1.
Somatic Motor Pathways|
Pyramidal & Extrapyramidal Tracts|
Descending Tracts of spinal cord
FATIMA WAHID MANGRIO
fatimawahid1234@gmail.com

2.
Somatic Motor Pathways
• Neural circuits in the brain and spinal cord
orchestrate all voluntary and involuntary movements.
• All excitatory and inhibitory signals that control
movement converge on the motor neurons.
• These neurons, also known as lower motor neurons
(LMNs), have their cell bodies in the brain stem and
spinal cord.
• From the brain stem, axons of LMNs extend through
cranial nerves to innervate skeletal muscles of the
face and head.

3.
• From the spinal cord, axons of LMNs extend through
spinal nerves to innervate skeletal muscles of the
limbs and trunk.
• Only LMNs provide output from the CNS to skeletal
muscle fibers. For this reason, they are also called
the final common pathway

4.
• Neurons in four distinct neural circuits, control of
movement by providing input to lower motor neurons.
1 Local circuit neurons
• Input arrives at lower motor neurons from nearby
interneurons called local circuit neurons. These
neurons are located close to the lower motor neuron
cell bodies in the brain stem and spinal cord.
• Local circuit neurons receive input from somatic
sensory receptors, such as nociceptors and muscle
spindles, as well as from higher centers in the brain.
They help coordinate rhythmic activity in specific
muscle groups, such as alternating flexion and
extension the lower limbs during walking.

5.
2 Upper motor neurons
• Both local circuit neurons and lower motor neurons
receive input from upper motor neurons (UMNs).
• Most upper motor neurons synapse with local circuit
neurons, which in turn synapse with lower motor
neurons. (A few upper motor neurons synapse directly
with lower motor neurons.)
• UMNs from the cerebral cortex are essential for the
execution of voluntary movements of the body.

6.
• Other UMNs originate in motor centers of the brain
stem: the red nucleus, the vestibular nucleus, the
superior colliculus, and the reticular formation. UMNs
from the brain stem regulate muscle tone,control
postural muscles, and help maintain balance and
orientation of the head and body.
• Both the basal nuclei and cerebellum exert influence
on upper motor neurons.

7.
3 Basal nuclei neurons
• Basal nuclei neurons assist movement by providing
input to upper motor neurons.
• Neural circuits interconnect the basal nuclei with
motor areas of the cerebral cortex (via the thalamus)
and the brain stem. These circuits help initiate and
terminate movements, suppress unwanted movements,
and establish a normal level of muscle tone.

8.
4 Cerebellar neurons
• Cerebellar neurons also aid movement by controlling
the activity of upper motor neurons.
• Neural circuits interconnect the cerebellum with
motor areas of the cerebral cortex (via the thalamus)
and the brain stem.
• A prime function of the cerebellum is to monitor
differences between intended movements and
movements actually performed.
• Then, it issues commands to upper motor neurons to
reduce errors in movement.
• The cerebellum thus coordinates body movements
and helps maintain normal posture and balance.

9.
Organization of Upper Motor Neuron
Pathways
• The axons of upper motor neurons extend from the brain
to lower motor neurons via two types of somatic motor
pathways—direct and indirect.
• Direct motor pathways provide input to lower motor
neurons via axons that extend directly from the cerebral
cortex.
• Indirect motor pathways provide input to lower motor
neurons from motor centers in the basal nuclei,
cerebellum, and cerebral cortex.

10.
Direct Motor Pathways
( Pyramidal pathway )
• Nerve impulses for voluntary movements propagate
from the cerebral cortex to lower motor neurons via
the direct motor pathways.
• The direct motor pathways, which are also known as
the pyramidal pathways, consist of axons that
descend from pyramidal cells. Pyramidal cells are
upper motor neurons with pyramid shaped cell bodies
located in the primary motor area and the premotor
area of the cerebral cortex (areas 4 and 6 ).
• The direct motor pathways consist of

11.
Corticospinal Pathways
• The corticospinal pathways conduct impulses for the
control of muscles of the limbs and trunk.
• Axons of upper motor neurons in the cerebral cortex
form the corticospinal tracts, which descend through
the internal capsule of the cerebrum and the cerebral
peduncle of the midbrain.
• In the medulla oblongata, the axon bundles of the
corticospinal tracts form the ventral bulges known as
the pyramids. About 90% of the corticospinal axons
decussate to the contralateral (opposite) side in the
medulla oblongata and then descend into the spinal
cord where they synapse with a local circuit neuron or
a lower motor neuron.

