Video: Somatosensory pathways

Have you ever wondered about how when you touch something, your brain knows about it almost immediately? If you think about it, there's quite a long distance from the tips of our fingers to our cerebral cortex. So, what's actually happening here to get this information across so efficiently?

The sensation of touch is being carried along a sequence of neurons with long axons that work as a real A-team, carrying that information from one to the next, until it reaches the brain. This relay team is fast; not speed-of-light fast, but fast enough that the journey seems like it didn't even happen. And in this tutorial, we're going to travel with them as we learn about the somatosensory pathways.

The somatosensory pathways start with sensory receptors. They pick up sensory information which could be external such as touch, pressure, pain, temperature, and vibration; or internal position sensors like proprioception.

The sensory receptors convert stimuli into electrical signals in turn generating action potentials which can then travel along the efferent neurons towards the spinal cord and the brain. This journey through the central nervous system could take different paths depending on the sensations being carried.

If we look at this transverse section of a spinal cord, we see gray matter in the middle with white matter around it. The white matter consists of myelinated ascending and descending axons, also known as tracts. The ascending tracts are sensory pathways, bringing information from the periphery towards the central nervous system, while the descending tracts go the other way, carrying motor information.

There are several ascending tracts such as the posterior funiculus-medial lemniscus pathway located in this region of white matter here called the posterior funiculus. There are spinocerebellar tracts, spinothalamic tracts, and other tracts such as the spinoreticular and spino-olivary tracts. Each tract is responsible for carrying a set of different sensations to the brain. In this tutorial, however, we'll be focused on exploring only the posterior funiculus-medial lemniscus pathway and the spinothalamic tracts.

Let's start with the posterior funiculus-medial lemniscus pathway. This tract gets part of its name from its location on the posterior funiculus of the spinal cord, which is also commonly known as the posterior column or dorsal column. Thus, this pathway is often called the posterior column-medial lemniscus or dorsal column-medial lemniscus pathway -- PCML or DCML for short.

This tract carries information regarding multiple sensory modalities. These include tactile sensations such as fine touch like when a butterfly rests on the back of your hand. It's involved in discriminative touch which includes tactile localization -- our ability to locate exactly where a stimulus is coming from. It also includes two-point discrimination -- our ability to perceive two separate points of stimuli as distinct entities.

In addition to touch, it also carries the sensations of pressure, vibration, and position senses, particularly, conscious proprioception. Conscious proprioception involves purposefully monitoring the positions of body parts like our limbs. For instance, if you were drawing at a desk and the lights were to go out or if you were to close your eyes, you'd know your elbow is bent. This is conscious proprioception, and we can thank the posterior funiculus-medial lemniscus pathway for it.

Now that we know what this pathway does, let's get a better understanding of how it's arranged.

Here we can see the key components of the pathway represented as a series of transverse sections taken at different levels through the central nervous system. If we start from the lowest one, this is a section of the spinal cord; in this case, specifically from the cervical region, picking up information from the upper limbs.

A section further down, like from the lumbar region, would receive information from the lower limbs. Higher up we have sections of the brainstem, the rostral medulla oblongata, caudal pons, the midbrain, and finally, a coronal section of the postcentral gyrus -- that's the somatosensory cortex, with a depiction of the sensory homunculus above. We'll understand what that is a little later on.

Notice that this pathway doesn't simply consist of one single continuous neuron. Rather, it has three separate neurons, known as the first, second, and third order neurons. Each neuron hands over information to the next at a synapse.

Let's start with the first order neurons. Like we saw earlier, they start at the sensory receptors which could be in the skin, muscles, and tendons. These are mechanoreceptors, responsible for tactile and position sensors. These first order neurons are specialized sensory neurons called pseudounipolar neurons, which have a peripheral process and a central process.

Information gets transported along this peripheral process to the cell bodies. That cell body is located in a structure called the spinal ganglion, also known as the posterior or dorsal root ganglion. This is an enlargement situated on the posterior root of the spinal nerve.

The central processes of these first order neurons enter the spinal cord at different levels, depending on the source of sensory information. For example, information from the hand will enter at a lower cervical level. The neurons enter the spinal cord via the posterior gray horn. Though they enter the gray matter, most of the fibers don't synapse here, but rather just pass through and enter the white matter, specifically, into the posterior funiculus.

The posterior funiculus is divided into two white matter tracts. These are the gracile fasciculus located medially, and the cuneate fasciculus situated laterally. The posterior funiculus has a somatotopic organization and information from the inferior parts of the body below the sixth thoracic spinal segment like from the lower limbs are carried in the gracile fasciculus. Think G for gracile and G for close to the ground.

Meanwhile, neurons from above the sixth thoracic spinal segment, like those carrying information from the trunk and upper limbs, can be found in the cuneate fasciculus. From here, the first order neurons continue up to the medulla oblongata. Note that they're staying on the ipsilateral side of the spinal cord. We'll see why that's important a little later on.

In the rostral medulla oblongata, both fasciculi synapse in gray matter nuclei.

