Nervous System Infections

Anatomy of the Nervous System
Central nervous system (CNS) Blood-brain barrier Peripheral nervous system (PNS) Cells of the nervous system Meningitis and encephalitis
 * Aponeurosis: flat, broad tendon layer that anchors the superficial layers of the skin
 * Periosteum: below aponeurosis; firmly encases the bones of the skull and provides protection, nutrition, and capacity for bone repair
 * Meninges: layers of membranes under the skull
 * Dura mater: layer closest to the bones of the skull
 * Arachnoid mater
 * Pia mater: delicate layer that firmly adheres to the convoluted surface of the brain
 * Subarachnoid space: space between arachnoid and pia mater filled with CSF
 * Cerebrospinal fluid (CSF): watery fluid produced by cells of the choroid plexus - areas in each ventricle of the brain that consist of cuboidal epithelial cells surrounding dense capillary beds
 * CSF serves to deliver nutrients and remove waste from neural tissues
 * blood vessels in the brain are less permeable than others
 * no normal microbiota in CSF
 * inhibits movement of drugs into the brain
 * pathogens must cross this barrier to affect the CNS by different virulence factors and mechanisms
 * Intercellular (paracellular): involves the use of microbial virulence factors, toxins, or inflammation-mediate processes t
 * o pass between the cells of the blood-brain barrier
 * Transcellular: pathogen passes through the cells of the blood-brain barrier using virulence factors that allow it to adhere to and trigger uptake by the vacuole- or receptor-mediated mechanisms
 * Leukocyte facilitated: Trojan-horse mechanism that occurs when a pathogen infects peripheral blood leukocytes to directly enter the CNS
 * Nonhematogenous: allows pathogens to enter the brain without encountering the blood-brain barrier; it occurs when pathogens travel along either the olfactory or trigeminal cranial nerves that lead directly into the CNS
 * not protected by bone, meninges, or a blood barrier, making it more susceptible to injury and infection
 * Neuropathy: tingling or numbness
 * microbial damage to peripheral nerves can lead to neuropathy
 * can also be caused by trauma or noninfectious causes (i.e., drugs, chronic diseases)
 * Glial (neuroglial) cells: assist in the organization of neurons, provide a scaffold for some aspects of neuronal function, and aid in recovery from neural injury
 * Neurons (nerve cells): transmit signals through the nervous system via electrochemical processes
 * Cell body (soma): metabolic center of the neuron, contains the nucleus and most organelles
 * Dendrite: branched extensions of the soma
 * Axon: transmits electrochemical signals through elaborate ion transport processes
 * Myelin sheath: surrounds and insulates the axon to increase the speed of electrochemical signal transmission
 * formed from the cell membranes of glial cells
 * Synaptic terminal: bulbed ends of the axon
 * Synapse: junctions where electrochemicals are exchanged
 * brain/meninges swelling can lead to intracranial pressure, which can cause severe damage
 * Meningitis: inflammation of the meninges
 * symptoms include severe headache, fever, photophobia, stiff neck, convulsions, and confusion
 * Encephalitis: inflammation of brain tissue
 * similar symptoms to meningitis in addition to lethargy, seizures, and personality changes
 * Meningoencephalitis: when inflammation affects both the meninges and the brain tissue
 * all 3 forms can lead to blindness, deafness, coma, and death
 * can be caused by microbial pathogens, head trauma, some cancers, and certain drugs that cause inflammation
 * lumbar puncture performed to obtain CSF sample and test for increased leukocytes and abnormal glucose and protein levels which indicate inflammation is a response to infection

Bacterial Diseases of the Nervous System
Bacterial meningitis: one of the most serious forms of meningitis
 * often gain access to the CNS through bloodstream after trauma or as a result of the action of bacterial toxins
 * bacteria can spread from upper respiratory tract
 * patients with head wounds or cochlear implant are at risk of meningitis
 * meningitis-causing bacteria often found in healthy people
 * Neisseria meningitidis, Streptococcus pneumoniae, and Haemophilus influenzae
 * spread by respiratory secretions
 * without systemic antibacterial therapy, fatality rate can be as high as 70%
 * 20% of survivors may be left with irreversible nerve damage or tissue destruction, resulting in hearing loss, neurologic disability, or loss of a limb