Dorsal Column-Medial Lemniscus Pathway to the Cortex

-In this lecture, we are going to describe the pathways for general sensation from the body, and when we say body we mean the limbs and the trunk.

The pathways for general sensation that reach the cerebral cortex are formed of thousands of sets of 3 neurons; first-order neurons, second-order neurons, and third-order neurons. Neither the first-order nor the third-order neurons cross the midline, which means that only second-order neurons cross the midline. And for the sensation to be described as conscious it has to reach the post-central gyrus, otherwise it is described as unconscious as you are going to see in the pathways of unconscious proprioception that go to the cerebellum which consist of sets of only two neurons. But now we’ll start with the conscious sensory pathways.

Nerve impulses for conscious proprioception and most tactile sensations ascend to the cerebral cortex along this pathway. Let’s start by describing its first-order neurons, then the second then the third:

1. First-order neurons : as you already know, the first-order neurons form the fibers of the Gracile tract and the Cuneate tract in this pathway. These sensory fibers arise from the dorsal root ganglia, which are formed of pseudo-unipolar nerve cells that have central branches and peripheral branches.

The central branches that enter the sacral, lumbar and lower thoracic spinal segments (Spinal segments below T6) will ascend in the medial part of the dorsal column of the spinal cord (the lowest being most medial) and they will form the gracile tract which will ascend and terminate in the gracile nucleus of the medulla. Whereas the fibers that

1

enter the spinal cord above the level of T6 will form the cuneate tract, and ascend lateral to the gracile tract to terminate in the cuneate nucleus of the medulla.

This figure shows the dorsal column-medial lemniscus pathway which transmits impulses for proprioception and different types of touch and vibration, which are carried as we said by the gracile and cuneate tracts.

2. Second-order neurons : The cell bodies of these neurons are located in the gracile nucleus or the cuneate nucleus of the medulla. The axons of these neurons will cross the midline in the medulla and ascend as the medial lemniscus. Because these fibers cross the midline, we call them internal arcuate fibers. And they terminate in the ventral posterolateral (VPL) nucleus of the thalamus. (Follow the neurons on the figure to the right.)

3. Third-order neurons : In the thalamus, the axon terminals of second-order neurons synapse with the third-order neurons, which project their

2

axons through the posterior limb of the internal capsule then through the corona radiata to the primary sensory area of the cerebral cortex (Postcentral gyrus.)

Damage to this pathway will lead to loss of sensation. As for where and which side, it depends on the site of the lesion.

•Lesions before the crossing (In Gracile and/or cuneate tracts):

- If the lesion is in the gracile tract on one side, patient will lose sensation below the level of (T6) in the ipsilateral side. Even if the lesion was in the cervical area (a tumor in the midline for example), in the beginning it will destroy the gracile only, so the patient will lose sensation in the lower part of the body although the lesion is in the cervical area. So damage of one of the gracile tracts at any level will lead to loss of sensation in the ipsilateral lower part of the body (below the level of T6.)

- If the lesion is in the cuneate tract on one side, it will lead to loss of sensation above the level of (T6) ipsilateral to the lesion.

- Damage to both Gracile and cuneate tracts on one side, will lead to loss of sensation in both above and below (T6) ipsilateral to the lesion.

•If the lesion is after the crossing (In the Medial Lemniscus):

For example if the lesion was in the right medial lemniscus at any level, it will result in loss of sensation in the contralateral side of the body (which is the

3

left side in this example) in both upper and lower parts of the body (Above and below [T6]).

In addition to losing sensation, the patient will also have sensory ataxia (disturbance of balance). It’s called “sensory ataxia” because the problem is in the sensory pathway. So the patient will have what we call “wide-based gait,” in which the patient separates the legs to improve balance to avoid falling.

There’s a test called “Romberg’s test,” in this test you ask the patient to put his feet together, this patient will depend on his vision to know the position of his lower limbs, then you ask the patient to close his eyes, so he won’t know the position of his lower limbs and he will start to swing, this is a positive Romberg’s test. This test is used to differentiate between sensory ataxia and motor or cerebellar ataxia which is caused by a lesion in the cerebellum or its connections. In motor ataxia, vision has no role, but in sensory ataxia the patient will improve when they open their eyes, and it becomes difficult when the eyes are closed.

