Nervous System Infections

## Anatomy of the Nervous System

**Central nervous system (CNS)**

* **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
	1. **Dura mater:** layer closest to the bones of the skull
	2. **Arachnoid mater**
	3. **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-brain barrier**

* 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
	1. **Intercellular (paracellular):** involves the use of microbial virulence factors, toxins, or inflammation-mediate processes t
	2. o pass between the cells of the blood-brain barrier
	3. **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
	4. **Leukocyte facilitated:** Trojan-horse mechanism that occurs when a pathogen infects peripheral blood leukocytes to directly enter the CNS
	5. **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

**Peripheral nervous system (PNS)**

* 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)

**Cells of the nervous system**

* **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

**Meningitis and encephalitis**

* 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