CHAPTER 8 Central Nervous System ANATOMY AND PHYSIOLOGY • The central nervous system (CNS) includes the brain and the spinal cord. • The CNS is composed of neurons (nerve cells) and neuroglia (the interstitial tissue) and extends peripherally through nerves that carry motor messages through efferent nerves to muscles and sensory messages from skin. • The brain consists of the • The brainstem, composed of 1.cerebrum (right and left the midbrain, pons, and hemispheres), 2.cerebellum, medulla oblongata, 3.diencephalon (including connects the cerebrum with the hypothalamus and the spinal cord. thalamus), and 4.brainstem. • The spinal cord originates as an extension of the medulla oblongata at the foramen magnum in the base of the skull. • It extends to approximately the level of the first or second lumbar vertebra and terminates with a coneshaped area called the conus medullaris • Spinal nerves beyond this point are referred to as the cauda equina •Both the brain and the spinal cord are covered by the meninges, which consist of three distinct layer 1.Dura mater has three major extensions: • (a) the falx cerebri, which divides the cerebral hemispheres. •(b) the falx cerebelli, which similarly divides the cerebellar hemispheres. •(c) the tentorium cerebelli, which separates the occipital lobe of the cerebrum from the cerebellum. 2. The arachnoid is the middle layer of the meninges and has the appearance of cobwebs. 3. The pia mater is innermost and adheres directly to the cortex of the brain and the spinal cord. •The subarachnoid space, at its deepest at the base of the brain, is located between the arachnoid and the pia mater. •It is filled with cerebrospinal fluid (CSF) to continuously bathe the brain and the spinal cord with nutrients and to cushion them against shocks and blows. •They may be further divided into anterior, posterior, and inferior horns, as well as a body and a trigone. •The third and fourth ventricles are midline structures connected to each other by the cerebral aqueduct (see Fig. 8-3, C). • After entering the cranial vault through the foramen magnum, the vertebral arteries converge to form the basilar artery. • The basilar artery and the internal carotid arteries form the circle of Willis (Fig. 8-4) to distribute oxygenated, arterial blood through various branches to all parts of the brain. • In the brain, they prevent passage of unwanted substances into the brain through a special function called the blood–brain barrier. • Neurons are the primary tissue comprising the nervous system, and they may vary greatly in size. • The three basic components of a neuron are the cell body, which is located within the CNS; dendrites, which carry nerve impulses; and axons, responsible for carrying impulses away from the cell body. • Most neurons have only one axon, which is covered by a delicate web of connective tissue of Schwann cells covered by a myelin sheath. • Myelin is a lipid substance that acts as an insulator and assists in nerve impulse transmission. • In addition to Schwann cells, neuroglias include astrocytes, oligodendrocytes, ependymal cells, and microglia (Table 8-1). CONGENITAL AND HEREDITARY DISEASE Meningomyelocele (Spina Bifida) •Is an incomplete closure of the vertebral canal that is particularly common in lumbarsacral area (Fig. 2-17). •Often, such patients have no visible abnormality or neurologic deficit, but failure of fusion of the two laminae is visible radiographically (spina bifida occulta). •Treatment of spina bifida is determined on the basis of the extent of the anomaly and requires the services of a variety of physicians. • spina bifida is a condition in which the bony neural arch that encloses and protects the spinal cord is not completely closed (Fig. 8-24). • Elevated α-fetoprotein (AFP) levels in the mother’s blood and on amniocentesis may allow spina bifida to be diagnosed prenatally. • The defect and soft tissue sac are confirmed with fetal sonography. • If only the meninges protrude, the condition is termed a meningocele (Fig. 8-25). •Myelocele is a protrusion of the spinal cord, minus its meningeal coverings, which may also be treatable surgically. •Meningomyelocele is the most common and most serious of possible conditions, affecting approximately one in every 800 infants and consisting of a protrusion of both the meninges and the spinal cord into the skin of the back (Figs. 8-26 and 8-27). • Hydrocephalus may affect as many as 90% of children diagnosed with meningomyelocele. Hydrocephalus • Hydrocephalus refers to an excessive accumulation of CSF within the ventricles and can be either congenital or acquired. • In noncommunicating hydrocephalus, an obstruction may occur congenitally or result from tumor growth, trauma (hemorrhage), or inflammation • Hydrocephalus may also occur from overproduction of CSF, although this is the least common cause. • Treatment may consist of surgery. In some cases, a shunt is surgically inserted to divert excess fluids. INFLAMMATORY AND INFECTIOUS DISEASE Meningitis •An inflammation of the meningeal coverings of the brain and spinal cord is termed meningitis. •It may be caused by bacteria, viruses, or other organisms that reach the meninges from elsewhere in the body via blood or lymph or may occur as a result of trauma and penetrating wounds •Bacterial infection is the most common cause of meningitis (Fig. 8-30). • Pathogens responsible for acute bacterial meningitis include meningococci, streptococci, and pneumococci. • Meningococcal meningitis is most common in infants, streptococcal meningitis in children, and pneumococcal meningitis in the adult population. • In addition, a few cases result from infection by the tubercle bacillus Mycobacterium tuberculosis • This type of bacterial meningitis is more difficult to diagnose because it does not have the same acute symptoms as those of the other types of bacterial meningitis. Acute bacterial meningitis may follow an upper respiratory infection or sore throat. Symptoms include •fever, •headache, •stiff neck, • vomiting, •changes in consciousness, with the patient becoming severely ill within 24 hours. •Treatment depends on the infecting agent, with amphotericin B being the preferred drug for all fungal infections. •Although the disease may progress at a slower rate, the outcome may still be fatal. Chronic meningitis is most commonly caused by fungi and is often seen in patients with acquired immunodeficiency syndrome (AIDS) or in individuals undergoing immunosuppressant drug therapy. • The symptoms are similar to those associated with acute meningitis; however, unlike in acute meningitis, the symptoms of chronic meningitis occur over weeks rather than days. Encephalitis • An infection of the brain tissue is termed encephalitis. In contrast to meningitis, which is most frequently a bacterial infection, encephalitis isusually viral in nature (Fig. 8-31) • Primary viral encephalitis may be caused by the arbovirus transmitted by mosquitoes during warm weather or by herpes simplex virus. • The symptoms and signs most commonly associated with encephalitis are headache, malaise, and coma. Brain Abscess • A brain abscess is an encapsulated accumulation of pus within the cranium resulting from a cranial infection, a penetrating head wound, or an infection spread through the bloodstream. • A brain abscess may also result from the direct spread of organisms associated with a complicated case of sinusitis, chronic otitis, or mastoiditis. •Brain abscesses are fatal unless • The symptoms are similar to they are treated with antibiotic those of encephalitis and include fever, headache, nausea therapy. •The specific antibiotic agent and vomiting, seizures, and depends on the infecting organism, personality changes. and treatment lasts approximately 4 to 8 weeks (Fig. 8-32).