There’s also the “Finger-to-Nose Test.” Normally it’s easy for a healthy person to touch the nose with the tip of the finger. But if there’s a lesion causing ataxia whether sensory or motor, it becomes a bit difficult to do that. So the patient will have a few unsuccessful attempts before actually being able to land his finger on his nose. But how can we use this test to differentiate between the sensory and motor ataxia? The answer is simply by involving vision in the test. In case of sensory ataxia, vision will help the patient to locate the position of his finger, so he will lead the movement of his finger with his vision to be able to put it on his nose. But if we ask him to close his eyes, it will be very difficult for him to perform this test successfully.

Now if the patient has motor ataxia, vision will play no role and it will be very difficult for the patient to perform the test whether the eyes are closed or opened.

4

Spinothalamic Pathways to the Cortex

Of course the patient will also lose the sense of discriminative touch in case of damage to the dorsal column, but not the simple (crude) touch, and the reason is because simple touch is also carried by the ventral (anterior) spinothalamic tract in addition to the dorsal column (Gracile + Cuneate).

The lateral spinothalamic tract conveys sensory impulses for pain and temperature; the anterior spinothalamic tract conveys impulses for tickle, itch, crude touch, pressure, and vibrations.

The doctor described the lateral spinothalamic tract which conveys pain and temperature impulses in details, then he briefly talked about the anterior spinothalamic tract because the neuron arrangement in the anterior spinothalamic tract is the same as the lateral, the only difference is the type of sensation it

conveys.

This pathway (Spinothalamic) is formed of first, second, and third-order neurons, and only the second-order neurons cross the midline just like in the previous pathway, let’s start with the lateral spinothalamic and go through its neurons one by one.

5

1. First-order neurons: these are the pain and temperature fibers of the spinal nerves. So in this pathway the first-order neurons are always sensory fibers of the spinal nerves, while the first-order neurons in the pathway for general sensation of the face are always sensory fibers of the cranial nerves.

These sensory fibers of the spinal nerves arise from the dorsal root ganglia and give central branches and peripheral branches. The peripheral branches run along the spinal nerves to terminate as free nerve endings (pain and temperature receptors). The central branches enter the spinal cord.

These sensory fibers (central branches) ascend for one or two spinal segments before they enter the dorsal horn (You will not find this point

illustrated in the previous figure) and then synapse with the second-order neurons. They ascend in the area between the dorsal horn and the dorsolateral sulcus to form a small tract called the dorsolateral tract of Lissauer. So the tract of Lissauer is formed of the central branches of pain and temperature fibers which are the first-order neurons of this pathway.

2. Second-order neurons: they arise from the dorsal horn, they cross the midline in the anterior white commissure to the contralateral side of the spinal cord where they ascend in the lateral white column as the lateral spinothalamic tract. This tract ascends through the medulla, pons, and midbrain to terminate in the thalamus within the Ventral Posterolateral (VPL) nucleus. So they arise in the dorsal horn on one side and terminate in the thalamus on the contralateral side.

3. Third order neurons: these are the thalamocortical neurons, which arise from the VPL, ascend in the corona radiata through the posterior limb of the internal capsule to reach the different parts of the postcentral gyrus.

6

Damage to this pathway at any level will lead to loss of pain and temperature sensations, and the affected area of the body depends on the site of the lesion.

Let’s take a few examples.

-For these examples the doctor used a self-drawn figure that I couldn’t get, so I drew one myself, and I used colors to make it easy to understand (Don’t worry I was in the lecture, so I kinda know what the figure should look like =P), you might wanna take a look at the colored version of the figure while reading the examples, you know, so you can get them right.

•This is the spinal segment C8 (look at the figure). Let’s assume the lesion was in the left lateral spinothalamic tract at the level of C6. This lesion will lead to loss of pain and temperature sensations in dermatomes of C8 and below not in C6 and below, although the lesion is in C6. What is the explanation for this?

It’s because of what we mentioned earlier, we said that when these pain and temperature fibers enter the spinal cord at a certain level (C8 in this example), they ascend for one or two segments (C6 in this example) before they join the second order neurons in the dorsal horn (which cross the midline to ascend in the lateral spinothalamic tract). So the loss of pain and temperature sensations will be in dermatomes of C8 and below, but not

7

above because the pain and temperature fibers that enter the spinal cord at the level of C6 will ascend to C4 before they synapse with second-order neurons to join the lateral spinothalamic tract.

*Dermatome: an area of the skin which is mainly supplied by one spinal segment.

So if the lesion is in the left lateral spinothalamic tract at C6, the damage resulting from that lesion will be in the contralateral side of the body; the right side (because the second-order neurons crossed the midline before they joined the lateral

spinothalamic tract), in the dermatomes below the lesion by one or two segments; in C8 and below.

