The
corticospinal or
pyramidal tract is a 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 anterior 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 corticobulbar tract is also considered to be a pyramidal tract, though it carries signals to motor neurons of the cranial nerve nuclei, rather than the spinal cord.
The neurons of the corticospinal tracts are referred to as pyramidal neurons. The name comes from the shape of the corticospinal tracts as they pass through the medulla, which somewhat resembles a pyramid.
The corticospinal tract is concerned specifically with discrete voluntary skilled movements, especially of the distal parts of the limbs. (Sometimes called "fractionated" movements)
The motor pathway
The corticospinal tract originates from
pyramidal cells in layer V of the
cerebral cortex. About half of its fibres arise from the
primary motor cortex. Other contributions come from the
supplementary motor area,
premotor cortex,
somatosensory cortex,
parietal lobe, and
cingulate gyrus. The average fibre diameter is in the region of 10μm; around 3% of fibres are extra-large (20μm) and arise from
Betz cells, mostly in the leg area of the primary motor cortex.
Upper motor neurons
The neuronal cell bodies in the
motor cortex, together with their axons that travel down through the
brain stem and
spinal cord are commonly referred to as
upper motor neurons. It should be noted however, that they do not project to muscles, and thus the term 'motor neuron' is somewhat misleading.
Some of 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 (
Corticopontine tract), and
medulla oblongata (
cortico-bulbar tract), decussating just before they reach their target nuclei. These are called
geniculate fibers. Many more motor cortex neurons, however, extend fibers all the way down to the
spinal cord (corticospinal tract).
Most of the corticospinal fibers (about 80%) cross over to the contralateral side in the medulla oblongata (pyramidal decussation). Those that cross in the medulla oblongata travel in the lateral corticospinal tract.
10% enter the lateral corticospinal tract on the same side.
The remainder of them (10%) cross over at the level that they exit the spinal cord, and these travel in the anterior corticospinal tract.
Whichever of these two tracts it travels in, a corticospinal 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, 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 (oculomotor, 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
Spinothalamic tract
Spinocerebellar tract
Visual pathway
Olfactory system
Posterior column pathway
Corticospinal tract damage
Damage to the descending motor pathways anywhere along the trajectory from the cerebral cortex to the lower end of the spinal cord gives rise to a set of symptoms called the "upper motor neuron syndrome". A few days after the injury to the upper motor neurons a pattern of motor signs and symptoms appears, including
spasticity, the decreased vigor (and increased threshold) of superficial
reflexes, a loss of the ability to perform
fine movements, and an
extensor plantar response known as the
Babinski sign.
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
References
External links
McGill
Category:Central nervous system pathways
Category:Motor system
