The spinocerebellar tract can be broken down into four specific sub-tracts, if you will, known as the dorsal spinocerebellar tract (DST), ventral spinocerebellar tract (VST), cuneocerebellar tract (CT), and the spino-olivary tract (goes to the olives first, inferior olives, and then to the cerebellum).
Remember, when looking at the name of a nerve, you can infer from which organ the nerve originates and where it terminates. What do you see when looking at spinocerebellar tract? Well, immediately, the name tells you that this is an ascending tract, since it goes from the spinal cord to the cerebellum.
Each of the more specific tracts that make up the spinocerebellar tract respond to prioprioception (proprioception is the subconscious awareness of the positioning of your limbs in space).
There is a collection of receptors and fibers that work closely with the spinocerebellar tract to transmit sensory information and motor plans required for reaction to stimuli between the central nervous system (CNS) and peripheral nervous system (PNS).
To produce a motor reaction to chemical, physical, and thermal stimuli, there are: muscle spindles. The muscle spindle is made up of: nuclear bag fibers (these respond to the stretching of skin and muscle tissue), nuclear chain fibers (respond to the beginning of a stretch), and Golgi tendon organs (respond to the stretching of tendons).
The Golgi tendon organs are also a part of the collection of sensory sensors that pick up touch and pressure. Additional information for touch & pressure comes from both sensory receptors and proprioceptors: the Meisner’s corpuscles, Ruffini corpuscles, and Pacinian corpuscles.
All of these pieces form the whole of each tract that compile to produce the entire spinocerebellar tract. Let’s take a closer look, section by section.
Routes of the Individual Spinocerebellar Tracts
Most of the spinocerebellar tracts travel wholly ipsilaterally, meaning they do not decussate, or cross, to the other side of the spinal cord at any point in the transmission of action potentials to the central nervous system. Information passed along the spinocerebellar tracts concerns proprioception, physical touch (reacting to physical, chemical, and thermal stimuli), and sensations of stretching of the skin and muscles.
The individual tracts that make up the whole of the spinocerebellar system are distinguished by the specific proprioceptor and/or sensory receptor the tract is connected to. This connection determines the route the action potential will take to reach the cerebrum and generate the proper motor response.
Dorsal Spinocerebellar Tract
This tract is also known as the posterior spinocerebellar pathway, or “Fleschig’s tract.” Let’s take a look at this route sensory information takes when being sent from the point of stimuli to the brain: the nerves of the DST first flow into the dorsal root ganglion, the first point of synapse and the stop just before entrance to the spinal cord.
The DST is known in this tract as the first order neuron. Secondly, the DSTtravels into the central process, where it synapses on a highly-specific nucleus in the Rexed Lamina VII.
The spinocerebellar tracts are arranged in columnar order, earning them their name “Clarke’s column.” The DST is located on spinal levels C8 down to L2 or L3, depending on how many spinal levels the nerve bundles ascend (normally two). The spinal level on which the nerves synapse before continuing onto the CNS is the second order neuron.
From the second order neuron, the tract will continue ipsilaterally into the dorsal part of the lateral white column and then ascend into the central nervous system
To innervate the cerebellar cortex, the nerves of the spinocerebellar tract pass through a white matter structure called the inferior cerebellar peduncle. Once through, the DST reaches the third order neuron, the cerebellar cortex. From spinal levels L2/3 downward, everything is carried by the ventral spinocerebellar tract.
Ventral Spinocerebellar Tract
The ventralspinocerebellar tract, also known as the anterior spinocerebellar pathway, carries both proprioceptive and cutaneous information from the lower body (spinal levels L5 to T12) and enters the cerebellum via the superior cerebellar peduncles. It is a part of the somatosensory system and runs in parallel with the dorsal spinocerebellar tract.
Structures below L3, down to coccygeal spinal level one, are innervated by the ventral spinocerebellar tract. The nerve fibers of this tract travel the following route: After receiving information of a stimulus from the peripheral process, an action potential is sent first to the dorsal root ganglion (this is a pseudounipolar neuron, and because it is the first point of synapse, it is the first order neuron)
From the first order neuron, the VST travels to the dorsal gray horn for another synapse event. This is now the second order neuron. After synapsing in the dorsal gray horn, the VST does something very different than the DST, and decussates to the contralateral side of the spinal cord before ascending. The VST then travels behind the superior cerebellar peduncles to decussate again to reach the other side of the cerebellum where it will synapse on third order neuron, finally transmitting its neuronal signals carrying information regarding proprioception, touch, and pressure.
