Ok, so in this tutorial, we're going to take a look at the spinal pathways.
So we're going to look at the different tracts that ascend and descend in the spinal cord.
So what we're looking at here is a cross section of the spinal cord, and I just want to recap
some basic anatomy of the spinal cord before going any further.
So we're looking at a transverse section of the spinal cord, so this is at the level C8,
and just the basics, so the "H" shaped grey stuff in the middle is the grey matter, and
everything peripheral to that is white matter.
So you can see these different coloured shapes lying in the periphery, and these colours
just represent different bundles of ascending and descending white matter tracts.
So you've got the grey "H" shaped matter in the middle and the white matter peripheral
to that, where the ascending and descending tracts are located.
So at the back you've got the dorsal horns, and at the front anteriorly, you've got the
ventral horns.
In the centre you've got the central canal.
And then you've got the dorsal median sulcus dorsally, and the ventral median fissure,
anteriorly - ventrally.
So as I mentioned you've got these bundles which are known as fasciculi, and in these
bundles you've got axons which run either up towards the brain, or down the spinal cord
towards the periphery.
So these fasciculi are either carrying information up or down the spinal cord, so you've got
ascending and descending tracts.
So these tracts are also referred to as fasciculi which just Latin for "small bundles".
So tracts which have similar function and go to and come from similar places are organised
into these small bundles.
Ascending tracts carry sensory information from the periphery to the brain, whereas descending
tracts carry information from the brain to the periphery.
So what kind of information do the ascending tracts carry?
Well if we think about what kind of information is presented peripherally to our senses, we
can think about the different modalities of sensation which might be conveyed from the
periphery, up towards our brain for processing and response.
So ascending tracts carry information including pain, temperature, tactile information which
ranges from coarse touch, to fine touch as well as vibration information, and you've
also got information from muscle and joint receptors.
So at this point, it's worth mentioning that not all sensory information is consciously
interpreted.
Information that is consciously processed and consciously experienced is conveyed to
the cerebral cortex, via these pathways.
Whereas the other information, which isn't consciously processed, and never actually
reaches the level of our consciousness, reaches different areas in the brain, so it doesn't
go to the cortex, it goes to other areas like the cerebellum, with proprioceptive information
- so information from muscles and joint receptors.
So why do we make this distinction between pathways that carry consciously processed
information and pathways that carry subconsciously processed information?
Well the answer is that the consciously processed pathways have this similar format of three
neurone sequence.
So you have a first order, a second order, and a third order neurone.
So the first order neurone is known as the primary afferent neurone, and this brings
information from the periphery into the spinal cord, via the dorsal horn.
So this primary afferent first order neurone has its cell bodies contained in the dorsal
root ganglion.
So what's important to know about the primary afferent neurone, or the first order neurone,
is that it remains ipsilateral.
So ipsilateral just means "on the same side".
So "ipse" in Latin, means the "self" or the "same".
So ipsilateral just means "same side".
So the first order neurone terminates either by forming a synapse in the dorsal horn of
the grey matter of the spinal cord, with the second order neurone, as I'm showing you on
the left hand side here.
So this second order neurone then crosses over to the other side of the spinal cord,
and ascends all the way to the thalamus, where it forms its synapse with the third order
neurone.
So on the right hand side of the screen, you can see the other path that the first order
neurone takes.
So it enters the dorsal horn and then it can ascend up into the medulla, where it forms
its synapse with the second neurone.
So again this second order neurone crosses over the midline and ascends up, on the other
side to the thalamus, where it terminates by forming a synapse with the third order
neurone.
So it's important to know that the first order neurone remains ipsilateral, and it terminates
by forming a synapse with the second order neurone.
And this termination can either occur in the grey matter of the spinal cord, or in the
medulla oblongata.
So both these terminations have been demonstrated on this diagram here.
And then just as I've showed you, the second order neurone in red, crosses over to the
other side of the spinal cord, or the other side of the brainstem in the medulla oblongata,
and this crossing over has a fancy name, which is known as "decussation".
So the second order neurone decussates, crosses over to the other side and then it ascends
to the thalamus, and this is where it forms a synapse with the final, third order neurone.
I'm just drawing this third order neurone in blue, and this third order neurone has
its cell body in the thalamus, and its axons pass to the somatosensory cortex, where the
peripheral sensation is consciously perceived.
So that's hopefully introduced you to the idea of first, second and third order neurones,
and the idea that primary afferent neurones, or first order neurones remain ipsilateral.
They synapse with second order neurones, which decussate and ascend contralaterally, so contralaterally
means "on the other side".
And then they terminate in the thalamus with synapses with the third order neurone, which
projects to the somatosensory cortex, for the perception of that peripheral stimulus.
So next what we're going to do, is take a look at a section of the spinal cord, and
the different ascending and descending tracts which run within the spinal cord.