Anatomy of the ascending spinal cord pathways

Chapters:

Watch video only

Intro0:00–0:41

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.

anatomy overview0:41–2:55

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.

Ascending tracts2:55–5:13

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.

Dorsal column-medial lemniscus5:13–7:58

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.

Quiz7:58–8:13

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.

Spinothalamic tract8:13–10:57

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.The first-order neurons of the spinothalamic tract originate from receptors within the skin.
The impulse travels within peripheral nerves to the dorsal root ganglion, where the cell bodies of the first-order sensory neurons are located.
Then the signal enters the spinal cord, where the axons of the anterior tract don't actually synapse right away, but first ascend one or two spinal levels before synapsing with the second-order neuronal cell bodies located in the ipsilateral dorsal horn of the gray matter.
On the other hand, the first-order neurons of the lateral tract typically synapse right away with the second order neurons at the same level they enter the spinal cord.
The axons of the second-order neurons decussate via the anterior white commissure of the spinal cord, and then both anterior and lateral spinothalamic tracts ascend through the contralateral white matter of the spinal cord to the thalamus, specifically to the ventral posterolateral - or VPL - nucleus, where they terminate by synapsing with the third-order neurons within this nucleus.
As the lateral spinothalamic tract ascends through the spinal cord, new fibers are added to the anteromedial aspect of the tract.
As a result, the sacral and lumbar fibers are located more posterior and laterally, whereas the thoracic and cervical fibers are somewhat more anterior and medial.
From the VPL nucleus of the thalamus, it’s pretty much the same for both tracts. The third-order neuronal axons ascend toward the cerebrum through the internal capsule to the primary somatosensory cortex where they synapse on the fourth-order neurons.Now let’s explore the collaterals of the spinothalamic tract.

Spinomesencephalic and Spinotectal tracts10:57–12:09

First, we have the spinomesencephalic tract, which carries information about pain to the brainstem, specifically the periaqueductal gray of the midbrain.
The first-order neuronal axons of this tract enter the spinal cord from the dorsal root ganglion, travel to the gray matter where they synapse on the cell bodies of its second-order neurons.
Axons of the second-order neurons of this tract then decussate and ascend in the anterolateral white column, terminating by synapsing with neurons in the periaqueductal gray.
The periaqueductal gray then initiates a cascade of synapses that terminates back on neurons within the spinal cord. This tract and system is important in helping to inhibit, or control, painful sensations.
There are also collaterals that terminate in the superior colliculus within the tectum of the midbrain, through the spinotectal tract.
The superior colliculus is an important visual reflex centre, with the spinotectal tract providing afferent information that results in reflexive eye and head movements toward the site of painful stimuli.
Next is the spinoreticular tract. Its first-order neurons enter the spinal cord from the dorsal root ganglion and enter the gray matter where they synapse with the cell bodies of the second-order neurons.

Spinoreticular tract12:09–12:55

Second-order neuronal axons of this tract ascend in the lateral white column, mixed with axons of the lateral spinothalamic tract.
Most of the fibers of the spinoreticular tract do not decussate. They terminate by synapsing with the cell bodies of neurons within the reticular formation in the medulla oblongata, the pons, and the midbrain.
Since the reticular formation plays a role in alertness and attention, the spinoreticular tract plays a role in directing attention towards painful stimuli.
All right, let’s take another pause. But in the meantime, what is the function of the lateral spinothalamic tract?The last of the ascending pathways are the spinocerebellar tracts.

Quiz12:55–13:07

Spinocerebellar tract13:07–14:55

Of the three tracts that send proprioceptive information to the cerebellum, there are two that are prominent in the white matter of the spinal cord, the anterior and posterior spinocerebellar tracts.
These are sensory pathways that send unconscious proprioception to the cerebellum, which in turn helps to maintain posture and coordinate voluntary movement.
The signals on these pathways originate in proprioceptive receptors such as muscle spindles and golgi tendon organs, and then travel along first order neuronal axons of peripheral nerves.
The signal enters the dorsal root ganglion and then the spinal cord where there’s a synapse with second order neurons located in the ipsilateral gray matter.
The fibers either decussate right away and create the anterior spinocerebellar tract just lateral to the lateral spinothalamic tract; or they remain on the ipsilateral side to form the posterior spinocerebellar tract.
The anterior spinocerebellar tract ascends to the superior cerebellar peduncle, passing through it and then decussating again before arriving at the ipsilateral cerebellar cortex where it synapses on neurons there.
On the other hand, the posterior spinocerebellar tract doesn’t decussate. Instead, it ascends to the inferior cerebellar peduncle, passes through it and arrives at the ipsilateral cerebellar cortex as well, ending its journey by synapsing on neurons in the cortex.
So, compared to other sensory tracts, spinocerebellar tracts have a three neuron chain and transmit information ipsilaterally between the body and the cerebellum, whereas other ascending tracts are contralateral in their target.

Quiz14:55–15:06

Okay, one last question before we recap. What is the function of the spinotectal tract?

Recap15:06–17:02

All right, as a quick recap... Ascending tracts are sensory pathways that carry information received from various sensory receptors, from the spinal cord up to the brain.
Most ascending tracts consist of three to four successively connected neurons that terminate in the cerebral cortex. First-order neurons carry information from sensory receptors through the dorsal root ganglion - where their cell bodies are found - to second-order neurons located in the spinal cord or brainstem.
These second order neurons then transmit sensory information to the third-order neurons in the thalamus, and then the fourth-order neurons located in the cerebral cortex.
The ascending tracts include the dorsal column, which transmits sensory information including vibration, conscious proprioception, and two-point discrimination.
The dorsal column consists of the fasciculus gracilis carrying sensations from the lower half of the body and the fasciculus cuneatus which transmits sensory information from the upper half of the body.
There is also the spinothalamic tract which consists of the lateral spinothalamic, and anterior spinothalamic tracts. The spinothalamic tract transmits information such as pressure, crude touch, pain, and temperature sensations.
The spinothalamic tract includes as collateral tracts: the spinomesencephalic tract, which helps in pain suppression; the spinotectal tract, which is involved in reflexive eye and head movements toward the site of painful stimuli and the spinoreticular tract which assists with attention to painful stimuli.
The last ascending tracts are the spinocerebellar tracts, which carry unconscious proprioceptive information from voluntary musculature to the