Chapter 12
The Central Nervous System
469
12
From the thalamus, impulses are forwarded to specific ar-
eas of the somatosensory cortex.
2.
Spinothalamic pathways.
Te
spinothalamic pathways
receive input from many different types of sensory recep-
tors and make multiple synapses in the brain stem. Tese
pathways consist of the
lateral
and
ventral (anterior)
spinothalamic tracts
(see Figure 12.31b and ±able 12.2).
Teir fibers cross over in the spinal cord.
Te fibers in these pathways primarily transmit im-
pulses for pain and temperature, but also for coarse touch
and pressure. All are sensations that we are aware of but
have difficulty localizing precisely on the body surface.
3.
Spinocerebellar pathways.
Te third ascending pathway
consists of the
ventral
and
dorsal spinocerebellar tracts
.
Tey convey information about muscle or tendon stretch to
the cerebellum, which uses this information to coordinate
skeletal muscle activity (see Figure 12.31a and ±able 12.2).
As noted earlier, these pathways do not contribute to con-
scious sensation. Te fibers of the spinocerebellar pathways
either do not decussate or else cross over twice (thus “undo-
ing” the decussation).
Descending Pathways and Tracts
Te descending pathways that deliver efferent impulses from
the brain to the spinal cord are divided into two groups: (1) the
direct pathways,
equivalent to the pyramidal tracts, and (2) the
indirect pathways
, essentially all others. Motor pathways involve
two neurons, referred to as the upper and lower motor neurons:
Upper motor neurons
are the pyramidal cells of the motor cor-
tex (see p. 433) and the neurons of subcortical motor nuclei.
Lower motor neurons
are the ventral horn motor neurons.
Tese directly innervate the skeletal muscles (their effectors).
We give a brief overview of these pathways on p. 471. See
Table 12.3
on p. 473 for more information.
and spinal nerves conduct somatic sensory impulses from the
rest of the body to the CNS. First-order neurons entering the
spinal cord are shown at the bottom of
Figure 12.31
.
Te cell bodies of
second-order neurons
(Figure 12.31, mid-
dle) reside in the dorsal horn of the spinal cord or in medul-
lary nuclei. Tey transmit impulses to the thalamus or to the
cerebellum where they synapse.
Tird-order neurons
have cell bodies in the thalamus (Fig-
ure 12.31, top). Tey relay impulses to the somatosensory
cortex of the cerebrum. (Tere are no third-order neurons in
the cerebellum.)
In general, somatosensory information travels along three
main pathways on each side of the spinal cord. ±wo of these
pathways (the
dorsal column–medial lemniscal
and
spinotha-
lamic pathways
) transmit impulses via the thalamus to the sen-
sory cortex for conscious interpretation. Collectively the inputs
of these sister tracts provide
discriminative touch
and
conscious
proprioception
. Both pathways decussate—the first in the me-
dulla and the second in the spinal cord.
Te third pathway, the
spinocerebellar pathway
, terminates in
the cerebellum, and does not contribute to sensory perception.
Let’s examine these pathways more closely.
1.
Dorsal column–medial lemniscal pathways.
Te
dorsal
column–medial lemniscal pathways
(lem-nis
9
kul; “rib-
bon”) mediate precise, straight-through transmission of
inputs from a single type (or a few related types) of sen-
sory receptor that can be localized precisely on the body
surface, such as discriminative touch and vibrations. Tese
pathways are formed by the paired tracts of the
dorsal
white column
of the spinal cord—
fasciculus cuneatus
and
fasciculus gracilis
—and the
medial lemniscus
.
Te medial lemniscus arises in the medulla and ter-
minates in the thalamus (Figure 12.31a and
Table 12.2
).
Ascending tracts
Descending tracts
Fasciculus gracilis
Dorsal
white
column
Fasciculus cuneatus
Dorsal
spinocerebellar tract
Lateral spinothalamic
tract
Ventral spinothalamic
tract
Ventral white
commissure
Lateral
corticospinal
tract
Lateral
reticulospinal tract
Ventral
corticospinal tract
Medial
reticulospinal tract
Rubrospinal tract
Vestibulospinal tract
Tectospinal tract
Ventral
spinocerebellar
tract
Figure 12.30
Major ascending (sensory) and descending (motor) tracts of the spinal
cord, cross-sectional view.
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