The development of vestibular system and related functions in mammals: impact of gravity

Abstract

This chapter reviews the knowledge about the adaptation to Earth gravity during the development of mammals. The impact of early exposure to altered gravity is evaluated at the level of the functions related to the vestibular system, including postural control, homeostatic regulation, and spatial memory. The hypothesis of critical periods in the adaptation to gravity is discussed. Demonstrating a critical period requires removing the gravity stimulus during delimited time windows, what is impossible to do on Earth surface. The surgical destruction of the vestibular apparatus, and the use of mice strains with defective graviceptors have provided useful information on the consequences of missing gravity perception, and the possible compensatory mechanisms, but transitory suppression of the stimulus can only be operated during spatial flight. The rare studies on rat pups housed on board of space shuttle significantly contributed to this problem, but the use of hypergravity environment, produced by means of chronic centrifugation, is the only available tool when repeated experiments must be carried out on Earth. Even though hypergravity is sometimes considered as a mirror situation to microgravity, the two situations cannot be confused because a gravitational force is still present. The theoretical considerations that validate the paradigm of hypergravity to evaluate critical periods are discussed. The question of adaption of graviceptor is questioned from an evolutionary point of view. It is possible that graviception is hardwired, because life on Earth has evolved under the constant pressure of gravity. The rapid acquisition of motor programming by precocial mammals in minutes after birth is consistent with this hypothesis, but the slow development of motor skills in altricial species and the plasticity of vestibular perception in adults suggest that gravity experience is required for the tuning of graviceptors. The possible reasons for this dichotomy are discussed.

Vestibular loss disrupts daily rhythm in rats

“…The signals are integrated and relayed to the supramesencephalic projections by the vestibular nucleus (10). Head movements and gravitational force changes are then encoded and integrated within the vestibular nuclei, which are formed from birth to adulthood in mammals (32). Despite the paucity of evidence of a monosynaptic connection, one main indirect neuronal pathway has been shown to exist from the vestibular nuclei to the SCN.…”

Mechanotransduction as an Adaptation to Gravity

“…These gravity receptors utilize a layer of calcium carbonate (otoconia) lying over the sensory receptor areas. The shearing force produced by the otoconia displaced against the stereocilia of the sensory hair cells allows the detection of linear accelerations, and gravity (72). …”

“…“All together, these results provide further evidence that the gravistatic sensory system has a genetically controlled phase of development and a stimulus-controlled phase for fine-tuning synaptic terminals. Therefore, the level of gravity plays a critical role in fine-tuning of axons and is required for appropriate development of the projections from graviceptors to the brain and spinal cord” (72). …”

The Neurovestibular Challenges of Astronauts and Balance Patients: Some Past Countermeasures and Two Alternative Approaches to Elicitation, Assessment and Mitigation

“…In fact, lack of vestibular function has been employed as an animal model for graviception in space (Jamon, 2014). It is likely that research into space adaptation syndrome could provide insights into clinical vestibular disorders such as vertigo (Clément and Ngo-Anh, 2013) and vice versa.…”