Thomas Thach and colleagues made the following observation (Thach et al. 1992) as summarized here: Myotomes are a group of muscle fibres innervated by a single spinal nerve composed of sensory and motor roots (see Fig. 1). They are represented mainly coronally in the cerebellar cortex and thus project parallel to the trajectory of the parallel fibers of the cerebellum, the fibres that originate from the most numerous neurons, i.e., the granular cells, in the brains of all mammals (Herculano-Houzel 2009). The parallel fibres project along the length of a lobule (see Fig. 2) and they are situated orthogonal to the climbing fibres (from the inferior olive) of the cerebellum. It is the climbing fibres (one per Purkinje neuron) that adjust the gain of the Purkinje neurons, which are central to efference-copy encoding (Bell et al. 1997; De Zeeuw 2021; Gallistel et al. 2022; Giovannucci et al. 2017; Loyola et al. 2019; Shadmehr 2020; Tehovnik et al. 2021; Wang et al. 2023). Since the parallel fibres are connected to the nuclear cells via Purkinje neurons, a beam of parallel fibres in combination with the climbing fibres control the modulation of the nuclear neurons (in the dentate, interpositus, and fastigial nuclei) to affect the synergistic activation of the muscles of a myotome, thereby controlling all multijointed movements in mammals including humans (Figs. 1 and 2).
The length of a parallel fibre (about 6 mm in monkeys and humans) determines their functional reach (Fig. 2). A 6-mm stretch of cerebellar lobule projects onto a 3-mm beam of cerebellar-nucleus neurons, which is the approximate width of a nucleus. Thus, a beam of Purkinje cells under the influence of a set of parallel/climbing fibres of the same origin and length affects a beam of nucleus neurons across an entire nucleus. Therefore, a nuclear beam would influence muscles across several joints in a limb or the muscles of the eyes, head, neck, arm, or leg. Beams also bridge and link the nuclei, e.g., the two fastigial nuclei coding for different sides of the body. As well, beams link the interpositus and fastigial nuclei for reflex sensitivity and locomotion, and they link the dentate and the interpositus nuclei for reflex sensitivity and reaching. Speech would be controlled by the interpositus and fastigial nuclei. Thus, the parallel fibres with their gain-controllers (the climbing fibres) have access to the entire musculature of the body, which are connected with the neocortex to permit control by consciousness. And this conscious control extends well beyond the necessary cerebellar neurons involved in task performance as evidenced by anatomy, neural recording, and optogenetics so that body posture can be altered to optimize task performance (Hasanbegović 2024).
Figure 1: Myotomes of a generic human (right panel) which represent the exclusive innervation of sensory-motor neurons as segregated in the spinal cord (left panel) spanning from the top of the cord (Cervical 1) to the bottom of the cord (Sacral 5). In coronal section, the cerebellum and its parallel fibres map onto the myotomes (Thach et al. 1992), thereby indicating that the ultimate function of the cerebellum is not cognition but rather sensorimotor control for the purpose of merging the long-term memory-capabilities of neocortex with the motor system (Hasanbegović 2024). Figure from figure 1 of https//physio-pedia.com/Pain_Management_ in_Spinal_Cord_Injury.
Figure 2: Overhead organization of the parallel fibres within a cerebellar lobule of a primate, i.e., the monkey. The inverted homunculi from center to peripheral represent the fastigial, interpositus, and dentate nuclei. Each beam depicts a functional motor unit that maps onto a specific myotome of the body. This arrangement allows for total motor control of the body. From figure 3 of Thach et al. (1992).