canal, mediate the same pattern of muscle activity. In other words, head tilt in the plane of the right posterior canal, and subsequent left anterior canal, results in excitation of the right superior oblique and superior rectus muscles, and the contralateral correlate of left inferior oblique and inferior rectus muscles. Inhibition occurs in the opposite pairs of eye muscles. The net effect is a downward and left torsional eye movement. Thus, eye movement occurs in the opposite direction of head movement to maintain eye position fixed on the target. We will revisit this phenomenon when discussing examination of canalithiasis. 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!
(up and down). The oblique muscles drive rotational eye movement, with the superior oblique generating movement down and in, and the inferior oblique generating eye movement up and out. When head movement occurs in the plane of the right posterior canal (head tipped back with rotation to the right) VOR pathways from the right posterior canal carry excitatory responses to the ipsilateral superior oblique muscle and contralateral inferior rectus muscle, and simultaneous inhibition to the ipsilateral inferior oblique muscle and contralateral superior rectus muscle. At the same time, the VOR pathways for the contralateral anterior canal, being the coplanar pair of the posterior 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
CERVICAL CONTRIBUTION TO POSTURAL CONTROL
Cervical reflexes (cervico-ocular, cervico-spinal, and cervicocollic) are mediated through cervical afferent inputs
to aid the vestibular system in maintaining gaze stabilization and postural orientation.
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 mechanism is not well understood, but it may be tapped into as a secondary gaze stabilization system when vestibular damage has occurred.
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 forward head motion 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
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
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.
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. Specific information from 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,
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