nervous

The central nervous system (CNS), which is composed of the

brain and spinal cord, functions as the master control center

of the body. The brain has several main parts, including the

following:

• The cerebrum—the largest, upper part of the brain—controls voluntary muscles, perception, and what people commonly call “thinking.”

• The lower cerebellum controls many involuntary body

movements, such as swinging the arms while walking.

• The brain stem, which connects the brain to the spinal cord,

has a number of functions, including control of breathing,

heart rate, and blood pressure.

The spinal cord extends down from the brain stem only as far

as the lumbar region (lower back). Below this region, a bundle

of nerves called the cauda equina1 extends from the spinal cord.

As its name suggests, this bundle resembles the tail of a horse.

Bones of the cranium, bones of the vertebral column, and

three layers of tissue called meninges2 (mě-nin’jēz) surround the

brain and spinal cord, providing support and protection from

external shock. The three meninges vary in their structure and

appearance (FIGURE 20.1b, c).

Lying next to the bones is the dura mater3 (dŭ’ră mā’ter), a

tough fibrous sheath that provides a strong, yet flexible, covering for the soft organs of the CNS. It also provides a barrier

against the spread of infections from the bones. Deep to the

dura mater is the arachnoid mater4 (ă-rak’noyd mā’ter), which

contains numerous branching fibers giving the appearance of

a spider’s web. The cavities between the fibers of the arachnoid

mater are collectively called the subarachnoid space. The internal layer, which is pressed close to the spinal cord and brain, is

the pia mater5 (pī’ă mā’ter). Blood vessels on top of the pia mater

supply the CNS with blood. The walls of these blood vessels are

composed of tightly joined cells that form the blood-brain barrier, which prevents most microbes and large molecules in the blood

from entering the subarachnoid space. Thus, blood infections

do not easily spread to the CNS, but unfortunately, neither do

many common antimicrobial drugs, such as penicillin, cephalosporins, tetracyclines, and aminoglycosides, making it more

difficult to treat infections of the CNS.

Fluid leaks from the blood into the subarachnoid space

that lines the brain. This watery fluid—called cerebrospinal fluid

(CSF)—circulates throughout the subarachnoid space of both

brain and spinal cord to bathe both organs. Arachnoid villi or

arachnoid granulations, which are knoblike extensions of the

arachnoid mater, extend into a blood-filled cavity at the top of

the cranium and return CSF to the blood.

Cerebrospinal fluid acts as a shock absorber; provides

nutrients, electrolytes, and oxygen to the nervous tissues;

and removes wastes. In a medical procedure called a lumbar

puncture (spinal tap), physicians remove a sample of CSF from

the region of the subarachnoid space surrounding the cauda

equina. They insert a needle through the skin

between two of the lumbar vertebrae, through the dura mater

and subdural space, and through the arachnoid mater to reach

the CSF in the subarachnoid space.

The peripheral nervous system (PNS) is composed of nerves

that transfer commands from the CNS to muscles and glands

throughout the body and provide information to the CNS concerning events in the body. Cranial nerves extend from the brain

through holes in the cranial bones, and spinal nerves extend

from the spinal cord through gaps between vertebrae. Branches

of nerves often merge together to form a nerve plexus (see

Figure 20.1a).

Functionally, there are three types of nerves: Sensory nerves

primarily carry signals toward the CNS. The optic nerves from

the eyes are examples. Motor nerves carry signals from the CNS

to other organs of the body, and mixed nerves carry signals both

toward and away from the CNS.

Cells of the Nervous System

The entire nervous system is composed of two basic types of

cells—neurons and supportive cells called neuroglia. The smaller neuroglia provide a supportive scaffolding, insulation, and nutritive support and phagocytize

microbes. The cytoplasmic membrane of a neuron generates an

electrical signal called an action potential or nerve impulse.

The nucleus of a neuron lies in a region of cytoplasm called

the cell body. Outside the CNS, a collection of many neurons’ cell

bodies is called a ganglion. Two types of fingerlike cytoplasmic

processes extend from a cell body: numerous, perhaps hundreds,

of short dendrites and a longer single axon. Within the cytoplasm

of an axon, the cytoskeleton transports substances by a process

known as axonal transport. Do not confuse dendrites and axons,

which are part of individual cells (neurons), with sensory and

motor nerves, which are bundles of thousands of cells.

