Chapter 12
The Central Nervous System
473
12
the
extrapyramidal system
because their nuclei of origin were
presumed to be independent of (“extra to”) the pyramidal tracts.
Tis term is still widely used clinically. However, pyramidal tract
neurons are now known to project to and influence the activity
of most “extrapyramidal” nuclei, so modern anatomists prefer
the term
indirect
, or
multineuronal
,
pathways
, or even just the
names of the individual motor pathways.
Indirect motor pathways are complex and multisynaptic.
Tey are most involved in regulating:
Axial muscles that maintain balance and posture
Muscles controlling coarse limb movements
Head, neck, and eye movements that follow objects in the
visual field
Table 12.3
Major Descending (Motor) Pathways and Spinal Cord Tracts
SPINAL CORD TRACT
LOCATION
(FUNICULUS)
ORIGIN
TERMINATION
FUNCTION
Direct (Pyramidal) Pathways
Lateral corticospinal
Lateral
Pyramidal neurons of motor
cortex of the cerebrum. Cross
over in pyramids of medulla.
By synapse with
ventral horn
interneurons that
influence motor
neurons and
occasionally with
ventral horn motor
neurons directly.
Transmits motor impulses from
cerebrum to spinal cord motor
neurons (which activate skeletal
muscles on opposite side of
body). A voluntary motor tract.
Ventral corticospinal
Ventral
Pyramidal neurons of motor
cortex. Fibers cross over at the
spinal cord level.
Ventral horn (as
above).
Same as lateral corticospinal
tract.
Indirect Pathways
Tectospinal
Ventral
Superior colliculus of midbrain
of brain stem (fibers cross to
opposite side of cord).
Ventral horn
(as above).
Turns neck so eyes can follow a
moving object.
Vestibulospinal
Ventral
Vestibular nuclei in medulla
of brain stem (fibers descend
without crossing).
Ventral horn
(as above).
Transmits motor impulses that
maintain muscle tone and
activate ipsilateral limb and trunk
extensor muscles and muscles
that move head. Helps maintain
balance during standing and
moving.
Rubrospinal
Lateral
Red nucleus of midbrain of brain
stem (fibers cross to opposite side
just inferior to the red nucleus).
Ventral horn
(as above).
In experimental animals,
transmits motor impulses
concerned with muscle tone
of distal limb muscles (mostly
flexors) on opposite side of
body. In humans, functions are
largely assumed by corticospinal
tracts.
Reticulospinal (ventral,
medial, and lateral)
Ventral and
lateral
Reticular formation of brain stem
(medial nuclear group of pons
and medulla). Both crossed and
uncrossed fibers.
Ventral horn
(as above).
Transmits impulses concerned
with muscle tone and many
visceral motor functions.
May control most unskilled
movements.
Many of the activities controlled by subcortical motor nuclei
depend heavily on reflex activity. Figure 12.32b illustrates one
of these tracts, the rubrospinal tract.
Overall, the
reticulospinal
and
vestibulospinal tracts
main-
tain balance by varying the tone of postural muscles (±able 12.3).
Te
rubrospinal tracts
control flexor muscles, whereas the
tec-
tospinal tracts
and the
superior colliculi
mediate head move-
ments in response to visual stimuli.
Check Your Understanding
23.
What is the explanation for the cervical and lumbar
enlargements of the spinal cord?
24.
Where are the cell bodies of the first-, second-, and third-
order sensory neurons in the spinothalamic pathway located?
For answers, see Appendix H.
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