as antibiotics and cancer drugs14.3 The Hindbrain and Midbrain The hindbrain has two subdivisions: the myelencephalon and metencephalon The myelencephalon becomes the medulla oblongata The medulla begins at the foramen magnum of the skull and extends 3 cm rostrally The anterior surface has a pair of ridges called the pyramids – wider at the rostral end, taper caudally, and are separated by a groove (anterior median fissure) Lateral to each pyramid is a prominent bulge called the olive Posteriorly, the gracile and cuneate fasciculi of the spinal cord continue as two ridges on the medulla All nerve fibers connecting the brain to the spinal cord pass through the medulla The ascending (sensory) fibers include the first-order sensory fibers of the gracile and cuneate fasciculi The second-order fibers form the medial lemniscus on each side These second-order fibers give rise to the thalamus, synapsing there with the third-order fibers that complete the path to the cerebral cortex The largest group of descending (motor) fibers is the pair of corticospinal tracts filling the pyramids on the anterior surfaceo These carry motor signals from the cerebral cortex on the way to the spinal cord, ultimately to skeletal muscles The medulla contains neural networks involved in a multitude of fundamental sensory and motor functions Signals for