Facts on File Encyclopedia of Health and Medicine

because only so many ions can be inside the cell
body, sodium ions leave. To restore itself to nega-
tive polarity the cell body activates a burst of
energy to “pump” more sodium ions out, at an
exchange rate of three sodium ions out for every
two potassium ions in.
Meanwhile, back at the synaptic terminals
another conversion is taking place. The synaptic
terminals contain tiny storage pockets called vesi-
cles that hold NEUROTRANSMITTER molecules. The
electrical impulse causes the synaptic vesicle to
release a neurotransmitter molecule. The neuro-
transmitter molecule crosses the synapse and
binds with a NEURORECEPTORon a dendrite of the
receiving neuron. The binding either causes or
blocks an action.
The longer the neurotransmitter molecules
remain in the synapse the more neuroreceptors
they can bind. As a safety mechanism, the neuro-
transmitter’s presence in the synapse stimulates
the synaptic vesicles to reuptake (recycle) the
remaining neurotransmitter, containing the
potential for binding. Further stimuli are then
necessary to continue. The neuron sending a mes-
sage is the presynaptic neuron; the neuron receiv-
ing a message is the postsynaptic neuron. Some
neurologic conditions affect the functioning of
presynaptic neurons and others affect the func-
tioning of postsynaptic neurons.

NEURONAL PATHWAYS:

THE BRAIN’S ENDLESS CAPACITY TO LEARN

The adult BRAIN contains 60 to 200 trillion

synapses. Synapses represent the networks of

axons that neurons develop to form neuronal

pathways, the routes by which neurons commu-

nicate with one another in expedited fashion.

Even though all the neurons the brain will ever

have are present at birth, the brain has an end-

less capacity to create new neuronal pathways

and thus “grow” its ability to learn.

Many medications work by inhibiting (block-
ing) or expediting the reuptake process to extend
or shorten, respectively, the time the neurotrans-
mitter is active. Selective serotonin reuptake
inhibitors (SSRIs), for example, are ANTIDEPRESSANT
MEDICATIONSthat work by blocking serotonin reup-
take. Serotonin is a neurotransmitter that facili-

tates neuron communication in areas of the brain

related to mood. The acetylcholinesterase

inhibitors, medications to treat ALZHEIMER’S DIS-

EASE, work by blocking the enzyme that breaks

down the neurotransmitter acetylcholine. This

action extends the presence of acetylcholine in the

synapses, increasing neuroreceptor binding.

Acetylcholine facilitates neuron communication in

areas of the brain that process cognitive functions

and memory. Medications may also masquerade

as neurotransmitters to bind with neuroreceptors.

Therapies for PARKINSON’S DISEASE, a degenerative

condition that results from depletion of the neuro-

transmitter DOPAMINEin the brain, are among the

most effective applications of this approach.

Health and Disorders of the Nervous System

The most significant health risks the nervous sys-

tem faces occur before birth. The most vulnerable

period in nervous system development takes place

before most women have missed a menstrual

period or suspect they are pregnant. Within the

first three weeks after conception, the rudimen-

tary nervous system, the neural tube, forms and

rapidly differentiates into the brain and spinal

cord. Numerous factors, environmental and

genetic, can disrupt this process to cause cephalic

disorders (structural defects of the brain) or SPINA

BIFIDA(structural defects of the spinal cord). CERE-

BRAL PALSYis the most common developmental dis-

turbance of the nervous system.

FOLIC ACID AND NEURAL DEVELOPMENT

Folic acid is crucial for proper development of

the NERVOUS SYSTEMearly in PREGNANCY, especially

at the time of CONCEPTION through the first

trimester. Numerous studies show that taking

folic acid supplements before and during preg-

nancy can prevent 70 percent of NEURAL TUBE

DEFECTS. Health experts recommend that all

women of childbearing age take 400 micrograms

of folic acid supplement daily regardless of

whether they are trying actively to become preg-

nant.

From birth through midlife, injury becomes the

most worrisome threat to the nervous system.

Young people are especially vulnerable to TRAU-

MATIC BRAIN INJURY(TBI) and SPINAL CORD INJURY; 80

The Nervous System 217