neurologic examination A series of basic diag-
nostic procedures that help the neurologist assess
an individual’s neurologic status. A fundamental
dimension of the neurologic examination is obser-
vation, through which the neurologist can gain
much information about a person’s mental status
(cognition and memory), motor function (balance,
coordination, STRENGTH), and sensory perception
(touch, vision, hearing). Specific procedures
include testing the reflexes, response to specific
sensory stimuli (soft touch, PAIN, vibration, partic-
ular smells or tastes), and the ability to carry out
directed movements (such as tracking the move-
ment of an object or touching specific body loca-
tions). The neurologist generally performs basic
tests ofVISUAL ACUITYand field of vision, hearing,
and examination of the inner EYE.
Basic assessment of cognitive function and
memory may include reading, spelling, and draw-
ing activities. Tasks such as unbuttoning and but-
toning, writing one’s signature, and handling
small objects test dexterity and fine motor move-
ment. The ways in which a person stands, walks,
and rises from and lowers to sitting and prone
positions help the neurologist further assess MUS-
CLE strength, coordination, and PROPRIOCEPTION
(awareness of the body’s position relative to its
environment). The neurologist may conduct other
evaluations and procedures, including more
extensive cognitive testing, depending on basic
findings. A basic neurologic examination may take
about 20 minutes for the neurologist to complete.
See also COGNITIVE FUNCTION AND DYSFUNCTION;
MEMORY AND MEMORY IMPAIRMENT; REFLEX.
neuron A NERVEcell. A neuron consists of a cell
body, nucleus containing the cell’s DNA, cytoplasm,
organelles (structures within the cytoplasm that
conduct functions such as energy production),
axons (fibers that carry nerve impulses away from
the neuron), and dendrites (rootlike structures
that receive the nerve impulses). A myelin sheath
(coating of myelin, a fatty substance) may enclose
a neuron’s axon, insulating it to maintain the
integrity of the nerve impulses the axon carries.
Dendrites do not have myelin sheaths. An axon
may extend several inches to several feet, while
dendrites remain close to the neuron’s cell body.
Axons and dendrites extend into spaces around
the neurons called synaptic corridors. The end of
the axon branches into numerous extensions
called presynaptic terminals. The gap between an
axon of one neuron and the dendrites of another
neuron is a synapse.
Neurons communicate (conduct nerve
impulses) through an electrochemical process
called action potential, an exchange of electrically
charged particles (ions) across the cell’s mem-
brane. A neuron at rest contains a negative charge
within its membrane though the environment
outside the cell membrane carries a positive
charge. A nerve impulse creates a surge or spike of
electrical energy that causes ion channels in the
cell membrane to open. At the same time the neu-
ron releases a chemical messenger, a NEUROTRANS-
MITTER.
The neurotransmitter binds with the appropri-
ate NEURORECEPTORon the dendrites of the neuron
intended to receive the nerve impulse. The type of
neuron—for example, motor or sensory—deter-
mines which neurotransmitters and neurorecep-
tors are present. As the neuron’s polarity changes
(the neuron achieves action potential), the electri-
cal impulse moves to the neuron. The process
takes only a minuscule fraction of a second to
complete. An enzyme then initiates reuptake
(recycling) of the neurotransmitter, setting the
stage for the next cycle. Neurons align in neuronal
pathways that facilitate the conduction of nerve
impulses for optimal efficiency.
Neurons are the oldest cells in the body. Most
develop in the early to middle stages of gestation,
with the final surge of neuron production taking
place shortly before birth. The body’s full comple-
ment of neurons is present at birth, the majority
of which survive and function for the course of a
full lifetime. The body cannot replace neurons
that die, and neurons do die at a fairly consistent
rate, barring injury or disease, of about one per
day throughout life.
See alsoCELL STRUCTURE AND FUNCTION.neuropathy Dysfunction of or damage to the
PERIPHERAL NERVES. Neuropathy may be temporary
or permanent and may result from numerous
causes, such as INFECTION, compression, degenera-
tive disease processes, and AUTOIMMUNE DISORDERS.
Neuropathy may involve only a single peripheral270 The Nervous System