USMLE® Step 1 Question of the Day: Spinal cord structure

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Learn how a skilled pathologist meticulously analyzes cross-sectional slides of the spinal cord, uncovering the significance behind the asterisk-marked structure. Discover the intricate functions woven into this essential neural component. Eager to delve deeper? Find out more about the fascinating realm of neural anatomy, unraveling the complexities that define spinal cord intricacies.

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

pathological view of the spinal cord

          Image reproduced from Wikimedia Commons

Which of the following best characterizes the function of this structure?

A. Inhibition of extensor motor neurons

B. Transmission of pain signals to the brain

C. Initiation of voluntary hand movements

D. Conduction of temperature sensation

E. Transmission of vibratory sensation

Scroll down for the correct answer!

The correct answer to today’s USMLE® Step 1 Question is…

E. Transmission of vibratory sensation

Before we get to the Main Explanation, let’s look at the incorrect answer explanations. Skip to the bottom if you want to see the correct answer right away!

Incorrect answer explanations

A. Inhibition of extensor motor neurons

Incorrect: The rubrospinal tract facilitates flexor and inhibits extensor motor neurons. However, the rubrospinal tract is located in the lateral white column instead of the dorsal white column.

B. Transmission of pain signals to the brain

Incorrect: The spinothalamic tract relays pain, temperature, crude touch, and pressure sensations to the brain. The spinothalamic tract is located within the anterior and lateral portions of the spinal cord.

C. Initiation of voluntary hand movements

Incorrect: The corticospinal tract facilitates voluntary motor control of the trunk and extremities. The fibers of the corticospinal pathway can be found in the anterior and lateral portions of the spinal cord. In contrast, the asterisk is located within the dorsal white column, which houses the dorsal column medial lemniscus tract.

D. Conduction of temperature sensation

Incorrect: The spinothalamic tract relays pain, temperature, crude touch, and pressure sensations to the brain. The spinothalamic tract is located in the anterior and lateral portions of the spinal cord.

Main Explanation

The asterisk points to the dorsal column medial lemniscus tract, one of the three main ascending tracts in the spinal cord. The other two tracts are the spinothalamic tract and spinocerebellar tract. All three tracts enter the spinal cord via the dorsal root.

The dorsal column medial lemniscus (DCML) tract transmits delicate sensations, including vibration, proprioception, two-point discrimination, and touch to the brain. Neurons in this tract ascend via the ipsilateral dorsal columns and decussate (crossover) within the nucleus gracilis and cuneatus of the medulla.

The spinothalamic tract transmits sensations, including crude touch, pain, pressure, and temperature to the brain. Neurons ascend ipsilaterally for 1-2 spinal segments, decussate in the anterior white commissure, and ascend via one of two tracts in the anterior or lateral spinal cord.Finally, there is the spinocerebellar tract, which transmits unconscious proprioceptive sensations to the brain. After entering the spinal cord, the neurons either decussate immediately and become part of the ventral spinocerebellar tract (which later decussates again in the superior cerebellar peduncle) or ascend ipsilaterally as the dorsal spinocerebellar tract.

image of the three main ascending tracts in the spinal cord

Major Takeaway

The dorsal column medial lemniscus (DCML) tract ascends in the ipsilateral dorsal column and decussates within the nucleus gracilis and cuneatus of the medulla. It relays sensations, including vibration, proprioception, two-point discrimination, and touch to the brain.

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