Okay, 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 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 then 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 the axons in 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 is just Latin for small bundles. So tracts which have similar function and go to and come from similar places are organized into these small bundles. 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 tracks 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.
So 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 muscle 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 neuron sequence. So you have a first order, a second order and a third order neuron. So the first order neuron is known as the primary afferent neuron and this brings information from the periphery into the spinal cord via the dorsal horn. So this primary afferent first order neuron has its cell bodies contained in the dorsal root ganglion.
So what's important to know about the primary afferent neuron or the first order neuron is that it remains ipsilateral. So ipsilateral just means on the same side. So ipse in Latin means the cell for the same. So ipsilateral just means same side. So the first order neuron terminates either by forming a synapse in the dorsal horn of the grey matter of the spinal cord cord with the second-order neuron as I'm showing you on the left-hand side here.
So this second-order neuron 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 neuron. So on the right-hand side of the screen, you can see the other path that the first-order neuron takes. So it enters the dorsal horn and then it can ascend up into the medulla where it forms its synapse. with this second-order neuron.
So again, this second-order neuron cross 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 neuron. So it's important to know that first order neuron remains ipsilateral and it terminates by forming a synapse with the second order neuron. 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 neuron 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 neuron 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 neuron.
So I'm just drawing this third order neuron in blue. And this third order neuron 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 neurons and the idea that primary afferent neurons or first order neurons. remain ipsilateral, they synapse with second-order neurons which decassate and ascend contralaterally, so contralateral means on the other side and then they terminate in the thalamus with synapses with the third-order neuron 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.