So, here comes the information about two point discrimination, vibration and conscious proprioception (very important stuff!!) over the central process of the alpha-beta axon. As the central process of the alpha-beta axon approaches the spinal cord it travels in what is called the medial division of the dorsal root (this medial group of fibers will be contrasted with other central processes that lie laterally in the dorsal root). Once in the dorsal funiculus, the alpha-beta axon takes off for the medulla, where it synapses. The medulla is the most caudal part of the brainstem (midbrain, pons, medulla), and it lies immediately rostral to the spinal cord. Remember--there has been no synapse in the dorsal root OR spinal cord. Also, the axon does NOT CROSS IN THE SPINAL CORD!! It terminates in the medulla on the same side (IPSILATERAL; ipsi = L., same; latus = side) as its cell body. Cells in nucleus gracilis and cuneatus project to the thalamus, the information is then relayed to somatosensory cortex for perception. More on this later!! Letís stick to the spinal cord for now.
There are two components to the dorsal column system, called fasciculus gracilis and fasciculus cuneatus (fasciculus = L., little bundle; gracilis = slender; cuneatus = wedge). The central process of the alpha-beta fiber travels within the fasciculus gracilis if it arises from dorsal root ganglia T7 and below. In contrast, if the central process of the alpha-beta fiber arises from cells in dorsal roots T6 and above (toward your head), it is part of fasciculus cuneatus (THINK: GRACILIS = LEG AND CUNEATUS = ARM).
Fasciculus gracilis and fasciculus cuneatus are thus comprised of the alpha-beta axons whose cell bodies lie in IPSILATERAL DORSAL ROOT GANGLIA. That is, the cell bodies are on the SAME SIDE as the fasciculi. I have mentioned that fibers in the dorsal column system DO NOT CROSS in the spinal cord and eventually synapse in the medulla. While we will cover the medulla later in the course, you might like to know that axons in fasciculus gracilis terminate in the ipsilateral (to the fasciculus) nucleus gracilis, while fibers in fasciculus cuneatus synapse in ipsilateral nucleus cuneatus (big surprise).
The fasciculus gracilis contains fibers from spinal cord levels lower than fasciculus cuneatus, and fasciculus gracilis lies MEDIAL to fasciculus cuneatus. This lower = medial spatial relationship holds not only for the two fasciculi, but also for the individual fibers in each fasciculi. For example, the most medially placed fiber in fasciculus gracilis arises from the coccygeal dorsal root and the most laterally placed arises from the T7 dorsal root. In the fasciculus cuneatus, the most medially placed fiber arises from dorsal root T6 and the most lateral arises from dorsal root C2 (remember from Gross Anatomy that C1 is purely motor, and therefore does not have a dorsal root ganglion?!!).