Chapter 12
The Central Nervous System
471
12
Table 12.2
Major Ascending (Sensory) Pathways and Spinal Cord Tracts
SPINAL CORD TRACT
LOCATION
(FUNICULUS)
ORIGIN
TERMINATION
FUNCTION
Dorsal Column–Medial Lemniscal Pathways
Fasciculus cuneatus and
fasciculus gracilis (dorsal
white column)
Dorsal
Central axons of sensory (first-
order) neurons enter dorsal root
of the spinal cord and branch.
Branches enter dorsal white
column on same side without
synapsing.
By synapse with
second-order neurons
in nucleus cuneatus
and nucleus gracilis
in medulla. Fibers of
medullary neurons
cross over and ascend
in medial lemniscus
to thalamus, where
they synapse
with third-order
neurons. Thalamic
neurons then
transmit impulses
to somatosensory
cortex.
Both tracts transmit sensory
impulses from general
sensory receptors of skin and
proprioceptors, which are
interpreted as discriminative
touch, pressure, and “body
sense” (limb and joint position)
in opposite somatosensory
cortex. Cuneatus transmits
afferent impulses from upper
limbs, upper trunk, and neck.
Gracilis carries impulses from
lower limbs and inferior body
trunk.
Spinothalamic Pathways
Lateral spinothalamic
Lateral
Interneurons (second-order
neurons) in dorsal horns. Fibers
cross to opposite side before
ascending.
By synapse with
third-order neurons
in thalamus. Thalamic
neurons then
convey impulses to
somatosensory cortex.
Transmits impulses concerned
with pain and temperature
to opposite side of brain for
interpretation by somatosensory
cortex.
Ventral spinothalamic
Ventral
Interneurons (second-order
neurons) in dorsal horns. Fibers
cross to opposite side before
ascending.
By synapse with
third-order neurons
in thalamus. Thalamic
neurons eventually
convey impulses to
somatosensory cortex.
Transmits impulses concerned
with crude touch and pressure
to opposite side of brain for
interpretation by somatosensory
cortex.
Spinocerebellar Pathways
Dorsal spinocerebellar*
Lateral
(dorsal part)
Interneurons (second-order
neurons) in dorsal horn on
same side of cord. Fibers ascend
without crossing.
By synapse in
cerebellum
Transmits impulses from trunk
and lower limb proprioceptors
on one side of body to same side
of cerebellum for subconscious
proprioception.
Ventral spinocerebellar*
Lateral
(ventral part)
Interneurons (second-order
neurons) of dorsal horn. Contains
crossed fibers that cross back to
the opposite side in the pons.
By synapse in
cerebellum
Transmits impulses from the
trunk and lower limb on the
same side of body to cerebellum
for subconscious proprioception.
*These spinocerebellar tracts carry information from the lower limbs and trunk only. The corresponding tracts for the upper limb and neck (rostral
spinocerebellar and others) are beyond the scope of this book.
Direct (Pyramidal) Pathways
Te direct pathways originate
mainly with the pyramidal neurons located in the precentral gyri.
Tese neurons send impulses through the brain stem via the large
pyramidal (corticospinal) tracts
(Figure 12.32a)
. Te direct path-
ways are so called because their axons descend without synapsing
from the pyramidal neurons to the spinal cord. Tere they synapse
either with interneurons or with ventral horn motor neurons.
Stimulation of the ventral horn neurons activates the skel-
etal muscles with which they are associated. Te direct pathway
primarily regulates fast and fine (or skilled) movements such as
doing needlework and writing.
Indirect Pathways
Te indirect pathways include brain
stem motor nuclei and
all motor pathways except
the pyramidal
pathways. Tese pathways were formerly lumped together as
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