Now, I want you to understand an example of an ascending pathway. Ascending pathways are relaying
sensory information up to the brain, to the primary somatosensory cortex. One of the ascending pathways
is the dorsal column – medial meniscal system. That is shown here in blue on this side of the cord and then
the opposite one would be in through here that’s unshaded. The dorsal column - medial meniscal system
is composed of two fasciculi. One lies medial as you see here. This is going to be the fasciculus gracilis.
This is conveying sensory information from the lower extremity. It will enter the most inferior part of the
spinal cord and then ascend through the spinal cord in this medial location. The one lateral to the fasciculus
gracilis is known as the fasciculus cuneatus. This is conveying sensory information from the upper extremity.
So information will come in to this fasciculus at the level of T6 and then all the way up through C2.
When this information comes in, it can’t go medial. That’s occupied by the fasciculus gracilis. So these
more superior points of entry have to stay on the lateral side of the dorsal columns. Another way to perhaps
distinguish which one’s lower and which one’s upper is that the gracilis, the G is found in leg. So the cuneatus
has to be the upper portion of the body relaying sensory information superiorly. What are the functions
of these ascending components that make up the dorsal column – medial meniscal system?
First, these fasciculi are conveying conscious proprioception to the primary somatosensory area.
Fine touch is also conveyed in this system as is two-point discrimination. In addition, vibration as well as
pressure are also being conveyed in this important pathway. The dorsal column – medial lemniscal system
is composed of three neurons, so a first order neuron, a second order neuron, and a third order neuron.
The pathway is going to involve the spinal cord, medulla, ventral posterolateral nucleus of the thalamus.
Then our destination is going to be the primary somatosensory cortex. Now, I want you to understand
the travels of these three neurons as they begin and then ultimately end at the primary somatosensory cortex.
The first order neuron that we see here is coming in from the right. Here’s a receptor that has stimulated the
first order neuron coming in to the right side of the dorsal column – medial lemniscal system. It’s going to
ascend one or two levels in the Lissauer's tract before actually entering the ipsilateral fasciculus gracilis
or cuneatus. If this is coming in from the lower extremity, this first order neuron is going to ascend in the
fasciculus gracilis. We see it continuing up to the medulla. Within the medulla, you have the nucleus
gracilis and the nucleus cuneatus. Since this is coming from the lower limb, this first order neuron will
synapse ipsilaterally in the nucleus gracilis. The second order neuron now, if we follow it, will cross over.
It decussates. It’s decussating to the left side of the medulla and entering the contralateral medial
lemniscus that we see up in through here. That’s a part of the brainstem. The second order neuron will
continue its ascent and then will synapse with the ventral posterolateral nucleus of the thalamus.
It synapses with the third order neuron that will then reach the primary somatosensory cortex.