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Pyramidal tracts

The most important motor pathways of the CNS.

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Transcript

Have you ever wondered how our brain gets a message or a signal to one of our muscles to tell it that it’s time to work? We all know that our muscles are, of course, innervated by nerves; however, nerves only run from the spinal cord to their respective muscles, right? How does the signal from the motor cortex in our brain make it all the way down to the spinal cord? Is there some kind of motion signal highway running from the brain that we should know of?

Well, actually there is. And today, I’m going to tell you more about just that. So, stay with me while we discuss the pyramidal tracts.

In this tutorial, we’re going to be taking a look at one of the major pathways of voluntary motor signals which are commonly known as the pyramidal tracts. And as I already mentioned, these tracts function to carry motor impulses from the cerebral cortex to the spinal cord. While we’re learning about this, we’re going to be using this illustration to help guide us through the various levels of the central nervous system. And as you can see, we’ve taken seven sections from the cerebrum, the brainstem and the spinal cord.

Our plan will be to first look at each of these regions in more detail and identify any structures of interest at each level. After we’ve familiarized ourselves with the anatomy involved, we’ll then take a look at the specific root which the pyramidal tracts take through these regions. So, let’s jump right into it now beginning with the cerebral structures.

Our first stop today is at the motor cortex where our motor signals primarily originate from. The wrinkled or the folded path of the brain which we see when we’re looking at it from the outside is the cerebral cortex. We know that certain areas of the cortex are designated to perform different functions such as the somatosensory cortex, the visual cortex or the gustatory cortex, which is responsible for taste.

The motor cortex is housed within the precentral gyrus of the frontal lobe towards the middle of the brain and controls the movement of the voluntary muscles. Within the primary motor cortex, the neurons that control specific regions of the body are somatotopically organized – meaning neurons related to similar regions are grouped together which is demonstrated by this conceptual topographical map across the top of your brain. This map is known as a homunculus. Some parts of your body have a greater portion of the motor cortex dedicated to them as they require or are capable of greater motor control. That’s why the hands and face of the homunculus are so much bigger than your feet and back.

We’re going to move now onto the next section of our illustration where we’ll look at some of the subcortical structures of the cerebrum. And this area, now highlighted in green, is part of the thalamus.

The thalamus is kind of like the traffic director of your brain. It’s a collection of gray matter organized into large nuclei, for example, the ventral posteromedial nucleus and the ventral posterolateral nucleus. Axons from either the cortex or the spinal cord synapse on cell bodies within the thalamus, and the thalamus then relays the message onto another part of the central nervous system.

The basal ganglia are a group of gray matter structures near the center of the brain responsible for the control of movement. Through both direct and indirect pathways, these nuclei influenced the activity of the motor cortex, and the nuclei of the basal ganglia are the putamen, the globus pallidus, and the caudate nucleus.

The putamen is the most lateral of the basal ganglia and it’s found deep to the insular cortex. The globus pallidus is just medial to the putamen and a thin strip of white matter called the medial medullary lamina divides the globus pallidus into an internal and external part. The caudate nucleus is a large structure divided up into three segments – a head near the genu of the corpus callosum, a body that passes over the thalamus, and a tail that curves around the thalamus to reach the amygdala. And in this section, we can see the head of the caudate nucleus.

Our next structure of interest is this large bundle of white matter known as the internal capsule. As you can see, it’s a V-shaped structure and is composed of an anterior limb which separates the thalamus from the head of the caudate nucleus and the posterior limb which separates the thalamus from the putamen. The internal capsule contains axons from fiber tracts passing either to or away from the cerebral cortex, and we’ll learn more about this later on in the tutorial, so be sure to keep note of this structure.

Finally, we have this long strip of gray matter interrupting the white matter of the insula, and this is known as the claustrum. It’s very thin and difficult to see in cross-section, and until recently, the function of the claustrum was a mystery, although scientists still aren’t completely sure. Current research indicates that the claustrum may play a role in consciousness.

And with that, we’re moving on from the cerebrum for now to the next section of our illustration which is, of course, the brainstem.

The brainstem is the middleman between the cerebrum, the cerebellum, and the spinal cord. Although most of the cranial nerve nuclei are located within the brainstem, it is largely made up of axons passing through on their way to another region of the nervous system. Such groups of axons or nerve fibers travelling to the same place within the CNS are called tracts and tend to make landmarks we can use to identify different regions of the brainstem when they are isolated in cross-section.

And over here, we’re looking at the midbrain. It’s the most superior or the most rostral part of the brainstem and is continuous with the thalamus, the hypothalamus, the epithalamus, and the subthalamus which are collectively called the diencephalon, and it’s involved in several processes relating to vision, hearing and motor control as well as alertness, arousal and temperature regulation. Let’s take a closer look at some of the unique landmarks used to identify parts of the midbrain.

Let’s start with these structures which are the most obvious feature of the midbrain, and these are the cerebral peduncles, sometimes called the crus cerebri. The cerebral peduncles are aligned nearly perpendicular to each other and create a small space called the interpeduncular fossa. The cerebral peduncles are large collections of white matter fiber bundles including the corticospinal, the corticobulbar and the corticopontine pathways, and we’ll learn more about some of these pathways a little bit later on in the tutorial.

