The pyramidal tract is a pathway of cortical efferent fibers composed of billions of myelinated axons which arise from pyramidal neurons (Betz cells) in the motor area of the frontal lobe. The long outgrowths, or axons, of these neurons form direct and indirect, descending pathways to the motor-reflex centers of the spinal cord, establishing synapse with nerve cells in the anterior horns of spinal cord; cortical information is transmitted over these pathways. The pyramidal tract does not exist in lower vertebrates as it is found only in mammals, where it forms the efferent portion of the motor analyzer, and it is most developed in Homo sapiens. When it is studied together with the afferent (ascending) fibers, it is called corticospinal tract.
The myelinated fibers of the pyramidal tract descend first directly towards the spinal cord; they also form branches, or collaterals, to the nuclei of the cranial nerves. They become a portion of the cerebral peduncles as they run down to the pons. At the lower border of the medulla oblongata, about 80% of these fibers decussate (cross over) to opposite side of medulla to form the lateral pyramidal tract; this crossing over of fibers continues in the upper portion of the spinal cord. Thus, in the spinal cord, the pyramidal tract divides into the lateral pyramidal tract (inderect), and anterior pyramidal tract (direct), which is the 20% of fibers that do not decussate, descending straight and lying anteriorly on both sides of the anterior median fissure of spinal cord. Here, the myelinated motor axons of pyramidal tract make synapse with alpha motor neurons in the anterior horns of the butterfly-shaped grey matter of spinal cord, from which motor or efferent axons come out of the spinal cord to combine with those of the posterior horns to form the spinal nerves.
In human beings, the pyramidal tract contains billions of myelinated axons, which cluster together to form thick, rapidly conducting fibers that are responsible for rapid phase movements and thin, slowly conducting fibers that are responsible for tonic contractions of muscles. The thick fibers are about 16 microns (μ) in diameter with a conduction rate as fast as 80 m/sec, whereas the thin fibers are about 4 μ in diameter with a conduction rate that ranges from 7 to 25 m/sec. Injuries to the pyramidal tract are manifested by paralysis, paresis, and pathological reflex activity. These disturbances may be compensated for by an intensification of activity in other descending systems, namely, those that belong to the extrapyramidal tract.
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| In red, the pathway of the pyramidal tract and their fibers end up in spinal cord. |