•Another case would be a lesion in the right C8 spinal nerve (One nerve), the patient will lose pain and temperature sensations in the dermatome supplied by that nerve on the same side of the lesion (ipsilateral).

•The last example is if there’s a lesion in the anterior white commissure. The most common cause for this lesion is called Syringomyelia. In syringomyelia the central canal starts to enlarge for unknown reasons, as a result, nerve cells and fibers in that area will be damaged, and the first structure to be destroyed by syringomyelia is the anterior white commissure. In the anterior white commissure, the second-order neurons on each side cross the midline to the other side, so damage in that area will result in loss of pain and temperature sensations bilaterally.

Let’s assume syringomyelia starts at C4, in the beginning, at the stage where it’s limited to only C4, the anterior white commissure in that segment will be destroyed. In which areas do you think the patient will lose pain and temperature sensation? The patient will lose sensation in the dermatomes C5 or C6 bilaterally, because as you know the fibers that cross the midline in C4 are connected to the fibers that are coming from one or two segments below. So in

8

Pathways for Unconscious Proprioception (Spinocerebellar Pathway)

the early stages, damage is restricted to only one spinal segment; which means loss of sensation in one dermatome bilaterally.

In the later stages of syringomyelia, the cavitation starts to get wider and destroys more structures. And it starts to spread upwards and downwards, in this late stage, it starts to involve more segments, thus more dermatomes will be affected.

This figure shows the pathway for simple (crude) touch (it’s the same figure as the one in the lateral spinothalamic tract section because both tracts are shown in

it).

Simple touch impulses are conveyed by the ventral (anterior) spinothalamic tract. Again this pathway has three-order neurons. The first-order neurons are sensory fibers of spinal nerves that carry simple touch impulses, the second-order neurons form the ventral spinothalamic tract, and third order neurons are the thalamocortical fibers. Crossing is made by second order neurons.

9

This is the last pathway we’re going to discuss. This pathway carries proprioceptive impulses to the cerebellum, unlike the conscious proprioception pathways which are carried to the thalamus and cerebral cortex. Although not consciously perceived, sensory impulses conveyed to the cerebellum along this pathway are critical for posture, balance, and coordination of skilled movements.

The pathways for unconscious proprioception are formed of sets of only two types of neurons; first-order neurons and second-order neurons. We have 3 cases in which the fibers take different paths to reach the cerebellum, and those paths depend on the level in which the sensory fibers enter the spinal cord.

I could not get any pictures or figures for this topic, I’m sorry.

•Sensory fibers entering between C8 and L2:

First-order neurons (which are sensory fibers of spinal nerves) that enter the spinal cord between C8 and L2 will terminate within the dorsal nucleus of Clarke.

Second-order neurons in this case are the fibers that form the dorsal spinocerebellar tract. In this case these fibers arise from the dorsal nucleus of Clarke, and ascend by forming the dorsal spinocerebellar tract in the lateral white column, then through the medulla ipsilaterally, to terminate in the ipsilateral side of the cerebellum. No crossing of the midline in this case.

•Sensory fibers entering below L2:

First-order neurons (sensory fibers of spinal nerves) that enter the spinal cord below L2 will not find the dorsal nucleus of Clarke; they will terminate outside this nucleus within the dorsal horn

Second-order neurons synapse with the first order neurons in the dorsal horn, and then cross the midline and ascend in the lateral white column forming the ventral spinocerebellar tract. So this tract crosses the midline after it synapses with first-order neurons in the ventral horn, then it ascends to the midbrain in the brain stem, where it crosses the midline yet again to terminate within the ipsilateral cerebellum. In this case the ventral spinocerebellar tract

10

crosses the midline twice, in the 1)spinal cord and in the 2)midbrain, and terminates in the ipsilateral cerebellum.

•Sensory fibers entering above C8:

First-order neurons (sensory fibers of spinal nerves) that enter the spinal cord above C8 will not find the dorsal nucleus of Clarke as well. These fibers will ascend to the medulla and terminate within a small nucleus called the Lateral (Accessory) cuneate nucleus (Cuneateconscious proprioception, Lateral [accessory]

cuneateUnconscious proprioception).

Second-order neurons in this case arise from the lateral cuneate nucleus in the medulla and go to the ipsilateral cerebellum. No crossing is made here.

Damage to the cerebellum on one side will lead to motor ataxia in the same side. And now you should be able to differentiate between motor ataxia which is caused by a lesion in the cerebellum or the spinocerebellar tracts and sensory ataxia. In a few words, in motor ataxia, vision will not improve balance, while in sensory ataxia it will.

Done By: Mohammad Al-Hazaimeh.

11