The Central Nervous System
From the thalamus, impulses are forwarded to speciﬁc ar-
eas of the somatosensory cortex.
receive input from many diﬀerent types of sensory recep-
tors and make multiple synapses in the brain stem. Tese
pathways consist of the
(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.
Te third ascending pathway
consists of the
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
equivalent to the pyramidal tracts, and (2) the
, 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
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
Te cell bodies of
(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.
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
In general, somatosensory information travels along three
main pathways on each side of the spinal cord. ±wo of these
dorsal column–medial lemniscal
) transmit impulses via the thalamus to the sen-
sory cortex for conscious interpretation. Collectively the inputs
of these sister tracts provide
. Both pathways decussate—the ﬁrst in the me-
dulla and the second in the spinal cord.
Te third pathway, the
, terminates in
the cerebellum, and does not contribute to sensory perception.
Let’s examine these pathways more closely.
Dorsal column–medial lemniscal pathways.
column–medial lemniscal pathways
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
of the spinal cord—
Te medial lemniscus arises in the medulla and ter-
minates in the thalamus (Figure 12.31a and
Major ascending (sensory) and descending (motor) tracts of the spinal
cord, cross-sectional view.