The neurons of the gracile fasciculus terminate in the gracile nucleus, again located medially, while the ones from the cuneate fasciculus terminate in the cuneate nucleus situated laterally. At these nuclei, the first order neurons synapse with the second order neurons, meaning, the presynaptic terminals of the first order neurons communicate with the postsynaptic membranes of the second order neurons handing over information.

The second order neurons emerging from the nuclei in the medulla are the internal arcuate fibers. This is where it gets interesting. They travel anteriorly and across the midline creating the decussation of the medial lemniscus, also called the lemniscal sensory decussation. Thus, they cross over to the opposite side of the medulla.

Thanks to this little crossover, sensory information is processed in the contralateral cortex. In other words, sensory information from the left side of the body gets processed in the right somatosensory cortex and vice versa.

After the sensory decussation, the second order neurons ascend in the contralateral brainstem as the medial lemniscus. The word lemniscus is Greek for “ribbon” and this ribbon-like pathway ascends all the way up to the thalamus in the diencephalon.

On its way to the thalamus, the pathway continues through the tegmentum of the midbrain where it's located posteromedial to the substantia nigra. Eventually, the second order neurons meet the third order neurons in the thalamus, which thus contains the cell bodies of the third order neurons.

The thalamus is considered a sensory relay station, sorting and distributing information to different areas of the brain. The thalamus comprises numerous nuclei and the one we need to look at for this pathway in particular is the ventral posterolateral nucleus, which forms a part of the ventroposterior complex.

This gray matter structure gives rise to neurons that traverse the internal capsule. The fibers carrying somatosensory information travel through the posterior third of the posterior limb of this white matter structure, following which they journey through the corona radiata, finally reaching their destination -- the postcentral gyrus.

The somatosensory cortex is located in the postcentral gyrus of the parietal lobe. It consists of Brodmann areas 3, 1, 2. It's also called somatosensory area 1 since posterior to it is somatosensory area 2, or the sensory association area with Brodmann areas 5 and 7. Area 2 is important for deeper processing of sensory information, receiving inputs from Area 1 -- the auditory and visual cortex.

But when we say somatosensory cortex, we're usually referring to somatosensory area 1 in the postcentral gyrus. It is somatotopically organized into what we call the sensory homunculus, and that's what we can see here.

The different parts of the body are represented in the part of the cortex where they get processed, and the more sensitive the body part, the larger its representation on the cortex, like the lips and fingers, for example. Sensation from areas like the feet are carried through the gracile fasciculus to the medial aspect of the cortex, while information from the hands comes through the cuneate fasciculus to this area here, on the anterolateral aspect.

Perhaps now, the complex name of the pathway makes more sense. The posterior funiculus is the location of these fibers in the spinal cord and the medial lemniscus starts from the medulla up to the thalamus.

Next up, we have the spinothalamic tracts.

There are two spinothalamic tracts to discuss. The anterior spinothalamic tracts carry crude non-discriminative touch and pressure sensations, while the lateral spinothalamic tracts carry pain and temperature sensations. Now these tracks look very similar to the posterior funiculus-medial lemniscus. They also have three segments -- first, second, and third order neurons. But, of course, there has to be something different, so let's take a closer look at what's special about them.

Again, our journey starts with the sensory receptors which carry information via the sensory neurons towards the spinal ganglia from where the first order neurons enter the spinal cord. Upon entering the gray matter, these neurons either ascend or descend a few vertebral levels and most of the first order neurons synapse with the second order neurons in multiple laminae of the spinal cord gray matter, including the gelatinous substance.

From here, the second order neurons take on the responsibility of carrying the information and they do something interesting here. They cross over obliquely to the contralateral side towards the anterior and lateral funiculi with the anterior spinothalamic tract, as the name suggests, located more anteriorly, and the lateral spinothalamic tract, situated more laterally. Thus, again, thanks to this decussation, sensation from the left side of the body is going to be processed in the right somatosensory cortex and vice versa.

Note that with the spinothalamic tracts, the crossover is occurring at the level of the spinal cord while with the medial lemniscus, it's occurring in the medulla oblongata.

Continuing the journey, the spinothalamic tracts ascend through the medulla oblongata where they're located between the inferior olivary nucleus and the spinal tract of the trigeminal nerve. During their further ascent, these two tracts are joined by the other ascending pathways such as the spinoreticular tracts and spino-olivary tracts, among others, together forming the anterolateral tract, also known as the spinal lemniscus.

The anterolateral tract travels through the pons and the midbrain to reach the ventral posterolateral nucleus of the thalamus. In the thalamus, the second order neurons synapse with the third order neurons, which can then carry the information through the posterior limb of the internal capsule and through the corona radiata to finally reach the somatosensory cortex in the postcentral gyrus.

The tactile sensations carried by the anterior spinothalamic tract usually reach the ventral posterolateral nucleus, however, some of the pain fibers from the lateral spinothalamic tract can reach the reticular formation and from there, they can project to the intralaminar nuclei of the thalamus. Thus, the afferent neurons successfully bring sensory information from the periphery to the brain, and these somatosensory pathways help us to perceive the world around us and within us.

I hope you enjoyed the journey, since that concludes this tutorial on the important somatosensory tracts.

Make sure you check out our other articles and study units we've got on the nervous system.