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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).
• Upper motor neuron lesions are
indicated by:
– Spasticity, increase in tone in the extensor muscles (lower
limbs) or flexor muscles (upper limbs)
– Clasp-knife response where initial resistance to movement is
followed by relaxation
– Weakness in the flexors (lower limbs) or extensors (upper
limbs), but no muscle wasting
– Increase Deep tendon reflex (DTR)
– Presence of Babinski sign
Descending Lesions cont.
• Damage to lower motor neurons, lower motor neurone lesions (LMNL)
causes:
– Decreased tone
– Decreased strength
And:
– Decreased reflexes in affected areas.
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 Brown-Séquard 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 Brown-Séquard 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 tracts in
spinal cord.
Spinal cord organization
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Spinal Cord Cross Section
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Spinal Cord, Roots & Nerves
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