Video: Posterior column-medial lemniscus pathway (PCML)

Just like Frodo in Lord of the Rings, the sensory information has a long way to travel from the peripheral regions to Mordor – I mean, the somatosensory cortex. And although there may not be any angry spiders, overly possessive ringbearers, or evil wizards trying to stop it along the way, each sensory signal has many stops, twists, and turns that are necessary to make it get to where it needs to be. Just like how Frodo travel to deliver a piece of mysterious ring to a flaming mountain far away from home.

Let’s start off on our own sensory adventure now as we explore the posterior column - medial lemniscus pathway.

We’ll get to know why this ascending sensory tract has such a complicated name in a few minutes, but first, let’s see exactly what topics we’ll tackle today. We’ll first look at the function of the pathway by reviewing what types of sensory information it carries to the cortex. We’ll then look at its position in the cross-section of the spinal cord. We’ll take a close look at how the first, second, and third order neurons travel in the PCML pathway, and along the way, we’ll look at the arrangement of fibers which carry the sensory information and where in the somatosensory cortex the information is received. Finally, we’ll take some look at some clinical notes.

The posterior or dorsal column - medial lemniscus, often called the PCML or DCML for short, is an ascending pathway in the brain and spinal cord that carry sensory information from receptors in the periphery of the body to the brain for processing. But, what sort of sensory information?

The PCML is a bit of a multitasker, as it carries information from a few different sensations. Firstly, it is associated with fine touch like when a little spider crawls up your arm. It also carries information about vibration, or oscillation. It’s also involved in the pathway of discriminative touch. So, what does that encompass exactly? It includes tactile localization or knowing exactly where you’re being touched. It’s also concerned with two-point discrimination which allows you to know that you’re being touched in more than one spot simultaneously. It includes recognizing the shape of the object you’re holding known as stereognosis. Finally, it carries information about conscious proprioception, which allows you to purposely recognize the position and movement in different parts of your body. For example, if you are dancing, you need to know where your limbs are any given time. This is different from unconscious proprioception; however, we’ll speak more about this later on in the tutorial.

So, we now understand the functions of this pathway, but its name may still seem daunting. Let’s take a look at why it’s named the way it is.

So, where did the name posterior column - medial lemniscus pathway come from? Well, there are a number of pathways that information from the outside world can travel to the brain and often they’re named according to the structures that they pass through – and that’s what we’ve done here.

We’re now looking at an illustration of a transverse section of the spinal cord with gray matter in the middle and white matter around the periphery. The white matter is made up of myelinated ascending and descending axons of nerve cells. This particular part of white matter here is called the dorsal or posterior column, and it carries ascending sensory information only. As I mentioned previously, information relating to fine touch, two-point discrimination, vibration, and proprioception ascend in the spinal cord here.

So, that’s half of the name. The other half refers to the medial lemniscus, which is a nucleus located in the brainstem, specifically, the medulla oblongata, which the tract passes through. As our pathways pass through both of these structures, it became named after them.

Okay, we know how the pathway is named, now let’s get into the nitty-gritty of it.

This is the overview schematic of the PCML that we saw before. This image illustrates a number of sections that have been taken through the central nervous system including two different levels of the spinal cord – one receiving information in the lumbar region here, which is represented by the blue neurons, and a more rostral spinal level, which is picking up information from the upper limb and enters the spinal cord at the cervical levels. You’ll notice that we’re looking at sections through the nervous system at various levels. D is the rostral medulla, C is the caudal pons, B is the midbrain, and finally, A is a coronal section of the primary sensory cortex with the sensory homunculus depicted above the cortex.

If we extend this image into the periphery to the source of cutaneous stimulation in the skin, we can see where the PCML pathway starts. It’s not a straightforward or continuous path and actually consists of three separate segments known as the first, second, and third order neurons, which are in sequence, so let’s examine them in a little more detail.

First, though, let’s have a quick recap of neuron anatomy, so that we know exactly what we’re talking about later on in the tutorial. We’re looking at an image of a neuron you’re probably really familiar with. The rounded structure is the cell body with its nucleus in the center. The short extensions here are known as dendrites. They connect to either other neurons or senses through synapses, which are the point of entry for electric impulses carrying information. These impulses are carried out of the cell through this long projection here known as the axon. It also terminates in a synapse.

An important type of neuron you’ll find at the PCML is the pseudounipolar neuron. Unlike multipolar, it only has a single process, which divides into a peripheral process and a central process. We’ll see this type of neuron come up again in just a moment.

Now we’re ready to take a look at this pathway starting with its first segment – the first order neuron.

