Applications - Quantitative Neurophysiology - page 109

effect which only occurs when two supercooled superconductors are separated by a thin isolating barrier.
The recording of magnetic potentials from the brain is called magnetoencephalography (MEG) and is
typically performed with an array of SQUIDs.
Just as currents generate a magnetic field, Faraday's law of induction allows a time varying magnetic
field to induce current to flow. Therefore, applying a strong time-varying magnetic field to the brain can
force current to flow in neurons. Unlike electrical fields which do not easily penetrate the insulating
scalp and skull, the magnetic field can extend deep into the brain, attenuated only by distance. The
stimulation of the brain through a magnetic field is called Transcranial Magnetic Stimulation (TMS).
A pulse or pair of pulses can be delivered that will cause a depolarization and firing of neurons. For
example, if the depolarization is in the occipital cortex, a patient will sense flashes of light. Repetitive
TMS (rTMS) has been shown to induce longer lasting changes and provides some interesting treatment
and research options. Clinically, rTMS may help alleviate some of the symptoms of migraines, stroke,
Parkinson's disease, and depression but the mechanisms are unknown. In the research laboratory, rTMS
is an effective and noninvasive way of “knocking out” a very specific region of the brain. By focusing a
magnetic field of a particular orientation and polarity, it is possible to hyperpolarize a small region of the
brain, rendering it unexcitable. If the subject is asked to perform a task that requires that region, they will
be incapable of completing the task. Unlike the images created by fMRI, rTMS provides much stronger
evidence that a particular region of the brain is actually used to perform a specific task.
10.4 DRUGANDGENETICTHERAPIES
Neurons make very heavy use of proteins and amino acids that function as neurotransmitters, enzymes and
ion channels. It is therefore not surprising that genetic mutations and changes in expression can have an
enormous impact on neural conduction. Any breakthroughs in genetic therapies or drug design/delivery
will therefore quickly find applications in neuroscience. Two mechanisms will most likely provide the
greatest benefit. First, when a mutation has caused a deformation of a protein, an engineered DNA
strand can be introduced to the body (typically through a viral vector) with a correction. The virus
then spreads the corrected DNA to neurons which begin producing copies of the corrected protein.
Alternatively, a synthetic drug can be designed to perform a desired function. In fact, the action of many
current drugs, both legal and illegal, function in a very similar manner. Second, the mechanisms that
control protein expression can be altered indirectly through the availability and effectiveness of second
messengers. Again, correction of defective second messengers may be accomplished through genetic or
pharmaceutical means. In the future, drug and genetic therapies may even be used in combination to
correct a defective protein and ensure proper expression.
One very serious issue with delivering drugs and genes to the central nervous system is the blood
brain barrier (BBB), which severely limits the transport of molecules to the brain. Two technologies may
lift this barrier. First, a better understanding of transport across the BBB may allow drug designers to
attach tags to the drug that fool transport mechanisms into carrying the drug across the membrane.
Alternatively, there may be a way to find small molecules that can diffuse through the BBB that have the
desired effect indirectly. Second, it should be possible to build implanted devices (similar to an insulin
pump) that can control the direct deliver drugs directly to the brain, therefore bypassing the blood brain
barrier.