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Anatomy of the descending spinal cord pathways

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Anatomy of the descending spinal cord pathways

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USMLE® Step 1 style questions USMLE

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A pathologist views cross-sectional slides of the brainstem and identifies the following structure (denoted by an asterisk):  


Which of the following best characterizes the function of the fibers passing through this structure?  

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The spinal cord is like a highway that enables two-way communication between the brain and the rest of the body. It contains neural pathways, called spinal cord pathways or tracts, which either ascend or descend depending on the information they are carrying. Both types of tracts are made up of neuronal axons that gather into long columns which are found inside the ventral, lateral and dorsal aspects of the spinal cord. The descending tracts are the motor pathways that tell various muscles in the body to contract, like when lifting a heavy weight.

Now, let’s start by looking at the anatomy of the spinal cord. Anteriorly, there is a deep midline depression called the ventral median fissure and, posteriorly, there is a more shallow midline depression called the dorsal median sulcus. Each half also has a ventrolateral sulcus, where ventral rootlets leave the spinal cord; and a dorsolateral sulcus, where dorsal rootlets enter the spinal cord. The ventral and dorsal rootlets fuse to form the ventral and dorsal roots, respectively Ventral rootlets and roots carry motor fibers that travel from the spinal cord to different organs and muscles, while their dorsal counterparts - with a sensory ganglion, called the dorsal root ganglion, attached to each dorsal root - carry sensory fibers from organs and receptors throughout the body to the spinal cord.

Now, on a transverse section, the spinal cord has an area of gray matter shaped like a capital “H” in the middle. The gray matter is subdivided into the gray commissure, which is the strip connecting the two halves of the spinal cord that surrounds the central canal; and the peripheral regions known as horns. There are two ventral, and two dorsal horns. Dorsal horns contain neuronal cell bodies that process information received from sensory fibers, entering the spinal cord from the dorsal roots and dorsal rootlets. On the other hand, the ventral horns contain cell bodies of motor neurons, with motor fibers,exiting through the ventral rootlets and ventral roots.

Alright, now let’s look at the descending somatic motor, pathways, which control the activity of skeletal muscles.. These pathways originate from either the cerebral cortex or the brainstem and have various termination points, depending on the pathway. Each pathway contains an upper motor neuron and a lower motor neuron. The upper motor neurons are found inside the cerebral cortex and various nuclei of the brainstem, while lower motor neurons are found inside the brainstem or the ventral horn of the spinal cord. From the ventral horns, axons from lower motor neurons leave the spinal cord through the ventral roots and travel within peripheral nerves to innervate the skeletal muscles of the body.

First up, there’s the corticospinal tract, which is a motor pathway that runs from the cerebral cortex to the spinal cord - hence the name “cortico-spinal”. This tract is responsible for the voluntary movements of the limbs and trunk.

The cell bodies of the first-order neurons of this pathway lie in the primary motor cortex. The axons then descend via the corona radiata, through the internal capsule, the cerebral peduncle of the midbrain, and the ventral aspect of the pons to reach the ventral aspect of the medulla.

At the medulla these fibres bundle together and create swellings known as the pyramids, and the majority of the corticospinal fibers decussate - or cross over to the opposite side - at the level of the caudal medulla, in what’s known as the pyramidal decussation. At this level, the corticospinal tract divides into the lateral corticospinal tract, and the anterior corticospinal tract. The decussated fibers form the lateral corticospinal tract, while the uncrossed fibers continue as the anterior corticospinal tract.

Both the lateral and the anterior corticospinal tracts descend through the spinal cord. The lateral corticospinal tract continues through the contralateral lateral column of the spinal cord, and synapse in the ventral horn on the lower motor neurons. Axons of the lower motor neurons leave the spinal cord through the ventral root to innervate muscles of the limbs, especially distal musculature that control fine movements. On a cross section of the spinal cord, the lateral corticospinal tract is organized based on the region of the spinal cord it is supplying; from lateral to medial, it is organized into sacral, lumbar, thoracic and cervical fibers. So, axons that control motor innervation of the upper limbs, terminate in the cervical region, and are therefore located more medially, whereas axons that control lower limb movements, terminate in the lumbar region and are more lateral.