Saladin Outline Ch.14 Page 27
Saladin 5e Extended Outline
Chapter 14
The Brain and Cranial Nerves
I. Overview of the Brain (pp. 515–520)
A. The brain of vertebrates has changed a great deal over evolutionary time; in average humans, the size of the brain is proportional to body size, not to intelligence. (p. 515)
B. The brain has been assigned major landmarks as reference points for its study. (pp. 515–518)
1. Two directional terms are rostral (“toward the nose,” or the forehead in upright humans) and caudal (“toward the tail,” or the spinal cord in humans).
2. The brain can be divided conceptually into the cerebrum, cerebellum, and brainstem.
a. The cerebrum is about 83% of the brain volume and consists of two cerebral hemispheres. (Fig. 14.1a)
i. Each hemisphere has thick folds called gyri separated by shallow grooves called sulci.
ii. The deep longitudinal fissure separates the right and left hemispheres.
iii. At the bottom of this fissure the hemispheres are connected by the corpus callosum. (Fig. 14.2)
b. The cerebellum, the second largest region of the brain containing over 50% of the brain’s neurons, occupies the posterior cranial fossa inferior to the cerebrum. (Fig. 14.1b, c)
c. The brainstem is that which remains of the brain if the cerebrum and cerebellum are removed.
i. Its major components, rostral to caudal, are the diencephalon, midbrain, pons, and medulla oblongata. (Figs. 14.1b, 14.2)
ii. It is oriented like a vertical stalk with the cerebrum perched on top in a living person; postmortem changes give it an oblique angle.
iii. The brainstem ends at the foramen magnum, and the CNS continues below this as the spinal cord.
C. The brain, like the spinal cord, is composed of gray matter and white matter. (p. 518) (Figs. 14.5, 14.6c)
1. White matter has a bright pearly white color due to myelin around its nerve fibers.
2. Gray matter has little myelin and a duller white color.
a. Gray matter forms a surface layer called the cortex over the cerebrum and cerebellum.
b. Deeper masses called nuclei are surrounded by white matter.
c. In most of the brain, the white matter lies deep to the cortical gray matter, opposite from their relation in the spinal cord.
d. White matter in the brain is composed of tracts, or bundles of axons.
D. Embryonic development of the brain produces the mature brain anatomy consisting of forebrain, midbrain, and hindbrain.(pp. 518–520) (Fig. 14.3)
1. The nervous system develops from ectoderm; early in the third week of development, a dorsal streak called the neuroectoderm appears along the embryo’s length and thickens to form the neural plate.
a. The neural plate gives rise to most neurons and all glial cells except microglia, which arise from mesoderm.
b. The neural plate sinks and its edges thicken, forming a neural groove with a raised neural fold.
c. The neural folds then fuse along the midline, beginning in the cervical (neck) region and progressing in both directions.
2. By four weeks of development, the neural tube has formed and closed, and it separates from the overlying ectoderm.
a. The neural tube grows lateral processes that later form motor nerve fibers.
b. The lumen becomes a fluid filled space that later constitutes the central canal of the spinal cord and ventricles of the brain.
3. As the neural tube develops, some ectodermal cells originally along the margin of the groove separate and form two neural crests on each side of the tube.
a. Neural crest cells give rise to the arachnoid mater and pia mater; most of the PNS including sensory and autonomic nerves, ganglia, and Schwann cells; and some other structures of the skeletal, integumentary, and endocrine systems.
4. By the fourth week, the neural tube exhibits three primary vesicles: the forebrain (prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon). (Fig. 14.4)
5. By the fifth week, the neural tube continues to subdivide into five secondary vesicles. (Fig. 14.4)
a. The forebrain becomes the telencephalon and diencephalon. (Fig. 14.4b)
i. The telenecephalon has a pair of lateral outgrowths that become the cerebral hemispheres.
ii. The diencephalon has a pair of small cuplike optic vesicles that become the retinas.
b. The midbrain is undivided and retains the name mesencephalon.
c. The hindbrain becomes the metencephalon and the myelencephalon.
