The corticospinal or pyramidal tract is a massive collection of axons that travel
between the cerebral cortex of the brain and the
spinal cord.
The corticospinal tract mostly contains motor axons. It actually consists of two separate
tracts in the spinal cord: the lateral corticospinal tract and the
medial corticospinal tract. An understanding of these tracts leads to an
understanding of why for the most part, one side of the body is controlled by the opposite side of the brain.
The motor pathway
The corticospinal tract originates from cells in layer V of the motor cortex.
Upper motor neurons
The motor neuron cell bodies in the motor cortex, together with their axons that travel
down the brain stem and spinal cord, are referred to as
upper motor neuron.
Decussation and synapses
The neuronal cell bodies in the motor cortex send long axons to the motor cranial nerve nuclei mainly of the contralateral side of the midbrain (cortico-mesencephalic tract), pons
(cortico-pontine tract), medulla
oblongata (cortico-bulbar tract); the bulk of these fibers, however, extend
all the way down to the spinal cord (corticospinal tract).
Whichever of these two tracts it travels in, a cortico-spinal axon will synapse with another neuron in the ventral horn. This
ventral horn neuron is considered a second-order neuron in this pathway, but is not part of the corticospinal tract itself.
From cerebral to motor neurons
The motor axons move closer together as they travel down through the cerebral white
matter, and form part of the posterior limb of the internal capsule.
The motor fibers continue down into the brainstem. The bundle of corticospinal axons is
visible as two column-like structures ("pyramids") on the ventral surface
of medulla oblongata - this is where the name pyramidal tract comes from.
After the decussation, the axons travel down the spinal cord as the lateral corticospinal tract. Fibers that do not cross over in the medulla oblongata travel down the separate anterior
corticospinal tract, and most of them cross over to the contralateral side in the spinal
cord, shortly before reaching the lower motor neurons.
Lower motor neurons
In the spinal cord, the axons of the upper motor neuron connect (most of them via interneurons, but to a lesser extent also via direct synapses) with the
lower motor neurons (LMNs), located in the ventral
horn of the spinal cord.
In the brain stem, the lower motor neurons are
located in the motor cranial nerve nuclei (occulomotor, trochlear, motor nucleus of the trigeminal nerve, abducens, facial, accessory, hypoglossal). The lower motor neuron axons leave the brain stem via motor cranial nerves and the spinal cord via anterior roots of the spinal nerves respectively,
end-up at the neuromuscular plate and provide motor innervation for voluntary
muscles.
Sensory pathways
Corticospinal tract damage
see upper motor neuron.
Extrapyramidal motor pathways
These are motor pathways that lie outside the corticospinal tract and are beyond voluntary control. Their main function is to
support voluntary movement and help control posture and muscle tone. See extrapyramidal
motor system.
Additional images
Dissection of brain-stem. Lateral view.
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Superficial dissection of brain-stem. Ventral view.
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External links
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Brain: rhombencephalon
(hindbrain) |
| Myelencephalon/medulla |
anterior/ventral: Arcuate nucleus of medulla • Pyramid (Decussation) •
Olivary body • Inferior olivary
nucleus • Anterior median fissure •
Ventral respiratory group
posterior/dorsal: VII,IX,X: Solitary/tract • XII, X: Dorsal • IX,X,XI: Ambiguus •
IX: Inferior salivatory nucleus • Gracile nucleus/Cuneate nucleus/Accessory cuneate nucleus • Area postrema •
Posterior median sulcus • Dorsal respiratory group
raphe/reticular: Sensory decussation • Reticular formation (Gigantocellular nucleus,
Parvocellular reticular nucleus, Ventral reticular nucleus, Lateral reticular
nucleus, Paramedian reticular nucleus) • Raphe nuclei (Obscurus, Magnus, Pallidus)
tracts: Corticospinal tract (Lateral, Anterior) •
Inferior cerebellar peduncle • Olivocerebellar tract • Spinocerebellar
(Dorsal, Ventral) • Spinothalamic tract •
PCML (Posterior external arcuate fibers, Internal
arcuate fibers, Medial lemniscus) • Extrapyramidal (Rubrospinal tract, Vestibulospinal tract, Tectospinal tract)
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| Metencephalon/pons |
anterior/ventral: Superior olivary nucleus • Basis pontis (Pontine nuclei, Middle cerebellar peduncles)
posterior/dorsal: Pontine tegmentum (Trapezoid body, Superior medullary velum,
Locus ceruleus, MLF,
Vestibulocerebellar tract, V
Principal Spinal &
Motor, VI, VII, VII: Superior salivary nucleus) •
VIII-c (Dorsal, Anterior)/VIII-v (Lateral, Superior, Medial, Inferior)
raphe/reticular: Reticular formation (Caudal pontine reticular nucleus, Oral
pontine reticular nucleus, Tegmental pontine reticular
nucleus, Paramedian pontine reticular formation) •
Median raphe nucleus
Apneustic center • Pneumotaxic
center
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| Metencephalon/cerebellum |
Vermis •
Flocculus • Arbor
vitae • Cerebellar tonsil • Inferior medullary velum
Molecular layer (Stellate cell, Basket
cell, Parallel fiber) • Purkinje cell layer (Purkinje cell) • Granule cell layer (Golgi cell) •
Mossy fibers • Climbing
fiber
Deep cerebellar nuclei (Dentate,
Emboliform, Globose, Fastigial) |
| Fourth ventricle |
apertures (Median, Lateral) • Rhomboid fossa (Vagal trigone, Hypoglossal trigone, Obex, Sulcus
limitans, Facial colliculus, Medial
eminence) • Lateral recess |
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