A thin dark line of cell bodies runs across the cerebral peduncles and this is the substantia nigra which translates literally into “black stuff”. The dark color is a result of neuromelanin contained within its cell bodies and the neurons of the substantia nigra release dopamine into the stratum to influence motor control. The cerebral peduncles and substantia nigra are closer to the ventral part of the midbrain, and if we move towards the dorsal part of the midbrain, we can see a small canal running through it – and this is the cerebral aqueduct.

The cerebral aqueduct receives cerebrospinal fluid from the third ventricle, and this fluid then passes inferiorly to the spinal canal. A small region of gray matter surrounds the cerebral aqueduct. This region is referred to as the periaqueductal gray and it contains neurons that projects to the raphe nuclei in the reticular system to modulate pain.

Moving in an inferior or caudal direction, the next cross-section that we’re going to be looking at is taken at the level of the pons, and it can be identified by this large bulge on its ventral or anterior surface called the basal pons and its large connections to the cerebellum. It also houses several of the cranial nerve nuclei. Like the midbrain, the pons also contains fibers of the ascending and descending white matter tracts and we’ll discuss this in more detail in just a few moments.

So, next stop is the smallest part of the brainstem and that is the medulla, also known as the medulla oblongata or the myelencephalon, and it’s the most inferior or caudal portion of the brainstem and is continuous with the spinal cord. Several small bulges and a uniquely-shaped nucleus provide landmarks to identify it and despite being the smallest part of the brainstem, the medulla is divided into two segments.

So, this over here is the rostral medulla, and rostral means “beak or nose” in Latin, and this part is called the rostral medulla because it is closer to the nasal region of the skull. Besides a few bulges on its ventral surface which are known as the pyramids of the medulla, the best structure to use in identifying the medulla is the olivary nucleus, and the olivary nucleus is a uniquely-shaped nucleus that appears convoluted in cross-sections, and the neurons in the olivary nucleus coordinate signals between the cerebellum and the spinal cord.

The most inferior part of the medulla is called the caudal medulla. And caudal means tail in Latin and so, therefore, the caudal medulla gets its name because it is closer to the tailbone or coccyx than the rostral medulla.

Continuing caudally from the medulla, our final section is taken from the spinal cord, which runs nearly the whole length of the spine from the foramen magnum of the skull right down to the lumbar vertebrae, and it relays information between the brain and the rest of the body. Much of the outer portions of the spinal cord is made up of white matter which is composed of nerve fibers or axons traveling in bundles either towards or from the brainstem, and the descending tracts are specifically indicated for you in pink and the central portion of the spinal cord features clusters of neuronal cell bodies which form its gray matter. And as you can see here, the gray matter of the spinal cord resembles a butterfly in cross-section.

So, besides the butterfly-shaped gray matter in its center, the spinal cord can be identified by its numerous nerves projecting laterally on either side, and these are known as spinal nerves and they contain fibers which synapse with both ascending and descending fiber tracts as they enter and exit the central nervous system. And it’s important to note that each spinal nerve is made up of two spinal nerve roots – the anterior root and the posterior root.

So, the anterior root of the spinal nerve contains axons which synapse with fibers originating from descending fiber tracts, and these axons control movement and automatic functions of the body like sweating and your heart rate. The posterior root of the spinal nerve contains axons that will synapse with fibers within the spinal cord belonging to ascending fiber tracts, and these axons transmits sensory information like temperature, pain and pressure back to the brain.

Alright now that we know our way around the brain and the spinal cord, let’s talk about some of the white matter tracts running through them. And in this tutorial, we’ll follow descending tracts which bring information away from the brain. The main descending tract which we’ll be discussing today is the corticospinal tract which carries axons primarily from the motor cortex of the brain to the spinal cord, where they synapse with motor neurons.

From the cerebral cortex, the fibers of the corticospinal tract descends through the subcortical white matter where they go in to pass through the posterior limb of the internal capsule, which we mentioned is located between the thalamus and the putamen. From here, the fibers of the corticospinal tract descend into the midbrain where they travel within the cerebral peduncles and then continue on to the pons before gathering into discrete bundles which form the pyramids of the rostral medulla.

And as you can see in the illustration, the fibers are somatotopically arranged with the more medial fibers belonging to the upper extremity and the more lateral fibers associated with the lower extremity. As they reached the caudal medulla, the fibers of the corticospinal tract separate into anterior and lateral divisions, and this occurs at the pyramidal decussation. And just a reminder, to decussate means to cross to the other side of the medulla.

This is the anterior corticospinal tract and axons within the anterior corticospinal tract remain ipsilateral which means being on the same side to the motor cortex they originated from into the spinal cord. And before these fibers synapse with interneurons in the spinal cord, they will cross over to the other side of the body anterior to the gray matter of the spinal cord. And these fibers provide motor innervation to your back as well as the proximal portions of the extremities. And they play an important role in maintaining posture.

The axons of the lateral corticospinal tract cross to the other side of the body in the medulla at the pyramidal decussation, and these fibers supply motor innervation to the distal parts of the extremities like your hand. This tract is a relatively recent evolutionary development and only found in mammals capable of performing skilled voluntary movements.