Bacterial meningitis involves inflammatory bacterial infection of

the meninges, commonly the pia mater and arachnoid mater

and, more rarely, the dura mater.

Signs and Symptoms

Bacterial meningitis is characterized by an increased number

of white blood cells in the CSF, sudden high fever, and intense

meningeal inflammation. The inflammation accounts for most

of the signs and symptoms: Swelling of the meninges retards

the normal flow of CSF, putting pressure on the underlying

organs. Inflammation of the cranial meninges typically produces severe headache, nausea, vomiting, pain, and in many

cases loss of various brain functions, leading to such conditions

as drowsiness, confusion, fretfulness, or irritability. Inflammation of the spinal meninges puts pressure on surrounding nerves and muscles, producing stiffness in the neck and

affecting sensory input and muscular control. When the brain

becomes infected—a condition called encephalitis—deafness,

blindness, drastic changes in the patient’s behavior, coma, or

death may result. Signs and symptoms of meningitis may rapidly develop; for example, meningococcal meningitis can kill

within six hours of the initial symptoms, allowing little time

for treatment.

A lumbar puncture reveals the normally clear CSF to be

milky in appearance because of the large number of bacteria and the increased number of white blood cells. Petechiae

(pe-tē’kē-ē)—small, dark purplish hemorrhages of blood vessels

in the skin—are sometimes present.

Pathogens and Virulence Factors

Researchers have shown that more than 50 species of bacteria

can cause meningitis. Among these are opportunistic members

of the normal microbiome, including species of Staphylococcus

(staf’i-1ō-kok’ŭs) and Streptococcus (strep-tō-kok’ŭs) as well as

Gram-negative enteric8 bacteria such as Escherichia coli (esh-ě-

rik’ē-ă ko’lī) and Klebsiella pneumoniae (kleb-sē-el’ă nū-mō’nē-ī).

However, five other species cause almost 90% of cases of bacterial meningitis. These are Neisseria meningitidis, Streptococcus

pneumoniae, Haemophilus influenzae, Listeria monocytogenes, and

Streptococcus agalactiae. All five of these bacteria have virulence

factors that allow them to resist phagocytosis and cause disease; the following sections detail these features.

Neisseria meningitidis Neisseria meningitidis (nī-se’rē-ă me-ninji’ti-dis) is one of only two species of Gram-negative cocci that

The terminal ends of axons have thousands of branches

that form junctions called synapses6 (sĭ-nap’sēz) with glands,

muscles, or other neurons. A synapse mediates transfer of a signal to a neighboring postsynaptic cell. In most synapses, there is

a synaptic cleft—an intercellular space about 40 nm wide

between the axon terminal and the postsynaptic cell. A synaptic cleft stops the transmission of electrical signals; therefore,

the signal between cells is chemical—an axon terminal releases

molecules called neurotransmitters into the synaptic cleft. A particular neurotransmitter may be stimulatory or inhibitory; that

is, it either (1) stimulates a muscle to contract, a gland to secrete,

or another neuron to carry a nerve impulse; or (2) inhibits such

activities.

Portals of Infection of the Central Nervous System

No openings allow microbial colonization of the central nervous

system; therefore, the CNS is an axenic (ā-zēn’ik) environment—

it has no normal microbiota. Pathogens may access the CNS

through breaks in the bones and meninges, through medical procedures such as spinal taps, or by traveling via axonal

transport in peripheral neurons to the CNS. Microbes carried

in the blood or lymph may penetrate the blood-brain barrier by

infecting and killing cells of the meninges, causing meningitis7

(men-in-jī’tis)—inflammation of the meninges. Some pathogens

gain access to the CNS when localized inflammation distorts

the cells of the blood-brain barrier, changing its permeability;

such change is more likely during chronic infection by many

pathogens. Circulation of cerebrospinal fluid can carry infective microbes throughout the cranial cavity and spinal column.

We will examine nervous system diseases caused by bacteria, viruses, fungi, protozoa, and prions. We begin by considering bacterial diseases.

Not only can bacteria infect cells of the nervous system, but

toxins released by bacteria growing elsewhere in the body can

also affect neurons. In the following sections, we consider disease examples of both types—leprosy, which is a disease of cells

found in the PNS, and botulism and tetanus, which involve

toxin production. However, the most common bacterial infection of the nervous system is bacterial meningitis, which we

consider next.