Spinal Pathways/Tracts – Part 1 – Introduction – Anatomy Tutorial

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.