Neurology: Spinothalamic & Dorsal Column Medial Lemniscus Pathways in 3D
Channel: About Medicine
Date: 2018-09-23

Sensory pathways of the nervous system, in simplified schematic 3D.

This video on sensory pathways covers the two major pathways: the spinothalamic tract, and the dorsal column medial lemniscus pathway.

What is the spinothalamic tract? What is the dorsal column medial lemniscus pathway? All these common questions and more answered in this video.

Looking for a spinothalamic pathway animation, or a dorsal column medial lemniscus animation? Look no further.

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What I’m drawing here is a sensory pathway.

From finger tip to brain, from sensory receptor to sensory cortex. Through a peripheral nerve, the spinal cord, the brainstem, the thalamus, always ending up on the opposite side of the brain. 

Now if we take everything else away, we see that the pathway is divided into three parts. That’s three separate neurons, the first, second and third order neurons.

The first order neuron picks up the information, and transfers it to the second order neuron somewhere in the spinal cord or brainstem. 

The second order neuron conveys that information to cell body of the third order neuron, which is usually in the thalamus. The third order neuron conveys it to the sensory cortex.

Before we go any further, some brief revision.

Neurons consist of axons and cell bodies. Individually these are microscopic, but when bunched together they appear white, and grey. 

In the central nervous system, i.e. the brain and the spinal cord, we call discrete chunks of grey matter nuclei. In the peripheral nervous system, i.e. everything apart from the brain and spinal cord, we call them ganglia.

The cell body of the first order neuron is packed into a ganglion called the dorsal root ganglion. There are heaps of these, one for every spinal nerve. It’s just a bunch of cell bodies, from nerves that relate to the same spinal nerve.


Now let’s talk about the individual pathways.

Ok, so let’s take the spinothalamic pathway first. I’m going to bring back in our bits and pieces. Then cut away the spinal cord at the level this nerve enters. Let’s say that’s T5, or the fifth thoracic spinal nerve.

FON has its CB in the DRG. The axon keeps going and drops its info on the CB of the SON, which will be sitting in the dorsal horn of the spinal cord’s grey matter.

Ok, so this is where the crossover happens for the spinothalamic pathway. There are two options for where it will cross over to, and which path it takes depends on the type of info that neuron is responsible for transmitting.

If that neuron is responsible for transmitting information about crude touch, it will move anteriorly, here.

If that neuron is responsible for transmitting information about pain and temperature, it will move laterally, over here.

These are the anterior and lateral spinothalamic tracts. Tract is just a word for bunch of axons.

If we zoom out to get our bearings, yeah cool that makes sense.

So our information has already crossed over, let’s say into the anterior spinothalamic tract.
So we continue up toward the thalamus, which contains the cell body of our third order neuron.

The thalamus is essentially a big nucleus. All grey matter.

So we found our cell body for the TON in the thalamus, it’s axon shoots to the sensory cortex to drop it off. If we remember our sensory homunculus - legs, arm, hand, face, tongue - that represents where the endpoints for the relevant sensory pathway.

That’s it for the spinothalamic pathway. Let’s move on to the dorsal column, medial lemniscus pathway.

Same again, our FON starts out here and has its CB in the DRG.

Instead of dropping its message here, the axon heads keeps on going, onward and up the spinal cord.

 It moves in here because all of this here, between the dorsal grey horns, is the dorsal column. 

It’s split down the middle of course, and this neuron will end up around hereish. Because, this half is split again into two separate parts, two separate bundles, one called gracile, and one called cuneate. If the nerves coming from the legs it’ll be in the gracile bundle, because gracilis is in the leg. If the nerve’s coming from the arms, or the upper body, it’ll be in the cuneate bundle.