1.
Descending pathways

2.
Types
• direct pathways
• indirect pathways .

3.
• Most of the indirect pathways, sometimes called the extrapyramidal
system, are involved in less precise control of motor functions,
especially those associated with overall body coordination and
cerebellar function such as posture.
• Some indirect pathways, such as those from the basal nuclei and
cerebellum, help in fine control of the direct pathways .

4.
Direct Pathways
• The direct pathways, also called the pyramidal (pi-rami-dal) system, are involved in
the maintenance of muscle tone and in controlling the speed and precision of skilled
movements.
• Direct pathways are so named because upper motor neurons in the
cerebral cortex, whose axons form these pathways, synapse directly
with lower motor neurons in the brainstem or spinal cord.

5.
• They are also called the pyramidal system
because the fibers of these pathways
primarily pass through the medullary
pyramids.
• They include the corticospinal tract, which
is involved in direct cortical control of
movements below the head,
• the corticobulbar tract, which is involved
in direct cortical control of movements in
the head and neck.

6.
Indirect Pathways
• The indirect pathways originate in upper motor neurons of the
cerebrum and cerebellum whose axons synapse in some
intermediate nucleus rather than directly with lower motor neurons.
• Axons from the upper motor neurons in these nuclei form the
indirect pathways. They do not pass through the pyramids or
through the corticobulbar tracts and, therefore, are sometimes
called extrapyramidal.
• rubrospinal,
• vestibulospinal
• reticulospinal tracts

7.
Parts of somatic nervous system
1- The sensory division
 It comprises all sensory
(afferent) nerves, the
sensory ascending
tracts, the thalamus and
sensory cortex.
 It serves the somatic
sensations from the skin,
muscles, tendons,
ligaments and
periosteum.

8.
Par ts of somatic ner vous system –
Cont.
2- The motor division
 It comprises of motor
cerebral cortex, the basal
ganglia, and the thalamus,
the reticular formation of the
brain stem, the descending
motor tracts and the motor
(efferent) nerves.
 This division controls the
voluntary muscular activity.

9.
Motor Neurons
• Motor neurons are split into two groups: Upper and
Lower motor neurons.
• Upper motor neurons originate in the motor region of
the cerebral cortex of the brain stem and carry motor
information down to the final common pathway. The
nerve fibers that descend in white matter from
supraspinal levels are segregate into nerve bundles –
descending tracts.
• The cell bodies of these neurons are some of the largest
in the brain, approaching nearly 100μm in diameter.

10.
Figure 16.4b
Locations of major descending motor tracts that contain axons of upper
motor neurons Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

11.
ANATOMICAL
ORGANIZATION
• FIRST ORDER- CEREBRAL CORTEX
• SECOND ORDER(INTERNUNCIAL
NEURON- ANTERIOR GRAY COLUMN
• THIRD ORDER(LOWERMOTOR
NEURON)- ANTERIOR GRAY COLUMN

12.
Figure 16.3a
Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

13.
Figure 16.3b
Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

14.
Figure 16.5
Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

15.
Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

16.
a) Originates at the
primary motor
cortex
– corticobulbar tracts
end at the motor nuclei
of CNs on the opposite
side of the brain
- most fibers crossover
in the medulla and
enter the lateral
corticospinal tracts
- rest descend in the
anterior corticospinal
tracts and crossover
after reaching target
segment in the SC
Figure 16.4a
Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

17.
• corticospinal tracts
– Axonsof pyramidal cells
situated in fifth layer of
cerbral cortex.
– One third –primary motor
cortex
– One third-secondary motor
cortex
• Descending fibers converge in
corona radiata
• Midbrain-basis pedunculi
• Pons-transverse
pontocerebellar fibers
• Medulla-pyramids
• Junctionof medulla and spinal
cord-decussating of fibers
• Lateral corticospinal tract

18.
DESCENDING
TRACTS OF
MOTOR
FIBERS

19.
Motor neurons
• The motor neurons situated in the anterior gray column send axon
to innervate muscles through anterior roots of spinal nerve. A
lower motor neuron's axon terminates on an effector
(muscle).
• Lower motor neurons are classified based on the type of
muscle fibre they innervate:
– Alpha motor neurons (α-MNs) innervate extrafusal muscle fibers.
– Gamma motor neurons (γ-MNs) innervate intrafusal muscle fibers,

20.
Descending Pathway Lesions
• An upper motor neuron lesion is a
lesion of the neural pathway above the
anterior horn cell or motor nuclei of the
cranial nerves.
• This is in contrast to a lower motor
neuron lesion, which affects nerve fibers
travelling from the anterior horn of the
spinal cord to the relevant muscle(s).

21.
Corticospinal tract leisons
• Babinski s sign is present
• Superfficial abdominal reflexes absent
• Loss of fine skilled movements

22.
• Great toe
becomes
dorsiflexed and
other toes fan
outward-positive
• Normally there is
plantar flexion of
all toes because
corticospinal
tract is intact

23.
Upper motor neuron lesions (extra pyramidal tract) are
indicated by:
– Spasticity, increase in tone in the extensor
muscles (lower limbs) or flexor muscles
(upper limbs) .As a result upperlimb is
maintained in flexion and lower limb in
extension

24.
– Clasp-knife response
where initial resistance
to movement is
followed by relaxation.
When passive
movement of a joint is
attempted there is
resistance owing to
spasticity of muscle.

25.
– Weakness in the flexors (lower limbs) or
extensors (upper limbs), but no muscle
wasting

26.
• Exaggerated abdominal reflexes

27.
Lower motor neuron leison
• Flaccid paralysis
• Atrophy of muscles
• Muscular fasiculations
• Muscular contracture

28.
Indirect (Extrapyramidal) System
• Includes the brain stem, motor nuclei, and all motor
pathways not part of the pyramidal system
• This system includes the rubrospinal, vestibulospinal,
reticulospinal, and tectospinal tracts
• These motor pathways are complex and multisynaptic, and
regulate:
– Axial muscles that maintain balance and posture
– Muscles controlling coarse movement of the proximal
portions of limbs
– Head, neck, and eye movement

29.
• LMN are known as the ‘final common
pathway’ because despite all the descending
tracts they are the only neurons that actually
physically contact the muscle fibers
• LMN’s get stimulated by corticospinal,
vestibulospinal, reticulospinal, rubrospinal
and tectospinal tracts, among others. The
cerebellum exerts a substantial influence on
many of them.

30.
Questions?

31.
• Draw labelled diagram showing ascending
tracts in spinal cord.

32.
• What will happen in leison of spinal
leminiscus in medulla oblongata.

33.
1
2
Dorsal root
4
3
3

34.
Spinal cord organization
5
6
7

35.
Spinal Cord Cross Section
4
3

36.
Spinal Cord, Roots & Nerves
1
2