26.1 Anatomy of the Nervous System

The nervous system is one of the most well-protected areas of the body, shielded by bone, membranes, and the blood-brain barrier. Understanding this anatomy is critical in microbiology because it explains why nervous system infections are relatively rare but devastatingly serious when they do occur.

Nervous System Anatomy and Infections

Key structures of the nervous system

The nervous system has two major divisions: the central nervous system (CNS) and the peripheral nervous system (PNS).

The CNS consists of the brain and spinal cord:

• The brain includes the cerebrum (higher cognitive functions like thought and memory), the cerebellum (coordination of movement), and the brainstem (regulation of vital functions like breathing and heart rate)
• The spinal cord transmits signals between the brain and the rest of the body and also controls simple reflexes independently

The PNS comprises all the nerves outside the CNS:

• Cranial nerves (12 pairs) emerge directly from the brain and innervate the head and neck, including facial muscles and sensory organs
• Spinal nerves (31 pairs) originate from the spinal cord and innervate the trunk and limbs

Three layers of protective membranes called the meninges cover the brain and spinal cord. From outermost to innermost:

• Dura mater: the tough, outermost layer
• Arachnoid mater: the middle layer with a web-like appearance. Between the arachnoid and pia mater lies the subarachnoid space, which is filled with cerebrospinal fluid (CSF)
• Pia mater: the delicate, innermost layer that adheres directly to the surface of the brain and spinal cord

The blood-brain barrier (BBB) is a selective barrier that tightly regulates what can pass from the bloodstream into the CNS. It's formed by tight junctions between endothelial cells lining the brain's blood vessels, with additional support from astrocyte foot processes and a basement membrane. This barrier is the main reason most bloodborne pathogens can't reach the brain.

Cellular components and communication

Neurons are the primary functional units of the nervous system. Each neuron has a cell body, dendrites (which receive signals), and an axon (which transmits signals away from the cell body). Neurons communicate at junctions called synapses, where chemical messengers called neurotransmitters carry signals from one neuron to the next.

Glial cells support and protect neurons. In the CNS, these include:

• Astrocytes: maintain homeostasis and contribute to the blood-brain barrier
• Oligodendrocytes: produce myelin, which insulates axons and speeds up signal transmission
• Microglia: the resident immune cells of the CNS, responsible for detecting and responding to pathogens or damage

Microglia are especially relevant in microbiology because they are the CNS's first line of immune defense when a pathogen breaches the barriers.

Nervous system organization and adaptation

• The autonomic nervous system regulates involuntary functions and is divided into the sympathetic ("fight or flight") and parasympathetic ("rest and digest") branches
• Neuroplasticity refers to the brain's ability to reorganize and form new neural connections, which allows for learning, adaptation, and some degree of recovery from injuries

Absence of normal microbiota

Unlike the skin or gut, the CNS is normally sterile. Several overlapping defenses maintain this:

• Physical barriers: The skull and vertebral column encase the brain and spinal cord in bone. The meninges add another protective layer that limits microbial access.
• The blood-brain barrier: Tight junctions between endothelial cells prevent most microorganisms and toxins from crossing into the CNS from the bloodstream.
• Chemical defenses in the CSF: Cerebrospinal fluid circulates through the subarachnoid space and ventricles, and it contains antibodies and some immune cells that can combat invading pathogens.
• Nutrient-poor environment: Compared to other body sites, the CNS has limited readily available nutrients, which restricts microbial growth even if a pathogen does gain entry.

Pathogen entry into the nervous system

Despite all these defenses, pathogens can reach the CNS through four main routes:

1. Penetrating injuries: Skull fractures, open wounds, or surgical procedures can directly introduce microorganisms past the physical barriers.
2. Spread from adjacent infections: Infections in nearby tissues can extend into the CNS. For example, sinusitis or otitis media (middle ear infection) can spread to the meninges and cause meningitis. Untreated dental abscesses can also reach the brain.
3. Hematogenous spread (via the bloodstream): Certain pathogens can cross the blood-brain barrier. Streptococcus pneumoniae and Neisseria meningitidis are bacterial causes of meningitis that use this route. The fungus Cryptococcus neoformans can also cross the BBB, particularly in immunocompromised patients (such as those with HIV/AIDS).
4. Retrograde axonal transport: Some viruses hijack the nerve fibers themselves, traveling along axons from the periphery to the CNS. The classic example is the rabies virus, which enters peripheral nerves at the bite site and travels to the brain, causing fatal encephalitis.

Symptoms of nervous system infections

The specific symptoms depend on which structures are affected:

Meningitis (inflammation of the meninges):

• Severe headache from meningeal irritation and increased intracranial pressure
• Neck stiffness (nuchal rigidity), where the patient cannot flex the neck forward
• Photophobia (sensitivity to light) due to meningeal irritation
• Fever as a systemic response to infection

Encephalitis (inflammation of the brain tissue itself):

• Headache from increased intracranial pressure
• High fever
• Altered mental status (confusion, disorientation) caused by brain dysfunction
• Seizures from abnormal electrical activity in the inflamed brain

Brain or spinal cord abscess (a localized collection of pus):

• Localized neurological deficits (weakness, sensory loss) that depend on where the abscess is located
• Fever and headache from the immune response and increased intracranial pressure

Neuritis or neuralgia (inflammation or dysfunction of peripheral nerves):

• Pain, tingling, or numbness in the area supplied by the affected nerve (for example, the facial nerve or sciatic nerve)

Paralysis or weakness can result from damage to motor nerves or CNS structures:

• Spinal cord damage can cause paralysis below the level of injury
• Brain damage (from stroke or trauma) can cause weakness or paralysis on one side of the body (hemiparesis)