Vestibulospinal pathways and vestibulospinal reflex The VSR utilizes three neural pathways as outputs from the vestibular system to the anterior horn cells of the skeletal muscles. These pathways are the medial vestibulospinal tract (MVST), which innervates muscles of the trunk and neck, the lateral vestibulospinal tract (LVST), innervating the extremities, and the reticulospinal tract. The reticulospinal tract is a highly collateralized yet poorly defined region of the spinal cord that receives input from all vestibular nuclei, as well as other sensory and motor inputs responsible for postural control (Herdman & Clendaniel, 2014). The LVST and MVST receive inputs from the otoliths and semicircular canals to mediate antigravity motor responses to maintain upright postural control and vertical orientation of
the head in response to head position changes with respect to gravity. The LVST primarily drives reactionary balance responses, such as ankle, hip, and stepping strategies, in response to changes in the center of mass. It is also responsible for eliciting protective extension responses, such as extending legs or arms, when the center of mass is displaced enough to generate head movement toward the ground. In addition, increased flexor activity is stimulated on the opposite side. For example, head and body tilt (or loss of balance) to the right would elicit right arm and leg extension, and left arm and leg flexion. It is clear to see that the primary role of the VSR is to protect the head from hitting the ground – a feature that is very compatible with surviving a fall!
CERVICAL CONTRIBUTION TO POSTURAL CONTROL
Cervical reflexes (cervico-ocular, cervico-spinal, and cervicocollic) are mediated through cervical afferent inputs to aid the Cervico-ocular pathway and cervico-ocular reflex (COR) Inputs from neck proprioceptors interact with the VOR system to aid in stabilizing images on the retina during slow head movements, or very low gain. The functional significance of this Cervicospinal pathway and cervicospinal reflex (CSR) Like the COR, the CSR (also known as the tonic neck reflex) receives inputs from neck proprioceptors to augment the role of the VSR in stabilizing the body during head motion. The pathways that mediate the CSR are an excitatory pathway from the lateral vestibular nucleus, and an inhibitory pathway from the reticular formation. The fundamental role of the CSR is to provide information to the CNS to differentiate head motion that occurs from head- on-neck versus head-on-body. For example, when the body tilts forward, the vestibular system registers Cervicocollic pathway and cervicocollic reflex (CCR) The CCR stabilizes the head on the body. Changes in neck position creating a stretch in neck muscles will result in a
vestibular system in maintaining gaze stabilization and postural orientation.
mechanism is not well understood, but it may be tapped into as a secondary gaze stabilization system when vestibular damage has occurred.
forward head motion in the vertical gravity-referenced direction, subsequently eliciting a protective extension response through the VSR in an effort to maintain postural control. Conversely, when the head is tilted forward on the neck, the same vestibular inputs register forward head motion, but in this case neck proprioceptors also register neck flexion motion. As a result, CSR and VSR inputs cancel one another and the head moves on the neck without unnecessary balance responses, thereby maintaining postural stability.
reflexive contraction of appropriate neck muscles to elicit head righting in the vertical plane.
SENSORY CONTRIBUTION TO POSTURAL CONTROL
Postural control is defined as the body’s ability to maintain the center of mass (COM) within the base of support (BOS) during quiet postures or positions (static postural control) as well as during movement (dynamic postural control; Shumway-Cook & Woollacott, 2011). Normal postural control requires integration of sensory information regarding body position and motion with respect to the environment to elicit effective motor responses. Vestibular system Up to this point, we have focused on the role of the vestibular system in maintaining postural control. It provides the CNS with information regarding rotational and linear head motion and position and is the only sensory structure that provides gravity-referenced information, to stabilize the head and trunk, achieve and maintain vertical orientation, and perceive self- motion. However, the vestibular inputs alone cannot help the Visual system The visual system provides information to the CNS regarding head and body motion and position with respect to the environment, as well as referencing verticality. It also contributes to maintaining postural control during quiet stance, as is seen with the occurrence of increased postural sway when the eyes are closed as compared with eyes open. Another example of the influence of visual inputs for postural control comes from an experiment by Lee and Lishman (1975) where they provided continual environmental oscillations (movement of walls and ceiling) and noted that the neurologically intact subjects
Specific information from vestibular, visual, and somatosensory systems provides the CNS with a different frame of reference regarding body position and motion. The effect of these systems working together triangulates sensory data, allowing the CNS, primarily the cerebellum, to minimize sensory conflict, thereby gaining a clear schema of the body’s orientation in space in order to drive appropriate postural responses. CNS distinguish the environmental context in which motion is occurring to determine if the body is moving through the environment (e.g., walking) versus moving with the environment (e.g., riding in a train), in order to elicit the appropriate postural responses. For that, the CNS requires information from the visual and somatosensory systems. exhibited an increased sway in response to movement of the environment. Consider a familiar example of standing on a train platform and watching a moving train pass in front of you. There is a sense that you are moving, despite maintaining a static standing position on the platform, which comes from the visually mediated sway that is occurring. This example also illustrates that the visual system is unable to accurately distinguish self- motion from motion of the environment. Thus, making that distinction requires additional information from the other sensory systems associated with postural control.
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