Chapter 10: Corticospinal Tract

The Corticospinal tract (CST)

Also known as the pyramidal tract, is a collection of axons that carry movement-related information from the cerebral cortex to the spinal cord. It forms part of the descending spinal tract system that originate from the cortex or brainstem

Upper and lower motor neurons

Are neurons that travel in the corticospinal tract they synapse on neurons in the spinal cord called the lower motor neurons,  which make contact with skeletal muscle to cause muscle contraction.

Major pathways that carry movements related from the brain to the spinal cord

Is the corticospinal tract. It has approximately 1 million nerve fibres (average conduction velocity of approximately 60m/s using glutamate as their transmitter substance)

Signaling along the corticospinal tract

Is involved in a variety of movements, including behaviors like walking and reaching, but it is especially important for fine finger movements e.g. writing, typing, or buttoning clothes.

CST represents

The highest order of motor function in humans and is most directly in control of fine, digital movements

After selective damage to the corticospinal tract

Patients are usually able to regain the ability to make crude movements (e.g. reaching) after a period of time, but they may be unable to fully recover the ability to make individual finger movements

Where does the CST originates?

It originates in several cortical areas, about half of these axons extend from neurons in the primary motor cortex, but others originate in the nonprimary motor areas of the brain as well as in regions of the parietal lobe like the somatosensory cortex.

How does the axons in the CST travel?

The axons that travel in the CST descend into the brainstem as part of large fiber bundles called the cerebral peduncles.

Pyramids

Is the two large collection that is formed own the tract continues to travel down the medulla. The pyramids create visible ridges on the exterior surface of the brainstem

What happens at the base of the pyramids?

Approximately 90% of the fibers in the corticospinal tract decussate, or cross over to the other side of the brainstem, in a bundle of axons called the pyramidal decussation.

Lateral corticospinal tract

Is formed when the fibers that have decussated. They will enter the spinal cord, and thus cause movement, on the side of the body that is contralateral to the hemisphere of the brain in which they originated.

What happens to the other 10% of the corticospinal tract fibers?

They do not decussate; they will continue down into the ipsilateral spinal cord; this branch of the corticospinal tract is known as the anterior (or ventral) corticospinal tract.

Most of the axons of the anterior corticospinal tract will decussate in the spinal cord just before they synapse with lower motor neurons.

The fibers of these two different branches of the corticospinal tract

Preferentially stimulate activity in different types of muscles.

Function of the lateral corticospinal tract 

It primarily controls the movement of muscles in the limbs

Function of the anterior corticospinal tract

Is involved with movement of the muscles of the trunk, neck, and shoulders

Of all corticospinal fibres ;

Aapproximately 20% terminate at thoracic levels, 25% at lumbosacral levels and 55% at cervical levels. Many of the fibres that originate from the motor cortex then terminate in the ventral horn of the spinal cord

Function of the CST

They include control of afferent inputs, spinal reflexes and motor neuron activity, the most important being the mediation of voluntary distal movements

Outputs from the primary motor cortex

Contribute to the CST, making connections to: excitatory monosynaptic alpha motor neurons;

polysynaptic connections onto gamma motor neurons (responsible for the control of muscle spindle length);

polysynaptic connections via interneurons within the spinal cord

Monosynaptic and polysynaptic

When the neurons are influenced directly by only one axon, they are called “monosynaptic,” and when indirectly, by many axons, they are known as “polysynaptic.”

Origin and termination of the CST neurons

• 30%-40% arise from the primary motor cortex.

• Rest of the fibers arise from the supplementary motor area (SMA), premotor cortex (PMA), parts of the somatosensory areas (S1 and S2) and parts of the posterior parietal cortex.

The fibers originating from the sensory cortex

Terminate in the dorsal horn of the spinal cord. Here they synapse with interneurons that receive input from somatosensory receptors and are thought to regulate information from peripheral receptors within the spinal cord.

Therefore, the CST may act as a ‘gate’, modulating or inhibiting information that is deemed useful or irrelevant