464
UNIT 3
Regulation and Integration of the Body
12
nuclei and later of the cerebral cortex. Its initial symptoms in
many are wild, jerky, almost continuous “flapping” movements
called
chorea
(Greek for “dance”). Although the movements ap-
pear to be voluntary, they are not. Late in the disease, marked
mental deterioration occurs. Huntington’s disease is progressive
and usually fatal within 15 years of onset of symptoms.
Te hyperkinetic manifestations of Huntington’s disease are
essentially the opposite of those of Parkinson’s disease (over-
stimulation rather than inhibition of the motor drive). Hunt-
ington’s is usually treated with drugs that block, rather than
enhance, dopamine’s effects. As with Parkinson’s disease, stem
cell implants may be a promising treatment in the future.
Check Your Understanding
20.
What is CSF? Where is it produced? What are its functions?
21.
What is a transient ischemic attack (TIA) and how is it
different from a stroke?
22.
Mrs. Lee, a neurology patient, seldom smiles, has a shuffling,
stooped gait, and often spills her coffee. What degenerative
brain disorder might she have?
For answers, see Appendix H.
The Spinal Cord
Describe the gross and microscopic structure of the spinal
cord.
List the major spinal cord tracts, and classify each as a
motor or sensory tract.
Gross Anatomy and Protection
Te spinal cord, enclosed in the vertebral column, extends
from the foramen magnum of the skull to the first or second
lumbar vertebra, just inferior to the ribs
(Figure 12.26)
. About
42 cm (17 inches) long and 1.8 cm (3/4 of an inch) thick, the
glistening-white
spinal cord
provides a two-way conduction
pathway to and from the brain. It is a major reflex center: Spinal
reflexes are initiated and completed at the spinal cord level. We
discuss reflex functions and motor activity of the cord in subse-
quent chapters. In this section we focus on the anatomy of the
spinal cord and its ascending and descending tracts.
Like the brain, the spinal cord is protected by bone, menin-
ges, and cerebrospinal fluid. Te single-layered
spinal dura
mater
(Figure 12.26c) is not attached to the bony walls of the
vertebral column. Between the bony vertebrae and the spinal
dura mater is an
epidural space
filled with a so± padding of
fat and a network of veins (see Figure 12.28a). Cerebrospinal
fluid fills the subarachnoid space between the
arachnoid
and
pia
mater
meninges.
Inferiorly, the dural and arachnoid membranes extend to
the level of S
2
, well beyond the end of the spinal cord. Te spi-
nal cord typically ends between L
1
and L
2
(Figure 12.26a). For
this reason, the subarachnoid space within the meningeal sac
inferior to that point provides a nearly ideal spot for remov-
ing cerebrospinal fluid for testing, a procedure called a
lumbar
mad cow disease. Current clinical trials focus on using the im-
mune system to clear away beta-amyloid peptide.
Another hallmark of Alzheimer’s disease is the presence of
neurofibrillary tangles
inside neurons. Tese tangles involve a pro-
tein called tau, which functions like railroad ties to bind microtu-
bule “tracks” together. In the brains of AD victims, tau abandons
its microtubule-stabilizing role and grabs onto other tau mole-
cules, forming spaghetti-like neurofibrillary tangles, which kill
the neurons by disrupting their transport mechanisms.
As the brain cells die, the brain shrinks. Particularly vulnerable
brain areas include the hippocampus and the basal forebrain, re-
gions involved in thinking and memory (see Figure 12.21). Loss
of neurons in the basal forebrain is associated with a shortage of
the neurotransmitter acetylcholine, and drugs that inhibit break-
down of acetylcholine slightly enhance cognitive function in AD
patients. Interestingly, a drug that blocks NMDA receptors, me-
mantine, also slightly improves thinking in more advanced stages
of AD, suggesting that glutamate excitotoxicity is also involved in
this disease.
Parkinson’s Disease
²ypically striking people in their 50s and 60s,
Parkinson’s dis-
ease
results from a degeneration of the dopamine-releasing neu-
rons of the substantia nigra. As those neurons deteriorate, the
dopamine-deprived basal nuclei they target become overactive.
Afflicted individuals have a persistent tremor at rest (exhibited
by “pill-rolling” movements of the fingers and wrist), a forward-
bent walking posture and shuffling gait, and a stiff facial expres-
sion. Tey are slow initiating and executing movement.
Te cause of Parkinson’s disease is still unknown, but multi-
ple factors may interact to destroy dopamine-releasing neurons.
Recent evidence points to abnormalities in certain mitochon-
drial proteins and protein degradation pathways. Te drug
L-dopa
helps to alleviate some symptoms. It passes through the
blood brain barrier and is then converted into dopamine.
However, as more and more neurons die off, ³-dopa becomes
ineffective. Mixing ³-dopa with drugs that inhibit the break-
down of dopamine (for example, deprenyl) can prolong its ef-
fectiveness. In addition, deprenyl by itself, early in the disease,
slows the neurological deterioration to some extent and delays
the need to administer ³-dopa for up to 18 months.
Deep brain stimulation via implanted electrodes shuts down
abnormal brain activity and can alleviate tremors. Tis treat-
ment (for patients who no longer respond to drug therapy) is
expensive and risky, but it works. Another possibility is to use
gene therapy to insert genes into adult brain cells, causing them
to secrete the inhibitory neurotransmitter GABA. GABA then
inhibits the abnormal brain activity just as the electrical stim-
ulation does. Replacing dead or damaged cells by implanting
stem cells is promising, but results to date are no better than
more conventional treatments.
Huntington’s Disease
Huntington’s disease,
a fatal hereditary disorder, strikes during
middle age. Mutant
huntingtin
protein accumulates in brain cells
and the tissue dies, leading to massive degeneration of the basal
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