Decussation of pyramids

The decussation of pyramids is a critical anatomical and functional landmark in the lower medulla, where approximately 90% of corticospinal fibers cross from one side of the brainstem to the other. These decussated fibers descend within the lateral white column of the spinal cord as the lateral corticospinal tract. Upon reaching their designated spinal levels, upper motor neurons in this tract synapse with alpha and gamma lower motor neurons in the anterior horns of the spinal cord. This motor tract is essential for the voluntary, skilled (or fine) movements of distal muscles.

The remaining fibers, which do not decussate at this point, continue uncrossed in the ventral aspect of the medulla and spinal cord, forming the anterior corticospinal tract. These fibers decussate at the level of their respective spinal cord segments, where they then synapse with lower motor neurons. This tract controls axial musculature, aiding in posture and balance.

Clinical Significance:

The decussation of pyramids is of paramount importance in understanding the clinical implications of corticospinal tract injuries. Lesions occurring above the decussation (e.g., in the cerebral cortex, internal capsule, midbrain, or pons) result in contralateral motor deficits (paralysis or weakness). In contrast, lesions located below the decussation (e.g., in the spinal cord) produce ipsilateral motor deficits.