Pyramidal and extrapyramidal tracts

In order for you to flex your bicep in the mirror, your brain and brainstem has to send a motor signal through the spinal cord to the muscles in the body.

These motor signals are carried through two tracts, the pyramidal and extrapyramidal tracts.

Neurons in the pyramidal tract are composed of upper motor neurons that directly innervate lower motor neurons in the anterior horn of the spinal cord.

Neurons in the extrapyramidal tract do not directly innervate lower motor neurons, but instead help coordinate muscle movement by indirectly activating or inhibiting groups of lower motor neurons through interneurons.

These groups of lower motor neurons usually innervate multiple muscles that share the same function, usually either flexors or extensors.

The pyramidal pathway is the primary pathway that carries out motor commands for voluntary movement. And it can be broken down into two main tracts, the corticospinal tract and the corticobulbar tract.

The corticospinal tract originates in the motor cortex where the cell bodies of the upper motor neurons are located.

The axons of these neurons travel together as fibers through the internal capsule to reach the brainstem where they form the medullary pyramids on the ventral surface of the brainstem.

At the level of the medulla, these fibers divide, and 90% of them form the lateral corticospinal tract which cross over to the opposite side of the medulla at the pyramidal decussation, while the remaining 10% of them form the anterior corticospinal tract which does not cross over just yet, and both tracts then travel through the spinal cord.

Neurons from the lateral corticospinal tract synapse on lower motor neurons in the anterior horn, while the neurons in the anterior corticospinal tract cross over in the spinal cord first before they synapse on the lower motor neurons in the anterior horn.