The cuneocerebellar tract regulates the transmission of sensory information traveling to and from the head, neck, and upper limbs. Essentially, it controls information being sent to any structures above the spinal level C8, up to C1.
The route the nerve fibers of the cuneocerebellar tract is nearly identical to the other individual sub-tracts of the spinocerebellar tract in the beginning, but takes a unique turn after the dorsal gray horn: Sensory information passed through the cuneocerebellar tract begins in the peripheral process (picking up touch, pressure, and proprioception, of course) onto the central process.
After it reaches the central process and enters the dorsal gray horn, the cuneocerebellar tract ascends ipsilaterally to ultimately reach the medulla oblongata, the structure within the midbrain.
After its ipsilateral journey up to the central nervous system structures, the cuneocerebellar tract travels into a structure inside of the medulla, called the accessory cuneate nucleus (also called the “external cuneate nucleus”).
Stop! Before we continue, here’s a fun and interesting reminder of the connections between the cuneocerebellar tract and a familiar old friend, the nucleus cuneatus (this is directly related to the dorsal column medial lemniscus pathway).
Recall that the nucleus cuneatus and nucleus gracilis are arranged in the following order, the outer nuclei being lateral (away from the midline of the body) and inner nuclei, medial (toward the midline of the body): N.C. N.G. N.G. N.C. These nuclei receive synapses from the dorsal column.
At one point, all of the fibers connected to these nuclei cross to form the internal arcuate fibers. Now, here’s where this all comes together to connect with the cuneocerebellar tract:
The accessory cuneate nucleus gives off fibers that move through the inferior cerebellar peduncles, these are known as the external arcuate fibers. Isn’t that so cool? We sure love seeing all the small interconnections of the major structures of the nervous system.
After these fibers travel through the inferior cerebellar peduncles, they innervate parts of the cerebellar cortex. It is suspected that the cuneocerebellar tract ascends through the fasciculus cuneatus. There is still much to be discovered about the route and synapse behavior of this tract, but this is the core of what researchers know so far.
Inferior Olivary Nuclei
The inferior olivary nuclei are positioned inferiorly to the internal arcuate fibers on either side of the spinal cord. This is another tract that is a little peculiar in the way it behaves, even though it also picks up sensory information regarding touch, pressure, and proprioception.
The tract connected to these nuclei begins at the proprioceptors connected to the peripheral process. The impulse is then sent to the central process, of course, and proceeds to the dorsal root ganglion, the site of the synapse onto the second order neuron.
The tract moves on to decussate through anterior white commissure to then ascend through the spinal cord to inferior olivary nucleus, where it will synapse again. Here’s the crazy part! The tract then decussates again before it enters the cerebellum through the inferior cerebellar peduncles.
Fibers that originate in the inferior olivary nuclei and terminate in the cerebellum are known as “climbing fibers.” These fibers innervate Purkinje cells and form many neuronal connections to provide particularly large action potentials. Further, their role in the inferior olivary nuclei has been discovered to not only play a significant role in motor activity, but also motor learning and memory. (Any fibers other than climbing fibers which are coming from the inferior olivary nucleus are known as “mossy fibers.” We’ll discuss these another time.)
The Role of the Spinocerebellar Tract The spinocerebellar pathway, as you’ve seen, is primarily responsible for the transmission of proprioceptive information from the peripheral nervous system (namely, muscle and joint proprioceptors). That said, it is composed of afferent neurons – those neurons responsible for taking information to the central nervous system, rather than away from it.
All of the functions of the spinocerebellar pathways are subconscious, happening at an involuntary level in order to inform the reactionary voluntary actions. Now that you understand how this process of sensory information processing works, you can move forward into a deeper understanding of the nature of the nervous system’s role in your perception and interaction with the world around you!
- Ninja Nerd Science. (2018, January 8). Ascending tracts | Spinocerebellar tract [Video file]. Retrieved from https://www.youtube.com/watch?v=6gqJMcNpNgA
- Palipana, D., & Hapugoda, S. (n.d.). Spinocerebellar tract | Radiology reference article | Radiopaedia.org. Retrieved December 3, 2019, from https://radiopaedia.org/articles/spinocerebellar-tract?lang=us
- ScienceDirect. (n.d.). Climbing fiber. Retrieved December 16, 2019, from https://www.sciencedirect.com/topics/neuroscience/climbing-fiber
- UT Health | Neuroanatomy Online. (n.d.). Lab 5 – Somatosensory, viscerosensory and spinocerebellar pathways. Retrieved from https://nba.uth.tmc.edu/neuroanatomy/L5/Lab05p15_index.html