Descending pathways
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.
Location of descending Pathways
• Lateral corticospinal tract
• Anterior corticospinal tract
ANATOMICAL ORGANIZATION
• FIRST ORDER- CEREBRAL CORTEX
• SECOND ORDER(INTERNUNCIAL
NEURON- ANTERIOR GRAY COLUMN
• THIRD ORDER(LOWERMOTOR
NEURON)- ANTERIOR GRAY COLUMN
Nerve pathways
Descending Tracts
Signal function
Tract
Corticospinal (pyramidal)
Fine voluntary motor control of the limbs. The
pathway also controls voluntary body posture
adjustments.
Rubrospinal
Involved in involuntary adjustment of arm position in
response to balance information; support of the body.
Reticulospinal (1) Pontine
Regulates various involuntary motor activities and
assists in balance (leg extensors). Some pattern
movements e.g. stepping
(2) Medullary
Inhibits firing of spinal and cranial motor neurons,
control of antigravity muscles.
Vestibulospinal (1) Medial
It is responsible for adjusting posture to maintain
balance (neck muscles).
(2) Lateral
It is responsible for adjusting posture to maintain
balance (body/lower limb).
Tectospinal
Controls head and eye movements, Involved in
involuntary adjustment of head position in response to
visual information.
Spinal cord organization
White matters
(nerve fibres)
lateral
dorsal
Sensory
inflow
Dorsal
horn
medial
Ventral
horn
Grey matters
(cell bodies)
ventral
Motor
commands
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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-basispedunculi
Pons-transverse
pontocerebellar fibers
Medulla-pyramids
Junctionof medulla and spinal
cord-decussating of fibers
Lateral corticospinal tract
DESCENDING
TRACTS OF
MOTOR
FIBERS
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,
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).
Corticospinal tract leisons
• Babinski s sign is present
• Superfficial abdominal reflexes absent
• Loss of fine skilled movements
• Great toe
becomes
dorsiflexed and
other toes fan
outward-positive
• Normally there is
plantar flexion of
all toes because
corticospinal
tract is intact
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
– 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.
– Weakness in the flexors (lower limbs) or
extensors (upper limbs), but no muscle
wasting
• Exaggerated abdominal reflexes
Lower motor neuron leison
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Flaccid paralysis
Atrophy of muscles
Muscular fasiculations
Muscular contracture
• Overall, upper
• lower motor neuron
motor neuron
lesions result in
lesions result in
hypotonia, no
hypertonia,
reflexes, flaccid
hyperreflexia,
paralysis, muscle
spastic paralysis,
atrophy
revival of the
Babinski reflex
(stroke lateral sole)
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
• 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.
Ascending Pathway Lesions
• Loss of sensory input from
relevant pathway
–E.g. Spinothalamic tract
• Unilateral lesion usually causes contralateral
anaesthesia (loss of sensation (pain and
temperature)). Anaesthesia will normally begin 1-2
segments below the level of lesion, affecting all
caudal body areas. This is clinically tested by using
pin pricks.
– If lesion is hemisection (halfway across the spinal
cord) (causing hemiplegia)) it is known as BrownSéquard syndrome.
• Brown-Séquard syndrome may be caused by a spinal cord
tumour, trauma (such as a gunshot wound or puncture
wound to the neck or back), ischemia (obstruction of a blood
vessel), or infectious or inflammatory diseases such as
tuberculosis, or multiple sclerosis.
– Any presentation of spinal injury which is an
incomplete lesion can be called a partial BrownSéquard or incomplete Brown-Séquard syndrome, so
long as it has characterized by features of a motor
loss on the same side of the spinal injury and loss of
sensation on the opposite side.
Questions?
• Draw labelled diagram showing ascending
tracts in spinal cord.
• What will happen in leison of spinal
leminiscus in medulla oblongata.
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Dorsal root
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Spinal cord organization
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Spinal Cord Cross Section
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Spinal Cord, Roots & Nerves
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