395
CHAPTER ELEVEN
Nervous System II
3.
Spinocerebellar
(spi
no-ser
e
˘-bel
ar)
tracts.
The
posterior
and
anterior spinocerebellar tracts
lie near the
surface in the lateral funiculi of the spinal cord (see F
g.
11.11). ±ibers in the posterior tracts remain uncrossed,
whereas those in the anterior tracts cross over in the
medulla. Impulses conducted on their F
bers originate
in the muscles of the lower limbs and trunk and then
travel to the cerebellum. These impulses coordinate
muscular movements.
Descending Tracts
The major descending tracts of the spi-
nal cord include the following:
1.
Corticospinal
(kor
tı˘-ko-spi
nal)
tracts.
The
lateral
and
anterior corticospinal tracts
occupy the lateral and
anterior funiculi, respectively (see F g. 11.11). Most of the
F bers of the lateral tracts cross over in the lower medulla
oblongata. Some F bers of the anterior tracts cross over
at various levels of the spinal cord
(f g. 11.13)
. The
corticospinal tracts conduct motor impulses from the
brain to spinal nerves and outward to various skeletal
muscles. Thus, they help control voluntary movements.
The corticospinal tracts are sometimes called
pyramidal tracts
after the pyramid-shaped regions in
the medulla oblongata through which they pass. Other
descending tracts are called
extrapyramidal tracts,
and
they include the reticulospinal and rubrospinal tracts.
2.
Reticulospinal
(re
˘-tik
u-lo-spi
nal)
tracts.
The
lateral
reticulospinal tracts
are in the lateral funiculi, whereas
the
anterior
and
medial reticulospinal tracts
are in
the anterior funiculi (see F
g. 11.11). Some F
bers in
the lateral tracts cross over, whereas others remain
uncrossed. Those of the anterior and medial tracts
remain uncrossed. Motor impulses transmitted on the
reticulospinal tracts originate in the brain and control
muscular tone and activity of sweat glands.
3.
Rubrospinal
(roo
bro-spi
nal)
tracts.
The F
bers of
the rubrospinal tracts cross over in the brain and pass
through the lateral funiculi (see F
g. 11.11). They carry
motor impulses from the brain to skeletal muscles, and
they coordinate muscles and control posture.
Table 11.3
summarizes the nerve tracts of the spinal
cord. Clinical Application 11.3 describes injuries to the spi-
nal cord.
A hemi-lesion of the spinal cord (severed on one side) aF
ecting the
corticospinal and spinothalamic tracts can cause Brown-Séquard syn-
drome. Ascending tracts cross over at diF
erent levels, so the injured
side of the body becomes paralyzed and loses touch sensation. The
other side of the body retains movement but loses sensations of pain
and temperature.
2.
Spinothalamic
(spi
no-thah-lam
ik)
tracts.
The
lateral
and
anterior spinothalamic tracts
are in the lateral
and anterior funiculi, respectively (see F
g. 11.11).
The lateral tracts conduct impulses from various body
regions to the brain and give rise to sensations of pain
and temperature. Impulses carried on F
bers of the
anterior tracts are interpreted as touch and pressure.
Impulses in these tracts cross over in the spinal cord
(F g. 11.12).
Cerebrum
(frontal
section)
Brainstem
(transverse
sections)
Spinal cord
(transverse section)
Sensory
cortex of
cerebrum
Midbrain
Pons
Medulla
Thalamus
Spinothalamic
tract
Fasciculus
cuneatus
tract
Sensory fibers
cross over
Sensory
impulse
from skin
temperature
or pain
receptors
FIGURE 11.12
Sensory impulses originating in skin touch receptors
ascend in the fasciculus cuneatus tract and cross over in the medulla
of the brain. Pain and temperature information ascends in the lateral
spinothalamic tract, which crosses over in the spinal cord.
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