As we’ve already seen, there is a number of different receptors under the epidermis and in muscles in attendance that are responsible for recording some types of mechanical stimulation in the skin. These specialized ends of sensory neurons are adapted to pick up certain types of stimuli and send it to the spinal cord via the first order neuron. These first order neurons will enter the spinal cord at a specific level of the spinal cord depending on the source of the sensory information.

In this example, the information is coming from the hand so we’ll enter at the lowest cervical level. These peripheral sensory neurons are pseudounipolar in morphology, which means that peripheral processes of these neurons stretch between the periphery of the body and the spinal ganglion of each posterior spinal nerve root. From here, the central process continues to the spinal cord.

The spinal ganglion, also commonly known as the dorsal root ganglion, is an entanglement on the posterior root of a spinal nerve and houses the cell bodies of all first order sensory neurons traveling between the periphery and the spinal cord, not just the ones in the PCML pathway.

The first order neurons concerned with proprioception, as you can see highlighted now, diverged within the spinal cord and take either one of two pathways. This is due to the fact that conscious and unconscious proprioception is carried along different pathways in the spinal cord and brain. Conscious proprioception refers to actions and movements which purposefully monitor the movement and position of our body – for instance, when dancing or drawing a picture. This type of information is carried along the PCML.

Unconscious proprioception, on the other hand, refers to movements which we do not think about. For example, we don’t think about the position of our legs when walking; we just walk. Once we’ve learned the skill, it becomes an unconscious task, so although we’re not thinking about the position of our body parts with these actions, our brain most certainly is. This type of information is not carried via the PCML, but through other tracts known as the spinocerebellar tracts.

If we now follow the central process of the same neuron in the PCML pathway, we can see its course into the spinal cord via the posterior gray horn. Though it enters gray matter, it doesn’t synapse here, but rather just passes through and enters the white matter. Here’s we’ve already seen. The sensory neurons in the PCML pathway occupy the posterior column.

The posterior column is further divided into two white matter tracts known as the cuneate fasciculus located laterally and a more medial gracile fasciculus. The posterior column has a somatotopic organization and information from the inferior parts of the body is carried in the gracile fasciculus. Think G for gracile and G for close to the ground.

Neurons from the trunk and upper limb can be found in the cuneate fasciculus. Once in the white matter of the spinal cord, the central processes of first order neurons extend all the way to the medulla oblongata staying on the ipsilateral side of the spinal cord where they entered. This means that the neuron or nerve cell that picks up information on the sole of your foot when you get tickled is the same neuron that goes the whole way to the base of the brain on the same side of the body. That one neuron is almost as tall as you are.

Now that we’ve covered the first order neurons of this pathway, what else could we move on to if not the second order neurons.

The connection between the first and second order neurons is in the rostral medulla oblongata. The neurons synapse in gray matter nuclei which correspond to the white matter tract they were travelling in. The nerve cells of the fasciculus gracilis terminate in the gracile nucleus, again, located medially, and the ones in the fasciculus cuneatus terminate in the cuneate nucleus more laterally. The nuclei mentioned contain the cell bodies of our second order neurons as well as presynaptic terminals of the central processes of the first order neurons.

The journey of the second order neurons start once their processes emerge from the nuclei. They become myelinated and travel as internal arcuate fibers anteriorly and across the midline at the lemniscal or sensory decussation. That’s right, they cross over to the opposite sides of the medulla. This is something worth remembering. In this pathway, all of the information that comes from the left side of the body is processed in the right somatosensory cortex and vice versa.

In all sensory pathways, it is the second order neuron that decussates, i.e., crosses the midline. After the lemniscal decussation, the second order neurons ascend in the brainstem forming the medial lemniscus. The word lemniscus is Greek for ribbon, which refers to its appearance of this medially located tract in the brainstem. This ribbon extends the whole way from the medulla oblongata to the thalamus in the diencephalon.

As the internal arcuate fibers decussate, the fibers from the gracile nucleus which are medial and carrying information from the lower limb become anterior in the medial lemniscus, while the fibers from the cuneate nucleus which are lateral and carry sensory impulses from the trunk and the upper limb become posterior in the medial lemniscus in the rostral medulla.

As we move more cranially and into the pons, the medial lemniscus changes orientation again so that now the fibers from the lower limb become lateral and the fibers from the trunk and the upper limb become medial – the opposite to their previous orientation.

As we move superiorly along the medial lemniscus pathway, let’s take a pitstop at the midbrain, which we can see highlighted now and point out a few structures to orientate ourselves.

Firstly, in the anterior aspect, we find the cerebral peduncles. Posterior to these is the substantia nigra over here. The medial lemniscus on either side can be found posteromedial to the substantia nigra, and finally, for completion, this structure here is the periaqueductal gray matter surrounding the opening to the fourth ventricle.

