When the entire neocortex is ablated in rodents, although they are still able to swim, all the limbs move continuously and asynchronously (Vanderwolf 2006; Vanderwolf et al. 1978). Normal animals by comparison swim with the forelimbs tucked under the chin and with the hindlimbs alternating in sequence as each hindlimb propels an animal forward. It is noteworthy that during swimming, the proprioceptive feedback is mainly from the muscle spindles, which measure muscle length (Gruner and Altman, 1980). Asynchronous movements also occur in rodents if large lesions are made of the midbrain (including the colliculi and tegmentum, Tehovnik 1982, also see Denney-Brown and Fischer 1976), which transect fibres projecting between the neocortex and the brain stem. When such connections are damaged in humans, a vegetative state is induced, especially if the ablation includes the pontine nuclei (Penfield 1975; Plum and Posner 1980), which support neocortical-cerebellar projections (Tehovnik, Patel, Tolias et al. 2021). Declarative information (of the neocortex) is merged with the motor information (of the cerebellum) using neocortical-cerebellar loops, which integrate the feedback from the muscle spindle proprioceptors to update the efference-copy code while perfecting movement sequences (Tehovnik, Hasanbegović, Chen 2024).
Animals (quadrupeds) whose spinal cord has been disconnected from the brain stem retain the step-cycle. This cycle depends on proprioceptive feedback even though the body must be supported, but a spinal cord/brain stem disconnection in primates compromises the step-cycle (Armstrong 1988; Grillner 2009). Unlike stepping, which depends on both muscle spindles and Gogi tendon organs, swimming (as mentioned) relies on the muscle spindles (Gruner and Altman, 1980). Genetically-modified rodents that were made to have deficient muscle spindles are still able to swim (Takeoka et al. 2014). If the spinal cord is transected, however, swimming is abolished, and these animals can no longer save themselves in water. This indicates that for the spinal cord to have full functionality for swimming, it must be connected to the neocortex and cerebellum, as well as to the muscle spindles. This guarantees that proprioceptive feedback is shared with neocortical-cerebellar loops to keep the efference-copy code updated (Tehovnik, Hasanbegović, Chen 2024).
Swimming is often used to prepare astronauts for zero gravity conditions. Much has been learned about both proprioceptive and vestibular adaptation by studying astronauts (Carriot et al. 2021; Demontis et al. 2017; Lawson et al. 2016). The adaptation of returning astronauts takes about 1 week and it depends on modifications of the efference-copy code of neocortical-cerebellar loops and these loops must be updated using current proprioceptive as well as vestibular feedback (Tehovnik, Hasanbegović, Chen 2024).
This is my answer based on my understanding of the question: Swimming/space travel depends on the proprioceptive muscle spindles?
Proprioceptive muscle spindles are essential for both swimming and space travel, though their roles differ significantly in each context. In swimming, these sensory receptors, located within muscles, detect changes in muscle length and send this information to the central nervous system. This feedback mechanism is crucial for swimmers as it helps them adjust their strokes, maintain balance, and execute precise movements. For instance, when a swimmer's arm extends during a stroke, the muscle spindles sense the change in muscle length and provide feedback to ensure the arm moves smoothly and efficiently. This continuous feedback loop is vital for maintaining proper form and preventing injuries, allowing swimmers to perform at their best.
In space travel, the absence of gravity presents unique challenges that make proprioception even more critical. Astronauts rely heavily on proprioceptive muscle spindles to navigate and perform tasks in a microgravity environment. Without the usual gravitational cues, astronauts must depend on their sense of body position and movement to orient themselves and carry out activities. For example, when an astronaut reaches for a tool, the muscle spindles help them understand the position of their arm and hand, enabling them to grasp the tool accurately. This ability to perceive and control body movements without gravity is essential for adapting to the unique conditions of space, such as floating and maneuvering in three dimensions.
In both swimming and space travel, proprioceptive muscle spindles are vital for effective motor control and spatial awareness. They ensure that individuals can perform complex tasks with precision and safety, whether it's gliding through water or navigating the vastness of space. By providing continuous feedback on muscle length and movement, these spindles play a key role in helping humans adapt to and excel in diverse environments.
Dear Alejandra Endrina,
Thank you for your very insightful response. I will share it with my colleagues. Ed Tehovnik.
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