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Section IV • Neuroscience

Half of brain and spinal cord tumors are metastatic. Some differences between primary and metastatic twnors are listed in Table IV-2-2:

Table IV-2-2. Tumors ofthe CNS and PNS

Primary

Metastatic

Poorly circumscribed

Well circumscribed

Usually single

Often multiple

Location varies by specific type

Usually located at the junction

 

between gray and white matter

Abbreviations: CNS, central nervous system; PNS, peripheral nervous system

Table IV-2-3. PrimaryTumors

Tumor

Glioblastoma multiforme (grade IV astrocytoma)

Astrocytoma

(pilocytic)

Oligodendroglioma

Features

•Most common primary brain tumor

•Highly malignant

•Usually lethal in 8-12 months

Benign tumor of children and young adults Usually in posteriorfossa in children

•Slow growing

•Long survival (average 5-10 years)

Ependymoma

•

Ependymal origin

 

Can arise in IV ventricle and lead to hydrocephalus

Medulloblastoma

Highly malignant cerebellar tumor

 

• A type of primitive neuroectodermal tumor (PNEl)

Meningioma

•

Second most common primary brain tumor

 

Dural convexities; parasagittal region

Schwannoma

Third most common primary brain tumor

 

• Most frequent location: CN VIII atcerebellopontine angle

 

• Hearing loss, tinnitus, CN V + VII signs

 

• Good prognosis after surgical resection

Retinoblastoma

 

Sporadic-unilateral

 

 

Familial-bilateral; associated with osteosarcoma

Craniopharyngioma

 

Derived from oral epithelium (remnants of Rathke pouch)

 

 

Usually children and young adults

Often calcified

Symptoms due to encroachment on pituitary stalk or optic chiasm

Benign but may recur

Pathology

•Can cross the midline via the corpus callosum ("butterfly glioma")

•Areas of necrosis surrounded by rows of neoplastic cells

(pseudopalisading necrosis) Rosenthal fibers lmmunostaining with GFAP

•"Fried-egg" appearance­ perinuclear halo

Rosettes and pseudorosettes

Blue, small, round cells with pseudorosettes

Attaches to the dura, compresses underlying brain without invasion

Microscopic-psammoma bodies

•Antoni A (hypercellular) and B (hypocellular) areas

•Bilateral acoustic schwannomas­

pathognomonic for neurofibromatosis type 2

Small, round, blue cells; may have rosettes

Histology resembles adamantinoma (most common tumor oftooth)

Abbreviation: GFAP, glial fibrillary acidic protein

348 MEDICAL

Chapter 2 • Central NervousSystem

ChapterSummary

•The neural tube forms 3 primary vesicles at its cranial end:

-Forebrain (prosencephalon)

-Midbrain

-Hindbrain (rhombencephalon)

•These primary vesicles then develop into 5 secondary vesicles that form the adult derivatives of the CNS.

-The telencephalon forms the cerebral hemispheres

-The diencephalon forms 4 thalamic derivatives

-The mesencephalon forms the midbrain

-The metencephalon forms the pons and cerebellum

-The myelencephalon forms the medulla

•The remainder of the neural tube forms the spinal cord. The lumen of the neural tube will develop into the ventricular system.

•The typical neuron is the multipolar neuron. It consists of a cell body (soma), multiple dendrites, and a single axon. Axons utilize anterograde and retrograde axonal transport to move subcellular elements to and from the soma. Skeletal motor neurons, preganglionic autonomic neurons, and glial cells develop from the neural tube. Glial cells include astrocytes, oligodendrocytes, microglia, and ependymal cells.

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CSF DISTRIBUTION, SECRETION, AND CIRCULATION

CSF fills the subarachnoid space and the ventricles ofthe brain. The average adult has 90 to 1 50 mL of total CSF, although 400 to 500 mL is produced daily. Only 25 mL of CSF is found in the ventricles themselves.

Approximately 70% of the CSF is secreted by the choroid plexus, which consists of glomerular tufts of capillaries covered by ependymal cells that project into the ventricles (the remaining 30% represents metabolic water production). The cho­ roid plexus is located in parts ofeach lateral ventricle, the third ventricle, and the fourth ventricle.

CSF from the lateral ventricles passes through the interventricular foramina of Monro into the third ventricle. From there, CSF flows through the aqueduct of Sylvius into the fourth ventricle. The only sites where CSF can leave the ventricles and enter the subarachnoid space outside the CNS are through 3 openings in the fourth ventricle, 2 lateral foramina of Luschka and the median foramen of Magendie.

Within the subarachnoid space, CSF also flows up over the convexity ofthe brain and around the spinal cord. Almost allCSF returns to the venous system by drain­ ing through arachnoid granulations into the superior sagittal dural venous sinus.

