In this USMLE Step neuroanatomy video I will be discussing the Posterior Column-Medial Lemniscus pathway starting from the sensors (Meissners, Pacinian, Spindle Fibers) to spinal cord, medulla oblongata, pons, midbrain, thalamus, internal capsule and the primary somatosensory cortex.
Dorsal column functions for proprioception, fine touch, 2 point discrimination and vibration sensation.
THe medulla oblangata is made up of pyramids and olives. From the medulla oblangata are two inferior cerebellar peduncles which go to the cerebellum. Above this is the pons and midbrain which do not play a very important role in the dorsal column-medial lemniscus pathway.
Above this is the Thalamus which is broken up into three parts, Anterior thalamus, Medial Thalamus and Ventral thalamus. The ventral thalamus is made up of Ventral anterior, Ventral Intermediate and Ventral Posterior which is split in Ventral posterior lateral and Ventral Posterior Medial.
The receptors involved in the dorsal column-medial lemsniscus pathway are the meissner's corpuscles, pacinian corpuscles and the spindle fibers and golgi tendon.
Initially it will go to the dorsal root ganglion and go into the gray matter of the spinal cord. Under T6 it will go into the Fasiculus Gracilis and moves up into the corresponding Fasciculus Gracilis. Once you go above the T6 vertebrae and new tract is added to the dorsal column called the fasciculus cuneatus which is added to both sides. Therefore there are four tracts now formed. The fasciculus cuneatus carries the infromation related to arms and upper trunk.
In the midbrain the name changes from fasciculus gracilis to nucleus gracilis and the name changes from fasciculus cuneatus to nucleus cuneatus. Therefore the dorsal column has name changes once in the midbrain. Here the second neurons are formed and this is the first time there is a synapse. Here two types of fibers are formed. The external arcuate fibers go into the inferior cerebellar peduncle and send information to the cerebellum about the proprioception. These fibers are ipsilateral which are important because the internal arcuate fibers will cross over in the medulla oblangata and form the medial lemniscus. Therefore it is no longer called the dorsal column.
In the pons the tracts move straight through and don't really do anything. Here the medial lemniscus does move lateraly a small amount as it goes up to the midbrain. Once in the midbrain it will go into the thalamus.
In the thalamus the medial lemniscus will go to the ventroposterior lateral aspect of the thalamus. Here the third degree neurons are formed and the neurons go to the posterior horn of the internal capsule. FInally it will expand as the corona radiata and go into the cerebrum in the post-central gyrus also known as the primary somatosensory cortex.
Hope you learned a lot from my neuroanatomy video looking at the Dorsal Column-Medial Lemniscus tract. Hope this prepares you well for the USMLE Step 1.
