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Spinal cord- 2
Descending tracts
Upper motor neurons in
primary motor cortex
Somatic motor nuclei
of brain stem
Skeletal muscle
Lower
motor
neurons
Somatic motor nuclei
of spinal cord
Skeletal muscle
In the somatic nervous system (SNS), an upper motor
neuron in the CNS controls a lower-motor neuron in the
brain stem or spinal cord. The axon of the lower-motor
neuron has direct control over skeletal muscle fibers.
Stimulation of the lower- motor neuron always has an
excitatory effect on the skeletal muscle fibers.
Visceral motor nuclei
in hypothalamus
Preganglionic neuron
Visceral effectors
Autonomic nuclei in
brain stem
Ganglionic
neurons
Autonomic nuclei
in spinal cord
In the autonomic nervous system (ANS), the axon of a
preganglionic neuron in the CNS controls ganglionic neurons
in the periphery. Stimulation of the ganglionic neurons may
lead to excitation or inhibition of the visceral effector
innervated
Preganglionic neuron
First order neuron: has its cell body in the cerebral cortex
Second order neuron: an interneuron situated in the anterior gray column
Third order neuron: the lower motor neuron in the anterior gray column and
Innervates the skeletal muscle through the anterior root of spinal nerve
In some instances the axon of the first order neuron terminates directly on the third
order neuron
Motor tracts
•There are two major descending tracts
•Corticospinal tract (pyramidal) : Conscious control of skeletal muscles
•Subconscious tract (extrapyramidal): Subconscious regulation of balance, muscle tone,
eye, hand, and upper limb position
UMN
LMN
To skeletal
muscles
Corticobulbar tract
Motor
nuclei of
cranial nerves
•Corticobulbar tracts: conscious control
over eye, jaw, and face muscles
•Lateral corticospinal tracts: conscious
control over skeletal muscles (mostly
distal muscles)
•Anterior corticospinal tracts:
conscious control over skeletal muscles
(mostly proximal muscles)
Lateral corticospinal tract
MESENCEPHALON
To skeletal
muscles
Motor
nuclei of
cranial nerves
MEDULLA
OBLONGATA
Pyramids
Lateral corticospinal
tract
To skeletal
muscles
Anterior
corticospinal
tract
Anterior corticospinal tract
Corticospinal tracts
 The corticospinal tracts are often called the pyramidal tracts because they form
pyramid-shaped enlargements on the anterior surface of the medulla
concerned with controlling skilled movements of the distal extremities (facilitation of
alpha and gamma motor neurons which innervate the distal flexor muscles )
The upper motor neurons of these tracts originate in the precentral gyrus of the
cerebral cortex
In medulla oblongata they form the medullary pyramids
Most of the fibers (85 percent) cross over (decussate) to the opposite side in the
pyramidal decussation, where they continue to descend in the lateral funiculus of the
spinal cord as the lateral corticospinal tract (LCST).
The tract descends all the way of spinal cord with fibers continually leaving it in order
to synapse on interneurons in the anterior gray horn. ( Some even synapse directly on
alpha and gamma motor neurons)
Those corticospinal fibers which do not decussate in the medulla continue
descending on the same (ipsilateral) side of the cord and become the anterior
corticospinal tract (ACST). This tract does not extend below the midthoracic level.
Fibers leave the tract at various levels to cross over in the anterior white
commissure to synapse on interneurons in the anterior gray horn.
The anterior corticospinal tract
acts on the proximal muscles of upper limb
(shoulder muscle) of the ipsilateral and
contralateral sides
corticospinal tract
for fine skilled movements
The Corticobulbar Tract
 This tract is composed of fibers originating
in the precentral gyrus of the lower quarter of
the motor cortex.
The descending fibers terminate in the
motor nuclei of cranial nerves III and IV in the
midbrain; V, VI. and VII in the pons; and IX, X,
XI, and XII in the medulla.
 The corticobulbar fibers from one side of
the brain project to the motor nuclei on both
sides of the brainstem (bilateral input)
The corticobulbar input is bilateral to :
1- 5th
2- Part of 7th ( which supplies UPPER facial muscles)
3- 9th,10th,11th
4- 12th !!!!
