Chapter 7

• Two component parts of the nervous system


  • Central Nervous System (CNS)
  • Peripheral Nervous System (PNS)
• Central Nervous System (CNS)


  Consisting of the brain and spinal cord
• Peripheral Nervous System (PNS)


  Contains the nerves that emanate from the brain and spinal cord to sense organs and to the periphery of the body
• Three important functions of the nervous system


  • Sensory function
  • Integrative function
  • Motor Function
• Sensory function


  Fulfilled by sensory receptors at the ends of peripheral nerves. They generate nerve impulses that are transmitted to the central nervous system.
• Integrative function


  The impulses are translated, or integrated, into sensation or thought, which in turn drives the motor function.
• Motor Function


  Necessarily involves structures outside of the nervous system, such as muscles and glands.
• Brain and Spinal Cord


  • Dense structures made up of cells called neurons
  • Both surrounded by bone
  • Brain is situated inside the skull
  • Spinal cord lies within the spinal column which is composed of a stack of interconnected bones called vertebrae
• Meninges


  The soft tissue of the brain and spinal cord is encased within a tough casing of three membranes
• Layers of the three meninges membranes, from outermost to innermost position

  • dura mater
  • arachnoid mater
  • pia mater
• Subarachnoid space


  The space under the arachnoid mater, filled with cerebrospinal fluid (CSF)
• Cerebrospinal fluid (CSF)


  Provides nutrition to the CNS, while also providing a liquid cushion for the sensitive brain and spinal cord
• Meningitis


  Infection of the meninges, where microorganisms can often be found in the CSF
• Components of the PNS


  • Nerves
  • Ganglia
• Ganglion


  A swelling in the nerve where the cell bodies of the neurons aggregate
• Nerves


  Bundles of neuronal axons that receive and transmit nerve signals
• Axons and Dendrites


  From adjacent neurons communicate with each other over a very small space, called a synapse
• Neurotransmitters


  Chemicals that are released from one cell and act on the next cell in the synapse
• Defenses of the nervous system: structural

  • The bony casings of the brain and spinal cord protect them from traumatic injury
  • The cushion of surrounding CSF also serves a protective function
  • The blood-brain barrier prohibits most microorganisms from passing into the central nervous system
• Blood-brain barrier


  The restricted permeability of blood vessels in the brain, which prohibits most microorganisms from passing into the central nervous system
• Immunologically privileged site


  The CNS is able to mount only a partial, or at least a different, immune response when exposed to immunologic challenge
• Specialized cells in the central nervous system


  • Microglia have phagocytic capabilities
  • Brain macrophages also exist in the CNS, although their activity is thought to be less than that of phagocytic cells elsewhere in the body
• There is no normal biota in either the CNS or PNS, and that finding microorganisms of any type in these tissues represents a deviation from the healthy state
• Viruses such as herpes simplex live in a dormant state in the nervous system between episodes of acute disease, but they are not considered normal biota
• Meningitis


  An inflammation of the meninges
• Meningitis


  • Excellent example of anatomical syndrome
  • Can be caused by different microorganisms and they produce a similar constellation of symptoms
  • The more serious forms of acute meningitis are caused by bacteria, but their entrance to the CNS is often facilitated by coinfection or previous infection with respiratory viruses
  • Meningitis in neonates is most often caused by different microorganisms than those causing the disease in children and adults
• Typical symptoms of meningitis


  • Severe headache
  • Painful or stiff neck
  • Fever
  • Nausea and vomiting
  • Photophobia (sensitivity to light)
  • Skin rashes may be present in specific types of meningitis
  • Increased number of white blood cells in the CSF
• Specific microorganisms may cause additional, and sometimes characteristic, symptoms of meningitis
• Acute and chronic meningitis


  Some microorganisms are more likely to cause acute meningitis, and others are more likely to cause chronic disease
• In a normal healthy person, it is very difficult for microorganisms to gain access to the nervous system. Those that are successful usually have specific virulence factors.
• Neisseria meningitidis


  Gram-negative diplococci (round cells occurring in joined pairs), commonly known as the meningococcus, often associated with epidemic forms of meningitis, causes the most serious form of acute meningitis and accounts for 15% to 20% of all meningitis cases
• Neisseria meningitidis infection


  1. Bacterium enters the body via upper respiratory tract

  2. Bacterium will then move into the blood

  3. Afterwards, it will rapidly penetrate the meninges

  4. It will produce symptoms of meningitis
• Meningococcemia


  • The most serious complications of meningococcal infection, due to the pathogen releasing endotoxin into the generalized circulation, which is a potent stimulus for certain white blood cells, leading to vascular collapse, hemorrhage, and crops of lesions called petechiae on the trunk and appendages
  • The bacterium has an IgA protease and a capsule, both of which counter the body's defenses
  • In a small number of cases, meningococcemia becomes an overwhelming disease with a high mortality rate
• Symptoms of severe meningococcemia


  • Fever higher than 40°C or 104°F
  • Sore throat
  • Chills
  • Delirium
  • Severe widespread areas of bleeding under the skin
  • Shock
  • Coma
  • Generalized intravascular clotting
  • Cardiac failure
  • Damage to the adrenal glands
  • Death can occur within a few hours
• Transmission of meningococcal meningitis


  • Meningococci do not survive long in the environment, so they are usually acquired through close contact with secretions or droplets
  • Meningococcal meningitis has a sporadic or epidemic incidence in late winter or early spring
  • The continuing reservoir of infection is humans who harbor the pathogen in the nasopharynx
  • The carriage state, which can last from a few days to several months, exists in 3% to 30% of the adult population and can exceed 50% in institutional settings
  • The highest risk groups are young children (6 to 36 months old) and older children and young adults (10 to 20 years old)
• Diagnosis of meningococcal meningitis


  1. Treatment is usually begun with this bacterium in mind until it can be ruled out

  2. Cerebrospinal fluid, blood, or nasopharyngeal samples are stained and observed directly for the typical gram-negative diplococci

  3. Cultivation may be necessary to differentiate the bacterium from other species

  4. Specific rapid tests are also available for detecting the capsular polysaccharide or the cells directly from specimens without culturing

  5. It is usually necessary to differentiate this species from normal Neisseria that also live in the human body and can be present in infectious fluids

  6. Specimens are streaked on Modified Thayer-Martin medium (MTM) or chocolate agar and incubated in a high CO2 atmosphere. Presumptive identification of the genus is obtained by a Gram stain and oxidase testing on isolated colonies.
• Natural immunization


  A sort of natural immunization occurs during the early years of life as one is exposed to the meningococcus and its close relatives
• Treatment of meningococcal meningitis


  1. Chemotherapy begins as soon as possible with one or more drugs, generally given in high doses intravenously

  2. Patients may also require treatment for shock and intravascular clotting

  3. Preventive therapy with rifampin or tetracycline may be warranted for close contacts
• Meningococcal vaccination


  Immunization begins at the age of 11 followed by a booster dose, and vaccines are also available for younger children and for adults over the age of 55 who are at high risk for infection
• Streptococcus pneumoniae


  Also referred to as the pneumococcus, causes the majority of bacterial pneumonias and meningitis, and is the most frequent cause of community-acquired meningitis