OK so in this part of the tutorial, what I'd like to do is introduce you to the idea of
the different ascending and descending spinal pathways.
So hopefully you will have watched the previous parts on the spinal pathways, and will understand
that the ascending tracts carry sensory information from the periphery to the brain, and the descending
tracts carry information from the brain to the periphery.
So what we are looking at here is a horizontal section of the cervical spinal cord, and you'll
recognise the "H" shaped grey matter in the middle and around it is the white matter.
And there's various different shapes that you'll see here, and these represent the different
ascending and descending spinal pathways.
So first we'll take a look at the ascending pathways.
So we'll look first at the dorsal columns, which convey fine touch, proprioception, and
vibration sense, from the periphery up to the brain.
So you've got the fasciculus gracilis, which I've highlighted in yellow, and you've got
the fasciculus cuneatus.
So just remember that fasciculus means small bundle, so the fasciculus gracilis carries
information from the lower limb, so it has axon fibres from the lower limb, whereas the
fasciculus cuneatus has fibres from the upper limb.
So the way I remember this, is that the letter "C" is before the letter "G" in the alphabet,
so "C" is higher, so your upper limbs are higher than your lower limbs.
So fasciculus cuneatus carries fibres from the upper limb, whereas fasciculus gracilis
carries fibres from the lower limb.
Now how do you remember which one is medial, and which one is lateral, so in yellow we've
got the fasciculus gracilis, and in green we've got the fasciculus cuneatus.
And how to remember this is that your arms are lateral to your legs, and you know from
the previous mnemonic that the fasciculus cuneatus carries from the upper limb.
So because your arms are lateral to your legs, the fasciculus cuneatus is lateral to the
fasciculus gracilis in the spinal cord.
So next we've got the spinothalamic tracts.
So these I've just highlighted in blue, and the spinothalamic tracts are also known as
the anterolateral tracts, as they sit anteriorly and laterally in the spinal cord, and some
texts will refer to distinct anterior and distinct lateral tracts, but almost invariably
there is some intermingling of fibres between the anterior and lateral tracts.
So on this diagram, they are represented as one tract.
So the anterolaterally tract is known as the spinothalamic tract and this relays information
to the somatosensory cortex regarding pain and temperature information, and also crude
touch.
So the lateral aspect of the spinothalamic tract carries pain and temperature, whereas
the anterior spinothalamic tract fibres carry crude touch information.
So the next ascending pathway is that of the spinocerebellar pathways.
So I've highlighted these both in purple and lighter purple.
So you've got the dorsal and ventral spinocerebellar tracts.
So these tracts relay information from muscle and joint receptors to the cerebellum, and
these tracts are examples of the pathways which carry subconsciously processed information.
Whereas the dorsal columns, the fasciculus gracilis and the fasciculus cuneatus and the
spinothalamic tracts, carry consciously processed information, which is processed in the somatosensory
cortex.
So these spinocerebellar tracts don't carry information that is consciously processed,
so they don't have the three neurone sequence which I described in the previous part of
the tutorial.
So basically you've got information coming via these tracts from muscle spindles and
Golgi tendon organs, and this information is relayed to the cerebellum and allows for
the control of posture, and to coordinate muscular movements.
So now we'll take a look at the descending tracts.
So these descending tracts start in the cerebral cortex and brainstem and descend down the
spinal cord to innervate muscles.
So they are involved in controlling movement, muscle tone, spinal reflexes, and also have
an ability to modulate some of the sensory information which is brought up by the ascending
tracts.
So essentially the ascending tracts bring in information from the environment relating
to sensory information, and the descending tracts carry information from the centre - the
brain, to act in response to this sensory information.
So it's useful to think of these descending tracts as pyramidal, or extra-pyramidal, and
pyramidal just refers to tracts that pass through the medullary pyramids.
So the pyramidal tracts are the corticospinal tracts.
You've got the lateral corticospinal tracts and the anterior corticospinal tracts, which
I've highlighted in orange.
So the corticospinal tracts are important to know about, because they convey voluntary
skilled movements, originating from the motor cortices of the brain.
So the lateral corticospinal tracts carry motor information destined for the limbs,
and it's important in skilled voluntary movement.
The ventral corticospinal tracts carry axial motor information - the axial muscles are
those central muscles which aren't limb muscles, essentially.
So you may start to notice the significance of the naming of these tracts.
So remember the descending tracts we talked about, the spinothalamic tracts and the spinocerebellar
tracts, you'll notice that the first part of the name of the tract relates to where
the tract begins, and the second part of the name of the tract relates to where that tract
ends.
So spinocerebellar - it starts in the spinal cord and it projects to the cerebellum.
And then spinothalamic - starts in the spinal cord, projects to the thalamus.
So you know that they are ascending tracts.
And now we've just talked about the corticospinal tract, so starts in the motor cortex, and
projects down into the spinal cord, so it's a descending tract.
So there's always clues and hints in the names.
So just to cover some of the extra-pyramidal tracts: you've got the rubrospinal tract,
which I've highlighted in red.
So this originates in the red nucleus of the midbrain tegmentum, and is responsible for
limb flexors.
And then you've got the tectospinal tract which I've highlighted in light purple here.
So it's seen here kind of intermingled with the medial longitudinal fasciculus, but the
tectospinal tract originates from the superior colliculus of the midbrain, so the clue is
in the name again - so the "tecto" part of this word refers to the tectum, which consists
of the superior colliculus and the inferior colliculus.
So fibres from the tectospinal tract mainly terminate in the cervical segment of the spinal
cord, and carry information responsible for movements in response to visual sensory information.
And then we've got the vestibulospinal tract - this is highlighted in bright pink, and
this originates in the vestibular nuclei in the pons and medulla.
So you've got a lateral vestibulospinal tract, and a medial vestibulospinal tract.
So highlighted here in pink is the lateral vestibulospinal tract.
So the vestibulospinal tract receives information from the bony labyrinths and the cerebellum,
relating to balance and posture, and this pathway allows for the control of extensor
motor neurones in order to maintain posture.
And then lastly, we've got the reticulospinal tract, which arises as the name suggests,
from the reticular formation of both the pons and the medulla.
So you've got a medial, or pontoreticulospinal tract, which is the one highlighted here in
green.
And then you also have the medullary reticulospinal tracts, which aren't shown here, but I'll
just draw them on the rough location of these fibres so you'd have the medullary reticulospinal
tract fibres in this region here.
So that's the lateral reticulospinal tract.
And this tract, the reticulospinal tract, is responsible for reflexes, muscle tone,
and is also involved with the respiratory and circulatory system.
So that's an overview of the different ascending and descending spinal cord tracts.