12.
• The 10% that remain on the ipsilateral (same) side
eventually decussate at the spinal cord levels where
they synapse with a local circuit neuron or lower
motor neuron.
• Thus, the right cerebral cortex controls most of the
muscles on the left side of the body, and the left
cerebral cortex controls most of the muscles on the
right side of the body.
• There are two types of corticospinal tracts:

13.
1 Lateral corticospinal tract
• Corticospinal axons that decussate in the medulla
form the lateral corticospinal tract in the lateral
white column of the spinal cord .
• These axons synapse with local circuit neurons or
lower motor neurons in the anterior gray horn of the
spinal cord.
• Axons of these lower motor neurons exit the cord in
the anterior roots of spinal nerves and terminate in
skeletal muscles that control movements of the distal
parts of the limbs.
• The distal muscles are responsible for precise, agile,
and highly skilled movements of the hands and feet.
Examples include the movements needed to button a
shirt or play the piano

14.
2 Anterior corticospinal tract.
• Corticospinal axons that do not decussate in the
medulla form the anterior corticospinal tract in the
anterior white column of the spinal cord
• At each spinal cord level, some of these axons
decussate via the anterior white commissure. Then,
they synapse with local circuit neurons or lower motor
neurons in the anterior gray horn.
• Axons of these lower motor neurons exit the cord in
the anterior roots of spinal nerves. They terminate in
skeletal muscles that control movements of the trunk
and proximal parts of the limbs.

15.
Corticobulbar Pathway
• The corticobulbar pathway conducts impulses for the
control of skeletal muscles in the head.
• Axons of upper motor neurons from the cerebral
cortex form the corticobulbar tract, which descends
along with the corticospinal tracts through the
internal capsule of the cerebrum and cerebral
peduncle of the midbrain.

16.
• Some of the axons of the corticobulbar tract
decussate; others do not. The axons terminate in the
motor nuclei of nine pairs of cranial nerves in the
brain stem: the oculomotor (III), trochlear (IV),
trigeminal (V), abducens (VI), facial (VII),
glossopharyngeal (IX), vagus (X), accessory (XI), and
hypoglossal (XII).
• The lower motor neurons of the cranial nerves convey
impulses that control precise, voluntary movements of
the eyes, tongue, and neck, plus chewing, facial
expression, speech, and swallowing.

17.
Indirect Motor Pathways
• The indirect motor pathways or extrapyramidal
pathways include all somatic motor tracts other than
the corticospinal and corticobulbar tracts.
• Axons of upper motor neurons that give rise to the
indirect motor pathways descend from various nuclei
of the brain stem into five major tracts of the spinal
cord and terminate on local circuit neurons or lower
motor neurons.

18.
• These tracts are the
1 rubrospinal
2 tectospinal
3 vestibulospinal
4 lateral reticulospinal
5 medial reticulospinal tracts

19.
RUBROSPINAL TRACT
• Conveys nerve impulses from red nucleus (which
receives input from cerebral cortex and cerebellum)
to contralateral skeletal muscles that govern precise,
voluntary movements of distal parts of upper limbs.
• The rubrospinal tract is small in humans and
terminates in the cervical cord. It is thought to be
responsible for taking over functions after
corticospinal tract injury and may also play a role in
flexor or decorticate posturing in the upper
extremities with lesions above the level of the red
nucleus in which the rubrospinal tract is preserved.

20.
Tectospinal & Reticulospinal Tract
• Conveys nerve impulses from superior colliculus to
contralateral skeletal muscles that reflexively move
head, eyes, and trunk in response to visual or auditory
stimuli.
• The final two descending motor pathways are the
tectospinal tract, terminating in the cervical cord,
and the reticulospinal tract, terminating along the
entire cord. The tectospinal tract is responsible for
coordination of head and eye movements, and this is
poorly understood in humans. The reticulospinal tract
is responsible for automatic posture and gait-related
movements.

21.
Vestibulospinal Tract
• Conveys nerve impulses from vestibular nucleus
(which receives input about head movements from
inner ear) to ipsilateral skeletal muscles of trunk and
proximal parts of limbs for maintaining posture and
balance in response to head movements.
• the vestibulospinal tract with medial fibers
terminating in the cervical and upper thoracic cord
and lateral portions, which are still considered part of
the medial system, terminating in the entire cord.
Vestibulospinal tracts are responsible for positioning
of the head and neck (the medial fibers) and balance
(the lateral fibers).

22.
Medial and lateral reticulospinal
Tract
• Conveys nerve impulses from reticular formation to
ipsilateral skeletal muscles of trunk and proximal
parts of limbs for maintaining posture and regulating
muscle tone in response to ongoing body movements.