396
UNIT THREE
Amyotrophic lateral sclerosis
(ALS or Lou Gehrig’s disease) may begin
with garbled speech, clumsiness, sudden fatigue, or limb weak-
ness. Fasciculations (muscle twitches) that resemble moving ropes
beneath the skin may prompt the person to seek medical attention.
But because ALS is a diagnosis of exclusion, identifying it may take a
year or more.
ALS a±
ects the upper and lower parts of the body, and progresses
faster if symptoms begin in the face or neck. Usually the battle is lost
two to ²
ve years following diagnosis, typically from respiratory fail-
ure. Using assistive breathing devices and a drug, Riluzole, can extend
life. The mind is often spared—one patient wrote a novel in his last
months, and another remained a brilliant songwriter.
In ALS, motor neurons degenerate in the spinal cord, brainstem,
and the cerebral cortex. ALS may be due to an inability of the motor
neurons or associated astrocytes to counter buildup of oxidative free
radicals, or the astrocytes may release a neurotoxin. Researchers have
provided conditions that induced stem cells from ALS patients. In cul-
ture these cells give rise to motor neurons and astrocytes, providing
peeks into the pathogenesis of this devastating disease. If researchers
can understand how the disease begins, perhaps treatments will follow.
PRACTICE
11
Describe the structure of the spinal cord.
12
What are ascending and descending tracts?
13
What is the consequence of ²
bers crossing over?
14
Name the major tracts of the spinal cord, and list the types of
impulses each conducts.
Cerebrum
(frontal
section)
Brainstem
(transverse
sections)
Spinal cord
(transverse section)
Midbrain
Pons
Medulla
oblongata
Corticospinal
tract
Motor cortex
of cerebrum
Motor fibers
cross over
Motor
impulse to
skeletal
muscle
TABLE
11.3
|
Nerve Tracts of the Spinal Cord
Tract
Location
Function
Ascending Tracts
1. Fasciculus
gracilis and
fasciculus
cuneatus
Posterior
funiculi
Conduct sensory impulses associated
with the senses of touch, pressure, and
body movement from skin, muscles,
tendons, and joints to the brain
2. Spinothalamic
tracts (lateral
and anterior)
Lateral and
anterior
funiculi
Conduct sensory impulses associated
with the senses of pain, temperature,
touch, and pressure from various
body regions to the brain
3. Spinocerebellar
tracts (posterior
and anterior)
Lateral
funiculi
Conduct sensory impulses required
for the coordination of muscle
movements from muscles of the lower
limbs and trunk to the cerebellum
Descending Tracts
1. Corticospinal
tracts (lateral
and anterior)
Lateral and
anterior
funiculi
Conduct motor impulses associated
with voluntary movements from the
brain to skeletal muscles
2. Reticulospinal
tracts (lateral,
anterior, and
medial)
Lateral and
anterior
funiculi
Conduct motor impulses associated
with the maintenance of muscle tone
and the activity of sweat glands from
the brain
3. Rubrospinal
tracts
Lateral
funiculi
Conduct motor impulses associated
with muscular coordination and the
maintenance of posture from the
brain
FIGURE 11.13
³Most³²
bers of the corticospinal tract originate in the
cerebral cortex, cross over in the medulla, and descend in the spinal
cord, where they synapse with neurons whose ²
bers lead to spinal
nerves supplying skeletal muscles. Some ²
bers cross over in the spinal
cord.
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