Another white matter tract I would like to briefly mention to you is the corticobulbar tract; however, instead of controlling muscles in your legs or arms, the neurons in the corticobulbar tract control muscles in the head and neck. And because they don’t need to travel lower than the neck, the axons in the corticobulbar tract do not enter the spinal cord. Instead, they synapse onto the nuclei of cranial nerves three, four, five, six, seven, nine, ten, eleven and twelve. So, basically, cranial nerves three to twelve minus number eight. And these cranial nerves contain motor fibers for the muscles of your eyes, face, the muscles of mastication, and some of the muscles of your neck.

Something to important to note is that the corticobulbar tract is sometimes called the corticonuclear tract and this tract has actually been renamed because the word “bulbar” refers to the medulla, however, not all fibers of the corticobulbar tract actually reached the medulla. And it’s more accurate to say, corticonuclear which instead refers to the cranial nerve nuclei.

Alright, thanks for sticking with me throughout this tutorial. Now, we are down to our clinical notes as per usual, and before we finish today, let’s have a look at the pyramidal tracts from a more clinical perspective.

So, you’ll remember at the very beginning of our tutorial, we mentioned that there are two main parts or sections to the journey of a motor signal. And the first is the neural pathway that is contained within the central nervous system, meaning from the motor cortex of the brain to the spinal cord which we learned includes the pyramidal tract. And the second part of the journey is from the spinal cord to the target muscle and this relates to our peripheral nerves.

Motor neurons found within the central nervous system are known as upper motor neurons while motor axons found in the peripheral nerves are known as lower motor neurons. So, what does this mean in a clinical context? Well, damage to the neurons or the nerve fibers anywhere between the motor cortex and the synapses in the spinal cord can cause a disorder known as upper motor neuron syndrome, and this can be a result of lesions along those pathways caused by stroke, spinal cord injury or multiple sclerosis, to name a few examples.

Symptoms of upper motor neuron syndrome are considerably different to those seen when damage occurs to the lower motor neuron pathway or to the peripheral nerve, and these include muscle weakness, decreased motor control in particular concerning fine or skilled movements, spasticity which means increased muscle tone, as well as exaggerated and sometimes repeating deep tendon reflexes which are known as hyperreflexia and clonus respectively.

Another sign of upper motor neuron syndrome is what’s known as the Babinski sign. And in normal conditions when we rub a sharp instrument from the heel to the ball of the foot, we get negative a plantar or Babinski reflex causing the toes to flex or point downwards.

When damage has occurred to an upper motor neuron, instead of the toes flexing, the great toe extends or points upwards while the remaining toes demonstrate fanning which is mainly caused by adduction, and this is known as a positive Babinski reflex.

In contrast, symptoms of damage to lower motor neurons include paralysis, decreased muscle tone, depressed or hyperactive deep tendon reflexes as well as muscle atrophy.

And now, we’ve reached the end of our tutorial. So, let’s quickly recap what we’ve learned today.

In this video, we used horizontal cross-sections taken from various regions within the central nervous system to learn about the anatomy of the pyramidal tracts. We first investigated some structures in the cerebrum and we looked at the motor cortex and the surface of the frontal lobes where most of our motor signals originate from. We then looked at some subcortical structures involved in the controlling of the direction of information flow, and these included the thalamus as well as the basal ganglia which is made up of the putamen, the globus pallidus, and the caudate nucleus. We also identified the internal capsule containing axons from ascending and descending fiber tracts, and the claustrum, a small strip of gray matter deep to the insular cortex.

Moving on from the cerebrum, we talked about the three parts of the brainstem, which included the midbrain which is closest to the cerebrum, the pons which features large connections to the cerebellum, and the caudal and rostral medulla. And the brought us into the spinal cord which is a long bundle of axons that relays information between the brain and the rest of the body.

Then we went over some of the major landmarks that characterized each of these regions, and on the midbrain, we looked at the cerebral peduncles that contain white matter tracts, substantia nigra that contains neurons that release dopamine, the cerebral aqueduct that contains cerebrospinal fluid, and periaqueductal gray which surrounds the cerebral aqueduct.

On the pons, we looked at one major landmark, the basal pons – a large bulge on the ventral surface of the pons. In the medulla, we looked at the olivary nucleus – a small nucleus just lateral to the pyramids. And next, we made our way into the spinal cord. And the spinal cord is most easily identified by the butterfly-shaped group of neuronal cell bodies in its center, and the presence of many spinal nerves projecting from its lateral surface. In each spinal nerve is made up of anterior and posterior roots carrying axons to and from the spinal cord.

Next, we focused our attention on the white matter tracts and their pathways beginning with the corticospinal tract which had the pyramidal decussation divided into the anterior corticospinal tract, and the lateral corticospinal tract that contain axons controlling muscles related to the trunk and extremities of the body respectively. And finally, we briefly looked at the corticobulbar tract which contains axons that synapse on motor cranial nerve nuclei.

And that concludes our tutorial for today. I hope you enjoyed it. Thanks so much for watching. Happy studying!

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