Chapter 26 The Nervous System
Structure and Function of the Brain and Nervous System
THE MENINGES
Dura: fibrous outer covering.
Pia: inner membrane adherent to brain and cord.
Arachnoid: middle membrane interposed between other two. Contains cerebrospinal
fluid and blood vessels.
THE BRAIN
Divided into cerebrum, cerebellum, and brain stem.
Surrounded by cerebrospinal fluid secreted by choroid plexus.
Ventricles: hollow interior cavities.
Blood supply from arterial circle (circle of Willis) at base of brain.
Venous blood returns into venous sinuses in dura.
VOLUNTARY MUSCLES
Muscle tone caused by reflex arcs.
Voluntary motor activity controlled by cortical neurons.
Development of the Nervous System
EARLY DEVELOPMENT
Neural plate becomes tube.
Forebrain forms cerebral hemispheres and diencephalon.
Midbrain and hindbrain form remainder of adult brain.
Mesoderm surrounding neural tube forms cranial cavity, vertebral bodies, and
surrounding structures.
Muscle Paralysis
FLACCID PARALYSIS
Destruction of motor neurons by disease.
Peripheral-nerve destruction.
SPASTIC PARALYSIS
Injury to cortical neurons stops voluntary control.
Reflex arc unaffected.
Tone increased and atrophy does not develop.
Closure Defects
ANENCEPHALY
Failure of normal development of brain and cranial cavity.
Multifactorial inheritance.
SPINA BIFIDA
Several types: differ in severity.
Most severe type associated with meningomyelocele.
PRENATAL DETERMINATION OF NEURAL
TUBE DEFECT
Alpha fetoprotein leaks from fetal blood into amnionic fluid through open neural tube
defect; high levels found in amnionic fluid.
Perform amniocentesis and measure alpha fetoprotein.
Hydrocephalus
FORMATION AND ABSORPTION OF CEREBROSPINAL FLUID
Secreted by choroid plexuses.
Flows through ventricles and exits through openings in roof of fourth ventricle.
Circulates around brain and spinal cord.
Absorbed into venous sinuses in dura.
CONGENITAL HYDROCEPHALUS
Caused by congenital obstruction of aqueduct or absence of openings in roof of fourth
ventricle.
Head enlarges as ventricles dilate because cranial structures have not fused.
ACQUIRED HYDROCEPHALUS
Obstruction of cerebrospinal fluid by tumor or adhesions blocking opening in fourth
ventricle.
Ventricles dilate but head does not enlarge because cranial structures are fused.
TREATMENT OF HYDROCEPHALUS
Shunt cerebrospinal fluid into venous system by tube extending from ventricles into
jugular vein or peritoneal cavity.
Pressure in ventricles falls and enlargement of ventricles is arrested.
Stroke
ENCEPHALOMALACIA
Thrombus or embolus in vessel leads to breakdown of brain tissue.
Cystic cavity forms.
May be caused by atherosclerosis of internal carotid artery in neck.
Thrombi form on ulcerated plaque.
Atheromatous debris or thrombi from plaque form emboli carried to brain.
Rarely, artery is completely occluded in neck.
Condition of artery in neck can be determined by x-ray studies using radiopaque
material.
Treated by carotid endarterectomy.
Stroke caused by a paradoxical embolus.
Uncommon complication in persons with patent foramen ovale.
High right atrial pressure may allow embolus from leg vein thrombus to traverse patent
foramen ovale into left atrium.
Systemic circulation carries embolus to brain and obstructs cerebral artery.
HEMORRHAGE
Ruptured artery in brain discharges blood under high pressure.
Hypertension predisposes.
Can be distinguished from encephalomalacia by CT scan.
MANIFESTATIONS OF STROKE
Depend on location and size.
Paralysis or sensory loss or both on opposite side of body.
REHABILITATION OF THE STROKE PATIENT
A program of graduated exercises and relearning.
Most patients can learn to walk; useful function of upper limbs less likely.
Transient Ischemic Attack (TIA)
MANIFESTATIONS AND TREATMENT
Brief episodes of neurologic dysfunction.
Dysfunction usually a result of embolization of material from plaque in carotid artery in
neck.
Treated by endarterectomy or medical therapy.
Vascular Dementia
MULTIPLE SMALL INFARCTS CAUSE PROGRESSIVE BRAIN DAMAGE WITH
MANIFESTATIONS SIMILAR TO ALZHEIMER DISEASE.
PERSONS WITH HYPERTENSION, DIABETES, ABNORMAL BLOOD LIPIDS AT
RISK.
PREVENTION AND SLOWING PROGRESSION REQUIRES MANAGEMENT OF
RISK FACTORS.
Cerebral Aneurysm
CONGENITAL ANEURYSM OF CIRCLE OF WILLIS
Congenital weakness in arterial wall allows lining (intima) to protrude.
Weakness is congenital but aneurysm develops in adult life.
Hypertension predisposes.
Rupture causes subarachnoid hemorrhage.
Treated by occluding aneurysm surgically.
ARTERIOSCLEROTIC ANEURYSM
Arteriosclerotic cerebral artery dilates and compresses adjacent brain.
Rupture very uncommon.
Infections of the Nervous System
MANIFESTATIONS
Systemic infection: fever, nonspecific symptoms.
Meningeal irritation: headache, stiff neck.
Infection of brain: alteration of consciousness, focal neurologic symptoms.
Abnormalities of spinal fluid: neutrophil increase in bacterial infection. Lymphocyte
increase in virus infection.
BACTERIAL AND FUNGAL MENINGITIS
Usually caused by pneumococcus, meningococcus.
