5. LOWER MOTOR NEURON (LMN):
• LOWER MOTOR NEURON (LMN): It is the
neuron situated in the anterior gray horn of
the spinal cord
• Its axon innervates skeletal muscle through
the anterior roots of the spinal nerve
• LMN constitutes the final common pathway to
the muscles
6. UPPER MOTOR NEURON (UMN)
• UPPER MOTOR NEURON (UMN): These are
the nerve fibers that descend in the white
matter from different supra-spinal centers like
medulla, pons, midbrain & cerebral cortex
• Collectively they form descending tracts
• UMNs constantly send the impulses from
higher centers to the LMNs
7. Anatomical organization of descending
tracts
• First-order neuron has the cell body in the
cerebral cortex
• Second-order neuron (an internuncial neuron) is
situated in the anterior gray column of the spinal
cord. It is usually short
• Third-order neuron is an LMN, situated in the
anterior gray horn. It innervates the skeletal
muscle through anterior root & spinal nerve
• Sometimes, 1st order neuron (UMN) terminates
directly on 3rd order neuron (LMN)
8. Functions of the descending tracts
• Corticospinal tracts: are concerned with
voluntary, discrete, skilled movements,
especially those of the distal parts of the limbs
• Reticulospinal tract: may facilitate or inhibit
voluntary movement or reflex activity
• Tectospinal tract: is concerned with reflex
postural movements in response to visual
stimuli and causes pupillodilatation reflex in
darkness
9. Functions of the descending tracts
• Rubrospinal tract: facilitates the activity of
flexor muscles & inhibits the activity of
extensor or antigravity muscles
• Vestibulospinal tract: is concerned with
postural activity associated with balance
• Olivospinal tract: may play some role in
muscular activity
• Descending autonomic fibers: control the
visceral activity
14. Cortico-spinal tracts
• Their cells arise from the cerebral cortex
Descending fibers converge in the corona
radiata Then pass through internal
capsule Pass through midbrain
To pons To medulla oblongata
15. Cortico-spinal tracts within medulla
oblongata
• In the medulla oblongata, many fibers are
grouped together to form a swelling known as
the pyramid
• Hence these fibers are called pyramidal tract
• At the junction of the medulla & spinal cord,
most of the fibers cross the midline at the
decussation of the pyramids & enter the
lateral white column of the spinal cord to form
the lateral corticospinal tract
16. Cortico-spinal tracts within medulla
oblongata
• The remaining fibers do not cross in the
decussation but descend in the anterior white
column of the spinal cord as the anterior
corticospinal tract
• The fibers of anterior corticospinal tract cross
the midline in the cervical and upper thoracic
regions and terminate in the anterior gray
horn
17.
18.
19. Cortico-spinal tracts
• Most corticospinal fibers synapse with the
internuncial neuron
• The remaining fibers synapse directly with the
motor neuron
• The corticospinal tracts are used in performing
rapid skilled movements
20. Reticulo-spinal tracts
• The reticular formation is formed by groups of
nerve cells scattered throughout the midbrain,
pons and medulla oblongata
• Pontine part of reticulospinal tract remain
uncrossed, and descend through the anterior
white column of spinal cord
• Medullary part of reticulospinal tract is
crossed & uncrossed, descend through the
lateral white column of the spinal cord
21. Functions of reticulo-spinal tracts
• They facilitate or inhibit the voluntary
movements and reflex activity
• Control the sympathetic outflow and sacral
parasympathetic outflow
22. Tectospinal tract
• Fibers arise from the nerve cells situated within
the superior colliculus of the midbrain
• Most of the fibers cross the midline immediately
& descends through the brain stem
• It descends through the anterior white column of
spinal cord close to the anterior median fissure
• Majority of fibers terminate in the anterior gray
horn in the upper cervical segments of the spinal
cord by synapsing with internuncial neuron
23. Functions of tectospinal tract
• These are concerned with reflex postural
movements in response to visual stimuli
24. Rubrospinal tract
• Red nucleus is situated in the tegmentum of the
midbrain at the level of the superior colliculus
• The fibers cross the midline at the level of the
red nucleus , descend as the rubrospinal tract
through the pons & medulla oblongata to enter
the lateral white column of the spinal cord
• The fibers terminate on the internuncial
neurons in the anterior gray horn of the cord
25. Functions of rubrospinal tract
• The red nucleus receives afferent impulses
from the cerebral cortex & cerebellum
• Therefore, the cerebral cortex & cerebellum
indirectly infuence the spinal cord through
this tract
• The tract facilitates the activity of the flexor
muscles and inhibits the activity of the
extensor or antigravity muscles
26. Vestibulospinal tract
• Vestibular nuclei are situated in the pons &
medulla oblongata beneath the floor of the 4th
ventricle
• These nuclei receive afferent fibers from the
inner ear through the vestibular nerve & from
the cerebellum
• The neurons of lateral vestibular nucleus give
rise to the axons that form the vestibulospinal
tract
27. Vestibulospinal tract
• The tract descends uncrossed through the
medulla and spinal cord in the anterior white
column
• The fibers terminate on the internuncial
neurons in the anterior gray horn
28. Functions of vestibulospinal tract
• The inner ear & the cerebellum, by means of
this tract, facilitate to maintain the balance of
the body by facilitating the extensor muscles
and inhibiting the flexor muscles
29. Olivospinal tract
• Arises from the inferior olivary nucleus
• Descends in the lateral white column of the
spinal cord
• Influences the activity of motor nucleus in the
anterior gray horn
30. Descending autonomic fibers
• Higher centers of the CNS which control the
autonomic activity are situated in:
1. Cerebral cortex
2. Hypothalamus
3. Amagdaloid complex
4. Reticular formation
31. Descending autonomic fibers
• The fibers arise from the neurons within
higher centers
• Cross the midline in brain stem
• Descend in the lateral white column of spinal
cord
• Synapse with the autonomic motor cells in the
lateral gray horn in the thoracic & upper
lumbar (sympathetic) and mid-sacral
(parasympathetic) levels of spinal cord
32.
33. Intersegmental tracts
• Short ascending and descending tracts that
originate and end within spinal cord
• Exist in anterior, lateral and posterior white
columns
• These tracts interconnect the neurons of
different segmental levels
• Important in intersegmental spinal reflexes
34. Decerebrate rigidity
• Spinal segmental reflexes are influenced by
higher centers through descending tracts
• In spinal shock, due to severance of spinal
cord, the segmental spinal reflexes are
depressed
• Spinal shock disappears within few weeks, the
segmental spinal reflexes return & muscle
tone returns, called decerebrate rigidity
35. Renshaw cells
• These are the internuncial neurons which make a
connection between the:
1. Collateral branch of LMN &
2. Axon of LMN itself
• These internuncial neurons (Renshaw cells)
provide feedback on the lower motor neurons,
hence inhibiting their activity
