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Descending Projection Systems
and Motor Functions of the
Spinal Cord
I.
II.
Functional anatomy of motor systems and descending pathways.
A. Components of the motor systems of the CNS.
B. Organization and function of the descending pathways (i.e.,
lateral/medial).
Regional anatomy of the descending systems.
A. Motor regions of the cerebral cortex.
1. Premotor (higher-order) cortical regions.
2. 1° motor cortex – input, somatotopic organization and
outflow.
B. Descending projections through internal capsule  forming the
Corticospinal Tract.
C. Organization of the Corticospinal tract in the base of the midbrain
(Basis Pedunculi) [+ start of rubrospinal tract].
D. Passage of corticospinal tract through Ventral Pons.
E. Nuclei of dorsal pons and medulla (start of vestibulospinal and
reticulospinal tracts).
F. Pathways through medulla (+ lateral corticospinal tract
decussation).
G. Input from the descending pathways within the spinal cord +
topographic organization of the grey matter and white matter.
I. Functional anatomy of the motor systems
and descending pathways
A. Motor systems of the CNS – 4 components of motor
systems (during movements of the leg and trunk by
regulating skeletal muscle).
1. Descending projections pathways.
2. motor neurons and interneurons.
3. basal ganglia.
4. cerebellum.
Our ‘tours’ through motor systems will be opposite from
what we experienced with sensory (order of cortex----
motor neurons).
Today, we will focus on the 1st 2 of the above. Individual
lectures will be presented on the basal ganglia and
cerebellum at a later date.
1st component: descending projection pathways (Fig. 10-1)
Motor Cortical Areas (Fig. 10-1)
2nd component; for muscles of the limbs and trunk motor neurons and interneurons located in: ventral
horn and internal zone of the spinal cord.
A parallel exists for the muscles of the head: cranial
nerve motor nuclei and reticular formation in the
brainstem – these are analogous to above areas.
1 function of the brainstem is to serve as the
“spinal cord for the head”.
3rd and 4th components: basal ganglia and
cerebellum do not project directly to motor
neurons, but rather, synapse on descending
pathways and have a very important influence.
Overview of function using an example
1. See a cup  1° visual cortex  higherorder in post parietal lobe (for id object).
2. Prefrontal association areas (maturation
and cognition – “get the idea”).
3. Premotor areas  “plan of action”
4. 1° motor cortex
5. Corticospinal tract – interneurons 
motor neurons (grasp the cup)
Cerebellum and bg also play a role in
feedback control and movement intention
– we will discuss shortly.
B. Organization and function of descending
pathways.
1. Motor control pathways
2. Paths that regulate somatic sensory processing:
somatic sensory relay nuclei (brainstem) and
dorsal horn neurons (spinal cord).
i.e., pain suppression triggering of motor reflexes
damage  spasticity).
3. Paths regulating ANS.
Cortex , amygdala, hypothal, brain stem 
preganglionic autonomic nerves of brainstem and
spinal cord.
7 Major descending motor control pathways from
cerebral cortex or brainstem nuclei:
From frontal cortex:
i. lateral corticospinal tract
ii. Ventral corticospinal tract
iii. Corticobulbar tract ( cranial n. motor nuclei).
From brainstem nuclei:
i. Rubrospinal tract
ii. Reticulospinal tract
iii. Vestibulospinal tract
iv. Tectospinal tract
Interneuron Connections: sometimes an
intermediary before motor neurons:
1.Segmental interneurons – short branches
within single spinal cord segment.
2.Propiospinal neurons – projects for multiple
spinal segments before synapsing onto
motor n. – long projections coordinating
movements of upper and lower limbs; may
transmit control signals lower, for some
paths ending in cervical areas.
Somatotopic organization-motor (Fig. 10-2)
From lateral
descending
paths
Lateral:
limbs
Medial:
Axial
LATERAL DESCENDING PATHWAYS
Lateral Corticospinal Tract:
(Fig. 10-3)
Starts at the 1° motor cortex ------
decussation zone and lateral motor
nuclei of cervical and lumbrosacral
cord.
Why segmented? Limbs.
LATERAL DESCENDING PATHWAYS
Rubrospinal Tract
(Fig. 10-3):
Red nucleus (magnocellular) 
midbrain decussation  lateral
intermediate zone and ventral horn.
Provides some additional (residual)
motor control
[more important input connection is
with cerebellum].
MEDIAL DESCENDING PATHWAYS
Medial (Ventral) Corticospinal
Tract (Fig. 10-4):
Axial and girdle muscles.
Ipsilateral ventral column  bilateral
projections to medial grey matter.
Controls especially for head, shoulder, and
upper trunk muscles.
MEDIAL DESCENDING PATHWAYS
Note the bilateral projections; cf., the lateral
pathways (which crossed).