The Subconscious Motor Tracts
•Consists of four tracts involved in monitoring the subconscious motor control
•Vestibulospinal tracts
•Tectospinal tracts
•Reticulospinal tracts
•Rubrospinal tracts
Extrapyramidal tracts arise in the brainstem, but are under the influence of the cerebral cortex
These motor pathways are complex and multisynaptic, and regulate:
• Axial muscles that maintain balance and posture
• Muscles controlling coarse movements of the proximal portions of limbs
• Head, neck, and eye movement
Vestibulospinal tracts
Vestibular nuclei are situated in the pons and medulla beneath the floor of 4th ventricle
•Send information from the inner ear to monitor position of the head
• balance by facilitate the activity of the extensor muscles
The tract descends uncrossed through medulla and through spinal cord in
the anterior white column
Terminates by synapsing with interneurons of anterior gray column
The inner ear and the cerebellum facilitate the activity of extensor muscles
and inhibit the activity of flexor muscles in association with the maintenance of balance
Vestibulospinal tract
• nerve cells in vestibular nucleus (in
the pons and medulla oblongata
– received afferents from inner ear
and cerebellum
• axons descend uncrossed
– through medulla and through the
length of spinal cord
• synapse with neuron in the anterior gray
column of the spinal cord
( balance by facilitate the activity of the
extensor muscles )
Tectospinal tracts
Concerned with reflex postural movements in response to visual stimuli
•Send information to the head, neck, and upper limbs in response to bright and sudden
movements and loud noises
•The tectum area consists of superior and inferior colliculi
•Superior colliculi: receives visual information
•Inferior colliculi: receives auditory information
Arise from nerve cells in the superior colliculus
Most of the fibers cross the midline soon after their origin
The tract descends in the anterior white column close to Anterior median fissure
Majority of fibers terminate in the anterior gray horn in the upper cervical segments
by synapsing with interneurons
Tectospinal tract
• nerve cells in superior colliculus of the
midbrain
• nerve fibres/ axons
– cross the mid line
• terminate in the anterior gray column
of upper cervical segments of spinal
cord
( responsible for reflex movement of
head & neck in response to visual stimuli )
Reticulospinal tracts
• influence voluntary movements and reflex activity by facilitating or inhibiting the
activity of alpha and gamma motor neurons
Reticular formation (RF): group of scattered nerve cells in the brain stem
From pons: axons of RF neurons descend uncrossed into the spinal cord
( pontine Reticulospinal tracts )
 descend in the anterior white column as the medial reticulospinal tract (MRST)
activate the axial and proximal limb extensors
From medulla : axons of RF neurons descend crossed and uncrossed into the spinal cord
( medullary Reticulospinal tracts )
 descend in the lateral white column as the lateral reticulospinal tract (LRST).
inhibit the axial and proximal limb extensors (and to a lesser degree it also excites axial
and proximal limb flexors)
The reticulospinal tracts exert both somatic and autonomic control
 Has also descending autonomic fibers ( providing a pathway by which the hypothalamus
can control the sympathetic and sacral parasympathetic outflow)
 Most of these fibers are derived from the lateral reticulospinal tract
Rubrospinal tracts
• facilitate the activity of flexors and
inhibit the activity of extensors
The axons of neurons in the
red nucleus cross over near
their point of origin and
descend contralaterally in
the lateral funiculus of the
cord adjacent to
the lateral corticospinal
tract
Terminates by synapsing with
interneurons of anterior gray
column
It has connections with
cerebral cortex and
cerebellum
Rubrospinal tract
• nerve cells in red nucleus
( tegmentum of midbrain at the level of superior colliculus )
• nerve fibres / axons
– cross the mid line
– descend as rubrospinal tract
• through pons and medulla oblongata
• terminate anterior gray column of spinal cord
( facilitate the activity of flexor muscles )
Summary of somatic motor control
•Cerebral cortex initiates voluntary movement
•Information goes to the basal nuclei and cerebellum
•These structures modify and coordinate the movements so they are performed in
a smooth manner
•Information goes from the basal nuclei and cerebellum back to the cerebral cortex
to constantly monitor position and muscle tone
Somatic Motor Control
Motor
Association
areas
Decision
in frontal
lobes
Basal
nuclei
Cerebellum
The planning stage: When a conscious decision is
made to perform a specific movement, information
is relayed from the frontal lobes to motor association
areas. These areas in turn relay the information to
the cerebellum and basal nuclei.
Motor
Association
areas
Primary
motor
cortex
Basal
nuclei
Cerebellum
Other nuclei of the medial
and lateral pathways
Corticospinal pathway
Motor activity
Lower
motor
neurons
Movement: As the
movement begins,
the motor
association areas
send instructions to
the primary motor
cortex. Feedback
from the basal
nuclei and
cerebellum
modifies those
commands, and
output along the
conscious and
subconscious
pathways directs
involuntary
adjustments in
position and
muscle tone