Bacteria carried to meninges in bloodstream. Less commonly spread from sinuses or
middle ear or introduced after cerebral injury.
Tuberculous and fungus infections less common.
Usually spread from lung.
VIRAL INFECTIONS
Meningitis: usually mild illness with complete recovery.
Encephalitis: more serious disease with occasional deaths and late complications.
Many cases caused by arboviruses that infect animals and are transferred to
humans by mosquitoes.
Herpes virus may cause severe destructive inflammation.
No specific treatment for most cases. Antiviral chemotherapy may be useful in
some cases of herpes infection.
Poliomyelitis:
Affects gray matter of spinal cord; damages motor neurons, causing paralysis.
Largely eradicated in developed countries by widespread immunization.
Postpolio syndrome:
Late onset of muscle weakness and atrophy many years after recovery from
poliomyelitis.
Results from degeneration of “overworked” neurons that reinnervated muscles
affected by poliomyelitis.
Creutzfeldt-Jakob Disease
Caused by small protein particle (prion) produced as a result of gene mutation.
Causes dementia and neurologic dysfunction.
Most cases sporadic, but transmissible by contaminated biologic products or infected
transplanted cornea or organs.
Invariably fatal. No treatment available.
MAD COW DISEASE
Prion disease affecting cows.
Cows became infected from animal feed mixed with protein-rich tissue obtained from
sheep infected with a prion disease called scrapie.
Cases of atypical (variant) Creutzfeldt-Jakob disease related to eating meat from
infected cows.
Alzheimer Disease
CHARACTERISTICS
Affects chiefly middle-aged and elderly.
Progressive mental deterioration and emotional disturbances.
Relentless progression. No treatment available.
ANATOMIC AND BIOCHEMICAL FEATURES
Reflect unknown disturbance in brain metabolism.
Thickening of neuron neurofilaments forming neurofibrillary tangles.
Clusters of thick, broken neurofilaments form neuritic plaques.
Brain enzyme deficiencies.
Multiple Sclerosis
PATHOGENESIS AND MANIFESTATIONS
Probably autoimmune disease in genetically predisposed individual.
Random foci of demyelination followed by glial scarring.
Neurologic symptoms depend on location of plaques.
MRI useful diagnostic test to demonstrate plaques in CNS.
Parkinson Disease
MANIFESTATIONS AND PATHOGENESIS
Rigidity of muscles and tremor.
As a result of decreased concentration of dopamine in central nervous system.
Symptoms relieved by L-dopa, which is converted into dopamine in brain.
Huntington Disease
MANIFESTATIONS AND PATHOGENESIS
Progressive hereditary autosomal dominant disease.
Abnormal gene contains too many CAG triplet repeats.
The greater the number of repeats, the earlier the onset of the disease.
Characterized by brain degeneration and involuntary movements.
Degenerative Diseases of Motor Neurons
Affect both upper and lower motor neurons.
Cause weakness, paralysis, respiratory problems.
Some cases are familial.
No specific treatment.
Tumors of Nervous System
PERIPHERAL NERVE TUMORS
Usually solitary: arise from Schwann cells.
Neuromas of cranial nerves often involve acoustic nerve and are difficult to remove
surgically.
Multiple nerve tumors occur in multiple neurofibromatosis.
Transmitted as Mendelian dominant trait.
Disfiguring skin nodules, thickened patches of skin, and focal hyperpigmentation
of skin.
Sarcoma arises from preexisting tumors in 10 to 15 percent of cases.
BRAIN TUMORS
Metastatic tumors common.
Meningioma: from arachnoid cells. Good prognosis.
Gliomas: including ependymoma and medulloblastoma. Many carry poor prognosis
because of deep location in brain. Treatment by surgery, radiation, and
chemotherapy.
SPINAL CORD TUMORS
Same types of tumors that arise in brain.
Tumors involving vertebral bodies may invade or compress cord.
Peripheral Nerve Disorders
PERIPHERAL NERVE INJURY
Traumatic injury: associated with lacerations, fractures, crush injury.
Nerve entrapment neuropathy.
External compression by fibrous band.
Median nerve commonly involved.
May require surgical release if no response to conservative treatment.
POLYNEURITIS (PERIPHERAL NEURITIS)
Sensory and motor dysfunction in “glove and stocking” distribution. Proximal sensation
and motor function preserved.
As a result of systemic disease, toxins, alcoholism.
Treatment of underlying disease may lead to improvement.
GUILLAIN BARRÉ SYNDROME (IDIOPATHIC POLYNEURITIS)
Patchy demyelination of nerves and nerve roots with mild inflammation and sometimes
axon degeneration.
An autoimmune reaction to myelin triggered by preceding viral infection.
Progressive weakness usually followed by complete recovery.
No specific treatment.
Neurologic Manifestations of Human Immunodeficiency Virus Infection
AS A RESULT OF HIV INFECTION OF NERVOUS SYSTEM
Acute viral meningitis.
AIDS encephalopathy—chronic and progressive.
Polyneuritis.
CAUSED BY OPPORTUNISTIC INFECTIONS OF NERVOUS SYSTEM
Many pathogenic organisms can infect nervous system directly.
Manifestations depend on location of infection and extent of damage to nervous system.
Herpes, cytomegalovirus, Cryptococcus neoformans, and Toxoplasma gondii are
commonly implicated organisms.
AIDS-RELATED TUMORS
Kaposi sarcoma, lymphoma, or other malignant tumors may metastasize to nervous
system.
Primary lymphoma of brain may occur.
Tumors respond poorly to treatment.