Normal CSF is a clearfluid, isotonic with serum (290-295 mOsm/L).

The pH of CSF is 7.33 (arterial blood pH, 7.40; venous blood pH, 7.36).

Sodium ion (Na+) concentration is equal in serum and CSF (:o:l38 mEq/L).

CSF has a higher concentration of chloride (CI-) and magnesium (Mg2+) ions than does serum.

CSF has a lower concentration ofpotassium (K+), calcium (Ca2+), and bicarbon­ ate (HC03) ions, as well as glucose, than does serum.

The Blood-Brain Barrier and the Blood-CSF Barrier

The chemical integrity of the brain is protected in a different way by 2 separate systems.

Chapter 3 • Ventricular System

Clinical Correlate

The concentration of protein (including all immunoglobulins) is much lower in the CSF as compared with serum.

Normal CSFcontains 0 to 4 lymphocytes or mononuclear cells per cubic millimeter. Although the presence ofa few monocytes or lymphocytes is normal, the presence of polymorphonuclear leukocytes

is always abnormal, as in bacterial meningitis.

Red blood cells (RBCs) are not normally found in the CSF but may be present after traumatic spinal tap or subarachnoid hemorrhage.

Increased protein levels may indicate a CNS tumor.

Tumor cells may be present in the CSF in cases with meningeal involvement.

Blood-brain barrier

The blood-brain barrier is formed by capillary endothelium connected by tight junctions. Astrocytes participate in the maintenance of the blood-brain barrier. They have numerous long processes with expanded vascular end-feet, or pedicels, which attach to the walls of capillaries.

Water diffuses across the blood-brain barrier readily, but glucose, the primary energy source of the brain, requires carrier-mediated transport. Active transport systems are capable of pumping weak organic acids, halides, and extracellular K+ out of the brain against their respective concentration gradients.

Blood-CSFbarrier

Tight junctions located along the epithelial cells of the choroid plexus form the blood-CSP barrier. Transport mechanisms are similar to those described for the blood-brain barrier, although the ability ofa substance to enter the CSF does not guarantee it will gain access to the brain.

MEDICAL 355

Section IV • Neuroscience

ChapterSummary

•The ventricular system is continuous throughout each part of the central nervous system (CNS) and contains cerebrospinal fluid (CSF), which provides

a protective bath forthe brain and spinal cord. The system consists of 2 lateral ventricles in the cerebral hemispheres, a third ventricle in the

midbrain, and a fourth ventricle in the pons and medulla. CSF is produced in the choroid plexuses ofthe lateral, third, and fourth ventricles. CSF leaves the fourth ventricle through the foramen of Magendie and the foramina of

Luschka to enter the subarachnoid space. From the subarachnoid space, CSF returns to the venous system by passing through arachnoid granulations into the superior sagittal dural venous sinus.

•Hydrocephalus results from excess volume and pressure of CSF, producing ventricular dilatation. Noncommunicating hydrocephalus is caused by obstruction to CSFflow inside the ventricular system, and communicating hydrocephalus is caused by oversecretion or reduced absorption of CSF.

356 MEDICAL

The Spinal Cord

4

GENERAL FEATURES

The spinal cord is housedin the vertebral canal. It is continuous with the medulla below the pyramidal decussation and terminates as the conus medullaris at the second lumbar vertebra of the adult. The roots of 31 pairs of spinal nerves arise segmentally from the spinal cord.

There are 8 cervicalpairsof spinal nerves (Cl through CS). The cervical enlarge­ ment (CS through T l ) gives rise to the rootlets that form the brachia! plexus, which innervates the upper limbs.

There are 12 thoracic pairs of spinal nerves (Tl through T l2). Spinal nerves emanating from thoracic levels innervate most of the trunk.

There are 5 lumbarpairs of spinal nerves (Ll through LS). The lumbar enlarge­ ment (Ll through S2) gives rise to rootlets that form the lumbar and sacral plex­ uses, which innervate the lower limbs.

There are 5 sacral pairs of spinal nerves (Sl through SS). Spinal nerves at the sacral level innervate part of the lower limbs and the pelvis.

There is one coccygeal pair of spinal nerves. The cauda equina consists of the dorsal and ventral roots ofthe lumbar, sacral, and coccygeal spinal nerves.

Inside the spinal cord, gray matter is centrally located and shaped like a butterfly. It contains neuronal cell bodies, their dendrites, and the proximal parts of ax­ ons. White matter surrounds the gray matter on all sides. White matter contains bundles of functionally similar axons called tracts or fasciculi, which ascend or descend in the spinal cord (Figures IV-4- 1 and IV-4-2).

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