Ascending Pathways: Dorsal Column Medial Lemniscal (DCML) System

00:00 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 lemniscal system that is shown here in blue on this side of the cord and then the opposite one will be in through here that's unshaded. The dorsal column medial lemniscal system is composed of 2 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, and 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. So, 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 is lower and which one is upper is that the gracilis, the "g" is found in leg, and 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 lemniscal system? First, these fasciculi are conveying conscious proprioception to the primary somatosensory area. Fine touch is also conveyed in this system as is 2-point discrimination.

02:44 In addition, vibration as well as pressure are also being conveyed in this important pathway.

02:56 The dorsal column medial lemniscal system is composed of 3 neurons, a 1st order neuron, a 2nd order neuron, and a 3rd order neuron, and the pathway is going to involve the spinal cord, medulla, ventral posterolateral nucleus, the thalamus, and then our destination is going to be the primary somatosensory cortex. So now I want you to understand the travels of these 3 neurons as they begin and then ultimately end at the primary somatosensory cortex.

03:39 The 1st order neuron that we see here is coming in from the right, so here is a receptor that has stimulated the 1st order neuron coming in to the right side of the dorsal column medial lemniscal system. It's going to ascend in the ipsilateral fasciculus gracilis or cuneatus. So if this is coming in from the lower extremity, this 1st order neuron is going to ascend in the fasciculus gracilis. We see it continuing up to the medulla and within the medulla you have the nucleus gracilis and the nucleus cuneatus, and since this is coming from the lower limb, this 1st order neuron will synapse ipsilaterally in the nucleus gracilis. The 2nd order neuron now, if we follow it, will cross over, it decussates, and it's decussating to the left side of the medulla through the and entering the contralateral medial lemniscus that we see up and through here that's a part of the brainstem. The 2nd order neuron will continue its ascent and then will synapse with the ventral posterolateral nucleus of the thalamus, and it synapses with the 3rd order neuron that will then reach the primary somatosensory cortex.