Syringomyelia with dilated central cord T2 hyperintensity

A 34-year-old woman presents with 2 years of progressive dissociated sensory loss in a cape-like distribution over the shoulders and upper back, with loss of pain and temperature sensation but preserved light touch and proprioception. She has painless burn injuries to her fingertips and progressive upper-limb weakness with wasting of small hand muscles. Examination reveals lower motor neuron weakness in C8–T1 myotomes and upper motor neuron signs in the legs. Spine MRI shows cerebellar tonsillar descent of 8 mm below the foramen magnum. The structure marked **B** in the diagram—a longitudinally extensive T2 hyperintense fluid-filled cavity within the central cervicothoracic cord—is responsible for her neurological deficits through interruption of which anatomical pathway?