So, let’s take a look at the complete journey of the second order neuron. We saw that the cell bodies were located in the gracile and cuneate nuclei in the medulla oblongata. Their axonal cell processes extend anteriorly to form the internal arcuate fibers which then cross the midline at the lemniscal decussation. They then proceeded cranially to form the medial lemniscus which ascends through the brainstem to terminate at the thalamus where they synapse with – you guessed it – the third order neurons.

The thalamus, where the third order neuron cell bodies lie, is a relay center for information in the sensory, motor, and limbic systems. Think of it like a central hub receiving loads of different signals and sending them off in all different directions. It is comprised of numerous nuclei and the one we’re concerned with in this pathway is the ventral posterolateral nucleus.

This gray matter structure gives rise to neurons that navigate their way into the internal capsule and fibers carrying somatosensory information travel through the posterior third of the posterior limb of this white matter structure where they enter the corona radiata to finally end their journey in the somatosensory cortex.

And that brings us to the last stop on the journey of our PCML – the somatosensory cortex.

The somatosensory cortex is located in the postcentral gyrus rostral or anterior to the sensory association cortex and caudal or inferior to the primary motor cortex. It is comprised of Brodmann areas three, one, and two. It is somatotopically arranged into what we call the sensory homunculus. This picture illustrates nicely that information from different parts of the body is carried to different parts of the somatosensory cortex for processing. For example, sensation from the feet is carried through the fasciculus gracilis to the medial aspect of the cortex, while information from the hands comes through the fasciculus cuneatus to this area here on the anterolateral aspect.

And that’s pretty much it when it comes to the posterior column - medial lemniscus. I hope the name makes a bit more sense now, too.

Before we wrap up, we, of course, have to take a little look at the clinical relevance of the PCML.

One of the most important conditions we can talk about in relation to the nervous system is, of course, an autoimmune disorder called multiple sclerosis, or simply MS. All the neurons in the white matter tracts of the CNS are myelinated. MS affects these and many other white matter tracts by breaking down the myelin surrounding the axons which in turn results in a reduction or complete loss of myelin, and as the disease progresses, the breakdown of the axons themselves. When the myelin covering is compromised, a neuron can no longer effectively conduct electrical signals along its length.

Let’s consider a unilateral lesion in the left side of the posterior column at the L4 vertebral level. Can you work out what the sensory deficit would be if a sclerotic lesion was to occur here? Which one would be affected, the ipsilateral or the contralateral side of the body?

To work this out, let’s first think about the root of the first order neuron. It travels on the same side of the spinal cord as the source of the sensation the whole way to the brainstem. So, then we can conclusively say that a lesion at the level of L4 would cause a loss of perception on the ipsilateral or same side because sensation wouldn’t be transmitted due to the lesion. The areas of sensory deficit would be localized to some or all of the L4 to S5 dermatomes, just in case you’re curious.

A sclerotic lesion can affect any white matter of the central nervous system, be it in the brain or the spinal cord. So, what about a lesion in the internal capsule? What side of the body is going to be affected now? If you guessed the contralateral side, you would be correct. The pathway has decussated by the time it reaches the internal capsule and, therefore, a lesion on the left side of the brain would be affecting the sensation from the right side of the body.

That wraps up our tutorial. But just to make sure we got all of our heads around the extent of this pathway, let’s run it through again.

So, we started off in the peripheral nervous system with the sensory receptors of the skin, muscles, and tendons that are recording vibration, proprioception, fine touch, and pressure. An action potential is initiated here in response to stimuli, travels along the first order neuron and enters the spinal cord via the posterior root of the gray matter. This same neuron enters the dorsal column of the white matter and ascends the spinal cord in either the fasciculus gracilis if it is coming from the lower limb or the fasciculus cuneatus if it is coming from the upper limb or trunk.

The first order neuron ascends to the caudal brainstem where it synapses with the second order neuron in the gracile and cuneate nuclei of the medulla oblongata. The second order neuron starts in these nuclei and moves anteriorly through the medial oblongata as the internal arcuate fibers before decussating and entering the contralateral medial lemniscus. It ascends within the medial lemniscus to the ventral posterolateral nucleus of the thalamus where it synapses with the third order neuron.

This short third order neuron travels from the thalamus through the internal capsule into the corona radiata and on to the appropriate part of the somatosensory cortex. To finish up our tutorial, we had a look at how multiple sclerosis would affect the PCML at different levels along its pathway.

That’s it, folks! Hope you enjoyed this tutorial and see you next time.