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Anatomy of the ascending spinal cord pathways

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Anatomy of the ascending 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 a two-way communication between the brain and other parts of the body. It contains neural pathways, called spinal cord pathways, or simply spinal tracts, which can be either ascending or descending. Both types of tracts are made up of neuronal axons that gather into long columns called funiculi, which are found inside the ventral, lateral and dorsal aspects of the spinal cord. Here, we’ll focus on the ascending tracts, which are sensory pathways, carrying sensory information from the body, like pain, up the spinal cord to the brain.

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.

Now, the gray matter in the T1 through L3 spinal cord segments has a small lateral projection known as the lateral horn. The lateral horn is located between the dorsal and ventral horns, and contains the cell bodies of preganglionic neurons of the sympathetic nervous system, which mediates the fight-or-flight response, including increasing heart rate and dilating bronchioles to increase air intake. The gray matter is surrounded by white matter, which consists of bundles of axons that carry information to and from the brain. These axons are organized into bundles called tracts or pathways.

Now, let’s take a closer look at the ascending tracts. They begin at the spinal cord, and travel in the white matter, all the way up to the brain. Most of these pathways are made up of a three neuron chain that terminate on neurons in the cerebral cortex. These cell bodies in the cerebral cortex can be considered the fourth neuron.

The first-order neurons are those found inside dorsal root ganglia. These neurons gather sensory input from different receptors, such as touch or pressure receptors, and send it to the second-order neurons found inside the spinal cord or brainstem. Second-order neurons then transmit the information to the third-order neurons located inside the thalamus, and then the thalamus relays the information to the cell bodies in the cerebral cortex, the fourth-order neurons.

While climbing up through the spinal cord to the brain, most ascending tracts cross over to the opposite side of the central nervous system, meaning that the left side of the brain receives sensory input from the right side of the body and vice versa. These crossings are called decussations and they happen at different levels of the CNS for each of these tracts.

Now, there are several ascending tracts to look at in the spinal cord’s white matter. First, there is the dorsal column; which carries sensations like fine - or discriminative - touch, vibration and proprioception - meaning sensing the position of body parts in space. Another tract is the spinothalamic tract, which consists of the lateral and anterior spinothalamic tracts. In general, the spinothalamic tract transmits information such as pressure sensations, crude touch, pain, and temperature sensations.

There is also the spinocerebellar tract, which is also divided into the anterior spinocerebellar and the posterior spinocerebellar tracts. Just like the dorsal column, the spinocerebellar tract transmits proprioceptive sensory information. The difference is that the spinocerebellar tract is mainly in charge of unconscious proprioception, like the amount of stretch in our muscles or the degree of flexion in our knees or elbows, which enables us to maintain posture and produce movements.

Let’s take a deep dive into ascending tracts, starting with the dorsal column, known fully as the dorsal column-medial lemniscus - or DCML - pathway. This is a sensory pathway that transmits delicate, discriminative sensations like vibration, pressure, proprioception, and two-point discrimination, which is the ability to tell if two different points on your body are being touched at the same time.

All this is made possible by receptors like mechanoreceptors found throughout the skin that sense touch, or proprioceptors, like the muscle spindles and Golgi tendon organs located in muscles and joints that sense body position. When these receptors are stimulated, impulses are sent along peripheral nerves that are made of the axons of first-order neurons. These axons travel to the dorsal root ganglion where the cell bodies of these first-order neurons reside. From these ganglia, short axons enter the ipsilateral dorsal aspect of the spinal cord, forming the dorsal column.

The dorsal column is divided into two parts; one is the fasciculus gracilis, which is located throughout the length of the spinal cord and is situated in the medial part of the dorsal column in the upper thoracic and cervical segments. The fasciculus gracilis contains axons carrying sensations from the lower parts of the body, generally from T7 and below. So it contains ascending fibers from the sacral, lumbar, and lower thoracic spinal nerves.

The second part of the dorsal column is the fasciculus cuneatus, which, starting at about T7, is located in the lateral part of the dorsal column. The fasciculus cuneatus contains ascending fibers carrying sensations from the upper parts of the body. Both the fasciculus gracilis and fasciculus cuneatus ascend in the ipsilateral spinal cord to the medulla where the nucleus gracilis and nucleus cuneatus are located. This is where both fasciculi terminate after their fibers synapse with the cell bodies of these second-order neurons.

Second-order neuronal axons now decussate and form a pathway called the medial lemniscus. The medial lemniscus ascends to the thalamus, more specifically to the ventral posterolateral - or VPL - nucleus where the fibers synapse on the cell bodies of the third-order neurons. Finally, the third-order neuronal axons from the thalamus ascend towards the cerebrum, through the internal capsule to the primary somatosensory cortex where they terminate on the fourth-order neurons, the final destination for the dorsal column-medial lemniscus pathway.

Now, let’s take a quick break, and see if you can remember which component of the dorsal column-medial lemniscus carries information from the lower body.

Next, let’s look at the spinothalamic system, which includes the spinothalamic tract as well as the spinomesencephalic, spinotectal, and spinoreticular tracts which are collaterals - or offshoots - of the spinothalamic tract. Note that the spinothalamic system is also referred to as the anterolateral system because of its position within the white matter of the spinal cord.

Let’s explore the spinothalamic tract first. Information transmitted in this pathway begins in the periphery, travels to the spinal cord and then all the way up to the thalamus and finally to the primary somatosensory cortex. The spinothalamic tract is divided into the anterior spinothalamic tract, which transmits pressure sensations and crude touch; and the lateral spinothalamic tract, which conveys pain and temperature sensations.