II. Meninges, Ventricles, Cerebrospinal Fluid, and Blood Supply (pp. 520–524)
A. The brain is enveloped in three connective tissue membranes, the meninges, which lie between the nervous tissue and bone. (pp. 520–521)
1. The three membranes of the meninges are the dura mater, arachnoid mater, and pia mater. (Fig. 14.5)
2. In the cranial cavity, the dura mater consists of two layers, the outer periosteal layer and the inner meningeal layer.
a. Only the meningeal layer continues into the vertebral canal, where it forms the dural sac.
b. The dura mater is pressed closely against the cranial bone, but is not attached except in limited places (foramen magnum, sella turcica, crista galli, and sutures).
c. In some places the two layers of the dura are separated by dural sinuses.
i. The superior sagittal sinus is found just under the cranium along the median line.
ii. The transverse sinus runs horizontally from the rear of the head toward each ear.
iii. These sinuses meet like an inverted T at the back of the brain and ultimately empty into the internal jugular veins.
d. In certain places, the meningeal layer of the dura folds inward to separate major parts of the brain.
i. The falx cerebri extends into the longitudinal fissure as a wall between the cerebral hemispheres.
ii. The tentorium cerebelli is like a roof over the posterior cranial fossa and separates the cerebellum from the cerebrum.
iii. The falx cerebelli partially separates the right and left halves of the cerebellum.
3. The arachnoid mater and pia mater are similar to those of the spinal cord.
a. The arachnoid mater is a transparent membrane over the brain surface. (Fig. 14.1c)
i. The subarachnoid space separates it from the pia mater below.
ii. In some places a subdural space separates it from the dura above.
b. The pia mater is a very thin, delicate membrane that follows all contours of the brain and sulci.
Insight 14.1 Meningitis
B. The brain has four internal chambers called ventricles that are filled with cerebrospinal fluid. (pp. 521–524) (Fig. 14.6)
1. The largest and most rostral are the lateral ventricles, which form an arc in each cerebral hemisphere.
2. The lateral ventricles connect to the third ventricle, a median space inferior to the corpus callosum, via the interventricular foramina.
3. A canal called the cerebral aqueduct leads from the third ventricle to the fourth ventricle, a triangular chamber between the pons and cerebellum.
4. The fourth ventricle narrows caudally to form the central canal that extends through the medulla oblongata into the spinal cord.
5. Each ventricle has a mass of blood capillaries on the floor or wall called a choroids plexus.
a. Ependyma is a type of neuroglia that resembles cuboidal epithelium.
b. It lines the ventricles and canals, covers the choroids plexuses, and produces cerebrospinal fluid.
6. Cerebrospinal fluid (CSF) is a clear, colorless liquid that fills the ventricles and canals of the CNS and bathes its external surface.
a. The brain produces about 500 mL of CSF per day, but it is constantly reabsorbed and only 100 to 160 mL is normally present at one time.
b. CSF production begins with filtration of blood plasma through the brain’s capillaries.
i. Ependymal cells modify this filtrate so that CSF has more sodium and chloride, but less potassium, calcium, and glucose and very little protein.
b. CSF is circulated through the CNS by its own pressure, by the beating of cilia on the ependymal cells, and by rhythmic pulsations of the brain produced by the heartbeat.
i. CSF secreted in the lateral ventricles flows through the interventricular foramina into the third ventricle and then down the cerebral aqueduct to the fourth ventricle. (Fig. 14.7)
ii. The third and fourth ventricles add more CSF.
c. A small amount of CSF fills the central canal of the spinal cord, but all of it escapes through three pores in the walls of the fourth ventricle: a median aperture and two lateral apertures.
i. These apertures lead into the subarachnoid space.
ii. CSF is reabsorbed in this space by the arachnoid villi.
7. CSF serves three purposes.
a. Buoyancy. The brain and CSF are similar in density; this buoyancy allows the brain to attain considerable size without being impaired by its own weight.
b. Protection. CSF helps prevent the brain from striking the cranium when the head is jolted; however, severe jolts may still be damaging, as in shaken baby syndrome and concussions from car accidents, boxing, etc.
c. Chemical stability. The flow of CSF rinses metabolic wastes away and homeostatically regulates the brain’s chemical environment.