Reticulospinal Tract (Fig 10-4):
Many terminate in cervical cord.
But, 2° projections to propiospinal neurons may
influence lower axial muscles also.
Pontine  ventral column.
Medullary reticular formation  lateral column
(ventrolateral).
Autonomic movements: posture and repetitive
movements.
Tectospinal Tract (Fig. 10-4):
From sup coll (deep)  coord head and eye
movements.
Vestibulospinal Tracts:
Lateral vestibular n.  lat
vestibulospinal tract (to all spinal
limbs).
Maintains balance.
Medial nucleus  med tracts for
control of head position (cervical
only).
Organization of Tracts
(Fig. 10-5)
Note the descending and ascending tracts and their relative
positions.
II. Regional Anatomy of Descending Systems
A. Motor regions of the Cerebral Cortex – in frontal
lobe.
1. Higher-order cortical regions:
- planning movement.
- integration of info from diverse cortex (see
earlier e.g.).
a. bg  VA (thal) 
supplementary motor area (bimanual coordination)  prefrontal cortex
b. cerebellum  VL (thal)  premotor cortex  reticulospinal
tracts (control of girdle muscles).
c. Cingulate motor areas (part of limbic system) – rhythmical
movements (e.g., pedaling a bike).
- may be important in triggering movements initiated by
emotions and drives.
II. Regional Anatomy of Descending Systems
Premotor regions
Somatic sensory
Thalamic
(VL) (VA)
Cerebellum bg
Precentral gyrus (thick layer of 5
pyramidal neurons provide major
output (large Betz cells)
Corticospinal tract
Premotor Areas (Lateral View) (Fig. 10-6)
Premotor Areas (Medial View) (Fig. 10-6)
Motor “homunculus” (Fig. 10-7)
Somatotropic organization – analogous to sensory cortex
Origins of motor cortical tracts
Note that the origins of cortical tracts are shown – somewhat segmented
B. Descending projections through Internal
capsule and corticospinal fibers (Fig. 10-9)
Posterior limb of the
IC contains the cortico
spinal tract;
Somatotropic
Organization is maintained.
Divisions of internal capsule (Fig. 10-10)
C. Pathways within midbrain (Fig. 10-11)
Corticospinal tract courses in base of midbrain (basis pedunculi).
Somatotopic organization is retained here.
Origin of Tectospinal tract (superior colliculus) and rubrospinal tract
(red nucleus).
Also, these tracts cross close to their origins in midbrain.
D. Pathways within Pons (Fig. 10-12)
Descending cortical fibres are no longer on ventral surface 
they dive deeper within the pons (and also branch up into fasciculi)
among pontine and pontocerebellar fibres.
Medullary Decussation
E. Nuclei of Dorsal Pons and Medulla: start of Vestibulospinal and
Reticulospinal tracts (Review pathways on earlier slides).
Functions: Reticular formation has multiple projections, which makes this
System highly integrative (e.g., between motor and sensory).
Analogous to Intermediate Zone of the spinal cord (See Fig. 10-16A).
F. Pathways through Medulla and
Decussation of Corticospinal Tract.
Fibres from cortex collect once again on
ventral surface, forming pyramids.
Decussations occur in medulla – specifically
of lateral corticospinal tract axons.
G. Spinal Cord
Inputs: motor pathways synapse on motor
neurons of ventral horn and interneurons
(segmental, propiospinal) of intermediate
zone.
Generalized organization of grey and white
matter (see Fig. 10-16A and the last slide in
this set).
Ventral horn: Rexed’s laminae (VIII, IX).
Intermediate zone: (laminae VII).
Spinal cord grey matter organization (Rexed’s
laminae) (Fig. 10-16A):
Motor Terminations at
various levels of the sc.
Note the descent of various
pathways in lateral and
ventral columns.
Medially descending and
terminating pathways 
Girdle and axial muscles.
Cross-overs and bilateral
connectivities: coord
contraction during postural
adjustments.
Lateral descending/terminating paths:
Distal limb muscles
Note: terminations only evident in cervical
and lumbrosacral segments (sensory limbs)
(Fig. 10-19)
Medial input = continuous
Lateral input - interrupted
H. Some notes about lesions of the
descending cortical pathways:
Lesions of posterior limb of internal capsule, ventral
brainstem, and sc:
on 1 side produce a sequential series of symptoms
(focused primarily on limb muscles --- Why?)
1. Flaccid paralysis (immed)  decr strength +
muscle tone due to corticospinal interruption.
2. Spasticity (a few weeks later)  incr muscle tone
+ exaggerated reflexes (Babinski sign) due to
reticulospinal and sensory input damage (decr
feedback).
White Matter: Pathway Generalizations
Review of descending and ascending tracts