Insight 14.2 Hydrocephalus
C. The blood supply to the nervous system is critically important, and the brain barrier system protects the brain from harmful agents in the blood. (p. 524)
1. The brain is only 2% of the adult weight, but it receives 15% of the blood and consumes 20% of the oxygen and glucose of the body.
a. A 10-second interruption in blood flow can cause loss of consciousness; 1 to 2 minutes, impairment of function; and 4 minutes irreversible brain damage.
2. The brain barrier system regulates what substances can get from the bloodstream into the tissue fluid of the brain.
a. The blood capillaries through the brain tissue is one point of entry, and it is protected by the blood–brain barrier (BBB) consisting of tight junctions between endothelial cells that form the capillary walls.
i. During development, astrocytes induce development of the tight junctions in these endothelial cells.
ii. Anything leaving the blood must therefore pass through the cells and not between them.
b. The choriod plexuses are another point of entry, and this is protected by the blood–CSF barrier formed by tight junctions between ependymal cells.
i. Tight junctions are absent from ependymal cells elsewhere, allowing exchange between brain and CSF.
2. The BBS is highly permeable to water, glucose, and lipid-soluble substances such as oxygen, carbon dioxide, alcohol, caffeine, nicotine, and anesthetics; it is slightly permeable to sodium, potassium, chloride, and waste produces urea and creatinine.
a. The BBS is an obstacle to delivery of medications such as antibiotics and cancer drugs.
b. Trauma and inflammation sometimes damage the BBS, allowing pathogens to enter the brain tissue.
c. In the third and fourth ventricles, circumventricular organs (CVOs) lack the barrier, and the blood has direct access to the brain.
i. CVOs allow the brain to monitor and respond to blood variables, but they also afford a route of invasion by HIV.
III. The Hindbrain and Midbrain (pp. 524–531)
A. Beginning caudally, the medulla oblongata of the adult hindbrain differentiates from the embryonic myelencephalon. (pp. 524–525)
1. The medulla begins at the foramen magnum and extends about 3 cm rostrally, ending at a groove between the medulla and pons. (Figs. 14.2, 14.8)
a. Externally, the anterior surface has a pair of ridges called the pyramids, which are wider at the rostral end, taper caudally, and are separated by the anterior median fissure.
b. Lateral to each pyramid is a bulge called the olive.
c. Posteriorly, the gracile fasciculi and cuneate fasciculi of the spinal cord continue as two pairs of ridges on the medulla.
2. All nerve fibers connecting the brain to the spinal cord pass through the medulla.
a. The ascending fibers include first-order sensory fibers of the two fasciculi, which end in the gracile and cuneate nuclei. (Fig. 14.9c)
i. These nuclei synapse with second-order fibers that decussate and form the medial lemniscus on each side.
ii. The second-order fibers rise to the thalamus, synapsing with third-order fibers that continue to the cerebral cortex.
iii. Near the cuneate nucleus, a continuation of the spinal posterior spinocerebellar tract carries sensory signals to the cerebellum.
b. The largest group of descending fibers is the pair of corticospinal tracts filling the pyramids on the anterior surface.
i. These carry motor signals from the cerebral cortex to the spinal cord, ultimately to stimulate skeletal muscles.
ii. About 90% of these fibers cross over at the pyramidal decussation near the caudal end of the pyramids; muscles below the neck are therefore controlled contralaterally. (Fig. 14.8a)
iii. A smaller bundle of descending fibers, the tectospinal tract, originates in the midbrain, passes through the medulla, and controls muscles of the neck.
3. The medulla contains neural networks involved in many sensory and motor functions.
a. Sensory functions include the sense of touch, pressure, temperature, taste and pain
b. Motor functions include chewing, salivation, swallowing, gagging, vomiting, respiration, speech, coughing, sneezing, sweating, cardiovascular and gastrointestinal control, and head, neck, and shoulder movements.
4. Signals enter and leave the medulla not only via the spinal cord but also through four pairs of cranial nerves that begin or end there: glossopharyngeal (CN IX), vagus, (CN X), accessory (CN XI), and hypoglossale (CN XII) nerves. (Table 14.1)